Introduction
Stemflow delivers precipitation to the plant root zone more efficiently via
preferential root paths, worm paths and soil macropores, compared with
throughfall, another important element of rainfall redistribution. The
double-funnelling effects of stemflow and preferential flow create “hot
spots” and “hot moments” by enhancing nutrient cycling rates at the
surface soil matrix (Mcclain et al., 2003; Johnson and Lehmann, 2006;
Sponseller, 2007), thus substantially contributing to the formation and
maintenance of “fertile islands” (Whitford et al., 1997),
“resource islands” (Reynolds et al., 1999) or “hydrologic islands”
(Rango et al., 2006). This effect is important for the normal function of
rainfed dryland ecosystems (Wang et al., 2011).
Shrubs are the representative plant functional type (PFT) in dryland
ecosystems. They have developed effective physiological drought tolerance by
reducing water loss, e.g., through adjusting their photosynthetic and
transpiration rate by regulating stomatal conductance and abscisic acid (ABA),
titling their osmotic equilibrium by regulating the concentration of
soluble sugars and inorganic ions, and removing free radicals (Ma et al.,
2004, 2008). The stemflow, a vital ecohydrological flux, is involved in
replenishing soil water at shallow and deep layers (Pressland, 1973),
particularly the root zone (Whitford et al., 1997; Dunkerley, 2000; Yang,
2010), even during light rains (Li et al., 2009). It might allow the endemic
shrubs to remain physically active during drought spells (Navar and Bryan,
1990; Navar, 2011). The stemflow is an important potential source for
available water at rainfed dryland ecosystem (Li et al., 2013). Therefore,
producing stemflow with a greater amount in a more efficient manner might be
an effective strategy to utilize precipitation by reducing the evaporation
loss (Devitt and Smith, 2002; Li et al., 2009), acquire water (Murakami,
2009) and withstand drought (Martinez-Meza and Whitford, 1996). However,
because stemflow occurs in a small amount, some studies neglected the
dynamics of stemflow yield by setting a fixed percentage of incident
precipitation in the range of 1–8 % (Dykes, 1997; Germer et al.,
2006; Hagy et al., 2006) or even ignored stemflow while computing the water
balance of terrestrial ecosystems (Llorens and Domingo, 2007; Zhang et al.,
2016). That underestimated its disproportionately high influence on
xerophytic shrub species (Andersson, 1991; Levia and Frost, 2003; Li, 2011).
Therefore, it is important to quantify the inter- and intra-specific
stemflow yield, to assess the stemflow production efficiency and to
elucidate the underlying bio-/abiotic mechanisms.
Stemflow yield includes the stemflow volume and depth, and it describes the
total flux delivered down to the base of a branch or a trunk. However,
stemflow data are unavailable for comparison of inter-specific differences
caused by variations in the branch architecture, the canopy structure, the
shrub species and the ecozone. Herwitz (1986) introduced the funnelling
ratio (FR), which expressed as the quotient of the volume of stemflow yield
and the product of the base area and the precipitation amount. It indicates
the efficiency with which individual branches or shrubs capture raindrops
and deliver the water to the root zone (Siegert and Levia, 2014). The FR
allows for a comparison of the inter- and intra-specific stemflow yield under
different precipitation conditions. However, the FR does not provide a good
connection between hydrological processes (e.g., rainfall redistribution)
and the plant growth processes (e.g., biomass accumulation and allocation).
Recently, Yuan et al. (2016) introduced the parameter of stemflow
productivity (SFP), expressed as the volume of stemflow yield per unit of
branch biomass. The SFP describes the efficiency by comparing the stemflow
yield of unit biomass increment at different sized branches. Hence, it is
necessary to combine the results of stemflow volume, depth, percentage of
incident precipitation, FR and SFP for a comprehensive description of the
inter- and intra-specific stemflow yield and efficiency at branch and shrub scales.
The precipitation amount has been generally recognized as the single most
influential rainfall characteristic affecting stemflow production (Clements,
1972; André et al., 2008; Van Stan II et al., 2014). However, as to biotic
mechanisms, although the canopy structure (Mauchamp and Janeau, 1993;
Crockford and Richardson, 2000; Pypker et al., 2011) and branch architecture
(Herwitz, 1987; Murakami, 2009; Carlyle-Moses and Schooling, 2015) have been
studied for years, the most important plant traits vary with location and
shrub species and have not yet been determined. The effects of the leaves
have been studied more recently at a smaller scale, e.g., leaf orientation
(Crockford and Richardson, 2000), shape (Xu et al., 2005), arrangement
pattern (Owens et al., 2006), pubescence (Garcia-Estringana et al., 2010),
area (Sellin et al., 2012), epidermis microrelief (Roth-Nebelsick et al.,
2012), amount (Li et al., 2016), or biomass (Yuan et al., 2016).
Comparisons of stemflow yield during summer (the growing or foliated season)
and winter (the dormant or defoliated season) generally indicate negative
effects of leaves because more stemflow occurs during the leafless period
(Dolman, 1987; Masukata et al., 1990; Neal et al., 1993; Mużyło et
al., 2012). However, both negligible and positive effects have also been
confirmed by Martinez-Meza and Whitford (1996), Deguchi et al. (2006) and
Liang et al. (2009). The validity of these conflicting findings has been
called into question as a result of the seasonal variation of meteorological
conditions and plant traits, e.g., wind speed (André et al., 2008),
rainfall intensity (Dunkerley, 2014a, b),
air temperature and consequent precipitation type (snow-to-rain vs. snow) (Levia and Underwood, 2004).
Moreover, they ignore the effects of the exposed stems at leafless period,
which substitute the leaves to intercept raindrops and might play a
significant role in stemflow production. Furthermore, although the rainfall
simulator made an identical and gradient change of rainfall
characteristics possible, the laboratory experiment ignored the dynamics of rainfall
characteristics and meteorological features (e.g., wind speed, vapor
pressure deficit, air temperature and humidity) during rainfall events
at field conditions. Therefore, a controlled field experiment with the
foliated and manually defoliated plants under the same stand conditions is
needed to resolve these uncertainties.
Location of the experimental stands and facilities for stemflow
measurements of C. korshinskii and S. psammophila at the
Liudaogou catchment in the Loess Plateau of China.
In this study, the branch stemflow volume (SFb), the shrub
stemflow depth (SFd), the stemflow percentage of the incident
precipitation amount (SF %), the SFP and the FR were measured in two
xerophytic shrub species (C. korshinskii and S. psammophila)
during the 2014 and 2015 rainy seasons. Furthermore, a
controlled field experiment with foliated and manually defoliated shrubs was
also conducted for the two shrub species during the 2015 rainy season. The
detailed objectives were to (1) quantify the inter- and intra-specific
stemflow yield (SFb, SFd and SF %) and
efficiency (SFP and FR) at different precipitation levels; (2) identify the
most influential meteorological characteristics affecting stemflow yield
and (3) investigate the biotic influential mechanism of plant traits
especially at the finer leaf scale. Given that only the aboveground
ecohydrological process was involved, we focused on stemflow in this study,
its interaction with soil moisture would be discussed in next study. The
achievement of these research objectives would advance our understanding of
the influential mechanism of stemflow production, its ecological importance
for dryland shrubs, and the significance of leaves from an ecohydrological perspective.
Materials and methods
Study area
This study was conducted at the Liudaogou catchment (110∘21′–110∘23′ E,
38∘46′–38∘51′ N) in Shenmu County in the Shaanxi Province of
China. It is 6.9 km2 and 1094–1273 m a.s.l. (above sea level). This
area has a semiarid continental climate with well-defined rainy and dry
seasons. The mean annual precipitation (MAP) between 1971 and 2013 is 414 mm,
with approximately 77 % of the annual precipitation amount occurring
from July to September (Jia et al., 2013). The potential evaporation is
1337 mm yr-1, and the mean annual temperature is 9.0 ∘C
(Zhao et al., 2010). The coldest and warmest months are January and July,
with an average monthly temperature of -9.7 and 23.7 ∘C,
respectively. The two soil types Aeolian sandy soil and Ust-Sandiic Entisol
dominate this catchment (Jia et al., 2011). Soil particles consist
of 11.2–14.3 % clay, 30.1–44.5 % silt and 45.4–50.9 % sand
in terms of the soil classification system of United States Department of
Agriculture (Zhu and Shao, 2008). The original plants are scarcely present,
except for very few surviving shrub species, e.g., Ulmus macrocarpa,
Xanthoceras sorbifolia, Rosa xanthina or
Spiraea salicifolia. The currently predominant shrub species
were planted decades ago, e.g., S. psammophila, C. Korshinskii
or Amorpha fruticosa, and the predominant grass
species include Medicago sativa, Stipa bungeana,
Artemisia capillaris, Artemisia sacrorum, etc. (Ai et al., 2015).
Two representative experimental stands of C. Korshinskii and
S. psammophila were established in the southwest of the Liudaogou
catchment in this study (Fig. 1). As the endemic shrub species in arid and
semiarid northern China, they were generally planted for wind proofing and
dune stabilizing. Both C. korshinskii and S. psammophila
are multi-stemmed shrubs that have an inverted cone canopy and no trunk,
with the branches running obliquely from the base. C. korshinskii
usually grows to 2 m and has pinnate compound leaves with 12–16 foliates in
an opposite or sub-opposite arrangement (Wang et al., 2013). The leaf of
C. korshinskii is concave and lanceolate shaped, with an acute leaf
apex and an obtuse base. Both sides of the leaves are densely sericeous with
appressed hairs (Liu et al., 2010). In comparison, S. psammophila
usually grows to 3–4 m, and has an odd number of strip shaped leaves with
2–4 mm in width and 40–80 mm in length. The young leaves are pubescent and
gradually become subglabrous (Chao and Gong, 1999). These two shrub species
were planted approximately 20 years ago, and the two stands shared a
similar slope of 13–18∘, a size of 3294–4056 m2 and an
elevation of 1179–1207 m a.s.l. However, the C. korshinskii
experimental stand has a 224∘ aspect with a loess ground surface,
whereas the S. psammophila experimental stand has a 113∘
aspect with a sand ground surface.
Field experiments
Field experiments were conducted during the rainy seasons of 2014 (1 July
to 3 October) and 2015 (1 June to 30 September) to measure the
meteorological characteristics, plant traits and stemflow. To avoid the
effects of gully micro-geomorphology on meteorological recording, we
installed an Onset® (Onset Computer Corp., Bourne, MA, USA)
RG3-M tipping bucket rain gauge (0.2 mm per tip) at each experimental stand.
Three 20 cm diameter rain gauges were placed around to adjust the inherent
underestimating of automatic precipitation recording (Groisman and Legates,
1994). Then, the rainfall characteristics, e.g., rainfall duration (RD; h),
rainfall interval (RI; h), the average rainfall intensity (I, mm h-1),
the maximum rainfall intensity in 5 min (I5, mm h-1),
10 min (I10, mm h-1) and 30 min (I30, mm h-1)
could be calculated accordingly. In this study, the
individual rainfall events were greater than 0.2 mm and separated by a
period of at least 4 h without rain (Giacomin and Trucchi, 1992).
Furthermore, a meteorological station was also installed at each experimental
stand to record other meteorological characteristics (Fig. 1), e.g., wind
speed (WS; m s-1) and wind direction (WD; ∘) (Model 03002, R. M. Young
Company, Traverse City, Michigan, USA), the air
temperature (T; ∘C) and humidity (H; %) (Model HMP 155,
Vaisala, Helsinki, Finland), and solar radiation (SR; kW m-2)
(Model CNR 4, Kipp & Zonen B. V., Delft, the Netherlands).
Moreover, raindrops attributes, including raindrop diameter (D, mm),
raindrop terminal velocity (V, m s-1), and raindrop
inclination angle from the vertical (A, ∘), were also computed to
investigate the possible effects of raindrop striking, the oblique and
wind-driven rain on stemflow yield and efficiency.
C. korshinskii and S. psammophila, as modular organisms
and multi-stemmed shrub species, have branches that seek their own survival
goals and compete with each other for light and water (Firn, 2004; Allaby,
2010). They were ideal experiment objects to conduct a stemflow study at the
branch scale. Therefore, we focused on branch stemflow and ignored the
canopy variance by experimenting on sample shrubs that had a similar canopy
structure. Four mature shrubs were selected for C. korshinskii
(designated as C1, C2, C3 and C4) and S. psammophila (designated as S1,
S2, S3 and S4) for the stemflow measurements. They had isolated
canopies, similar intra-specific canopy heights and areas, e.g., 2.1 ± 0.2 m
and 5.1 ± 0.3 m2 for C1–C4, and 3.5 ± 0.2 m and
21.4 ± 5.2 m2 for S1–S4. We measured the morphological characteristics
of all the 180 branches of C1–C4 and all the 261 branches of S1–S4,
including the branch basal diameter (BD; mm), branch length (BL; cm) and
branch inclination angle (BA; ∘). The leaf area index (LAI) and
the foliage orientation (MTA, the mean tilt angle of leaves) were measured
using the LiCor® (LiCor Biosciences Inc., Lincoln, NE, USA)
2200C plant canopy analyzer approximately twice a month.
A total of 53 branches of C. korshinskii (17, 21, 7, 8 for the BD
categories of 5–10, 10–15, 15–18 and > 18 mm,
respectively) and 98 branches of S. psammophila (20, 30, 20 and
28 branches at the BD categories of 5–10, 10–15, 15–18 and
> 18 mm, respectively) were selected for stemflow
measurements following these criteria: (1) no intercrossing stems, (2) no
turning point in height from branch tip to the base (Dong et al., 1987)
and (3) representativeness in amount and branch size. Stemflow was collected
using aluminum foil collars, which were more accurate than the spiral tubes
because the tubes outlet were more liable to be blocked by vegetation litter
(Wright, 1977; Durocher, 1990). The collar was fitted around the entire
branch circumference and close to the branch base and sealed by neutral
silicone caulking (Fig. 1). Nearly all sample branches were selected on the
skirts of the crown, which were more convenient for installation and
limited the amount of shading by other branches lying above.
Associated with the limited external diameter of foil collars, they
minimized the accessing of the precipitation and throughfall (both free and
released). A 0.5 cm diameter PVC hose led the stemflow to lidded containers.
The collars and hoses were checked periodically against any leakage and
blockage. The stemflow was measured within 2 h after the rainfall
ended during the daytime; if the rainfall ended at night, we took the
measurement early the next morning. After completing measurements, we returned
stemflow back to the branch base to mitigate the unnecessary drought stress
for the sample branches. By doing so, we tried best to mitigate the
influences of the precipitation and throughfall, which might lead to an
overestimation of stemflow yield and efficiency. Nevertheless, these errors
might not be eradicated at field conditions after all. The careful
experiment practices were especially needed in this study, and more
thoughtful experiment designs were required in future studies.
The controlled field experiment for stemflow yield between the
foliated and manually defoliated shrubs.
The controlled field experiment with foliated and manually defoliated shrubs
was conducted during the 2015 rainy season for C. korshinskii (five
rainfall events from 18 to 30 September) and for S. psammophila
(10 rainfall events from 2 August to 30 September) (Fig. 2).
Considering the workload to remove all the leaves of 85 branches on
C. korshinskii (designated as C5) and 94 branches on S. psammophila
(designated as S5) nearly twice a month, only one shrub
individual was selected with similar intra-specific canopy height and area
(2.1 m and 5.8 m2 for C5, 3.3 m and 19.9 m2 for S5) as other
foliated experimental shrubs. A total of 10 branches of C5 (3, 3 and
4 branches at the BD categories 5–10, 10–15 and > 15 mm),
and 17 branches of S5 (4, 5 and 7 branches at the BD categories 5–10,
10–15 and > 15 mm) were selected for stemflow measurements.
According to the in situ measurement of branch morphology and the laboratory
measurement of biomass, these sample branches had similar BD, BL, BA and biomass of leaves
(BML)
with those in the foliated shrubs (C1–C4 and S1–S4) (see the values at
Sect. 3.2). Given a limited
amount of sample branches and rainfall events, the experimental results were
just used for a comparison with those of the foliated shrubs, but not for a
statistical analysis with meteorological characteristics and plant traits.
If not specifically stated, it is important to notice that the stemflow yield
and efficiency in this study referred to those of the foliated shrubs.
Another three shrubs of each species were destructively measured for biomass
and leaf traits. They had similar canopy heights and areas as those of the
shrubs for which the stemflow was measured, and were designated as C6–C8
(2.0–2.1 m and 5.8–6.8 m2) and S6–S8 (3.0–3.4 m and 15.4–19.2 m2).
Therefore, the development of allometric models could be developed for
estimating the corresponding biomass and leaf traits of C1–C5 and S1–S5
(Levia and Herwitz, 2005; Siles et al., 2010a, b; Stephenson et al., 2014).
A total of 66 branches for C6–C8 and 61 branches for S6–S8 were measured
once during mid-August for the biomass of leaves and stems (BML and BMS, g),
the leaf area of the branches (LAB, cm2), and the leaf numbers of the
branches (LNB), when the shrubs showed maximum vegetative growth. The BML
and BMS were weighted after oven-drying for 48 h. The detailed
measurements have been reported in Yuan et al. (2016). The validity of the
allometric models was verified by measuring another 13 branches of C6–C8 and
14 branches of S6–S8.
Calculations
The raindrop attributes (D, V and A) were calculated on the basis of the
best-fit equations developed from rainfall intensity and wind speed (Laws
and Parsons, 1943; Gunn and Kinzer, 1949; Herwitz and Slye, 1995; Van Stan II
et al., 2011; Carlyle-Moses and Schooling, 2015).
D=2.23⋅(0.03937⋅I)0.102V=(3.378⋅ln(D))+4.213tanA=WS/V,
where D is the average raindrop diameter (mm), V is the terminal raindrop
velocity (m s-1), A is the raindrop inclination angle from
the vertical (∘), I is the average intensity (mm h-1) and
WS is the average wind speed (m s-1).
Biomass and leaf traits were estimated by allometric models as an
exponential function of BD (Siles et al., 2010a, b; Jonard et al., 2006):
PTe=a⋅BDb,
where a and b are constants, and
PTe refers to the estimated plant traits BML, BMS,
LAB and LNB. The other plant traits could be calculated accordingly,
including individual leaf area of branch (ILAB = 100 ⋅ LAB/LNB, mm2),
and the percentage of stem biomass to that of branch (PBMS == BMS/(BML + BMS) ⋅ 100 %,
%). Furthermore, the total stem-surface area of individual
branch (SA) was computed representing by that of the main stem, which was
idealized as the cone (SA = π ⋅ BD ⋅ BL/20, cm2). Therefore,
specific surface area represented with LAB (SSAL = LAB/(BML + BMS),
cm2 g-1) and in SA (SSAS = SA/(BML + BMS),
cm2 g-1) could be calculated. It was important to
notice that this method underestimated the real stem-surface area by
ignoring the collateral stems and assuming main stem as the standard corn,
and therefore the SA and SSAS would not feed into the statistical analysis, but rather be
applied
to reflect a general correlation with SFb in this study.
In this study, stemflow yield was defined as the stemflow volume production
of branch (hereafter “stemflow production”, SFb, mL), the
equivalent water depth on basis of shrub canopy area (hereafter “stemflow
depth”, SFd, mm) and the stemflow percentage of the incident
precipitation amount (hereafter “stemflow percentage”, SF %, %):
SFd=10⋅∑i=1nSFbi/CASF%=SFd/P⋅100%,
where SFbi is the stemflow volume of branch i (mL),
CA is the canopy area (cm2), n is the number of branches and P is the
incident precipitation amount (mm).
SFP (mL g-1) was expressed as the
SFb (mL) of unit branch biomass (g) and represented the
stemflow efficiency of different-sized branches in association with a biomass
allocation pattern:
SFP=SFb/(BML+BMS).
The FR was computed as the quotient of SFb
and the product of P and BBA (branch basal area; cm2) (Herwitz, 1986).
The value of (P ⋅ BBA) equals to the precipitation amount that would have
been caught by the rain gauge occupying the same basal area in a clearing. A
FR with a value greater than 1 indicated a positive effect of the canopy on
the stemflow yield (Carlyle-Moses and Price, 2006):
FR=10⋅SFb/(P⋅BBA).
Data analysis
A Pearson correlation analysis was performed to test the relationship
between SFb and each of the meteorological characteristics (P,
RD, RI, I, I5, I10, I30, WS, T, H, SR, D, V and A) and plant
traits (BD, BL, BA, LAB, LNB, ILAB, BML, BMS and PBMS). Significantly
correlated variables were further tested with a partial correlation analysis
for their separate effects on SFb. Then, the qualified
variables were fed into a stepwise regression with forward selection to
identify the most influential bio-/abiotic factors (Carlyle-Moses and
Schooling, 2015; Yuan et al., 2016). Similar to a principal component
analysis and ridge regression, stepwise regression was commonly used because
it got a limited effect of multicollinearity (Návar and Bryan, 1990;
Honda et al., 2015; Carlyle-Moses and Schooling, 2015). Moreover, we
excluded variables that had a variance inflation factor (VIF) greater than 10
to minimize the effects of multicollinearity (O'Brien, 2007), and kept
the regression model having the least AIC (Akaike information criteria) values and largest R2.
The separate contribution of individual variables to
stemflow yield and efficiency was computed by the method of variance
partitioning. The same analysis methods were also applied to identify the
most influential bio-/abiotic factors affecting SFP and FR. The level of
significance was set at 95 % confidence interval (p = 0.05).
The SPSS 20.0 (IBM Corporation, Armonk, NY, USA), Origin 8.5 (OriginLab
Corporation, Northampton, MA, USA) and Excel 2013 (Microsoft Corporation,
Redmond, WA, USA) were used for data analysis.
Results
Meteorological characteristics
Stemflow was measured at 36 rainfall events in this study, 18 events (209.8 mm)
in 2014 and 18 events (205.3 mm) in 2015, which accounted for 32.7 and
46.2 % of total rainfall events, and 73.1 and 74.9 % of total
precipitation amount during the experimental period of 2014 and 2015,
respectively (Fig. 3). There were 4, 7, 10, 5, 4 and 6 rainfall events at
precipitation categories of ≤ 2, 2–5, 5–10, 10–15, 15–20, and > 20 mm,
respectively. The average rainfall intensity of
incident rainfall events was 6.3 ± 1.5 mm h-1, and the
average value of I5, I10 and I30 were 20.3 ± 3.9,
15.0 ± 2.9 and 9.2 ± 1.6 mm h-1, respectively. RD and RI were averaged
5.5 ± 1.1 and 63.1 ± 8.2 h. The average T, H, SR, WS and WD
were 16.5 ± 0.5 ∘C, 85.9 ± 2.2 %, 48.5 ± 11.2 kw m-2,
2.2 ± 0.2 m s-1 and 167.1 ± 13.9, respectively. As to the raindrop attributes, D, V and A were
averaged 1.8 ± 0.4 mm, 6.1 ± 0.1 m s-1 and 19.6 ± 1.2∘, respectively.
Meteorological characteristics of rainfall events for stemflow
measurements during the 2014 and 2015 rainy seasons.
Verification of the allometric models for estimating the biomass and
leaf traits of C. korshinskii. BML and BMS refer to the biomass of
the leaves and stems, respectively, and LAB and LNB refer to the leaf area
and the number of branches, respectively.
Species-specific variation of plant traits
Allometric models were developed to estimate the biomass and leaf traits of
the branches of C. korshinskii and S. psammophila measured
for stemflow. The estimation quality was verified by linear regression. As
shown in Fig. 4, the regression of LAB, LNB, BML and BMS of C. korshinskii
had an approximately 1 : 1 slope (0.99 for the biomass indicators
and 1.04 for the leaf traits) and an R2 value of 0.93–0.95.
According to Yuan et al. (2016), the regression of S. psammophila
had a slope of 1.13 and an R2 of 0.92. Therefore, those allometric models were appropriate.
Comparison of leaf traits, branch morphology and biomass indicators
of C. korshinskii and S. psammophila.
Plant traits
C. korshinskii (categorized by BD, mm)
S. psammophila(categorized by BD, mm)
5–10
10–15
15–18
> 18
Avg. (BD)
5–10
10–15
15–18
> 18
Avg. (BD)
Leaf traits
LAB (cm2)
1202.7
2394.5
3791.2
5195.2
2509.1 ± 1355.3
499.2
1317.7
2515.2
3533.6
1797.9 ± 1118.0
LNB
4787
11 326
20 071
29 802
12 479 ± 8409
392
1456
3478
5551
2404 ± 1922
ILAB (mm2)
25.4
21.3
18.9
17.5
21.9 ± 3.0
135.1
93.1
72.6
64.3
93.1 ± 27.8
SSAL (cm2 g-1)
22.8
17.3
14.3
12.6
18.2 ± 0.5
18.4
13.6
10.8
8.6
12.7 ± 0.4
SSAS (cm2 g-1)
3.4
2.3
1.9
1.6
2.5 ± 0.1
10.4
5.4
3.3
1.9
5.1 ± 0.3
Branch
BD (mm)
8.17
12.49
16.61
20.16
12.48 ± 4.16
7.91
12.48
16.92
19.76
13.73 ± 4.36
morphology
BL (cm)
137.9
160.3
195.9
200.7
161.5 ± 35.0
212.5
260.2
290.4
320.1
267.3 ± 49.7
BA (∘)
63
56
63
64
60 ± 18
64
63
51
60
60 ± 20
SA (cm2)
176.8
314.1
508.6
630.7
326.1 ± 20.6
268.0
514.1
827.7
1312.3
711.0 ± 38.9
Biomass
BML (g)
13.9
19.0
30.2
41.4
19.9 ± 10.8
5.4
18.0
40.0
61.3
27.9 ± 20.7
indicators
BMS (g)
62.9
121.4
236.4
375.8
141.1 ± 110.8
23.0
81.4
188.5
295.5
130.7 ± 101.4
PBMS (%)
82.0
86.3
88.7
90.0
85.6 ± 3.1
80.8
81.8
82.5
82.8
81.9 ± 0.8
Note: LAB and LNB are leaf area and number of branch, respectively.
ILAB is individual leaf area of branch. SSAL and SSAS are the specific surface
area representing with LAB and SA, respectively. BD, BL and BA are average branch
basal diameter, length and angle, respectively. SA is the surface area of stems.
BML and BMS are biomass of leaves and stems, respectively. PBMS is the percentage
of stem biomass to that of branch. The average values mentioned above are
expressed as the means ± SE.
C. korshinskii had a similar average branch size and angle, but a
shorter branch length than did S. psammophila, e.g., 12.5 ± 4.2 mm
vs. 13.7 ± 4.4 mm, 60 ± 18∘ vs. 60 ± 20∘, and
161.5 ± 35.0 cm vs. 267.3 ± 49.7 cm, respectively. Regarding branch
biomass accumulation, C. korshinskii had a smaller BML (an average of 19.9 ± 10.8 g) and a
larger BMS (an average 141.1 ± 110.8 g) than did S. psammophila
(an average of 27.9 ± 20.7 and 130.7 ± 101.4 g,
respectively). Both the BML and BMS increased with increasing branch size
for these two shrub species. When expressed as a proportion,
C. korshinskii had a larger PBMS than did S. psammophila in all the
BD categories. The PBMS-specific difference increased with an increasing
branch size, ranging from 1.2 % for the 5–10 mm branches to 7.2 % for
the > 18 mm branches.
An increase in LAB and LNB, and a decrease in ILAB, SSAL and SSAS were
observed with increasing branch size for these two shrub species. But at
each BD level, C. korshinskii had on average a larger LAB
(2509.1 ± 1355.3 cm2), LNB (12 479 ± 8409) and SSAL
(18.2 ± 0.5 cm2 g-1), but a smaller ILAB (21.9 ± 3.0 mm2)
and SSAS (2.5 ± 0.1 cm2 g-1) than did
S. psammophila (1797.9 ± 1118.0 g, 2404 ± 1922,
12.7 ± 0.4 cm2 g-1, 93.1 ± 27.8 mm2 and
5.1 ± 0.3 cm2 g-1, respectively) (Table 1). The
inter-specific differences in the leaf traits decreased with increasing
branch size. The largest difference occurred for the 5–10 mm branches, e.g.,
LNB and LAB were 12.2-fold and 2.4-fold larger for C. korshinskii,
and ILAB was 5.3-fold larger for S. psammophila.
In the controlled field experiment, the defoliated sample branches of
C. korshinskii and S. psammophila had similar branch
morphology and BML with those of the foliated branches. The average BD, BL,
BA and BML were 10.5 ± 4.4 mm, 168.5 ± 39.5 cm, 65 ± 15∘
and 22.2 ± 11.6 g in C5, and 14.8 ± 6.4 mm,
258.6 ± 39.0 cm, 50 ± 23∘ and 27.3 ± 22.1 g in S5, respectively.
Stemflow yield of the foliated and defoliated C. korshinskii and S. psammophila
In this study, stemflow yield was expressed as SFb on the
branch scale and SFd and SF % on the shrub scale. For the
foliated shrubs, SFb was averaged 290.6 and 150.3 mL for
individual branches of C. korshinskii and S. psammophila,
respectively, per incident rainfall events during the 2014 and 2015 rainy
seasons. The SFb was positively correlated with the branch
size and precipitation for these two shrub species. As the branch size
increased, SFb increased from the average of 119.0 mL for the
5–10 mm branches to 679.9 mL for the > 18 mm branches of
C. korshinskii and from 43.0 to 281.8 mL for the corresponding
BD categories of S. psammophila. However, with increasing
precipitation, a larger intra-specific difference in SFb was
observed, which increased from the average of 28.4 mL during rains ≤ 2 mm
to 771.4 mL during rains > 20 mm for C. korshinskii
and from 9.0 to 444.3 mL for the corresponding precipitation categories
of S. psammophila. The SFb varied significantly for
different rainfall characteristics and plant traits. The average
SFb of 2375.9 mL occurred for the > 18 mm branches
of C. korshinskii during rains > 20 mm in the 2014 and
2015 rainy seasons. However, the SF of 6.8 mL occurred for the 5–10 mm branches
during rains ≤ 2 mm. Comparatively, a maximum and minimum
SFb of 2097.6 and 1.8 mL occurred for S. psammophila
under similar bio-/abiotic conditions.
C. korshinskii produced a larger SFb than did
S. psammophila for all BD and precipitation categories, and the
inter-specific differences in SFb also varied substantially
with the rainfall characteristics and the plant traits. A maximum difference
of 4.3-fold larger for the SFb of C. korshinskii was
observed for the > 18 mm branches during rains ≤ 2 mm at the
2014 and 2015 rainy seasons. As the precipitation increased, the
SFb-specific difference decreased from 3.2-fold larger for
C. korshinskii during rains ≤ 2 mm to 1.7-fold larger during
rains > 20 mm. The largest SFb-specific difference
occurred for the 5–10 mm branches in almost all precipitation categories,
but no clear trend of change was observed with increasing branch size (Table 2).
Comparison of stemflow yield (SFb, SFd and
SF %) between the foliated C. korshinskii and S. psammophila.
Intra- and inter-specific
Stemflow
BD
Precipitation categories (mm)
Avg.
differences
indicators
categories
≤ 2
2–5
5–10
10–15
15–20
> 20
(P)
(mm)
Intra-specific differences in
SFb (mL)
5–10
10.7
29.8
73.5
109.9
227.6
306.1
119.0
C. korshinskii (CK)
10–15
26.0
64.0
166.1
236.0
478.6
689.7
262.4
15–18
44.3
103.3
279.9
416.6
826.0
1272.3
464.5
> 18
69.5
145.4
424.4
631.4
1226.9
1811.7
679.9
Avg. (BD)
28.4
67.3
180.6
264.6
529.2
771.4
290.6
SFd (mm)
n/a
0.1
0.2
0.6
0.9
1.9
2.6
1.0
SF % (%)
n/a
5.8
6.6
8.8
7.5
10.1
8.9
8.0
Intra-specific differences in
SFb (mL)
5–10
2.8
8.9
28.8
47.2
66.5
120.0
43.0
S. psammophila (SP)
10–15
7.6
23.2
76.6
134.6
188.3
353.5
121.8
15–18
12.0
35.9
121.6
223.4
319.4
592.6
201.5
> 18
16.2
52.3
165.5
289.2
439.6
860.4
281.8
Avg. (BD)
9.0
28.0
91.6
162.2
234.8
444.3
150.3
SFd (mm)
n/a
< 0.1
0.1
0.5
0.9
1.3
2.2
0.8
SF % (%)
n/a
0.7
3.0
6.1
6.8
7.2
7.9
5.5
Inter-specific differences
SFb
5–10
3.8
3.3
2.6
2.3
3.4
2.6
2.8
(the ratio of the stemflow yield
10–15
3.4
2.8
2.2
1.8
2.5
2.0
2.2
of CK to that of SP)
15–18
3.7
2.9
2.3
1.9
2.6
2.2
2.3
> 18
4.3
2.8
2.6
2.2
2.8
2.1
2.4
Avg. (BD)
3.2
2.4
2.0
1.6
2.3
1.7
1.9
SFd
n/a
8.5
2.2
1.3
1.0
1.5
1.2
1.3
SF %
n/a
8.3
2.2
1.4
1.1
1.4
1.1
1.4
Note: BD is the branch basal diameter; P is the precipitation
amount; CK and SP are the abbreviations of C. korshinskii and
S. psammophila, respectively. n/a means not applicable.
SFd and SF % averaged 1.0 mm and 8.0 % per incident
rainfall events during the 2014 and 2015 rainy seasons for individual
C. korshinskii shrubs, and 0.8 mm and 5.5 % for individual
S. psammophila shrubs, respectively. These parameters increased
with increasing precipitation, ranging from 0.09 mm and 5.8 % during rains
≤ 2 to 2.6 mm and 8.9 % during rains > 20 mm for
C. korshinskii, and from less than 0.01 mm and 0.7 % to 2.2 mm
and 7.9 % for the corresponding precipitation categories of
S. psammophila, respectively. Additionally, the individual
C. korshinskii shrubs had a larger stemflow yield than did
S. psammophila in all precipitation categories. The differences in
SFd and SF % maximized as an 8.5- and 8.3-fold larger for
C. korshinskii during rains ≤ 2 mm and decreased with
increasing precipitation to 1.2- and 1.1-fold larger during rains > 20 mm.
While comparing the intra-specific difference of SFb between
different leaf states, SFb of the defoliated S. psammophila
was 1.3-fold larger than the foliated S. psammophila
on average, ranging from 1.1-, 1.0- and 1.4-fold larger for
the 5–10, 10–15 and > 15 mm branches, respectively. A
larger difference was noted during lighter rains (Table 3). On the contrary,
SFb of the defoliated C. korshinskii was averaged
2.5-fold smaller than the foliated C. korshinskii at all
rainfall events. Except for a 1.2-fold larger at the 5–10 mm branches, the
3.3-fold smaller SFb was measured at the 10–15 mm and
> 15 mm branches of the defoliated C. korshinskii as opposed to
the foliated C. korshinskii (Table 3). While comparing the
SFb-specific difference at the same leaf states, a smaller
SFb of the foliated S. psammophila was noted than that
of
the foliated C. korshinskii. However, SFb of the
defoliated S. psammophila was 2.0-fold larger than that of the
defoliated C. korshinskii on average at nearly all BD categories
except for the 5–10 mm branches (Table 3).
Comparison of stemflow yield (SFb) of the foliated and
manually defoliated C. korshinskii and S. psammophila.
Leaf
BD
C. korshinskii
S. psammophila
SFb (CK)/SFb (SP)
states
categories
Incident precipitation amount (mm)
Avg.
Incident precipitation amount (mm)
Avg.
Incident precipitation amount (mm)
Avg.
(mm)
1.7
6.7
6.8
7.6
22.6
(P)
1.7
6.7
6.8
7.6
22.6
(P)
1.7
6.7
6.8
7.6
22.6
(P)
Foliated
5–10
12.9
85.1
93.0
77.7
254.8
104.7
3.6
32.1
55.1
40.6
140.7
46.9
3.6
2.7
1.7
1.9
1.8
2.2
10–15
28.6
197.0
274.6
190.1
694.3
276.9
10.1
67.7
141.5
119.6
351.4
130.8
2.8
2.9
1.9
1.6
2.0
2.1
> 15
51.0
382.3
616.0
370.7
1225.7
529.1
16.6
112.5
279.9
272.9
721.3
279.6
3.1
3.4
2.2
1.4
1.7
1.9
Avg. (BD)
30.2
221.5
317.5
211.4
708.8
297.9
11.9
82.4
191.6
178.6
489.6
186.6
2.5
2.7
1.7
1.2
1.4
1.6
Defoliated
5–10
17.3
87.3
116.7
85.7
264.7
114.3
4.8
22.3
46.7
43.5
152.7
52.4
3.6
3.9
2.5
2.0
1.7
2.2
10–15
11.0
50.0
65.3
50.0
151.0
65.5
12.0
72.4
159.2
118.2
396.8
129.0
0.9
0.7
0.4
0.4
0.4
0.5
> 15
14.7
105.5
183.3
102.7
504.0
182.0
28.2
177.8
460.1
326.0
947.3
358.7
0.5
0.6
0.4
0.3
0.5
0.5
Avg. (BD)
13.2
83.4
121.8
79.4
306.6
120.9
17.9
110.2
288.6
198.4
626.3
223.3
0.7
0.8
0.4
0.4
0.5
0.5
SFb (def)/
5–10
1.3
1.0
1.3
1.1
1.0
1.2
1.3
0.7
0.8
1.1
1.1
1.1
n/a
n/a
n/a
n/a
n/a
n/a
SFb (fol)
10–15
0.4
0.3
0.2
0.3
0.2
0.3
1.2
1.1
1.1
1.0
1.1
1.0
n/a
n/a
n/a
n/a
n/a
n/a
> 15
0.3
0.3
0.3
0.3
0.4
0.3
1.7
1.6
1.6
1.2
1.3
1.4
n/a
n/a
n/a
n/a
n/a
n/a
Avg. (BD)
0.4
0.4
0.4
0.4
0.4
0.4
1.5
1.3
1.5
1.1
1.3
1.3
n/a
n/a
n/a
n/a
n/a
n/a
Note: BD is the branch basal diameter; P is the precipitation amount;
SFb (def)/SFb (fol) refers to the ratio between branch
stemflow volume of the foliated and manually defoliated shrubs; and SFb
(SP)/SFb (CK) refers to the ratio between branch stemflow volume
of S. psammophila and C. korshinskii; N/A refers to not
applicable.
Stemflow efficiency of C. korshinskii and S. psammophila
With the combined results of SFP and FR, stemflow efficiency was assessed
for C. korshinskii and S. psammophila. SFP averaged
1.95 and 1.19 mL g-1 for individual
C. korshinskii and S. psammophila branches, respectively,
per incident rainfall events during the 2014 and 2015 rainy seasons (Table 4).
As precipitation increased, SFP increased from 0.19 mL g-1
during rains ≤ 2 mm to 5.08 mL g-1 during rains > 20 mm for C. korshinskii, and from
0.07 to 3.43 mL g-1 for the
corresponding precipitation categories for S. psammophila. With an
increase in branch size, SFP decreased from 2.19 mL g-1
for the 5–10 mm branches to 1.62 mL g-1 for the
> 18 mm branches of C. korshinskii, and from 1.64 to
0.80 mL g-1 for the corresponding BD categories of S. psammophila. Maximum SFP values
of 5.60 and 4.59 mL g-1 were recorded
for C. korshinskii and S. psammophila, respectively.
Additionally, C. korshinskii had larger SFP than did
S. psammophila for all precipitation and BD categories. This inter-specific
difference in SFP decreased with increasing precipitation from 2.7-fold
larger for C. korshinskii during rains ≤ 2 mm to 1.5-fold
larger during rains > 20 mm, and it increased with increasing
branch size: from 1.3-fold larger for C. korshinskii for the 5–10 mm
branches to 2.0-fold larger for the > 18 mm branches.
FR averaged 173.3 and 69.3 for the individual branches of C. korshinskii
and S. psammophila per rainfall events during the 2014 and
2015 rainy seasons, respectively (Table 5). As the precipitation
increased, an increasing trend was observed, ranging from the average FR of 129.2
during rains ≤ 2 mm to 190.3 during rains > 20 mm for
C. korshinskii and from the average FR of 36.7 to 96.1 during the
corresponding precipitation categories for S. psammophila. FR
increased with increasing BA from the average of 149.9 for the ≤ 30∘
branches to 198.2 for the > 80∘ branches of C. korshinskii and from the average
of 55.0 to 85.6 for the corresponding BA categories of
S. psammophila. Maximum FR values of 276.0 and 115.7 were recorded for
C. korshinskii and S. psammophila, respectively.
Additionally, C. korshinskii had a larger FR than
S. psammophila for all precipitation and BA categories. The inter-specific
difference in FR decreased with increasing precipitation from 3.5-fold
larger for C. korshinskii during rains ≤ 2 mm to 2.0-fold
larger during rains > 20 mm, and it decreased with an increase in
the branch inclination angle: from 2.7-fold larger for C. korshinskii
for the ≤ 30∘ branches to 2.3-fold larger for the > 80∘ branches.
Bio-/abiotic influential factors of stemflow yield and efficiency
For both C. korshinskii and S. psammophila, BA was the
only plant trait that had no significant correlation with SFb
(r < 0.13, p > 0.05) as indicated by the
Pearson correlation analysis. The separate effects of the remaining plant
traits were verified by the partial correlation analysis, but BL, ILAB and
PBMS failed this test. The rest of plant traits, including BD, LAB, LNB, BML
and BMS, were regressed with SFb using the forward selection
method. Biomass was finally identified as the most important biotic
indicator that affected stemflow, which behaved differently in
C. korshinskii for BMS and in S. psammophila for BML. The same
methods were applied to analyze the influence of meteorological
characteristics on SFb of these two shrub species. Tested by
the Pearson correlation and partial correlation analyses,
SFb related significantly with P, I10, RD and H for
C. korshinskii, and with P, I5, I10, I30 for
S. psammophila. The stepwise regression finally identified the
precipitation amount as the most influential meteorological characteristics
for the two shrub species. Although I10 was another influential factor
for C. korshinskii, it only made a 15.6 % contribution to the
SFb on average.
Relationships of branch stemflow volume (SFb), shrub
stemflow depth (SFd) and stemflow percentage (SF %) with
precipitation amount (P) for C. korshinskii and S. psammophila.
Comparison of stemflow productivity (SFP) between the foliated
C. korshinskii and S. psammophila.
Intra- and inter-specific
BD
Precipitation categories (mm)
Avg.
differences
categories
≤ 2
2–5
5–10
10–15
15–20
> 20
(P)
(mm)
Intra-specific differences in
5–10
0.20
0.56
1.37
2.04
4.18
5.60
2.19
C. korshinskii (CK)
10–15
0.19
0.47
1.20
1.72
3.47
4.96
1.90
(mL g-1)
15–18
0.17
0.38
1.05
1.55
3.08
4.74
1.73
> 18
0.15
0.35
1.00
1.46
2.95
4.35
1.62
Avg. (BD)
0.19
0.47
1.21
1.78
3.60
5.08
1.95
Intra-specific differences in
5–10
0.11
0.34
1.10
1.83
2.51
4.59
1.64
S. psammophila (SP)
10–15
0.08
0.25
0.82
1.43
1.98
3.72
1.29
(mL g-1)
15–18
0.05
0.16
0.53
0.97
1.40
2.61
0.88
> 18
0.05
0.15
0.47
0.82
1.25
2.44
0.80
Avg. (BD)
0.07
0.23
0.76
1.31
1.84
3.43
1.19
Inter-specific differences
5–10
1.8
1.7
1.3
1.1
1.7
1.2
1.3
(the ratio of the SFP values
10–15
2.4
1.9
1.5
1.2
1.8
1.3
1.5
of CK to that of SP)
15–18
2.8
2.4
2.0
1.6
2.2
1.8
2.0
> 18
3.0
2.3
2.1
1.8
2.4
1.8
2.0
Avg. (BD)
2.7
2.0
1.6
1.4
2.0
1.5
1.6
Note: BD is the branch basal diameter; P is the precipitation amount;
CK and SP are the abbreviations of C. korshinskii and S. psammophila, respectively.
SFb and SFd had a good linear relationships with
the precipitation amount (R2 ≥ 0.93) for both shrub
species (Fig. 5). The > 0.9 and > 2.1 mm rains were
required to start SFb for C. korshinskii and
S. psammophila, respectively. This was close to the 0.8 and
2.0 mm precipitation threshold calculated with SFd. Moreover, the
precipitation threshold increased with increasing branch size. The
precipitation threshold values were 0.7, 0.7, 1.4 and 0.8 mm for
the 5–10, 10–15, 15–18 and > 18 mm branches of
C. korshinskii, and 1.1, 1.6, 2.0 and 2.4 mm for the
branches of S. psammophila, respectively.
SF % of the two shrub species were inversely proportional to the
precipitation amount. As the precipitation increased, it gradually
approached asymptotic values of 9.1 and 7.7 % for
C. korshinskii and S. psammophila, respectively. As shown in Fig. 5,
fast growth was evident during rains ≤ 10 mm, but SF % slightly
increased afterwards for both shrub species.
Precipitation amount was the most important factor affecting SFP and FR for
C. korshinskii and S. psammophila, but the most important
biotic factor was different. BA was the most influential plant trait that
affected FR of these two shrub species at all precipitation levels. ILAB was
the most important plant trait affecting SFP during rains ≤ 10 mm of
these species. However, during heavier rains > 15 mm, BD and PBMS
were the most significant biotic factors for C. korshinskii and
S. psammophila, respectively. For these two shrubs species, it was
leaf trait (ILAB) and branch traits (biomass allocation pattern and branch
size) that played bigger roles on SFP during lighter rains ≤ 10 mm and
heavier rains > 15 mm, respectively. Therefore, it seemed that
the rainfall interception process of leaves controlled SFP during the
lighter rains, which functioned as the water resource to produce stemflow.
Nevertheless, while water supply was adequate during heavier rains, the
stemflow delivering process of branches might be the bottleneck.
Comparison of the funnelling ratio (FR) for the foliated C. korshinskii
and S. psammophila.
Intra- and inter-specific
BA
Precipitation categories (mm)
Avg.
differences
categories
≤ 2
2–5
5–10
10–15
15–20
> 20
(P)
(∘)
Intra-specific differences in
≤ 30
100.2
127.7
168.1
125.3
193.1
170.3
149.9
C. korshinskii (CK)
30–60
125.9
133.8
178.5
157.8
205.2
182.1
164.7
60–80
135.5
148.9
192.5
165.8
217.0
188.6
176.1
> 80
133.2
167.4
205.5
182.6
276.0
226.1
198.2
Avg. (BA)
129.2
144.8
187.7
162.3
219.6
190.3
173.3
Intra-specific differences in
≤ 30
32.6
37.3
52.0
59.0
65.8
85.2
55.0
S. psammophila (SP)
30–60
34.5
43.4
65.7
70.6
77.7
92.3
64.8
60–80
37.8
47.9
78.0
78.4
82.3
97.7
72.4
> 80
44.9
55.0
93.5
94.7
94.1
115.7
85.6
Avg. (BA)
36.7
46.0
72.6
75.3
80.5
96.1
69.3
Inter-specific differences
≤ 30
3.1
3.4
3.2
2.1
2.9
2.0
2.7
(the ratio of the FR values
30–60
3.7
3.1
2.7
2.2
2.6
2.0
2.5
of CK to that of SP)
60–80
3.6
3.1
2.5
2.1
2.6
1.9
2.4
> 80
3.0
3.0
2.2
1.9
2.9
2.0
2.3
Avg. (BA)
3.5
3.2
2.6
2.2
2.7
2.0
2.5
Note: BA is the branch inclined angle; P is the precipitation amount;
CK and SP are the abbreviations of C. korshinskii and S. psammophila, respectively.
Discussion
Differences of stemflow yield and efficiency between two shrub species
C. korshinskii produced stemflow in a larger quantity compared with
S. psammophila in all precipitation categories, particularly at the
5–10 mm young shoots during light rains ≤ 2 mm (Table 2). Although the
greatest stemflow yield was observed during rains > 20 mm for the
two shrub species, the inter-specific differences of SFb,
SFd and SF % were the highest at 3.2-, 8.5- and 8.3-fold larger
for C. korshinskii during rains ≤ 2 mm, respectively.
Additionally, C. korshinskii had a 2.8-fold larger
SFb than did S. psammophila for the 5–10 mm branches.
The average FR of C. korshinskii and S. psammophila were 173.3
and 69.3 per individual rainfall during the 2014 and 2015 rainy season
in this study, which agreed well with 156.1 (Jian et al., 2014) and 153.5
(Li et al., 2008) for C. korshinskii at the western Loess Plateau of
China, and 69.4 (Yang et al., 2008) for S. psammophila at the Mu Us
sandland of China. These two shrub species had a larger FR than those of
many other endemic xerophytic shrubs at water-stressed ecosystems, e.g.,
Tamarix ramosissima (24.8) (Li et al., 2008), Artemisia sphaerocephala (41.5) (Yang et al., 2008), Reaumuria soongorica (53.2)
(Li et al., 2008), Hippophae rhamnoides (62.2) (Jian et al.,
2014). Therefore, both C. korshinskii and S. psammophila
employed precipitation in an efficient manner to produce
stemflow, and C. korshinskii produced stemflow even more
efficiently for all precipitation categories particularly during rains
≤ 2 mm (Table 5). The higher stemflow efficiency of C. korshinskii
was also supported by SFP in all the precipitation and BD categories (Table 4).
In conclusion, compared with S. psammophila, C. korshinskii
produced stemflow at a greater amount and in a more efficient
manner. Moreover, the SFb-specific difference was the largest during
lighter rains. Dryland shrubs generally experienced several wetting–drying
cycles (Cui and Caldwell, 1997) when rains were sporadic. As an important
source of rhizosphere soil moisture at dryland ecosystems (Dunkerley, 2000;
Yang, 2010; Návar, 2011; Li et al., 2013), a considerable amount of
stemflow could be produced by various species and infiltrated into deep soil
during heavier rains. However, during lighter rains, the larger amount
stemflow produced in more efficient manner might benefit xerophytic shrubs,
as more soil moisture could be recharged especially at the root zone.
Therefore, in addition to quantify the soil moisture recharge, a thorough
study was required to depict the stemflow infiltration process, particularly
at the water-stressed environment.
Effects of precipitation threshold to produce stemflow
Precipitation below the threshold wet the canopy and finally evaporated, and
therefore
it theoretically did not generate stemflow. The precipitation threshold
varied with species and ecozones, for instance, 2.5 mm for the xerophytic
Ashe juniper communities in central Texas, USA (Owens et al., 2006) or
5 mm for xerophytic shrubs (S. psammophila, Hedysarum scoparium, A. sphaerocephala and Artemisia ordosica) at
the Mu Us sandland of China (Yang, 2010). Generally, for many xerophytic
shrub species, it generally ranges in 0.4–2.2 mm (Belmonte Serrato and Romero Diaz, 1998;
Li et al., 2008; Wang et al., 2013; Zhang et al., 2015). In this study, at
least the 0.9 and 2.1 mm rainfalls were necessary to initiate stemflow in
C. korshinskii and S. psammophila. That fell in the threshold range
of 0.4–1.4 mm for C. korshinskii (Li et al., 2009; Wang et al.,
2013), and agreed well with 2.2 mm for S. psammophila in the Mu Us
sandland (Li et al., 2009).
Scant rainfall prevailed in arid and semiarid regions. The light rains took
lead in events amount but ranked near the bottom in total precipitation
amount among different precipitation categories (Owens et al., 2006; Yang,
2010; Jian et al., 2014). In this study, the rains ≤ 2 mm accounted for
45.7 % of all the rainfall events and 7.2 % of the precipitation amount
during the 2014 and 2015 rainy seasons. C. korshinskii produced
stemflow at more rainfall events (71 events) than those of
S. psammophila (51 events) during the experimental period, which could be
partly explained by their different precipitation threshold. Because of the
2.1 mm threshold, S. psammophila produced the limited amount of
stemflow during 20 rainfall events of 1–2 mm, which took 21.3 % of all
rainfall events during the rainy season. Comparatively, stemflow yield
during rains of 1–2 mm was an extra benefit for C. korshinskii, for a
smaller precipitation threshold of 0.9 mm on average. Despite of a small
amount of stemflow during light rains, the soil moisture replenishment and
the resulting ecological responses were not negligible for dryland shrubs
and the peripheral arid environment (Li et al., 2009). A 2 mm summer rain
might stimulate the activity of soil microbes, resulting in an increase of
soil nitrate in the semiarid Great Basin in western USA (Cui and Caldwell,
1997), and a brief decomposition pulse (Austin et al., 2004). The summer
rains ≥ 3 mm were usually necessary to elevate rates of carbon fixation
in some higher plants in southern Utah, USA (Schwinning et al., 2003), or
for biological crusts to have a net carbon gain at eastern Utah, USA
(Belnap et al., 2004). That benefited the formation and maintenance of the
“fertile islands” (Whitford et al., 1997), “resource islands” (Reynolds
et al., 1999) or “hydrologic islands” (Rango et al., 2006).
Therefore, a smaller precipitation threshold might entitle
C. korshinskii with more available water at the root zone, because stemflow
functioned as an important source of available moisture in dryland
ecosystems (Dunkerley, 2000; Yang, 2010; Návar, 2011; Li et al., 2013).
That agreed with the findings of Dong and Zhang (2001) that
S. psammophila belonged to the water-spending paradigm from the aspect of leaf
water relations and anatomic features, and the finding of Ai et al. (2015)
that C. korshinskii belonged to the water-saving paradigm and had
larger drought tolerance ability than S. psammophila from the
aspect of root anatomical structure and hydraulic traits.
Effects of leaf traits on stemflow yield
Leaf traits had been recently reported for a significant influence on
stemflow (Carlyle-Moses, 2004; Garcia-Estringana et al., 2010). The factors,
such as a relatively large LNB (Levia et al., 2015; Li et al., 2016), a
large LAB (Li et al., 2015), a high LAI (Liang et al., 2009), a big BML
(Yuan et al., 2016), a scale-like leaf arrangement (Owens et al., 2006), a
small ILAB (Sellin et al., 2012), a concave leaf shape (Xu et al., 2005), a
densely veined leaf structure (Xu et al., 2005), an upward leaf orientation
(Crockford and Richardson, 2000), leaf pubescence (Garcia-Estringana et al.,
2010) and the leaf epidermis microrelief (e.g., the non-hydrophobic leaf
surface and the grooves within it) (Roth-Nebelsick et al., 2012), together
resulted in retaining a large amount of precipitation in the canopy,
supplying water for stemflow yield and providing a beneficial morphology
that enables the leaves to function as a highly efficient natural water
collecting and channeling system.
According to the documentation at Flora of China (Chao and Gong,
1999; Liu et al., 2010) and the field observations in this study,
C. korshinskii had more beneficial leaf morphology for stemflow yield
than did S. psammophila, owing to a lanceolate and concave leaf
shape, a pinnate compound leaf arrangement and a densely sericeous pressed
pubescence (Fig. 6). Additionally, experimental measurements indicated that
C. korshinskii had a larger MTA, LAB, LNB and LAI (an average of
54.4∘, 2509.1 cm2, 12 479 and 2.4, respectively) and a
smaller ILAB (an average of 21.9 mm2) than did S. psammophila
(an average of 48.5∘, 1797.9 cm2, 2404, 1.7 and 87.5 mm2,
respectively). The concave leaf shape, upward leaf orientation (MTA)
and densely veined leaf structure (ILAB) (Xu et al., 2005) provided
stronger leaf structural support in C. korshinskii for the
interception and transportation of precipitation, particularly during highly
intense rains. Therefore, in addition to the leaf morphology,
C. korshinskii was also equipped with more beneficial leaf structural features
for stemflow yield.
A controlled field experiment was conducted for the foliated and manually
defoliated C. korshinskii and S. psammophila
simultaneously for the 2015 rainy season. Nevertheless, contradictory results
were reached in this study. SFb of the foliated
C. korshinskii was 2.5-fold larger than the defoliated
C. korshinskii on average (Table 3), which seemed to demonstrate an overall
positive effect of leaves affecting stemflow yield. However, it
contradicted with the average 1.3-fold larger SFb of the
defoliated S. psammophila than the foliated
S. psammophila. Despite of the identical stand conditions, meteorological
characteristics and plant traits, except for the leaf states, the changing
interception area for raindrops, were not taken into account. It could be
roughly represented by leaf area and stem-surface area at the foliated and
defoliated state, respectively, which was generally ignored at many previous
studies (Dolman, 1987; Masukata et al., 1990; Neal et al., 1993;
Martinez-Meza and Whitford, 1996; Deguchi et al., 2006; Liang et al., 2009;
Mużyło et al., 2012). The changing interception area at different leaf
states might explain the seemingly contradictory results. For comparing the
inter-specific SFb, the normalized area indexes of SSAL and
SSAS was analyzed in this study. At the foliated state, a 1.4-fold larger
SSAL of the C. korshinskii was corresponded to a 1.6-fold larger
SFb than that of S. psammophila. But at
the defoliated state, a 2.0-fold larger SSAS of S. psammophila
corresponded to a 1.8-fold larger SFb than that of
C. korshinskii (Tables 1 and 3). Indeed, it greatly
underestimated the real stem-surface area of individual branches by ignoring
the collateral stems and computing SA with the surface area of the main
stem, which was assumed as a standard cone, in addition to a not big enough
sample size of branches and rainfall events measured in this controlled
field experiment. However, the positive relations of SFb with
SSAL and SSAS at different leaf states might shed light on the long-standing
discussion about leaf's effects on stemflow, which suggested some relevant
plant traits that might need to be considered for better understanding the
influential mechanism of stemflow yield. Although an identical
meteorological features, stand conditions and similar plant traits were
guaranteed, the experiment by comparing stemflow yield between the foliated
and defoliated periods might provide no feasible evidence for leaf's effects
(positive, negative or neglectable) affecting stemflow yield, if the newly
exposed branch surface at the defoliated period and the resulting rainfall
intercepting effect were not considered.
Comparison of leaf morphologies of C. korshinskii and S. psammophila.
Conclusions
Compared with S. psammophila, C. korshinskii produced a
larger amount of stemflow more efficiently during different sized rains. An
average 1.9-, 1.3-, 1.4-, 1.6- and 2.5-fold larger in C. korshinskii
was observed for the branch stemflow volume (SFb), the shrub
stemflow depth (SFd), the shrub stemflow percentage (SF %),
the stemflow productivity (SFP) and the stemflow FR,
respectively. The inter-specific differences in stemflow yield
(SFb, SFd and SF %) and the production
efficiency (SFP and FR) were maximized for the 5–10 mm branches and during
rains ≤ 2 mm. The smaller threshold precipitation (0.9 mm for
C. korshinskii vs. 2.1 mm for S. psammophila), and the beneficial
leaf traits might be partly responsible for the superior stemflow yield and
efficiency in C. korshinskii.
Precipitation amount had the largest influence on both stemflow yield and
efficiency for the two shrub species. BA was the most influential plant
trait on FR. For SFb, stem biomass and leaf biomass were the
most influential plant traits in C. korshinskii and
S. psammophila, respectively. But for SFP, leaf traits (the individual leaf
area) and branch traits (branch size and biomass allocation pattern) had a
larger influence in these two shrub species during lighter rains ≤ 10 mm
and heavier rains > 15 mm, respectively.
By comparing SFb between the foliated and manually defoliated
shrubs simultaneously at the 2015 rainy season, a contradiction was noted:
the larger stemflow yield of C. korshinskii at the foliated state,
but the larger stemflow yield of S. psammophila at the defoliated
state. That corresponded to the inter-specific difference of the specific
surface area representing by leaves (SSAL) and stems (SSAS) at different
leaf states, respectively. It shed light on the feasibility of experiments
by comparing stemflow yield between the foliated and defoliated periods,
which might provide no convincing evidence for leaf's effects (positive,
negative or neglectable) affecting stemflow yield, if the newly exposed
branch surface at the defoliated period and the resulting rainfall
intercepting effects were not considered.