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 寒旱区科学  2017, Vol. 9 Issue (5): 495-502  DOI: 10.3724/SP.J.1226.2017.00495 0

### Citation

Zhang YF, Wang XP, Pan YX, et al. 2017. Intrastorm stemflow variability of a xerophytic shrub within a water-limited arid desert ecosystem of northern China. Sciences in Cold and Arid Regions, 9(5): 495-502. DOI: 10.3724/SP.J.1226.2017.00495.
[复制英文]

### Correspondence to

YaFeng Zhang, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences. No. 320, West Donggang Road, Lanzhou, Gansu 730000, China. E-mail: zhangyafeng1986@gmail.com

### Article History

Accepted: December 30, 2016
Intrastorm stemflow variability of a xerophytic shrub within a water-limited arid desert ecosystem of northern China
YaFeng Zhang , XinPing Wang , YanXia Pan , Rui Hu
Shapotou Desert Research and Experiment Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China
Abstract: An increasing number of studies in recent years has elucidated distinguishable effects of stemflow on hydrology and biogeochemistry within a variety of ecosystems. Nonetheless, no known studies have investigated the temporal variability of stemflow volume within discrete rainfall events for xerophytic shrubs. Here, stemflow was monitored at 5-min intervals using a tipping-bucket rain gage during the 2015 growing season for a xerophytic shrub (Caragana korshinskii) within a water-limited arid desert ecosystem of northern China. We characterized the stemflow temporal variability, along with rainfall, and found the temporal heterogeneity of rainfall clearly affected the timing of stemflow inputs into basal soil within discrete rainfall events. The rainfall threshold value for stemflow generation is not a constant value but a range (0.6~2.1 mm, with an average of 1.1 mm) across rainfall events and is closely associated with the antecedent dry period. Time lags existed between the onset of rainfall and the onset of stemflow, and between rainfall peaks and stemflow peaks. Our findings are expected to be helpful for an improved process-based understanding of the temporal stemflow yield of xerophytic shrubs within water-limited arid desert ecosystems.
Key words: stemflow    temporal variability    xerophytic shrub    antecedent dry period    time lag

1 Introduction

Precipitation is partitioned into interception loss, stemflow, and throughfall by vegetation canopies (Crockford and Richardson, 2000; Llorens and Domingo, 2007); this pattern alters the horizontal and vertical distribution of precipitation within shrub communities, greatly increases the variability of soil-moisture recharge, and plays a pivotal role in vegetation survival and reproduction in water-limited desert ecosystems (e.g., Martinez-Meza and Whitford, 1996; Johnson and Lehmann, 2006; Navar, 2011; Wang et al., 2011 ; Li et al., 2013 ; Zhang YF et al., 2013 , 2016a; Levia and Germer, 2015).

Stemflow is the portion of precipitation that is intercepted by leaves, twigs, and branches, and eventually delivered to the basal soil via trunks or stems. Stemflow has been reported to be a function of a suite of biotic and abiotic factors, including tree species and architecture (Levia and Frost, 2003; Barbier et al., 2009 ; Zimmermann et al., 2015 ; Zhang et al., 2017 ); leaf shape, number, and angle (Staelens et al., 2008 ; Levia et al., 2013 ); branch angle (Herwitz, 1987; Martinez-Meza and Whitford, 1996; Park and Cameron, 2008); bark structure (Levia and Herwitz, 2005; Van Stan et al., 2016 ); incident rainfall amount, intensity, duration, and angle (Crockford and Richardson, 2000; Dunkerley, 2014a; Zhang et al., 2015 ); air temperature (Levia and Herwitz, 2000; Andre et al., 2008 ); relative humidity (Andre et al., 2008 ); wind speed and direction (Xiao et al., 2000 ; Van Stan et al., 2014 ); vapor pressure deficit (Van Stan et al., 2014 ); antecedent dry period (Germer et al., 2010 ; Siegert and Levia, 2014); and the possible interactions among these factors (Levia and Frost, 2003).

An increasing number of studies in recent years has elucidated the distinguishable effects of hydrologically and chemically enriched stemflow on the hydrology, biogeochemistry, and ecology within a variety of ecosystems (Levia and Germer, 2015; Spencer and van Meerveld, 2016; Zhang YF et al., 2016b ). Those studies are mostly based on the monitoring of stemflow on a rainfall-event scale, weekly scale, or even larger scale. To date, only a couple of studies were conducted on the intrastorm scale in forest ecosystems (Durocher, 1990; Reid and Lewis, 2009; Germer et al., 2010 ; Levia et al., 2010 ); and no known studies have investigated the intrastorm stemflow variability of xerophytic shrubs. The finer intrastorm scale, e.g., with a frequency of 5-min, allows us to evaluate the stemflow dynamics, along with rainfall, in real time and is of critical importance to an improved characterization of hydrologic and biogeochemical cycling in vegetated terrestrial ecosystems (Durocher, 1990; Levia et al., 2010 ). As such, detailed process-based knowledge of the heterogeneity of stemflow yield from individual shrubs within discrete rainfall events is lacking and merits great attention.

The objectives of our study are (1) to characterize the temporal variability of stemflow of a xerophytic shrub within discrete rainfall events and (2) to quantify the threshold values of rainfall for stemflow generation. Achievement of the two objectives is also important for gaining a better understanding of the dynamic soil-water redistribution under shrub canopies.

2 Material and Methods 2.1 Study site

Field measurements were carried out during the 2015 growing season at Shapotou Desert Research and Experiment Station of the Chinese Academy of Sciences (37°32′N, 105°02′E, an elevation of 1,300 m above sea level), southeastern fringe of the Tengger Desert in northwestern China. Long-term (1956 to 2011) mean annual precipitation is 188 mm, with 80% of the rain falling between July and September. Most storms are of low intensity and short duration, with around 80% of the rainfall intensities ≤5 mm/h (Zhang et al., 2015 ). The groundwater is 50~80 m below the surface and cannot be accessed by plant roots. Potential evapotranspiration is approximately 2,500 mm during the growing season (April to October), resulting in a large annual moisture deficit. Annual mean air temperature is 9.6 °C, with a monthly mean of –6.9 °C in January and 24.3 °C in July. Annual mean wind velocity is approximately 2.8 m/s.

Caragana korshinskii is a multiple-stemmed, deciduous perennial shrub with an inverted cone shape; it is widely distributed in arid and semi-arid areas of northern China. C. korshinskii has been used successfully in revegetation for protecting the Baotou–Lanzhou Railway against encroaching sand dunes in the Shapotou area (Li et al., 2006 , 2012). Its stems are smooth, leaves are pinnately compound and opposite or sub-opposite, and each pinna has 5 to 8 pairs of ovate leaflets (7~8 mm in length and 2~5 mm in width).

2.2 Experimental setup

We selected a healthy adult shrub of C. korshinskii to measure intrastorm stemflow, as can be seen in Figure 1. The targeted shrub was 290 cm in height, with a canopy area (approximated as an ellipse) of 10 m2 (208 cm in the east–west direction and 153 cm in the north–south direction). It had 14 stems, with a basal area (the sum of the cross-sectional area of the individual stems) of 63.5 cm2. Aluminum foil collars were fitted around the entire circumference of the individual stems. Each collar was connected to a short section of polyvinyl chloride hose (0.8 cm in inner diameter). Stemflow was diverted through the collars into a covered tipping-bucket rain gage (GeoPrecision GmBH, Ettlingen, Germany; with an inner diameter of 16 cm and a resolution of 0.1 mm) that functioned as a stemflow gage (Figure 1). The stemflow gage recorded data at 5-min intervals and was covered with a tile shield to prevent throughfall from dripping into the gage. The stemflow collection system (Figure 1) was checked periodically for blockage or leakage.

 Figure 1 Stemflow collection system for C. korshinskii

Stemflow volume (SFv, mL) was computed using the following function suggested by Levia et al. (2010) :

 ${\rm{S}}{{\rm{F}}_{\rm{v}}} = \left( {{\rm{D}}/10} \right) \times {\rm{A}}$ (1)

where D is the depth equivalent (mm) recorded by the stemflow gage, and A is the orifice area (cm2) of the stemflow gage.

Stemflow depth (SFd, mm) was calculated as

 ${\rm{S}}{{\rm{F}}_{\rm{d}}} = {\rm{S}}{{\rm{F}}_{\rm{v}}}/{\rm{CA}}$ (2)

where SFv is the stemflow volume (L), and CA is the canopy area (m2).

Another tipping-bucket rain gage of the same type as the one recording stemflow was installed to measure rainfall in an open area approximately 50 m from the study plot. Individual events were separated by at least 6 h without rainfall, an interval that is commonly adopted in the literature (Dunkerley, 2015; Wang et al., 2016 ).

3 Results 3.1 Stemflow in relation to rainfall

During the study period from May to September 2015, stemflow was recorded after 13 individual rainfall events of a cumulative 81.4 mm. A significantly positive linear relationship was found between individual rainfall depth and individual stemflow depth (Figure 2a). Stemflow averaged 6.1%; it first increased with rainfall depth and then began to decrease slightly after a threshold value of 12 mm (Figure 2b). Detailed information about rainfall characteristics and stemflow on a rainfall event basis can be seen in Table 1.

 Figure 2 Relationship between rainfall and (a) stemflow and (b) stemflow percentage

Table 1 Rainfall characteristics and stemflow of C. korshinskii on a rainfall-event basis
3.2 Temporal variability of stemflow

Figure 3 illustrates the temporal variation of stemflow in response to rainfall amount at 5-min intervals. The temporal heterogeneity of rainfall clearly affects the timing of stemflow inputs into basal soil during discrete rainfall events. Larger amounts of rainfall were followed by increases in stemflow yield. For all events, time lags between rainfall peaks and stemflow peaks can be observed. Moreover, stemflow usually continued for less than 1 h (except for the event on July 8), and very small quantities were collected once rainfall had ceased.

 Figure 3 Synchronicity between the timing of rainfall and stemflow (SF) volume at 5-min intervals

A time lag existed between the onset of gross rainfall and the onset of stemflow. This time lag averaged 75 min, with a range of 15 to 280 min. No clear relation was found between the time lag and the rainfall amount (Figure 4a), whereas the time lag decreased with increasing rainfall intensity before the onset of stemflow, following a power function (Figure 4b).

 Figure 4 Time for the onset of stemflow (Tsf) in relation to the rainfall (a) amount and (b) intensity before the onset of stemflow
3.3 Stemflow generation in relation to the antecedent dry period

Figure 5 shows cumulative stemflow and the corresponding cumulative rainfall depth. Clearly, an initial wetting was required for stemflow generation; and the rainfall needed for stemflow generation ranged from 0.6 to 2.1 mm, with an average of 1.1 mm (Figure 5b). The slope of the regression between cumulative stemflow (mL) and cumulative rainfall (mm) in Figure 5a suggests the stemflow production efficiency per millimeter of individual rainfalls (mL/mm); a steeper slope corresponded to a higher stemflow production efficiency.

 Figure 5 Cumulative stemflow in relation to cumulative rainfall depth. Figure 5b is an enlarged view of the shaded area at the bottom left of Figure 5a

Events with short antecedent dry periods (ADP) needed less rainfall to initiate stemflow for a threshold ADP <5 d; and after this threshold, rainfall for stemflow generation hardly seemed to change with increasing ADP ( Figure 6). It should be noted that we define ADP as the time period between two rainfall events that can generate stemflow. Moreover, a negative but not statistically significant linear relationship (r=−0.43, P=0.12) can be found between rainfall amount and rainfall intensity before stemflow generation (data not shown).

 Figure 6 Rainfall for stemflow generation as a function of the antecedent dry period
4 Discussion

This study is the first time the high-resolution tipping-buck rain gage was used to characterize the temporal variability of stemflow within discrete rainfall events for xerophytic shrubs within water-limited arid desert ecosystems. The experiment permitted the investigation of some temporal aspects of stemflow generation for xerophytic shrubs that have rarely been shown before. Our results demonstrated that the stemflow generation reflected almost perfectly the rainfall dynamics once an initial canopy-storage capacity had been exceeded (Figure 3). By examining the real-time cumulative stemflow in relation to cumulative rainfall depth (Figure 5), we determined that the rainfall threshold value for stemflow generation was not constant but was differential (ranging from 0.6 to 2.1 mm, with an average of 1.1 mm) among individual rainfall events. The threshold value for a given tree/shrub species is a constant value for a given study in the literature, as this value is typically interpolated according to the fitted function between rainfall depth and stemflow depth (such as the fitted function in Figure 2a), a practice that obscures the true nature of the rainfall threshold value for stemflow generation and inevitably lacks accuracy. Our results thus provide new insight into the rainfall threshold value for stemflow generation. Nevertheless, the range of rainfall threshold values is comparable to that reported for shrubs in other arid and semi-arid areas. For instance, Martinez-Meza and Whitford (1996) reported a rainfall threshold value of 1.3 mm for Larrea tridentata and Flourensia cernua, and 1.8 mm for Prosopis glandulosa in New Mexico; Li et al. (2009) found a rainfall threshold value of 1.1 mm for Salix psammophila and 1.2 mm for Hedysarum scoparium in Mu Us sandy land of northern China; Llorens and Domingo (2007) summarized that the rainfall threshold value ranged from 1 to 3 mm among different Mediterranean shrub species. Moreover, we found that the threshold value had a close relationship with ADP, being well fitted by a power function (Figure 6). Events with short ADP needed less rainfall to initiate stemflow for a threshold ADP <5 d; and after this threshold, rainfall required for stemflow generation seemed to become stable with increasing ADP. A longer ADP usually suggests a drier shrub canopy and hence more rainfall required for initially wetting it for stemflow generation; however, as ADP increases and exceeds a threshold, the drying degree of a shrub canopy would hardly change. Similarly, based on the simple correlation, Germer et al. (2010) noticed that ADP had a negative influence on stemflow generation of babassum palms (Orbignya phalerata) in the southwestern Amazon basin of Brazil; Siegert and Levia (2014) observed the tree trunks of American beech (Fagus grandifolia) and yellow poplar (Liriodendron tulipifera) were primed for stemflow generation when the preceding rainfall events were most recent.

Between the onset of rainfall and the onset of stemflow, a time lag ranging from 15 to 280 min was observed between the onset of rainfall and the onset of stemflow, which had no clear relation with rainfall amount but was well associated with the rainfall intensity before stemflow generation (Figure 4). Tsf first sharply decreased and then gently decreased with increasing rainfall intensity, suggesting that the rainfall intermittency played a significant role, as a longer rainfall intermittency (normally corresponding to a lower rainfall intensity) means a longer time for canopy rewetting (Zhang ZS et al., 2016 ). This finding somewhat accords with that of Dunkerley (2014b), who conducted a laboratory rainfall-simulation experiment to examine the effects of rainfall intensity on stemflow production of an artificial olive tree (Olea europaea) and found that the temporal variation of rainfall intensity at the intrastorm scale is a key factor affecting the stemflow fraction. Time lags were also found between peak rainfall and peak stemflow (Figure 3), which may depend on rainfall amount (as a certain amount of water is required to fill the canopy storage) and on the travel time of stemflow down branches and stems toward the stemflow gage (Germer et al. 2010 ).

The temporal dynamics of stemflow in response to rainfall showed that stemflow continued for less than 1 h, and very small quantities were collected once rainfall has ceased. Our result is in line with that of Durocher (1990), who observed that, after the cessation of rainfall in northwestern California, little water was stored in excess of the storage capacity of the stem elements for red oaks (Quercus rubra) about 40 years old. However, much longer stemflow times were reported for some other tree species. For example, after a rainfall event, stemflow continued to drain for redwood (Sequoia sempervirens) and Douglas fir (Pseudotsuga menziesii) up to 48 h, as reported by Reid and Lewis (2009), and for babassu palms up to 11 h, as reported by Germer et al. (2010) . C. korshinkii in our study and red oak in Durocher's study (1990) have smooth bark that benefits stemflow generation, whereas rough, fibrous barks with a higher water-storage capacity of redwood and Douglas fir in Reid and Lewis (2009) would keep the preferred flow paths wetter over longer time periods. For babassu palms, Germer et al. (2010) attributed the stemflow interval to the sponge effect of the canopy soil that develops within the interspace of the palm stem and palm frond base. Thus, one should be cautious when drawing conclusion on this issue.

5 Conclusions

High temporal resolution data (5-min) from C. korshinskii instrumented with a tipping-bucket rain gage allow us to evaluate the intrastorm stemflow dynamics. By characterizing the real-time variability of stemflow along with rainfall, we found that the temporal heterogeneity of rainfall clearly affects the timing of stemflow inputs into basal soil for discrete rainfall events. New insights into the rainfall threshold value for stemflow generation are given in our study by identifying the differential threshold values among individual rainfall events. It is also the first time we quantified the relationship between ADP and the rainfall required for stemflow generation. More research on the temporal variability of the stemflow of various shrub species during discrete rainfall events is necessary to get an improved understanding of intrastorm stemflow dynamics and its role in the temporal and spatial variation of soil moisture around a shrub's basal areas.

Acknowledgements:

This study was supported by the National Natural Science Foundation of China (41530750, 41501108, 41371101) and the CAS "Light of West China" Program. The authors would like to express their gratitude to the associate editor and two anonymous reviewers for their constructive comments in improving the manuscript.

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