Interannual variation in sap flow response in three xeric shrub species to periodic drought
Introduction
Global climate models (GCMs) project a continuous increase in the frequency and severity of extreme drought with global climate warming, particularly in arid and semiarid regions of the world. Arid to semiarid regions cover nearly half of the earth's total land-surface area (Huang et al., 2017). There is presently convincing evidence, both from long-term historical observations and modelled scenarios, that precipitation regimes will shift and contribute to more frequent episodes of intense drought during the 21st century (Groisman & Knight, 2008; IPCC, 2007, 2013). Increases in frequency, intensity, and duration of drought will adversely affect the global and regional cycling of water, energy balance, and structure and functioning of ecosystems (Sherwood and Fu, 2014; Donat et al., 2016). Variations in transpiration, as an important component of local hydrology, couples the water and carbon (C) cycles (Piao et al., 2010; Niu et al., 2011; Wang et al., 2014; Kropp et al., 2017; Miner et al., 2017). In such circumstances, one would question how desert plants could effectively maintain their primary functions in drought-prone areas.
Episodic rainfall events of variable timing and scale can trigger rapid pulses of plant activity (Schwinning and Sala, 2004). Desert plants are often exposed to nominal amounts of meteoric water (usually < 5 mm) during small rainfall events (Sala and Lauenroth, 1982; Zhao and Liu, 2010). A significant fraction of this water is directly evaporated into the atmosphere, because such events only saturate the uppermost layers of the soil complex (Beatley, 1974). In contrast, large rainfall events are known to recharge the soil profile up to a 100-cm depth. This post-rainfall soil-water status can usually be maintained for several weeks (Sala and Lauenroth, 1982). Soil water recharging increases available soil moisture and stimulates transpiration, particularly after reaching a specified rainfall threshold (Ogle and Reynolds, 2004; Potts et al., 2006; Zeppel et al., 2008; Raz-Yaseef et al., 2012). Consequently, an comprehensive understanding of the role of precipitation and associated controls on plant-water consumption is key to the future management of dryland ecosystems.
Desert shrubs have evolved remarkable adaptations to mitigate stress caused by water deficits by adjusting their morphological and physiological characteristics (Xia et al., 2008; Huang et al., 2011; Amissah et al., 2015). The effect of drought on desert shrubs can be lessened to some extent by growing smaller leaves, reducing stomatal conductance, and tapping subsurface water with vast vertical rooting systems (McAdam et al., 2016; Brito et al., 2017; Fan et al., 2017). Moreover, drought resistance has been shown to be closely related to xylem hydraulic conductivity (Ennajeh et al., 2008). Sap flow rate measurements have been shown to provide a reasonable foundation for the analysis of acclimation processes in desert plants (Pataki et al., 2000).
Long-term environmental studies linked to plant physiological traits are fundamental to understanding vegetation response with ongoing climate change. Plant sap flow responses could more precisely explain the water-consumption rates and tradeoffs between water supply and demand (Lei et al., 2010; Ma et al., 2011). Temporally, soil water recharging accelerates sap flow rates (Oren et al., 1996; Zheng and Wang, 2015). In contrast, insufficient soil water usually delays the water transport to plant leaves, which may cause the stomata to close and increase the hydraulic resistance between plant roots and the soil (Fan et al., 2017; Chen et al., 2018). Unlike their drought-sensitive counterparts, drought-insensitive shrubs are less influenced by soil water deficits (Du et al., 2011).
Sap flow rates have been studied at multiple temporal scales, crossing diurnal to seasonal, to intraannual scales. Generally, these studies were constrained by existing atmospheric conditions [e.g., solar radiation (Rs) and water vapor pressure deficit (VPD)] and available soil water (Bovard et al., 2005; Ma et al., 2007; Guo et al., 2010; Chen et al., 2014). However, the elements of water dynamics in desert plants, especially as they concern the quantitative understanding of interannual variation in water consumption remain largely unknown. Interannual development of the hydrological cycle is generally distinct from the better-studied intraannual variation in environmental factors, although both types of variation occur along a continuum of altered temporal patterns. Sap flow response to precipitation may vary for different plant habitats, species (Liu et al., 2011), plant-water consumption relationships, and plant functional types (Ewers and Oren, 2000). Better insights into the interannual relationship between limited soil water resources and species-specific sap flow regimes could help enhance our understanding of long-term acclimation processes and related feedbacks in the environmental system.
Earlier studies have examined response of plant primary processes to environmental change (e.g., response of photosynthesis to drought) by manipulative experiments (Sofo et al., 2009; Balachowski et al., 2016). Such studies, however, are not designed to provide a mechanistic understanding of physiological acclimation in plants. Consequently, it is essential to investigate plant response to environmental change before, during, and after disturbance (Niinemets, 2010; Grant et al., 2014).
In this framework, the analysis of plant sap flow response to environmental change is a useful approach to retrospectively assess plant species resistance (i.e., innate ability to cope with environmental disturbance), recovery (ability to recover from disturbance; Ingrisch & Bahn, 2018), and resilience (ability to return to pre-disturbance conditions; Holling, 1996) to short- and long-term drought. Soil drought reduces sap flow rates, especially in shallow-rooted plants (Kume et al., 2007). The inability of plants to recover after a severe drought may cause their growth and vigor to decline, increasing their likelihood of dying (Rogers et al., 2018). The present study focuses on understanding the effects of periodic drought on desert shrubs with a specific reference to plant resistance, resilience, and species acclimation to soil-water deficits. A challenge associated with this research is to define quantitatively the sensitivity of different plant species to periodic drought. To attain the mechanistic understanding of interannual variation, we hypothesize that (i) interannual variation in Js was directly controlled by PPT-induced-VWC and VWC-induced-LAI, which highlights the importance of PPT in our study area; and (ii) rainfall amount and timing, coincidental with drought were the key sub-factors in controlling interannual variation in Js.
Our objectives of the research were to: (i) understand the water-consumption strategies invoked in three common desert-shrub species, under contrasting rainfall events and associated volumetric soil water content (VWC); (ii) determine the primary mechanisms behind drought-induced reduction in sap flow rates and its response to biophysical drivers at interannual timescales; and (iii) evaluate species’ suitability in averting desertification over the longer term. This study will contribute to a better understanding of the effects of drought on the physiological acclimation and water conservation in the three desert-shrub species.
Section snippets
Study site and shrub species
The study was carried out at Yanchi Research Station (37°42′31″N, 107°13′47″ E) of Beijing Forest University. The site is located in Ningxia, near the southern edge of the Mu Us Desert, northwest China, about 1,530 m above mean sea level. The prevailing climate of the greater study area is temperate semiarid, where episodic rainfall events of highly variable annual amounts are common. The large interannual variation of rainfall fluctuates between 145–587 mm, whereas mean annual precipitation is
Variation in hydrometeorological variables
Daily variation in hydrometeorological variables is shown in Fig. 1a–g for the growing seasons (1 May to 30 September) of 2012-–2017. Seasonal patterns in Ta, Rs, VPD, and LAI were similar among the six years, peaking in mid-summer (i.e., the July–August period; Fig. 1a–d). Interannual variation in Ta, Rs, and VPD during the growing-season were generally minor over the six years (dotted lines, Fig. 1a–c), with an interannual mean (± standard deviation) of 17.5±0.35°C, 236.4±6.38 W m−2, and
Comparison of Js with other species
Interannual variation in Js (May–September, 2012-2017) for the three species (0.15–0.33 g cm−2 d−1) fell within the annual ranges reported for plants in semiarid regions of China (Zhao and Liu, 2010; Chen et al., 2014; Zheng and Wang, 2015; Qian et al., 2015; Huang and Zhang, 2016; Zha et al., 2017; Wu et al., 2018). When compared with other species (Table 3), Js at our site was generally lower. These dissimilarities can often be related to the amount of PPT received (annual mean of 287 mm) at
Conclusions
Interannual variation in Js was largely controlled by annual mean PPT-induced-VWC and associated LAI during the growing season. However, controls of interannual variation in Js differed seasonally, with VWC and PPT being more important during the leaf expanded phase and Rs, during the leaf senescence phase of the shrubs. Regarding competitive ability among the three species, H. mongolicum with overall lower sensitivity to drought, suggests a strong capacity to conserve water under drought
Declaration of Competing Interest
The authors have declared that there is no conflict of interests
Acknowledgement
The research was supported by grants from the National Natural Science Foundation of China (NSFC: 32071842, 32071843, 31670710, 31670708), by the Fundamental Research Funds for the Central Universities (No. 2015ZCQ-SB-02), and by the National Key Research and Development Program of China (no. 2016YFC0608100). The U.S.-China Carbon Consortium (USCCC) supported this work by way of helpful discussion and the exchange of scientific ideas. We thank X. W. Yang, S. J. Liu, G. P. Chen, and C. Zhang for
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