Characteristic of Stomatal Conductance and Optimal Stomatal Behaviour in an Arid Oasis of Northwestern China

Stomatal conductance (gs), the process that governs plant carbon uptake and water loss, is fundamental to most Land Surface Models (LSMs). With global change accelerating, more attention should be paid to investigating stomatal behavior, especially in extremely arid areas. In this study, gas exchange measurements and environmental/biological variables observations during growing seasons in 2016 and 2017 were combined to investigate diurnal and seasonal characteristics of gs and the applicability of the optimal stomatal conductance model in a desert oasis vineyard. The results showed that the responses of gs to environmental factors (photosynthesis active radiation, PAR; vapor pressure deficit, VPD; and temperature, T) formed hysteresis loops in the daytime. The stomatal conductance slope, g1, a parameter in the unified stomatal optimal model, varied in different growing seasons and correlated with the soil-to-leaf hydraulic conductance (KL). These results indicated the potential bias when using a constant g1 value to simulate gs and highlighted that the water-use strategy of oasis plants might not be consistent throughout the entire growing season. Our findings further help to achieve a better understanding of stomata behavior in responding to climate change and encourage future efforts toward a more accurate parameterization of gs to improve the modeling of LSMs.

Hyposensitive canopy conductance renders ecosystems vulnerable to extreme droughts

Increased drought intensity with rising atmospheric demand for water (hereafter VPD) increases the risk of tree mortality worldwide. Ecosystem-scale water-use strategy (WUSe), quantified here by canopy stomatal sensitivity to VPD (Sc), is increasingly recognized as a factor in drought-related ecosystem dysfunction. However, the links between Sc and ecosystem adaptation to and stability following droughts are poorly established. We examined how Sc regulates carbon sequestration, identifying ecosystems potentially susceptible to drought-induced mortality based on data from the global flux network, remote-sensing products, and plant functional-traits archive. We found that Sc is higher where ecosystem water availability is low in arid regions, reflecting conservative WUSe (i.e., hypersensitivity), but ecosystems of all regions converge on permissive WUSe (i.e., hyposensitivity) under ample water supply. During extreme droughts, hyposensitive and hypersensitive ecosystems achieved similar net ecosystem productivity employing considerably different structural-functional strategies. However, hyposensitive ecosystems, risking their hydraulic system with permissive WUSe, did not recover from extreme droughts as quickly. Models predicting current performance and future distributions of vegetation types should account for the greater vulnerability of hyposensitive ecosystems to intensifying atmospheric and soil drought.

Water use strategy of Ammopiptanthus mongolicus community in a drought year on the Mongolian Plateau

Aims In desert ecosystems, water is a restricting factor for plant growth and vegetation dynamics. The relatively stable water source from deep soil profile or groundwater is important for plant survival during drought. Understanding water use strategy of endangered species, in desert ecosystem is essential for their conservation and restoration such as Ammopiptanthus mongolicus on the Mongolian Plateau. Methods The stable isotope method of δD and δ 18O was used to examine the main water sources of A. mongolicus and two companion shrubs, e.g. Artemisia ordosica and Artemisia xerophytica. The contribution of different water sources to each species was calculated by IsoSource model. Leaf δ 13C was used to compare long-term water use efficiency of three shrubs. Soil moisture and root distribution of three shrubs was measured to explain plant water use strategy. Important Findings The results showed that A. mongolicus relied on groundwater and 150–200 cm deep soil water, with the former contributing to almost half of its total water source. Artemisia ordosica mainly used 150–200 cm deep soil water, but also used shallow soil water within 100 cm in summer and autumn. Artemisia xerophytica mainly used 150–200 cm deep soil water and groundwater, with the latter contributing to about 30%–60% of its total water source. The three shrubs had dimorphic or deep root systems, which are in accord with their water sources. The WUE in the evergreen shrub A. mongolicus was higher than in two deciduous Artemisia shrubs, which may be an adaptive advantage in desert ecosystem. Therefore, groundwater is an important water source for the endangered shrub A. mongolicus in a drought year on Mongolian Plateau. Ammopiptanthus mongolicus and two Artemisia shrubs competed for deep soil water and groundwater.