Drought-Induced Mortality Is Related to Hydraulic Vulnerability Segmentation of Tree Species in a Savanna Ecosystem

Shubin Zhang,Guojing Wen,Daxin Yang

doi:10.3390/f10080697

Abstract

Vulnerability segmentation (VS) has been widely suggested to protect stems and trunks from hydraulic failure during drought events. In many ecosystems, some species have been shown to be non-segmented (NS species). However, it is unclear whether drought-induced mortality is related to VS. To understand this, we surveyed the mortality and recruitment rate and measured the hydraulic traits of leaves and stems as well as the photosynthesis of six tree species over five years (2012–2017) in a savanna ecosystem in Southwest China. Our results showed that the NS species exhibited a higher mortality rate than the co-occurring VS species. Across species, the mortality rate was not correlated with xylem tension at 50% loss of stem hydraulic conductivity (P50stem), but was rather significantly correlated with leaf water potential at 50% loss of leaf hydraulic conductance (P50leaf) and the difference in water potential at 50% loss of hydraulic conductance between the leaves and terminal stems (P50leaf-stem). The NS species had higher Huber values and maximum net photosynthetic rates based on leaf area, which compensated for a higher mortality rate and promoted rapid regeneration under the conditions of dry–wet cycles. To our knowledge, this study is the first to identify the difference in drought-induced mortality between NS species and VS species. Our results emphasize the importance of VS in maintaining hydraulic safety in VS species. Furthermore, the high mortality rate and fast regeneration in NS species may be another hydraulic strategy in regions where severe seasonal droughts are frequent.

Highlights

In the context of climate change, longer and more intense droughts have been projected [1], and widespread vegetation mortality will be induced by ongoing drought events [2,3], which may drive rapid shifts of vegetation structure and species composition [4,5]
P50leaf-stem and P50leaf across species (Figure 2b,c). These results demonstrated that Vulnerability segmentation (VS) plays a crucial role in drought avoidance or tolerance in VS species; in contrast, NS species with higher MR displayed a higher RR compared with VS species, which was associated with photosynthesis (Figure 3; Table 3)
Our results suggested that VS plays a crucial role in drought avoidance or tolerance in VS species, in contrast, NS species with high mortality rates and fast regeneration, adopt risky hydraulic safety strategies and exhibit high hydraulic conductivity and photosynthesis rates

Summary

Introduction

In the context of climate change, longer and more intense droughts have been projected [1], and widespread vegetation mortality will be induced by ongoing drought events [2,3], which may drive rapid shifts of vegetation structure and species composition [4,5]. The long-distance water transport pathway may be impaired by air-seeded embolisms in xylem conduits [6,7,8]. Prolonged drought may induce hydraulic failure, which is related to canopy dieback, and even widespread plant mortality [9,10,11]. Plants employ a range of adaptive strategies to maintain hydraulic safety during drought events. During a midday water deficit, stomatal control regulates leaf transpiration and maintains the diurnal integrity of the water transport pathway [12,13]. Leaves are more vulnerable to drought-induced embolism than stems due to positive differences in the water potential at 50% loss

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