Abstract
Stable hydraulic conductivity in forest trees maintains healthy tree crowns and contributes to productivity in forest ecosystems. Drought conditions break down this relationship, but the mechanisms are poorly known and may depend on drought severity. To increase the understanding of changes in hydraulic conductivity during drought, we determined hydraulic parameters in Populus euphratica Oliv. (P. euphratica) in naturally arid conditions and in a simulated severe drought using a high-pressure flow meter. The results showed that leaf-specific hydraulic conductance (LSC) of leaf blades was less variable in mild drought, and increased significantly in severe drought. Plants attempted to maintain stability in leaf blade LSC under moderate water stress. In extreme drought, LSC was enhanced by increasing hydraulic conductance in plant parts with less hydraulic limitation, decreasing it in other parts, and decreasing leaf area; this mechanism protected the integrity of water transport in portions of tree crowns, and induced scorched branches and partial mortality in other parts of crowns. We conclude that limitation in water supply and elastic regulation of hydraulic characteristics may drive the mortality of tree branches as a result of severe drought. Evaluation of adaptive water transport capacity in riparian plants in arid areas provides a scientific basis for riparian forest restoration.
Highlights
River basins in the arid inland region of China exhibit downstream oases formed by riparian forests
The hydraulic characteristics of P. euphratica were studied based on data analysis of whole root, whole shoot, and branch components in riparian tree species
Plants strive to maintain the stability of leaf-specific hydraulic conductance (LSC) of leaf blades under mild water stress
Summary
River basins in the arid inland region of China exhibit downstream oases formed by riparian forests. (P. euphratica), a dominant riparian tree species, plays a vital role in maintaining ecosystem functions in these oases [1,2,3]. Extensive branch scorching and mortality are commonly found in the crowns of otherwise healthy P. euphratica trees, indicating a degrading ecological function. The reasons for canopy mortality are unclear, but may be related to water transport within the trees and how that changes in drought conditions. The flow of water through plants has important effects on plant hydraulics, growth, structure, function, and ecology [4]. Past studies have shown that hydraulic properties are related to plant drought resistance [5]. Plant responses to drought include combinations of characteristics that allow plants to cope; these are xylem cavitation, morphological characteristics, photosynthetic responses, Forests 2019, 10, 407; doi:10.3390/f10050407 www.mdpi.com/journal/forests
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