Deepening Rooting Depths Improve Plant Water and Carbon Status of a Xeric Tree during Summer Drought

Xin-Jun Zheng,Xue Wu,Gui-Qing Xu,Yan Li

doi:10.3390/f10070592

Abstract

Exploring the effects of drought on trees of different sizes is an important research topic because the size-dependent mortality pattern of the major dominant species significantly affects the structure and function of plant communities. Here we studied the physiological performance and non-structural carbohydrates (NSCs) dynamics of a small xeric tree species, Haloxylon ammodendron (C.A.Mey.) of different tree size with varying rooting depth, during summer drought. We measured predawn (Ψpd) and midday (Ψm) leaf water potential, osmotic potential at saturated turgor (π100), and turgor lost point (Ψtlp), stomatal conductance (gs) at noon, maximum photochemical efficiency of photosystem II (Fv/Fm) in the morning, and NSCs concentration, from June–September. Our results demonstrated that the summer drought reduces the overall performance of physiological traits of the small young trees more than the larger adult trees. Ψpd, gs and Fv/Fm dropped larger in the small-diameter groups than the larger diameter groups. Substantial osmotic adjustments were observed in small size individuals (with lower π100 and Ψtlp) to cope with summer drought. Furthermore, mean concentration of NSCs for the leaf and shoot were higher in September than in July in every basal stem diameter classes suggested the leaf and shoot acted as reserve for NSC. However the root NSCs concentrations within each basal stem diameter class exhibited less increase in September than in the July. At the same time, the small young tress had lower root NSCs concentrations than the larger adult tree in both July and September. The contrasting root NSC concentrations across the basal stem diameter classes indicated that the roots of smaller trees may be more vulnerable to carbon starvation under non-lethal summer drought. The significant positive relationship between rooting depth and physiological traits & root NSCs concentration emphasize the importance of rooting depth in determining the seasonal variation of water status, gas exchange and NSCs.

Highlights

Climate change has increased the frequency and severity of drought over global terrestrial ecosystems [1]
For convenience of presentation and discussion, we define the small young trees: With average rooting depth of 165 ± 0.10 m and 2.64 ± 0.12 m in the basal stem diameter classes of 0–1 cm and 1–2 cm respectively; intermediate-sized trees: With average rooting depth of 4.68 ± 0.38 m in the basal stem diameter class of 2–4 cm; and the large adult trees: With average rooting depth of 6.82 ± 0.15 m and 7.29 ± 0.00 m in the basal stem diameter classes of 4–8 cm and >8 cm, respectively
The contrasting root non-structural carbohydrates (NSCs) concentration across the basal stem diameter classes indicated that the roots of smaller trees may be more vulnerable to carbon starvation under non-lethal summer drought (Figure 7). It appeared that the root NSCs rather than shoot and leaf NSCs was more suitable for characterizing risk of carbon starvation for this desert plant species at different developmental stages

Summary

Introduction

Climate change has increased the frequency and severity of drought over global terrestrial ecosystems [1]. The size-dependent mortality pattern is an important aspect of plant population dynamics and has important influence on the structure and composition of forest, which can be very sensitive to climatic change [5]. Their size and structural complexity increase [11], trees with contrasting size may response differently to external environmental change [12,13], inducing different physiological responses to variation in resource availability [10]. The competition for underground resource is usually considered to be size-symmetric [5,16] Under these circumstances, the quantity of available resources enhances proportionally with individual dimension [10], which means that biomass allocation changed with tree dimension may have a fundamental impact on size-dependent tree mortality patterns. The belowground biomass allocation and the dimensions of the root systems were the key in determining the scope of the resource that plants foraging and the species competitive advantage [17,18]

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Conclusion