摘要:理解干旱与树木个体发育阶段或大小之间的相互作用关系是一个重要的优先研究方向，因为依赖于个体大小的特定死亡率分布格局对生态系统的结构和功能将产生重要的影响。以广泛分布于古尔班通古特沙漠南缘的主要建群种梭梭（Haloxylon ammodendron）为研究对象，通过测定越夏生长期间不同个体大小梭梭黎明前同化枝水势（Ψ_pd）、压力-容积（P-V）曲线、最大气孔导度（G_s）、叶绿素荧光（F_v/F_m），调查不同个体大小梭梭根系分布特征和根深，将越夏生长期间土壤水分状况、植物生理性状与根深相结合，分析不同大小梭梭对干旱胁迫的响应差异。结果表明：（1）夏季干旱胁迫显著降低了梭梭幼苗生理性状的特征值：同成年植株相比，梭梭幼苗黎明前同化枝水势（Ψ_pd）、PSⅡ反应中心最大光化学效率（F_v/F_m）、饱和膨压渗透势（π₁₀₀）、膨压消失点渗透势（Ψ_TLP）显著降低；（2）梭梭生理性状与根深之间存在线性关系，根深决定不同大小梭梭的水分状况（Ψ_pd）进而主导了越夏生长期间的其他生理性状表达（F_v/F_m、π₁₀₀、Ψ_TLP）；（3）不同大小的梭梭对干旱胁迫的响应差异是植物根系分布特征和生境相互作用的结果，根深在梭梭应对夏季干旱胁迫中起重要作用。总之，梭梭幼苗根系发育不完全（浅根系），只能利用降水带来的有限的浅层土壤水，对干旱胁迫响应敏感；而成年梭梭植株能最大限度的利用深层土壤水分，减少表层土壤干旱对其产生的不利影响。



Abstract:Understanding the interaction between drought and ontogenetic stage or size of trees is an important research priority because the mortality pattern that depends on individual size of tree has an important impact on the structure and function of forest ecosystems. Haloxylon ammodendron, one of the widely distributed small trees in the southern margin of Gulbantonggute Desert, was taken as the target plant species in this study. We measured the predawn leaf water potential (Ψ_pd), PV curve, maximum stomatal conductance (G_s), maximum photochemical efficiency (F_v/F_m) of the H.ammodendron with different tree sizes (represent by basal stem diameter) during the growing season. To ascertain root distribution and rooting depth of different tree sizes, we carried out root investigation by digging method. The responses of different tree sizes surrogated by the basal stem diameters of H.ammodendron to drought were analyzed by combining gravimetric soil water content, plant physiological traits and rooting depth. Our results showed that summer drought stress significantly constrained the physiological traits performance of the small young trees with basal stem diameter of 0-1 cm and 1-2 cm, more than the larger adult trees with basal stem diameter of 4-8 cm and >8 cm. Compared with the adult trees, the Ψ_pd、F_v/F_m、osmotic potential at full turgor(π₁₀₀)、osmotic potential at turgor lost point(Ψ_TLP) of the young trees dropped significantly. Across the basal stem diameter groups, rooting depths were closely related to plant physiological traits. The contrasting physiological traits performance during the summer drought among the tree size groups of H.ammodendron was the result of the interaction between plant root distribution characteristics and the soil water conditions. Rooting depth dominated plant from which soil depth and corresponding soil volume to obtain water, and then determined the plant water status and thus the physiological response characteristics of different tree sizes during summer drought. Hence, rooting depth appeared to be a critical trait at the basis of tree resistance and resilience to extreme drought events and may be the most important traits defending trees with different size from drought-induced mortality. For the small young trees, their root systems were not fully developed (shallow rooting depth), which could only use the limited shallow soil water input by precipitation and thus was sensitive to drought stress. For the adult and older trees, their root systems were well developed and could fully use the deep soil moisture hence reduce the adverse effects of surface soil drying. Desert shrubs of different ages or sizes have different rooting depths and may experience different soil moisture environments. Further research should give more attention on how individuals of different sizes respond to and adapt to the contrasting soil water conditions.





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