Abstract

In semi-arid ecoregions, trees are restricted to river valley floodplains where river water supplements the limited precipitation. To characterize the associated diurnal and seasonal dynamics in hydrology and water relations, we studied narrowleaf cottonwoods (Populus angustifolia) along a prairie river in Canada. From June through August, the shallow soil moisture was depleted but moisture remained higher above the alluvial groundwater table, which dropped to 2.3 m along with river recession. Throughout the summer, with the daily rise in temperature and insolation, foliar stomatal conductance (g_s) and transpiration (E) increased to midday and then fell, thus maintaining the midday leaf water potential (Ψ_md) above ~−1.7 MPa. This Ψ_md approximated the water potential associated with 12% loss of xylem conductivity due to cavitation for branches (P₁₂); the Ψ_md and P₁₂ varied independently across eight trees, providing differences in relative hydraulic risk. Sap flux density (F_d) was measured with thermal dissipation probes, and revealed diurnal patterns similar to foliar E. In contrast to our expectation, the daily F_d maxima were consistent through the summer despite the seasonal recession in water supply. Canopy conductances (G_S), derived from F_d, sapwood area and canopy area, declined with vapor pressure deficit (D) and fell slightly in late summer along with stomatal sensitivity, which reflects the magnitude of decrease in G_S with increasing D. For spatial up-scaling, satellite-derived near-infrared reflectance of vegetation revealed the woodland phenology, with leaf expansion from May through June and gradual decline in photosynthetic productivity through the summer. Thus, the phreatophytic cottonwoods: (i) sustained substantial water use and productivity through the warm and dry summer, by (ii) accessing shallow soil moisture and then deeper alluvial groundwater, and (iii) providing diurnal stomatal regulation, to (iv) avoid xylem cavitation and (v) maintain fairly constant hydraulic conductance.

中文翻译：

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树木如何在干燥的气候中茁壮成长：河岸白杨树林中的昼夜和季节性水文和水关系

摘要

在半干旱生态区，树木仅限于河谷泛滥平原，河水补充了有限的降水。为了描述水文和水关系中相关的昼夜和季节动态，我们研究了加拿大草原河流沿岸的窄叶杨木 ( Populus angustifolia )。从 6 月到 8 月，浅层土壤水分被耗尽，但水分仍然高于冲积地下水位，随着河流的退缩，地下水位下降到 2.3 m。整个夏季，随着气温和日照的逐日升高，叶片气孔导度（g _s）和蒸腾作用（E）到正午先升高后下降，从而维持正午叶片水势（Ψ _md）高于~-1.7 MPa。这个 Ψ _{md近似于与由于树枝的空化（} P ₁₂ ）导致木质部电导率损失 12% 相关的水势；Ψ _md和P ₁₂在八棵树上独立变化，提供了相对水力风险的差异。用热耗散探头测量树液通量密度 ( F _d )，并揭示了类似于叶面E的昼夜模式。与我们的预期相反，尽管供水出现季节性衰退，但整个夏季的每日F _{d最大值是一致的。}冠层电导 ( _GS )，源自F _d, 边材面积和冠层面积随着蒸气压亏缺 ( D ) 下降, 夏末随着气孔敏感性略有下降, 反映了G _S随着D的增加而下降的幅度. 对于空间放大，卫星衍生的植被近红外反射揭示了林地物候，从 5 月到 6 月叶片扩张，整个夏季光合生产力逐渐下降。因此，浮生三叶杨：（i）通过（ii）获取浅层土壤水分和更深的冲积地下水，以及（iii）提供昼夜气孔调节，（iv）避免在温暖干燥的夏季维持大量的用水和生产力木质部空化和 (v) 保持相当恒定的水力传导。