Instituto de Ecología, Universidad Nacional Autónoma de México, Apartado Postal 70-275, 04510 Mexico DF, Mexico.
Oecologia. 2010 May;163(1):25-33. doi: 10.1007/s00442-009-1520-9. Epub 2009 Dec 3.
In the evergreen shrubland vegetation of Mexico (mexical), most of the species are sclerophyllous woody plants with steep leaf angles. This architectural pattern has been interpreted as a strategy to cope with water shortages and high radiation. However, the current association between evergreenness and steep leaf angles across mexical plant species could be the result of an adaptive association achieved through correlated evolutionary change between both traits or, alternatively, may be the result of common evolutionary ancestry. In this study, we quantified leaf angle in 28 dominant species under a phylogenetic framework and evaluated the functional implications of the observed range of leaf angles in terms of leaf temperature, water potentials and transpiration by combining manipulative experiments restraining leaves horizontally with microclimatic and stomatal conductance measurements in selected species and energy balance calculations. Horizontally restrained leaves exhibited reduced water potentials and stomatal conductances, and significantly increased temperatures and transpiration rates. Steeply inclined leaves operated near air temperatures and could sustain relatively high stomatal conductances during the dry season since they were associated with low transpiration rates. Phylogenetic analyses showed that steep leaf angles evolved in a correlated fashion in evergreen species. The functional consequences of leaf angle together with the phylogenetic analysis indicate the adaptive nature of this trait which allows the evergreen species to cope with arid conditions and therefore to persist within the mexical community.
在墨西哥的常绿灌木植被（mexical）中，大多数物种都是具有陡峭叶片角度的硬叶木本植物。这种结构模式被解释为一种应对水分短缺和高辐射的策略。然而，目前 mexical 植物物种中常绿性和陡峭叶片角度之间的关联可能是通过这两个特征之间的相关进化变化而实现的适应性关联的结果，或者可能是共同进化起源的结果。在这项研究中，我们在系统发育框架下量化了 28 个优势物种的叶片角度，并通过结合在选定物种中限制叶片水平的操作实验与微气候和气孔导度测量以及能量平衡计算，评估了观察到的叶片角度范围在叶片温度、水势和蒸腾方面的功能意义。水平限制的叶片表现出降低的水势和气孔导度，并且叶片温度和蒸腾速率显著增加。陡峭倾斜的叶片在空气温度附近运作，并且由于与低蒸腾速率相关，在旱季期间能够维持相对较高的气孔导度。系统发育分析表明，陡峭的叶片角度在常绿物种中以相关的方式进化。叶片角度的功能后果以及系统发育分析表明，这种特征具有适应性，使常绿物种能够应对干旱条件，从而在 mexical 群落中得以持续存在。
