Shestakova Tatiana A, Aguilera Mònica, Ferrio Juan Pedro, Gutiérrez Emilia, Voltas Jordi
Department of Ecology, University of Barcelona, Avda Diagonal 645, E-08028 Barcelona, Spain.
Department of Crop and Forest Sciences-AGROTECNIO Center, University of Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain.
Tree Physiol. 2014 Aug;34(8):819-38. doi: 10.1093/treephys/tpu037. Epub 2014 May 28.
Identifying how physiological responses are structured across environmental gradients is critical to understanding in what manner ecological factors determine tree performance. Here, we investigated the spatiotemporal patterns of signal strength of carbon isotope discrimination (Δ(13)C) and oxygen isotope composition (δ(18)O) for three deciduous oaks (Quercus faginea (Lam.), Q. humilis Mill. and Q. petraea (Matt.) Liebl.) and one evergreen oak (Q. ilex L.) co-occurring in Mediterranean forests along an aridity gradient. We hypothesized that contrasting strategies in response to drought would lead to differential climate sensitivities between functional groups. Such differential sensitivities could result in a contrasting imprint on stable isotopes, depending on whether the spatial or temporal organization of tree-ring signals was analysed. To test these hypotheses, we proposed a mixed modelling framework to group isotopic records into potentially homogeneous subsets according to taxonomic or geographical criteria. To this end, carbon and oxygen isotopes were modelled through different variance-covariance structures for the variability among years (at the temporal level) or sites (at the spatial level). Signal-strength parameters were estimated from the outcome of selected models. We found striking differences between deciduous and evergreen oaks in the organization of their temporal and spatial signals. Therefore, the relationships with climate were examined independently for each functional group. While Q. ilex exhibited a large spatial dependence of isotopic signals on the temperature regime, deciduous oaks showed a greater dependence on precipitation, confirming their higher susceptibility to drought. Such contrasting responses to drought among oak types were also observed at the temporal level (interannual variability), with stronger associations with growing-season water availability in deciduous oaks. Thus, our results indicate that Mediterranean deciduous and evergreen oaks constitute two clearly differentiated functional groups in terms of their carbon and water economies, despite co-existing in a wide range of environments. In contrast, deciduous oaks form a rather homogeneous group in terms of climate sensitivity.
确定生理反应如何在环境梯度上构建，对于理解生态因素以何种方式决定树木生长状况至关重要。在此，我们研究了沿干旱梯度在地中海森林中共存的三种落叶栎（欧洲水青冈（拉马克）、矮生栎、柔毛栎）和一种常绿栎（冬青栎）的碳同位素判别（Δ(13)C）和氧同位素组成（δ(18)O）信号强度的时空模式。我们假设，对干旱的不同应对策略会导致功能组之间气候敏感性的差异。这种差异敏感性可能会在稳定同位素上留下不同的印记，这取决于对年轮信号的空间或时间组织进行分析。为了检验这些假设，我们提出了一个混合建模框架，根据分类学或地理标准将同位素记录分组为潜在的同质子集。为此，通过不同的方差协方差结构对碳和氧同位素在年份间（时间层面）或地点间（空间层面）的变异性进行建模。从所选模型的结果中估计信号强度参数。我们发现落叶栎和常绿栎在其时间和空间信号组织上存在显著差异。因此，针对每个功能组独立研究了它们与气候的关系。虽然冬青栎的同位素信号对温度状况表现出较大的空间依赖性，但落叶栎对降水的依赖性更强，这证实了它们对干旱的更高敏感性。在时间层面（年际变异性）也观察到栎树类型对干旱的这种不同反应，落叶栎与生长季水分可利用性的关联更强。因此，我们的结果表明，尽管地中海地区的落叶栎和常绿栎在广泛的环境中共存，但就其碳和水分经济而言，它们构成了两个明显不同的功能组。相比之下，落叶栎在气候敏感性方面形成了一个相当同质的群体。
