Abstract

Semiarid forests in the southwestern USA are generally restricted to mountain regions where complex terrain adds to the challenge of characterizing stand productivity. Among the heterogeneous features of these ecosystems, topography represents an important control on system-level processes including snow accumulation and melt. This basic relationship between geology and hydrology affects radiation and water balances within the forests, with implications for canopy structure and function across a range of spatial scales. In this study, we quantify the effect of topographic aspect on primary productivity by observing the response of two codominant native tree species to seasonal changes in the timing and magnitude of energy and water inputs throughout a montane headwater catchment in Arizona, USA. On average, soil moisture on north-facing aspects remained higher during the spring and early summer compared with south-facing aspects. Repeated measurements of net carbon assimilation (A_net) showed that Pinus ponderosa C. Lawson was sensitive to this difference, while Pseudotsuga menziesii (Mirb.) Franco was not. Irrespective of aspect, we observed seasonally divergent patterns at the species level where P. ponderosa maintained significantly greater A_net into the fall despite more efficient water use by P. menziesii individuals during that time. As a result, this study at the southern extent of the geographical P. menziesii distribution suggests that this species could increase water-use efficiency as a response to future warming and/or drying, but at lower rates of production relative to the more drought-adapted P. ponderosa. At the sub-landscape scale, opposing aspects served as a mesocosm of current versus anticipated climate conditions. In this way, these results also constrain the potential for changing carbon sequestration patterns from Pinus-dominated landscapes due to forecasted changes in seasonal moisture availability.

中文翻译：

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地形影响半干旱山地森林的季节性初级生产力的特定物种模式。

摘要

美国西南部的半干旱森林通常局限于山区，复杂的地形增加了林分生产力特征的挑战。在这些生态系统的异质性特征中，地形代表着对系统级过程（包括积雪和融雪）的重要控制。地质学与水文学之间的这种基本关系会影响森林内的辐射和水平衡，并影响整个空间尺度上的冠层结构和功能。在这项研究中，我们通过观察美国亚利桑那州一个山区水源流域的两个主要原生树种对能源和水输入的时间和大小以及输入量的季节性变化的响应来量化地形对初级生产力的影响。一般，在春季和初夏，朝北地区的土壤湿度高于朝南地区。重复测量净碳同化（甲_净）显示，黄松C.劳森是这种差异敏感，而花旗松（Mirb。）佛朗哥没有。无论哪个方面，我们都观察到了物种水平的季节性差异模式，尽管在这段时间内孟席斯体育运动者更有效地利用了水，但黄松体育运动保持着更大的A_净值直至秋季。结果，这项对孟席斯疟原虫分布的南部范围的研究表明，该物种可以提高水利用效率，以应对未来的变暖和/或干燥，但相对于干旱程度更高的生产速率较低。改编绿脓杆菌。在亚景观尺度上，相反的方面充当了当前与预期气候条件的缩影。这样，由于预测的季节性水分供应变化，这些结果也限制了以松树为主的景观改变固碳模式的潜力。