Sala Anna
Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA.
Oecologia. 2006 Aug;149(1):1-11. doi: 10.1007/s00442-006-0420-5. Epub 2006 Apr 26.
As trees grow tall and the resistance of the hydraulic pathway increases, water supply to foliage may decrease forcing stomata to close and CO2 uptake to decline. Several structural (e.g. biomass allocation) and physiological adjustments, however, may partially or fully compensate for such hydraulic constraints and prevent limitations on CO2 uptake and growth. The degree to which trees compensate for hydraulic constraints as they grow tall may depend on the costs and benefits associated with hydraulic compensation according to their ecology and life history. Because later successional Rocky Mountain conifers are more shade tolerant, optimization of CO2 uptake as trees grow tall and shade increases may confer greater benefits than in earlier successional species. If so, higher compensation for hydraulic constraints is expected in later successional species relative to co-occurring earlier successional species. I have examined height-related changes of crown stomatal conductance on a leaf area basis (G(LA)) and leaf to sapwood ratios (A(L):A(S)) for five conifer species in the northern Rocky Mountains. Species were arranged in pairs, each pair consisting of an early and late successional species. For high elevations I used, respectively, whitebark pine (Pinus albicaulis) and subalpine fir (Abies lasiocarpa); for mid-elevations, western larch (Larix occidentalis) and Douglas-fir (Pseudotsuga menziesii); for lower elevations, ponderosa pine (Pinus ponderosa) and Douglas-fir. A(L):A(S) either decreased (subalpine fir, ponderosa pine), remained constant (Douglas-fir, western larch) or increased (whitebark pine) with tree height. As hypothesized, earlier successional species (ponderosa pine, whitebark pine and western larch) exhibited significantly stronger decreases of G(LA) with tree height relative to their later successional pairs (Douglas-fir and subalpine fir), which fully compensated for height-related hydraulic constraints on G(LA). A life history approach that takes into account the optimization of size- and species-specific ecological functions may also help researchers better understand biomass allocation and hydraulic function in trees.
随着树木长高，水力传导路径的阻力增加，叶片的水分供应可能减少，迫使气孔关闭，二氧化碳吸收量下降。然而，一些结构上的（如生物量分配）和生理上的调整可能会部分或完全补偿这种水力限制，防止二氧化碳吸收和生长受到限制。树木在长高过程中补偿水力限制的程度可能取决于与其生态和生活史相关的水力补偿的成本和收益。由于晚更新世落基山针叶树更耐荫，随着树木长高和遮荫增加，优化二氧化碳吸收可能比早期更新世物种带来更大的益处。如果是这样，相对于同时出现的早期更新世物种，预计晚更新世物种对水力限制的补偿会更高。我研究了落基山北部五种针叶树基于叶面积的树冠气孔导度（G(LA)）和叶与边材比（A(L):A(S)）与树高相关的变化。物种成对排列，每对由一个早期和一个晚期更新世物种组成。对于高海拔地区，我分别使用了白皮松（Pinus albicaulis）和亚高山冷杉（Abies lasiocarpa）；对于中海拔地区，使用了西部落叶松（Larix occidentalis）和花旗松（Pseudotsuga menziesii）；对于低海拔地区，使用了黄松（Pinus ponderosa）和花旗松。A(L):A(S)随着树高的增加要么降低（亚高山冷杉、黄松），保持不变（花旗松、西部落叶松），要么增加（白皮松）。如所假设的，相对于它们的晚期更新世配对物种（花旗松和亚高山冷杉），早期更新世物种（黄松、白皮松和西部落叶松）随着树高增加G(LA)的下降显著更强，而晚期更新世配对物种完全补偿了与树高相关的对G(LA)的水力限制。一种考虑到大小和物种特定生态功能优化的生活史方法也可能有助于研究人员更好地理解树木中的生物量分配和水力功能。
