Duursma R A, Kolari P, Perämäki M, Nikinmaa E, Hari P, Delzon S, Loustau D, Ilvesniemi H, Pumpanen J, Mäkelä A
Department of Forest Ecology, University of Helsinki, P.O. Box 27, FIN-00014, Finland.
Tree Physiol. 2008 Feb;28(2):265-76. doi: 10.1093/treephys/28.2.265.
The effect of drought on forest water use is often estimated with models, but comprehensive models require many parameters, and simple models may not be sufficiently flexible. Many tree species, Pinus species in particular, have been shown to maintain a constant minimum leaf water potential above the critical threshold for xylem embolism during drought. In such cases, prediction of the relative decline in daily maximum transpiration rate with decreasing soil water content is relatively straightforward. We constructed a soil-plant water flow model assuming constant plant conductance and daily minimum leaf water potential, but variable conductance from soil to root. We tested this model against independent data from two sites: automatic shoot chamber data and sap flow measurements from a boreal Scots pine (Pinus sylvestris L.) stand; and sap flow measurements from a maritime pine (Pinus pinaster Ait.) stand. To focus on soil limitations to water uptake, we expressed daily maximum transpiration rate relative to the rate that would be obtained in wet soil with similar environmental variables. The comparison was successful, although the maritime pine stand showed carry-over effects of the drought that we could not explain. For the boreal Scots pine stand, daily maximum transpiration was best predicted by water content of soil deeper than 5 cm. A sensitivity analysis revealed that model predictions were relatively insensitive to the minimum leaf water potential, which can be accounted for by the importance of soil resistance of drying soil. We conclude that a model with constant plant conductance and minimum leaf water potential can accurately predict the decline in daily maximum transpiration rate during drought for these two pine stands, and that including further detail about plant compartments would add little predictive power, except in predicting recovery from severe drought.
干旱对森林水分利用的影响通常用模型来估算，但综合模型需要许多参数，而简单模型可能灵活性不足。许多树种，尤其是松树，已被证明在干旱期间能将叶片水势维持在高于木质部栓塞临界阈值的恒定最低水平。在这种情况下，预测随着土壤含水量降低每日最大蒸腾速率的相对下降相对简单。我们构建了一个土壤 - 植物水流模型，假设植物导度和每日最低叶片水势恒定，但从土壤到根系的导度可变。我们用来自两个地点的独立数据对该模型进行了测试：一个北方苏格兰松（Pinus sylvestris L.）林分的自动枝条室数据和液流测量数据；以及一个海岸松（Pinus pinaster Ait.）林分的液流测量数据。为了关注土壤对水分吸收的限制，我们将每日最大蒸腾速率表示为相对于在具有相似环境变量的湿润土壤中所能获得的速率。比较结果是成功的，尽管海岸松林分显示出我们无法解释的干旱遗留效应。对于北方苏格兰松林分，土壤深度超过5厘米处的含水量能最好地预测每日最大蒸腾量。敏感性分析表明，模型预测对最低叶片水势相对不敏感，这可以由干燥土壤的土壤阻力的重要性来解释。我们得出结论，一个具有恒定植物导度和最低叶片水势的模型可以准确预测这两个松林分在干旱期间每日最大蒸腾速率的下降，并且除了预测从严重干旱中恢复外，纳入关于植物各部分的更多细节增加的预测能力很小。
