Key Laboratory Soil and Water Conservation and Desertification Combating, Ministry of Education, College of Soil and Water Conservation, Beijing Forestry University, Beijing, People's Republic of China.
PLoS One. 2012;7(10):e47882. doi: 10.1371/journal.pone.0047882. Epub 2012 Oct 31.
The functional convergence of tree transpiration has rarely been tested for tree species growing under urban conditions even though it is of significance to elucidate the relationship between functional convergence and species differences of urban trees for establishing sustainable urban forests in the context of forest water relations.
METHODOLOGY/PRINCIPAL FINDINGS: We measured sap flux of four urban tree species including Cedrus deodara, Zelkova schneideriana, Euonymus bungeanus and Metasequoia glyptostroboides in an urban park by using thermal dissipation probes (TDP). The concurrent microclimate conditions and soil moisture content were also measured. Our objectives were to examine 1) the influence of tree species and size on transpiration, and 2) the hydraulic control of urban trees under different environmental conditions over the transpiration in response to VPD as represented by canopy conductance. The results showed that the functional convergence between tree diameter at breast height (DBH) and tree canopy transpiration amount (E(c)) was not reliable to predict stand transpiration and there were species differences within same DBH class. Species differed in transpiration patterns to seasonal weather progression and soil water stress as a result of varied sensitivity to water availability. Species differences were also found in their potential maximum transpiration rate and reaction to light. However, a same theoretical hydraulic relationship between G(c) at VPD = 1 kPa (G(cref)) and the G(c) sensitivity to VPD (-dG(c)/dlnVPD) across studied species as well as under contrasting soil water and R(s) conditions in the urban area.
CONCLUSIONS/SIGNIFICANCE: We concluded that urban trees show the same hydraulic regulation over response to VPD across varying tree size and environmental conditions and thus tree transpiration could be predicted with appropriate assessment of G(cref).
尽管在森林水分关系的背景下阐明功能趋同与城市树木物种差异之间的关系对于建立可持续的城市森林具有重要意义，但对于生长在城市条件下的树种，其树木蒸腾的功能趋同很少得到测试。
方法/主要发现：我们使用热耗散探针（TDP）测量了城市公园中包括雪松、榉树、卫矛和水杉在内的四个城市树种的树干液流。同时还测量了同期的小气候条件和土壤水分含量。我们的目标是检验 1）树种和大小对蒸腾的影响，以及 2）在不同环境条件下，树木对蒸腾的水力控制，蒸腾由树冠导度代表的 VPD 表示。结果表明，树干胸径（DBH）与树冠蒸腾量（E(c)）之间的功能趋同并不可靠，无法预测林分蒸腾，而且在同一 DBH 类内存在物种差异。由于对水分可用性的敏感性不同，物种在蒸腾模式上存在差异，表现为对季节性天气变化和土壤水分胁迫的响应。还发现了物种之间在潜在最大蒸腾速率和对光的反应上的差异。然而，在所研究的物种以及在城市地区不同的土壤水分和 R(s)条件下，VPD = 1 kPa 时的 G(c)（G(cref)）与 G(c)对 VPD 的敏感性（-dG(c)/dlnVPD）之间存在相同的理论水力关系。
结论/意义：我们得出结论，城市树木在不同的树体大小和环境条件下对 VPD 的响应表现出相同的水力调节，因此可以通过对 G(cref)的适当评估来预测树木蒸腾。
