Department of Integrative Biology, University of California, Berkeley, CA 94720, USA.
Tree Physiol. 2010 Oct;30(10):1260-72. doi: 10.1093/treephys/tpq064. Epub 2010 Jul 14.
Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (V(S)), transpiration per unit leaf area (E(L)) and stomatal conductance per unit leaf area (G(S)) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (A(S):A(L)) in the tallest trees. Both E(L) and G(S) declined as soil moisture decreased and vapor pressure deficit (D) increased throughout the growing season with a greater decline in shorter trees. Under high soil moisture and light conditions, reference G(S) (G(Sref); G(S) at D = 1 kPa) and sensitivity of G(S) to D (-δ; dG(S)/dlnD) significantly decreased with increasing height. The close relationship we observed between G(Sref) and -δ is consistent with the role of stomata in regulating E(L) and leaf water potential (Ψ(L)). Our results confirm that increasing tree height reduces gas exchange of treetop foliage and thereby contributes to lower carbon assimilation and height growth rates as S. sempervirens approaches maximum height.
随着树木接近最大高度，树冠层受到由重力引起的水分胁迫的限制越来越大。在这里，我们研究了在一个生长季节中，12 棵未受抑制的海岸红杉（Sequoiadendron sempervirens (D. Don) Endl.）高达 68-113 米的树冠顶部的高度对季节性和昼夜蒸腾液流动态的影响。平均树梢液流速度（V(S)）、单位叶面积蒸腾量（E(L)）和单位叶面积气孔导度（G(S)）随着高度的增加而显著降低。这些液流差异与最高树木中意想不到的树梢边材面积与叶面积比（A(S):A(L)）的降低有关。随着生长季节的进行，在土壤水分减少和水汽压亏缺（D）增加的情况下，E(L)和 G(S)都下降了，短树的下降幅度更大。在高土壤水分和光照条件下，参考气孔导度（G(Sref)；D = 1 kPa 时的 G(S)）和气孔导度对 D 的敏感性（-δ；dG(S)/dlnD）随着高度的增加而显著降低。我们观察到的 G(Sref)和-δ之间的密切关系与气孔在调节 E(L)和叶片水势（Ψ(L)）方面的作用一致。我们的结果证实，随着红杉接近最大高度，树高的增加会降低树冠树叶的气体交换，从而导致较低的碳同化和较高的生长速率。
