Ewers B E, Mackay D S, Samanta S
Department of Botany, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA.
Tree Physiol. 2007 Jan;27(1):11-24. doi: 10.1093/treephys/27.1.11.
We investigated interannual variability of canopy transpiration per unit ground area (E (C)) and per unit leaf area (E (L)) across seven tree species in northern Wisconsin over two years. These species have previously been shown to be sufficient to upscale stand-level transpiration to the landscape level during one growing season. Our objective was to test whether a simple plant hydraulic model could capture interannual variation in transpiration. Three species, wetland balsam fir (Abies balsamea (L.) Mill), basswood (Tilia Americana L.) and speckled alder (Alnus rugosa (DuRoi) Spreng), had no change in E (C) or E (L) between 2000 and 2001. Red pine (Pinus resinosa Ait) had a 57 and 19% increase in E (C) and E (L), respectively, and sugar maple (Acer saccharum Marsh) had an 83 and 41% increase in E (C) and E (L), respectively, from 2000 to 2001. Quaking aspen (Populus tremuloides Michx) had a 50 and 21% decrease in E (C) and E (L), respectively, from 2000 to 2001 in response to complete defoliation by forest tent caterpillar (Malascoma distria Hüber) and subsequent lower total leaf area index of the reflushed foliage. White cedar (Thuja occidentalis L.) had a 20% decrease in both E (C) and E (L) caused by lowered surface water in wetlands in 2001 because of lower precipitation and wetland flow management. Upland A. balsamea increased E (L) and E (C) by 55 and 53%, respectively, as a result of release from light competition of the defoliated, overstory P. tremuloides. We hypothesized that regardless of different drivers of interannual variability in E (C) and E (L), minimum leaf water potential would be regulated at the same value. Minimum midday water potentials were consistent over the two years within each of the seven species despite large changes in transpiration between years. This regulation was independently verified by the exponential saturation between daily E (C) and vapor pressure deficit (D) and the tradeoff between a reference canopy stomatal conductance (G (S)) and the sensitivity of G (S) to D, indicating that trees with high G (S) must decrease G (S) in response to atmospheric drought faster than trees with low G (S). Our results show that models of forest canopy transpiration can be simplified by incorporating G (S) regulation of minimum leaf water potential for isohydric species.
我们在两年时间里，对威斯康星州北部七种树木单位地面面积（E(C)）和单位叶面积（E(L)）的冠层蒸腾作用的年际变化进行了调查。此前已表明，在一个生长季节中，这些树种足以将林分水平的蒸腾作用扩大到景观水平。我们的目标是测试一个简单的植物水力模型是否能够捕捉蒸腾作用的年际变化。2000年至2001年期间，湿地香脂冷杉（Abies balsamea (L.) Mill）、椴树（Tilia Americana L.）和斑点桤木（Alnus rugosa (DuRoi) Spreng）这三个树种的E(C)或E(L)没有变化。红松（Pinus resinosa Ait）的E(C)和E(L)分别增加了57%和19%，糖枫（Acer saccharum Marsh）的E(C)和E(L)分别增加了83%和41%。由于受到森林天幕毛虫（Malascoma distria Hüber）的完全落叶以及随后重新长出的树叶总叶面积指数较低的影响，2000年至2001年期间，颤杨（Populus tremuloides Michx）的E(C)和E(L)分别下降了50%和2l%。2001年，由于降水量减少和湿地水流管理，湿地水位降低，导致美国白松（Thuja occidentalis L.）的E(C)和E(L)均下降了20%。由于落叶的上层颤杨的光照竞争解除，高地香脂冷杉的E(L)和E(C)分别增加了55%和53%。我们假设，无论E(C)和E(L)年际变化的驱动因素如何，最小叶水势都将被调节到相同的值。尽管不同年份之间蒸腾作用变化很大，但七个树种中每个树种在这两年内的中午最小水势都是一致的。通过每日E(C)与水汽压差（D）之间的指数饱和关系以及参考冠层气孔导度（G(S)）与G(S)对D的敏感性之间的权衡，独立验证了这种调节，这表明气孔导度高的树木必须比气孔导度低的树木更快地响应大气干旱而降低G(S)。我们的结果表明，对于等水物种，通过纳入最小叶水势的G(S)调节，可以简化森林冠层蒸腾作用模型。
