Oren R, Sperry J S, Ewers B E, Pataki D E, Phillips N, Megonigal J P
School of the Environment, Duke University, 27708, Durham, NC, USA.
The Duke University Wetland Center, 27708, Durham, NC, USA.
Oecologia. 2001 Jan;126(1):21-29. doi: 10.1007/s004420000497. Epub 2001 Jan 1.
We measured the xylem sap flux in 64-year-old Taxodium distichum (L.) Richard trees growing in a flooded forest using Granier-type sensors to estimate mean canopy stomatal conductance of the stand (G ). Temporal variations in G were investigated in relation to variation in vapor pressure deficit (D), photosynthetic photon flux density (Q ), and the transpiration rate per unit of leaf area (E ), the latter variable serving as a proxy for plant water potential. We found that G was only weakly related to Q below 500 µmol m s (r =0.29), but unrelated to Q above this value. Above Q =500 µmol m s and D=0.6 kPa, G decreased linearly with increasing E with a poor fit (r =0.31), and linearly with lnD with a much better fit (r =0.81). The decrease of G with lnD was at a rate predicted based on a simple hydraulic model in which stomata regulate the minimum leaf water potential. Based on the hydraulic model, stomatal sensitivity to D is proportional to stomatal conductance at low D. A hurricane caused an ~41% reduction in leaf area. This resulted in a 28% increase in G at D=1 kPa (G ), indicating only partial compensation. As predicted, the increase in G after the hurricane was accompanied by a similar increase in stomatal sensitivity to D (29%). At night, G was ~20% of the daytime value under non-limiting light (Q >500 µmol m s). However, stomatal sensitivity to D decreased only to ~46% (both reductions referenced to pre-hurricane daytime values), thus having more than twice the sensitivity expected based on hydraulic considerations alone. Therefore, non-hydraulic processes must cause heightened nighttime stomatal sensitivity to D.
我们使用Granier型传感器测量了生长在水淹森林中的64年生落羽杉树的木质部汁液流量，以估算林分的平均冠层气孔导度（G）。研究了G的时间变化与水汽压差（D）、光合光子通量密度（Q）以及单位叶面积蒸腾速率（E）变化的关系，后者作为植物水势的替代变量。我们发现，在低于500 μmol m⁻² s⁻¹时，G与Q的相关性较弱（r = 0.29），但高于此值时与Q无关。在Q = 500 μmol m⁻² s⁻¹且D = 0.6 kPa以上时，G随E的增加呈线性下降，拟合度较差（r = 0.31），而随lnD呈线性下降，拟合度较好（r = 0.81）。G随lnD的下降速率与基于简单水力模型预测的速率一致，在该模型中气孔调节叶片最小水势。基于水力模型，气孔对D的敏感性在低D时与气孔导度成正比。一场飓风导致叶面积减少了约41%。这使得在D = 1 kPa时G增加了28%（Gₘₐₓ），表明只是部分补偿。正如预测的那样，飓风过后G的增加伴随着气孔对D敏感性的类似增加（29%）。在夜间，在非限制光照（Q > 500 μmol m⁻² s⁻¹）下，G约为白天值的20%。然而，气孔对D的敏感性仅下降到约46%（两次下降均以飓风前白天值为参考），因此其敏感性是仅基于水力考虑预期值的两倍多。所以，非水力过程必定导致夜间气孔对D的敏感性增强。
