Sala A, Carey E V, Keane R E, Callaway R M
Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA.
Tree Physiol. 2001 Jul;21(11):717-25. doi: 10.1093/treephys/21.11.717.
In subalpine forests of the northern Rocky Mountains, fire exclusion has contributed to large-scale shifts from early-successional whitebark pine (Pinus albicaulis Engelm.) to late-successional subalpine fir (Abies lasiocarpa (Hook.) Nutt.), a species assumed to be more shade tolerant than whitebark pine and with leaf to sapwood area ratios (A(L):A(S)) over twice as high. Potential consequences of high A(L):A(S) for subalpine fir include reduced light availability and, if hydraulic sufficiency is maintained, increased whole-tree water use. We measured instantaneous gas exchange, carbon isotope ratios and sap flow of whitebark pine and subalpine fir trees of different sizes in the Sapphire Mountains of western Montana to determine: (1) whether species-specific differences in gas exchange are related to their assumed relative shade tolerance and (2) how differences in A(L):A(S) affect leaf- and whole-tree water use. Whitebark pine exhibited higher photosynthetic rates (A = 10.9 micromol x m(-2) x s(-1) +/- 1.1 SE), transpiration rates (E = 3.8 mmol x m(-2) x s(-1) +/- 0.7 SE), stomatal conductance (g(s) = 166.4 mmol x m(-2) x s(-1) +/- 5.3 SE) and carbon isotope ratios (delta13C = -25.5 per thousand +/- 0.2 SE) than subalpine fir (A = 5.7 micromol x m(-2) x s(-1) +/- 0.9 SE; E = 1.4 mmol x m(-2) x s(-1) +/- 0.3 SE; g(s) = 63.4 mmol x m(-2) x s(-1) +/- 1.2 SE, delta13C = -26.2 per thousand +/- 0.2 SE; P < 0.01 in all cases). Because subalpine fir had lower leaf-area-based sap flow than whitebark pine (QL = 0.33 kgx m(-2) x day(-1) +/- 0.03 SE and 0.76 kg x m(-2) x day(-1) +/- 0.06 SE, respectively; P < 0.001), the higher A(L):A(S) in subalpine fir did not result in direct proportional increases in whole-tree water use, although large subalpine firs used more water than large whitebark pines. The linear relationships between tree size and daily water use (r2 = 0.94 and 0.97 for whitebark pine and subalpine fir, respectively) developed at the Sapphire Mountains site were applied to trees of known size classes measured in 12 natural subalpine stands in the Bob Marshall Wilderness Complex (western Montana) ranging from 67 to 458 years old. Results indicated that the potential for subalpine forests to lose water by transpiration increases as succession proceeds and subalpine fir recruits into whitebark pine stands.
在落基山脉北部的亚高山森林中，禁止火灾导致了从早期演替的白皮松（Pinus albicaulis Engelm.）到晚期演替的亚高山冷杉（Abies lasiocarpa (Hook.) Nutt.）的大规模转变，亚高山冷杉被认为比白皮松更耐荫，其叶面积与边材面积比（A(L):A(S)）是白皮松的两倍多。亚高山冷杉高A(L):A(S)的潜在后果包括光照可用性降低，如果维持水力充足，则整棵树的水分利用增加。我们在蒙大拿州西部的蓝宝石山脉测量了不同大小的白皮松和亚高山冷杉树的瞬时气体交换、碳同位素比率和液流，以确定：（1）气体交换的物种特异性差异是否与其假定的相对耐荫性有关；（2）A(L):A(S)的差异如何影响叶片和整棵树的水分利用。白皮松的光合速率（A = 10.9微摩尔·平方米⁻²·秒⁻¹±1.1标准误）、蒸腾速率（E = 3.8毫摩尔·平方米⁻²·秒⁻¹±0.7标准误）、气孔导度（g(s) = 166.4毫摩尔·平方米⁻²·秒⁻¹±5.3标准误）和碳同位素比率（δ¹³C = -25.5‰±0.2标准误）均高于亚高山冷杉（A = 5.7微摩尔·平方米⁻²·秒⁻¹±0.9标准误；E = 1.4毫摩尔·平方米⁻²·秒⁻¹±0.3标准误；g(s) = 63.4毫摩尔·平方米⁻²·秒⁻¹±1.2标准误，δ¹³C = -26.2‰±0.2标准误；所有情况下P < 0.01）。由于亚高山冷杉基于叶面积的液流低于白皮松（分别为QL = 0.33千克·平方米⁻¹·天⁻¹±0.03标准误和0.76千克·平方米⁻¹·天⁻¹±0.06标准误；P < 0.001），亚高山冷杉较高的A(L):A(S)并未导致整棵树的水分利用直接成比例增加，尽管大型亚高山冷杉比大型白皮松消耗更多水分。在蓝宝石山脉站点建立的树大小与每日水分利用之间的线性关系（白皮松和亚高山冷杉的r²分别为0.94和0.97）被应用于在鲍勃·马歇尔荒野复合体（蒙大拿州西部）12个天然亚高山林分中测量的已知大小等级的树木，这些林分年龄从67岁到458岁不等。结果表明，随着演替的进行以及亚高山冷杉进入白皮松林分，亚高山森林通过蒸腾作用失水的潜力增加。
