Marshall John D, Monserud Robert A
Department of Forest Resources, University of Idaho, Moscow, ID 83843, USA.
Tree Physiol. 2006 Aug;26(8):1055-66. doi: 10.1093/treephys/26.8.1055.
The stable oxygen isotope ratio (delta(18)O) of tree-ring cellulose is jointly determined by the delta(18)O of xylem water, the delta(18)O of atmospheric water vapor, the humidity of the atmosphere and perhaps by species-specific differences in leaf structure and function. Atmospheric humidity and the delta(18)O of water vapor vary seasonally and annually, but if the canopy atmosphere is well mixed, atmospheric characteristics should be uniform among co-occurring trees. In contrast, xylem water delta(18)O is determined by the delta(18)O of water being drawn from the soil, which varies with depth. If co-occurring trees draw water from different soil depths, this soil-water delta(18)O signal would be manifest as differences in delta(18)O among the trees. We examined the variation in tree ring delta(18)O, over eight decades during the 20th Century, among three species co-occurring in natural forest stands of the northern Rocky Mountains in the USA. We sampled 10 Douglas-firs (Pseudotsuga menziesii (Mirb.) Franco var. glauca), 10 ponderosa pines (Pinus ponderosa Laws.) and seven western white pines (Pinus monticola Dougl.). As expected, variation in atmospheric conditions was recorded in the delta(18)O of the cellulose produced in a given year, but observed climatic correlations with delta(18)O were weak. Significant correlations with June climate data included: daily maximum temperature (r = 0.29), daily minimum temperature (r = -0.25), mean temperature (r = 0.20), mean daily precipitation (r = -0.54), vapor pressure deficit (r = 0.32) and solar radiation (r = 0.44). Lagged effects were observed in Douglas-fir and western white pine. In these species, the delta(18)O of a given annual ring was correlated with the delta(18)O of the previous ring. Ponderosa pine showed no significant autocorrelation. Although the species means were correlated among years (r = 0.67 to 0.76), ponderosa pine was consistently enriched in delta(18)O relative to the other species; differences were close to 2 per thousand and they are steadily increasing. Relative to the mean for the three species, ponderosa pine is becoming steadily more enriched (-1.0 per thousand). In contrast, Douglas-fir is being steadily depleted and western pine is intermediate, with an enrichment of 0.5 per thousand. Because all trees were exposed to the same atmospheric conditions, the differences in delta(18)O observed between species are likely due either to differences in the depth of water extraction or leaf function. If the former, presumably ponderosa pine has steadily taken up more water from near the soil surface and Douglas-fir has shifted uptake to a greater depth. If the latter, we suggest the pronounced changes in leaf-water delta(18)O are a result of changes in leaf structure and function with tree size and age.
树木年轮纤维素的稳定氧同位素比率（δ¹⁸O）由木质部水的δ¹⁸O、大气水汽的δ¹⁸O、大气湿度共同决定，或许还受叶片结构和功能的物种特异性差异影响。大气湿度和水汽的δ¹⁸O随季节和年份变化，但如果冠层大气充分混合，共存树木间的大气特征应是一致的。相反，木质部水的δ¹⁸O由从土壤中汲取的水的δ¹⁸O决定，而这会随深度变化。如果共存树木从不同土壤深度取水，这种土壤水δ¹⁸O信号将表现为树木间δ¹⁸O的差异。我们研究了20世纪八十多年间，美国北落基山天然林分中共存的三个树种的树木年轮δ¹⁸O变化。我们采集了10棵花旗松（Pseudotsuga menziesii (Mirb.) Franco var. glauca）、10棵黄松（Pinus ponderosa Laws.）和7棵西部白松（Pinus monticola Dougl.）的样本。正如预期，特定年份产生的纤维素的δ¹⁸O记录了大气条件的变化，但观测到的气候与δ¹⁸O的相关性较弱。与6月气候数据的显著相关性包括：日最高温度（r = 0.29）、日最低温度（r = -0.25）、平均温度（r = 0.20）、日平均降水量（r = -0.54）、水汽压亏缺（r = 0.32）和太阳辐射（r = 0.44）。在花旗松和西部白松中观察到了滞后效应。在这些树种中，特定年轮的δ¹⁸O与前一年轮的δ¹⁸O相关。黄松未显示出显著的自相关性。尽管各物种的均值在年份间相关（r = 0.67至0.76），但相对于其他物种，黄松的δ¹⁸O始终富集；差异接近2‰，且在稳步增加。相对于三个物种的均值，黄松的富集程度正在稳步增加（-1.0‰）。相比之下，花旗松在稳步贫化，西部白松处于中间水平，富集程度为0.5‰。由于所有树木都暴露在相同的大气条件下，物种间观察到的δ¹⁸O差异可能是由于水分提取深度或叶片功能的差异。如果是前者，推测黄松可能一直在从土壤表层附近吸收更多水分，而花旗松已将吸水转移到更深的深度。如果是后者，我们认为叶片水δ¹⁸O的显著变化是叶片结构和功能随树木大小和年龄变化的结果。
