Growth decline and divergent tree ring isotopic composition (13C and 18O) contradict predictions of CO2 stimulation in high altitudinal forests
Growth decline and divergent tree ring isotopic composition (13C and 18O) contradict predictions of CO2 stimulation in high altitudinal forests
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DOI：
10.1111/gcb.12170
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发表时间：
2013-06-01
影响因子：
11.6
通讯作者：
Horwath, William R.
中科院分区：
文献类型：
--
作者：
Gomez-Guerrero, Armando;Silva, Lucas C. R.;Horwath, William R.
人类引起的大气组成变化预计将影响陆地生物群的初级生产力。最近在许多森林生态系统中观察到了生产力的变化，但低纬度上树线森林仍有待调查。在这里，我们使用树干年代学方法和同位素分析来研究生长在墨西哥中部高海拔森林中的冷杉和哈特韦吉松树生产力的变化及其生理基础。在从太平洋到墨西哥湾的纵向样带(横向火山轴)上选择了六个地点，在那里从3200到4000毫升的高度对成熟的优势树木进行了采样。共分析了60棵Ar树和84棵Ph树，以描述自20世纪初以来的生长(年分辨率)和同位素组成(年代分辨率)的变化。我们的结果表明，在上个世纪的前半个世纪中，断面积增量(BAI)开始普遍增加。然而，自20世纪50年代以来，白垩纪的数量显著减少，20世纪80年代后，无论是物种还是跨地点，白垩纪的数量都出现了显著下降。我们发现，由于水分利用效率的提高(2060%)，大气中的13C分辨率持续下降，与大气中二氧化碳的上升相吻合。然而，13C辨别力的变化并没有伴随着树轮18O的变化，这表明不同地点和物种在水源或吸收策略上的差异。我们的结果表明，CO2刺激不足以抵消变暖诱导的干旱胁迫，但其他胁迫因素，如渐进性营养限制，也可能导致生长下降。未来的研究应该探索资源限制(水与营养)在调节高海拔生态系统对大气变化的反应方面的独特作用。
Human-induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low-latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high-elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000masl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual-resolution) and isotopic composition (decadal-resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood 13C discrimination, resulting from increasing water use efficiency (2060%), coinciding with rising atmospheric CO2. Changes in 13C discrimination were not followed, however, by shifts in tree ring 18O, indicating site- and species-specific differences in water source or uptake strategy. Our results indicate that CO2 stimulation has not been enough to counteract warming-induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high-elevation ecosystems to atmospheric change.