Department of Geography, University of Missouri, Columbia, Missouri 65211, USA.
Ecology. 2012 Jul;93(7):1614-25. doi: 10.1890/11-1220.1.
Given the widespread and often dramatic influence of climate change on terrestrial ecosystems, it is increasingly common for abrupt threshold changes to occur, yet explicitly testing for climate and ecological regime shifts is lacking in climatically sensitive upper treeline ecotones. In this study, quantitative evidence based on empirical data is provided to support the key role of extrinsic, climate-induced thresholds in governing the spatial and temporal patterns of tree establishment in these high-elevation environments. Dendroecological techniques were used to reconstruct a 420-year history of regeneration dynamics within upper treeline ecotones along a latitudinal gradient (approximately 44-35 degrees N) in the Rocky Mountains. Correlation analysis was used to assess the possible influence of minimum and maximum temperature indices and cool-season (November-April) precipitation on regional age-structure data. Regime-shift analysis was used to detect thresholds in tree establishment during the entire period of record (1580-2000), temperature variables significantly Correlated with establishment during the 20th century, and cool-season precipitation. Tree establishment was significantly correlated with minimum temperature during the spring (March-May) and cool season. Regime-shift analysis identified an abrupt increase in regional tree establishment in 1950 (1950-1954 age class). Coincident with this period was a shift toward reduced cool-season precipitation. The alignment of these climate conditions apparently triggered an abrupt increase in establishment that was unprecedented during the period of record. Two main findings emerge from this research that underscore the critical role of climate in governing regeneration dynamics within upper treeline ecotones. (1) Regional climate variability is capable of exceeding bioclimatic thresholds, thereby initiating synchronous and abrupt changes in the spatial and temporal patterns of tree establishment at broad regional scales. (2) The importance of climate parameters exceeding critical threshold values and triggering a regime shift in tree establishment appears to be contingent on the alignment of favorable temperature and moisture regimes. This research suggests that threshold changes in the climate system can fundamentally alter regeneration dynamics within upper treeline ecotones and, through the use of regime-shift analysis, reveals important climate-vegetation linkages.
鉴于气候变化对陆地生态系统的广泛而深远的影响，生态系统经常会发生突然的阈值变化，但在气候敏感的高山林线生态交错带中，明确测试气候和生态系统的状态变化却很少见。在这项研究中，基于经验数据的定量证据提供了支持，证明了外在的、气候诱导的阈值在控制这些高海拔环境中树木建立的时空模式方面起着关键作用。利用树木年代学技术，沿着落矶山脉的一个纬度梯度（约北纬 44 度至 35 度）重建了高山林线生态交错带 420 年的再生动态历史。相关分析用于评估最小和最大温度指数以及凉爽季节（11 月至 4 月）降水对区域年龄结构数据的可能影响。状态转变分析用于检测整个记录期间（1580 年至 2000 年）树木建立的阈值、与 20 世纪建立显著相关的温度变量以及凉爽季节的降水。树木的建立与春季（3 月至 5 月）和凉爽季节的最低温度显著相关。状态转变分析确定了 1950 年（1950-1954 年龄组）区域树木建立的突然增加。与这一时期同时发生的是凉爽季节降水的减少。这些气候条件的一致变化显然引发了建立的突然增加，这在记录期间是前所未有的。这项研究得出了两个主要发现，强调了气候在控制高山林线生态交错带再生动态方面的关键作用。（1）区域气候变异性能够超过生物气候阈值，从而在广泛的区域尺度上引发树木建立的时空模式的同步和突然变化。（2）气候参数超过临界阈值并引发树木建立的状态变化的重要性似乎取决于有利的温度和水分模式的一致。这项研究表明，气候系统中的阈值变化可以从根本上改变高山林线生态交错带的再生动态，并且通过使用状态转变分析，揭示了重要的气候-植被联系。
