Paleoenvironments and Chronoecology, Ecole Pratique des Hautes Etudes, Montpellier, France.
PLoS One. 2010 Aug 30;5(8):e12480. doi: 10.1371/journal.pone.0012480.
Forecasting the effects of global changes on high altitude ecosystems requires an understanding of the long-term relationships between biota and forcing factors to identify resilience thresholds. Fire is a crucial forcing factor: both fuel build-up from land-abandonment in European mountains, and more droughts linked to global warming are likely to increase fire risks.
To assess the vegetation response to fire on a millennium time-scale, we analyzed evidence of stand-to-local vegetation dynamics derived from sedimentary plant macroremains from two subalpine lakes. Paleobotanical reconstructions at high temporal resolution, together with a fire frequency reconstruction inferred from sedimentary charcoal, were analyzed by Superposed Epoch Analysis to model plant behavior before, during and after fire events.
We show that fuel build-up from arolla pine (Pinus cembra) always precedes fires, which is immediately followed by a rapid increase of birch (Betula sp.), then by ericaceous species after 25-75 years, and by herbs after 50-100 years. European larch (Larix decidua), which is the natural co-dominant species of subalpine forests with Pinus cembra, is not sensitive to fire, while the abundance of Pinus cembra is altered within a 150-year period after fires. A long-term trend in vegetation dynamics is apparent, wherein species that abound later in succession are the functional drivers, loading the environment with fuel for fires. This system can only be functional if fires are mainly driven by external factors (e.g. climate), with the mean interval between fires being longer than the minimum time required to reach the late successional stage, here 150 years.
Current global warming conditions which increase drought occurrences, combined with the abandonment of land in European mountain areas, creates ideal ecological conditions for the ignition and the spread of fire. A fire return interval of less than 150 years would threaten the dominant species and might override the resilience of subalpine forests.
预测全球变化对高海拔生态系统的影响需要了解生物群与胁迫因子之间的长期关系，以确定弹性阈值。火灾是一个关键的胁迫因子：欧洲山区的土地废弃导致燃料堆积，以及与全球变暖相关的更多干旱，都可能增加火灾风险。
为了评估千年时间尺度上的植被对火灾的响应，我们分析了来自两个亚高山湖泊的沉积植物大化石记录的林分-局部植被动态证据。高时间分辨率的古植物重建与从沉积木炭推断的火灾频率重建一起，通过叠加时段分析进行分析，以模拟火灾事件前后的植物行为。
我们表明，火绒松（Pinus cembra）的燃料堆积总是先于火灾，火灾后迅速增加桦树（Betula sp.），然后在 25-75 年后增加石南科植物，在 50-100 年后增加草本植物。欧洲落叶松（Larix decidua）是与火绒松共同占据亚高山森林的天然优势种，对火灾不敏感，而火灾后 150 年内火绒松的丰度发生了变化。植被动态的长期趋势很明显，其中后来演替的物种是功能驱动者，为火灾提供燃料，使环境负载增加。如果火灾主要由外部因素（如气候）驱动，且火灾之间的平均间隔大于达到后期演替阶段所需的最短时间（此处为 150 年），那么该系统才具有功能性。
目前增加干旱发生的全球变暖条件，加上欧洲山区土地的废弃，为火灾的点火和蔓延创造了理想的生态条件。少于 150 年的火灾回返期将威胁优势物种，可能会破坏亚高山森林的弹性。
