Interactions between local and global drivers determine long-term trends in boreal forest understory vegetation

Aim: Global change effects on forest ecosystems are increasingly claimed to be context dependent, indicated by interactions between global and local environmental drivers. Most examples of such context dependencies originate from temperate systems, while limited research comes from the boreal biome. Here we set out to test if interactions between climate warming, nitrogen deposition, land-use change resulting in increasing forest density and soil pH drive long-term changes of understory vegetation in boreal forests. Location: Sweden Time period: 1953-2012 Major taxa studied: Vascular plants Methods: We used long-term (50 years) National Forest Inventory data on forest understory vegetation in Sweden to model the combined effects of climate warming, nitrogen deposition, increase in forest density (tree basal area) and soil pH. Results: Our results identify increasing temperature, nitrogen deposition and denser, shadier forest conditions as main drivers of understory vegetation changes during this time period. More importantly, we found that these effects varied with local conditions, i.e. that the change towards a more nitrophilic understory vegetation was more pronounced at low than high soil pH. Forest density was an important modulator of nitrogen deposition and temperature increase, with effects generally decreasing with density. Decreased cover of ericaceous dwarf shrubs was driven by both forest density and nitrogen deposition, with a stronger effect at low than at high pH. Main conclusions: Our results highlight that to understand forest ecosystems´ response to global change, and to make adequate management decisions to mitigate the effects of global change, we need to understand how changes in local environmental factors (forest density and soil pH) interact with global-scale drivers (nitrogen deposition and climate warming). Neglecting such interactions will lead to incorrect estimations of effects. In our case, we would e.g. have underestimated the eutrophication effects on acid soils, which constitute a considerable part of the boreal biome. --

研究目的：学界日益认为全球变化对森林生态系统的影响具有情境依赖性，其核心体现为全球尺度与局地尺度环境驱动因子间的交互作用。目前此类情境依赖性的研究案例多源自温带生态系统，而针对寒带生物群区（boreal biome）的相关研究较为匮乏。本研究旨在检验气候变暖、氮沉降、土地利用变化导致的森林密度升高，以及土壤pH值之间的交互作用，是否驱动了寒带森林林下植被的长期动态变化。
研究区域：瑞典
研究时段：1953年—2012年
研究类群：维管植物（Vascular plants）
研究方法：本研究采用瑞典长达50年的国家森林清查（National Forest Inventory）数据中的林下植被数据，构建模型以解析气候变暖、氮沉降、森林密度（以树木胸高断面积表征）升高以及土壤pH值的综合影响。
研究结果：本研究结果显示，气温升高、氮沉降以及森林密度增加、林下光照减弱的林分条件，是该时段内林下植被变化的主要驱动因子。更为关键的是，上述驱动效应存在显著的局地环境差异：即向着更偏好氮素的林下植被群落的转变，在低土壤pH值样地中表现得更为显著。森林密度是氮沉降与气温升高效应的重要调节因子，其整体调控强度随森林密度升高而减弱。杜鹃花科矮生灌木的盖度降低同时受到森林密度与氮沉降的驱动，且在低土壤pH值条件下的抑制效应更强。
主要结论：本研究结果表明，若要准确理解森林生态系统对全球变化的响应，并制定合理的管理决策以减缓全球变化的负面影响，我们需要厘清局地环境因子（森林密度与土壤pH值）与全球尺度驱动因子（氮沉降与气候变暖）之间的交互作用机制。忽略此类交互作用将导致效应评估出现偏差。以本研究为例，若未考虑交互效应，我们会低估酸性土壤（这类土壤在寒带生物群区中占比可观）的富营养化影响。