Division of Biological Sciences and Wildlife Biology Program, University of Montana, 32 Campus Drive, Missoula, Montana, 59812, USA.
Denali National Park and Preserve, P.O. Box 9, Denali Park, Alaska, 99755, USA.
Ecology. 2019 May;100(5):e02660. doi: 10.1002/ecy.2660. Epub 2019 Mar 22.
The expansion of shrubs and trees across high-latitude ecosystems is one of the most dramatic ecological manifestations of climate change. Most of the work quantifying these changes has been done in small areas and over relatively recent time scales. These land-cover transitions are highly spatially variable, and we have limited understanding of the factors underlying this variation. We use repeat photography to generate a data set of land-cover changes in Denali National Park and Preserve, Alaska, stretching back a century and spanning a range of edaphic, topographic, and climatic conditions. Most land-cover classes were quite stable, with low probabilities of transitioning to other land-cover types. The advance of woody vegetation into low-stature tundra, and the spread of conifer trees into shrub-dominated areas, were both more likely at low elevations and in areas without permafrost. Permafrost also reduced the likelihood of herbaceous vegetation transitioning to woody cover. Exceptions to the general trend of relative stability included nearly all (96%) of the broadleaf forest-dominated areas being invaded by conifers, an expected successional trajectory, and many open gravel river bars (17.8%) transitioning to thick shrubs. These floodplain areas were distinctly not at equilibrium, as only 0.1% of shrub-dominated areas converted to gravel. Warming temperatures in coming decades and concomitant declines in the extent of permafrost are predicted to enhance the spread of woody vegetation in Denali further, but only by ~3%. Land-cover transitions, notably the rapid advance of trees and shrubs observed in other studies, could be less likely and more spatially heterogeneous here than in other high-latitude systems.
灌木和树木在高纬度生态系统中的扩张是气候变化最显著的生态表现之一。大多数量化这些变化的工作都是在小范围内进行的，而且时间跨度相对较近。这些土地覆盖的转变具有高度的空间变异性，我们对导致这种变化的因素知之甚少。我们使用重复摄影来生成阿拉斯加德纳里国家公园和保护区的土地覆盖变化数据集，该数据集可以追溯到一个世纪前，涵盖了一系列土壤、地形和气候条件。大多数土地覆盖类型都非常稳定，向其他土地覆盖类型转变的可能性较低。木本植被向低海拔矮苔原的推进，以及针叶树向以灌木为主的地区的扩散，在低海拔和无永久冻土地区的可能性更大。永久冻土也降低了草本植被向木本植被转变的可能性。除了相对稳定的一般趋势之外，还包括几乎所有（96%）宽叶林主导地区都被针叶树入侵，这是一个预期的演替轨迹，以及许多开阔的砾石河漫滩（17.8%）转变为茂密的灌木。这些洪泛区显然没有达到平衡，因为只有 0.1%的灌木主导地区转变为砾石。未来几十年气温升高和永久冻土范围的相应下降预计将进一步增强德纳里的木本植被的扩散，但增幅仅为~3%。土地覆盖的转变，尤其是在其他研究中观察到的树木和灌木的快速推进，可能不如其他高纬度系统那样不太可能和空间异质性更大。
