Pathways of tundra encroachment by trees and tall shrubs in the western Brooks Range of Alaska
Pathways of tundra encroachment by trees and tall shrubs in the western Brooks Range of Alaska
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DOI：
10.1111/ecog.05015
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发表时间：
2020-02
期刊：
影响因子：
5.9
通讯作者：
A. Terskaia;R. Dial;P. Sullivan
中科院分区：
文献类型：
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作者：
A. Terskaia;R. Dial;P. Sullivan
气候变化预计将增加北极木本植物的丰富度，但变化的幅度、空间格局和路径仍不确定。我们比较了历史正射影像(1952和1979)和高分辨率卫星图像(2015)，以研究60年来白云杉和高大灌木(Salix spp.，Alnus spp.)丰度的变化。阿拉斯加西北部的阿加索克河附近。我们在5500公顷的研究区域内设立了3000个随机点，将其划分为9个盖度类型。为了考察地形对树木丰富度的控制，我们用来自数字海拔模型的预报器来拟合多项式对数线性模型，并将北极冻土带、高山冻土带和树木作为分类响应变量的水平。在1952年至2015年间，被归类为北极和高山冻土带的点分别减少了31%和15%。高灌木增加了86%，乔木与高灌木混交增加了385%，森林增加了84%。具有高灌木的冻土带很少过渡到森林。最好的多项式模型解释了71%的盖度变化，包括海拔、坡度和坡度与北度的交互作用。树线被定义为出现树木的概率等于冻土带的高度。2015年平均树线海拔为202米，相当于6-8月的平均气温&11°C，比全球树线海拔的6-7°C等温线高4°C。我们的结果显示乔木和高大灌木的数量急剧增加，质疑气温作为树线高度预测因子的普遍性，并提出了两条相互排斥的植被变化途径，因为获得高大灌木的冻土带很少过渡到森林。冻土带转变为高大的灌木和森林，对北极的碳循环、地表能量交换和野生动物栖息地具有重要的、潜在的相反影响。
Climate change is expected to increase woody vegetation abundance in the Arctic, yet the magnitude, spatial pattern and pathways of change remain uncertain. We compared historical orthophotos photos (1952 and 1979) with high‐resolution satellite imagery (2015) to examine six decades of change in abundance of white spruce Picea glauca and tall shrubs (Salix spp., Alnus spp.) near the Agashashok River in northwest Alaska. We established ~3000 random points within our ~5500 ha study area for classification into nine cover types. To examine physiographic controls on tree abundance, we fit multinomial log‐linear models with predictors derived from a digital elevation model and with arctic tundra, alpine tundra and ‘tree’ as levels of a categorical response variable. Between 1952 and 2015, points classified as arctic and alpine tundra decreased by 31% and 15%, respectively. Meanwhile, tall shrubs increased by 86%, trees mixed with tall shrubs increased by 385% and forest increased by 84%. Tundra with tall shrubs rarely transitioned to forest. The best multinomial model explained 71% of variation in cover and included elevation, slope and an interaction between slope and ‘northness’. Treeline was defined as the elevation where the probability of tree presence equaled that of tundra. Mean treeline elevation in 2015 was 202 m, corresponding with a June–August mean air temperature > 11°C, which is > 4°C warmer than the 6–7°C isotherm associated with global treeline elevations. Our results show dramatic increases in the abundance of trees and tall shrubs, question the universality of air temperature as a predictor of treeline elevation and suggest two mutually exclusive pathways of vegetation change, because tundra that gained tall shrubs rarely transitioned to forest. Conversion of tundra to tall shrubs and forest has important and potentially contrasting implications for carbon cycling, surface energy exchange and wildlife habitat in the Arctic.