Resurgence of warmth-demanding tree species and a common pending thermophilization in subalpine and northern boreal Sweden-an ecological signal of post-Little Ice Age climate improvement

Abstract

Largely consistent with general predictions and earlier empirical studies, it appears that post-Little Ice Age climate warming has started to affect large-scale biogeographic patterns in northern Sweden. Long-term monitoring in subalpine and adjacent regions reveals sparse spread of broadleaved thermophilic tree species. Saplings of Quercus robur, Ulmus glabra, Acer platanoides, Alnus glutinosa and Betula pendula have responded to recent climate warming by jump-dispersal in the order of 50-300 km northwards and 500-800 m upwards, relative to their natural range limits. Consistent with treeline rise by boreal tree species, the thermophilies have reinvaded regions where they grew during the warmest phase of the Holocene, 9500-8000 years ago, but were subsequently extirpated by the Neoglacial cooling. Confined to the past 20 years or so, the unique observations of recent termophilies comply with background climate data, i.e. warming of all seasons. These results may contribute to more realistic vegetation models by stressing that the distributions of certain plant species are able to track climate warming without substantial migrational lag. Hitherto, vegetation and climate evolution appear to be well within the frames of natural dynamics during the postglacial era, although mechanisms may differ.