Abstract
Ongoing global climate change is driving widespread shifts in species distributions. Trends show frequent upwards shifts of treelines, but information on changes in montane forest below the treeline and in the tropics and subtropics is limited, despite the importance of these areas for biodiversity and ecosystem function. Patterns of species shifts in tropical and subtropical regions are likely to be more complex and individualistic than global averages suggest due to high species diversity and strong influence of competition, alongside direct climatic limitations on distributions. To address the question of how subtropical montane tree species are likely to move as climate changes, we used an extensive national forest inventory to estimate distribution shifts of 75 tree species in Taiwan by comparing the optimum elevation and range edges of adults and juveniles within species. Overall there was a significant difference in optimum elevation of adults and juveniles. Life stage mismatches suggested upward shifts in 35% of species but downward shifts of over half (56%), while 8% appeared stable. Upward elevation shifts were disproportionately common in high elevation species, whilst mid to low elevation species suggested greater variation in shift direction. Whilst previous research on mountain forest range shifts has been dominated by work addressing changes in treeline position, we show that although high elevation species shift up, below the treeline species may shift individualistically, heralding widespread changes in forest communities over coming decades. The wide variation of responses indicated is likely driven by individual species responses to interacting environmental factors such as competition, topography and anthropogenic influences across the broad range of forest types investigated. As global environmental changes continue, more detailed understanding of tree range shifts across a wide spectrum of forests will allow us to prepare for the implications of such changes for biodiversity, ecosystem function and dependent human populations.
Highlights
Ongoing global climate change is contributing towards widespread shifts in species distributions (Parmesan and Yohe 2003, Chen et al 2011, Pecl et al 2017, Lenoir et al 2020), with movements to higher latitudes and elevations expected as species track warming climates (Gosz 1992, MoruetaHolme et al 2015)
There was a significant difference in the optimum elevation between life stages (V = 679.5, p = 0.002), with the juvenile median 50 m lower than the adult median
While it is generally expected that species should shift to higher elevations in response to climate change (Chen et al 2011), we found highly variable life stage mismatches in range optima and upper and lower range edge positions, with a tendency for juveniles to be located at lower elevations than their adult counterparts
Summary
Ongoing global climate change is contributing towards widespread shifts in species distributions (Parmesan and Yohe 2003, Chen et al 2011, Pecl et al 2017, Lenoir et al 2020), with movements to higher latitudes and elevations expected as species track warming climates (Gosz 1992, MoruetaHolme et al 2015). Evidence is accumulating of substantial shifts of montane forest tree distributions in response to climatic pressures (Harsch et al 2009). The impacts of shifts can vary considerably across species ranges (Wallingford et al 2020), with upwards shifts of trees threatening many endemic species frequently found at high elevations (Jump et al 2012). Patchy coverage of assessments of montane forest shifts retards our understanding of the impacts changes are likely to have on biodiversity, ecosystem function and dependent human populations from local to global scales (but see Elsen et al 2020). Accurate information on species distribution shifts is, urgently needed for improved bioclimatic modelling and to inform decisions on management, conservation and policy
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