Montane forest expansion at high elevations drives rapid reduction in non-forest area, despite no change in mean forest elevation

Aim At the elevational limit of forest distribution, montane forests show diverse responses to environmental change with upward shifts, increased tree density and lateral expansion reported. To enable informed analysis of the consequences forest advance will have on montane biodiversity, we quantify changes in the area and elevation of the tree line ecotone and identify how patterns of forest advance are modified by topography and over time. Location Central Mountain Range, Taiwan. Time period 1963-2016. Major taxa studied Montane Forests. Methods Changes in the area and elevation of montane forest at the tree line ecotone were quantified using a stratified random sample of aerial photography captured in 1963, 1980, 2001 and 2016. Weighted estimates of habitat area and elevation for each time step were used to quantify the influence of slope aspect and inclination on tree line ecotone change and identify how the rate of habitat change varies over time. Results Non-forest area declined by 29% between 1963 and 2016 driven by a 295.0 ha increase in forest area within the study region. Despite no change in mean forest elevation, the mean elevation of establishing forest has increased at a rate of 2.17 m/yr. Changes in forest area and elevation are spatially variable, driven by the complex montane topography. East and south facing slopes show the largest gains in forest area and 0-20 degrees slopes show an increasing rate of forest establishment up to 2016, while slopes facing west or with incline > 46 degrees show negligible change. Main conclusions Climate-linked montane forest expansion in the Central Mountain Range in Taiwan is dominated by infilling rather than increases in forest elevation. Forest expansion has significantly reduced non-forest habitat area in this endemic species-rich region. However, considerable terrain-dependent variation in forest advance occurs, offering the potential that non-forest species will continue to persist at high elevations with reduced population size.