Data from: Topography constrains the climatic response of treeline migration in Taiwan’s subalpine forests

Treelines are moving upslope, but the rates and drivers differ among
different regions globally. Many studies have examined the relationship
between treeline movement and climate change, particularly rising
temperature, while the role of topographical factors has received much
less attention, despite the longstanding recognition of its importance. We
examined treeline dynamics from 1980 to 2021 in two mountain ranges of
Taiwan, the Yushan Mountain Range (YMR) and the Central Mountain Range
(CMR), in relation to topography and climate using multi-temporal aerial
imagery and U-Net deep-learning segmentation. Treeline elevation advanced
approximately 60 m in the two mountains and was accompanied by forest area
expansion of 197 ha in the YMR and 158 ha in the CMR, which was mainly due
to upward forest movement, not tree densification. Elevation, increases in
growing-season temperature and minimum temperature, and growing-season
precipitation were important predictors of forest area expansion. In
contrast, elevation, increases in growing-season precipitation and maximum
temperature, and slope steepness were key predictors of treeline movement.
Treeline upslope migration was approximately two times greater during the
1980–2001 period (2.1 m yr-1 in the YMR and 1.6 m yr-1 in the CMR)
compared to the 2001–2021 period (1.0 m yr-1 in the YMR and 0.75 m yr-1 in
the CMR), likely because forests already occupied most of the suitable
habitat, and topographic factors constrained upward movement. Our study
highlights the importance of accounting for both climatic and topographic
factors in understanding treeline responses to climate change. In
addition, our results indicate that treeline advance in mountain island
forests, such as those in Taiwan, may be approaching an elevational limit,
above which further upward migration is likely to exhibit a more muted
response to further warming.