Abstract
Knowledge of intra-annual stem growth dynamics across environmental gradients is important for advancing our ability to understand the adaptability and vulnerability of subtropical tree species to future climate change. To assess the effects of seasonal drought on intra-annual stem growth, stem radial variation of Taiwan pine (Pinus taiwanensis Hayata) was monitored with band dendrometers for two years along an elevation transect from 921 to 1402 m in the Lushan Mountains, a transect that covers the contrasting climatic growing conditions for Taiwan pine in southeastern China. We found that the onset of stem growth was nearly synchronous across the transect, in early April 2017 and in late March 2018, whereas large elevational differences were observed for the end of the growing season, which was much earlier at lower elevations. Tree stems frequently rehydrated during the dry growing seasons at the two higher elevations, suggesting that seasonal drought had minor influence on the offset of high-elevation stem growth. A substantial and continuous tree water deficit of low-elevation Taiwan pine was detected during dry seasons, leading to an early growth cessation in late July in both years. Tree water status (reflected by tree water deficit) revealed a higher sensitivity to precipitation and soil water content across wet- and dry-seasons at the lowest elevation than at high elevations, indicating that low-elevation stem radial growth was highly dependent on moisture variables over the whole growing season. Due to the influences of seasonal drought on growth cessation and rates, Taiwan pine produced a rather narrow annual growth at the lowest site, whereas high-elevation Taiwan pine could benefit from the optimal wet-season environmental conditions and the reactivation of cambial activity during dry seasons. Our findings suggest that the more frequent and intensive drought episodes in the future will reduce tree growth of Taiwan pine at the dry edge, probably resulting in upward shifting of the optimal elevation for Taiwan pine in subtropical China.
Highlights
Climate change has led to temperature increase and more frequent and severe drought events, which is expected to pose major challenges for forest growth, plant species composition and ecosystem services [1]
The observed seasonal shift in growth-limiting factors from temperatures in wet season to precipitation and soil water content in dry season at the two higher elevations corresponds to our previous findings [21], whereas a higher growth-moisture dependency over the whole growing season was found at the lowest elevation
Our elevational transect observation of intra-annual stem growth of Taiwan pine confirmed that temperature was the main driver triggering the onset of stem growth, and elevation-specific temperature thresholds shaped the uniform onset of stem growth over elevations
Summary
Climate change has led to temperature increase and more frequent and severe drought events, which is expected to pose major challenges for forest growth, plant species composition and ecosystem services [1]. Many previous tree-ring-based investigations of conifers have demonstrated complicated growth-climate relationships in subtropical forests, varying from a positive correlation with winter half-year temperature [3,4,5,6,7,8] to a positive response to summer-season precipitation [9,10,11,12]. Such contradictions limit our ability to predict how subtropical tree growth responds to future climate changes. In-depth investigations on the intra-annual stem growth dynamics and its physiological basis are urgently needed
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