Intra-Annual Growth Dynamics and Environmental Response of Leaves, Shoots and Stems in Quercus serrata on Lushan Mountain, Subtropical China

Primary and secondary growth of trees are key components of carbon sequestration in forest ecosystems. However, the temporal relationships between primary and secondary growth as well as their responses to environmental variations are still poorly understood. Herein, we continuously measured the intra-annual leaf, shoot and stem growth of Quercus serrata for two years on Lushan Mountain, southeastern China. Our results showed that shoots were ranked as the first organ to initiate, peak and cease growth, rather than leaves and stems. Moreover, the phenological stages of shoot growth were negatively associated with those of leaves and stems, whereas there was a weak positive correlation in phenological events between leaves and stems. These temporal connections in phenological events between primary and secondary growth suggest a prioritized carbon allocation to shoot growth and a high dependence of stem growth on carbon from newly developing leaves. Although stem growth started earlier in response to the warmer spring in 2018 compared to the colder spring in 2017, no significant difference in annual increment was observed between years, which was related to the more severe drought condition during the dry season in 2018. At the intra-annual scale, different organs generally had a consistent growth response to temperature variables but showed a divergent response to vapor pressure deficit. Despite a relatively short observational period and potential bias in spatial representativeness, our data provide nuanced knowledge on seasonal growth dynamics in primary and secondary of broadleaved species, underlining the importance of jointly considering intra-seasonal variabilities of environmental conditions in order to correctly predict tree growth response to climate change in subtropical regions.