Understanding how intra-annual stem growth responds to atmospheric and soil conditions is essential for assessing the impact of climate extremes on forest productivity. However, it is unclear to what extent species growth patterns and climate responses are coordinated within a community.

In a study published in New Phytologist, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators from Wageningen University and Research examined intra-annual stem growth and its climatic drivers for 14 common tree species in a monsoon montane subtropical forest. They aimed to identify the role of atmospheric conditions and soil moisture in driving intra-annual stem growth variation - both timing and magnitude - of a tree community in a montane subtropical forest.

The researchers monitored stem growth at 10-min resolution of 27 trees of 14 species over five years in a subtropical evergreen broadleaf forest in the Ailao Mountains. They quantified diel and seasonal stem growth patterns, tested the extent to which observed growth patterns were consistent across study species, and tested underlying climatic drivers.

They found that seasonal growth patterns in this tree community were mainly driven by temperature and VPD, and that stem growth during the day mainly occurred before dawn associated with low vapor pressure deficit (VPD). Species differed widely in absolute stem growth rates, but diel and seasonal growth patterns were strikingly similar. Trees grew mainly before dawn when VPD was low. Over a year, trees grew mainly during May to August when climate conditions were warm and humid.

Growth-climate analyses at different temporal scales confirmed that atmospheric conditions (VPD and temperature) strongly determined when stem growth occurred during the day and the year. However, the rate of stem growth was mainly determined by temperature, and much less by moisture conditions. Temperature not only favored stem growth on a seasonal scale, but also increased the diel growth rate when moisture availability permitted growth.

"We present one of the first community-level studies of intra-annual stem growth in a species-rich forest," said FAN Zexin of XTBG.

The study was supported by the National Natural Science Foundation of China and the China Scholarship Council.

Ailaoshan subtropical forest. (Image by XTBG)