Influenced by the monsoon climate, drought events in winter and spring are significant limiting factors for tree growth in the southwestern region of China. However, investigations of intra-annual xylem formation and radial growth have been scarce in southwest China, and less is known about how different tree species with varying growth phenologies respond to winter and spring droughts.

In a study published in Tree Physiology, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators studied the differences in radial growth and xylem formation of two common tree species in the tropical montane evergreen broad-leaved forest of Xishuangbanna in 2020 and 2021, as well as their relationship with climatic factors.

The researchers simultaneously observed wood formation and stem diameter changes in a deciduous tree species Betula alnoides and an evergreen tree species Schima wallichii during two different hydrothermal pre-monsoon seasons, with high-precision growth meters and microcore method.

They analyzed the relationships between weekly climate variables before sampling and the enlarging zone width or wall-thickening zone width, as well as weekly radial increments and climate factors in the pre-monsoon period during two climatologically contrasting years.

It was found that wood formation and radial increments in 2020 were delayed by three months and one month for B. alnoides and S. wallichii, respectively, when its pre-monsoon was warmer and drier than in 2021. Wood formation and radial increments significantly decreased in B. alnoides but not in S. wallichii in 2020.

The thicknesses of the enlarging zone and wall thickening zone of S. wallichii were positively correlated with relative humidity, minimum, and mean temperatures but negatively correlated with vapor pressure deficit from 2020 to 2021.

Moreover, the weekly diameter growth of both species was positively correlated with precipitation and relative humidity, and negatively correlated with vapor pressure deficit and maximum temperature. This suggested that the stem diameter growth of both species is mainly limited by water availability.

“Our study suggests that the winter and spring drought in the southwest region has had a significant impact on the radial growth of trees in this area, with species that start growing earlier being more affected,” said FU Peili of XTBG, corresponding author of the study.