Seasonal pattern of stem radial growth of Salix matsudana and its response to climatic and soil factors in a semi-arid area of North China

Salix matsudana is an important tree species for shelterbelt and barren hill afforestation in North China. Knowledge of the seasonal pattern of the stem radial growth (SRG) of S. matsudana and its response to environmental factors is crucial for understanding the tree growth and climate relationships in the increasingly warm and dry climate of North China. To this end, a field trial was conducted in a semi-arid area of North China, and the stem radial dynamics of eight sample trees at two sites (four trees per site) with different soil moisture regimes, as well as the meteorological and edaphic conditions, were monitored from May to October in 2016 and 2019. During the growing season, the daily mean air temperature was 1.69 °C higher in 2019 than in 2016, and the total precipitation was 179.2 mm lower in 2019 than in 2016. The results showed that the seasonal pattern of S. matsudana SRG was divided into the initial stage (stage 1), the rapid growth stage (stage 2), and ending stage (stage 3) in terms of the threshold of 0.02 mm day−1 (daily growth rate). The initiation of stage 2 in 2019 (day of year (DOY) 126) was 48 days earlier than that in 2016 (DOY 174). However, the date on which the maximum growth rate was reached occurred also 34 days earlier, indicating the co-occurrence of the positive effect of the higher temperature in 2019 than in 2016 on growth initiation in stage 1 as well as the negative consequences of the water deficit on tree growth in stage 2. Higher cumulative growth of S. matsudana at the high soil moisture site was monitored relative to the low soil moisture site throughout the trial, which could be attributed to differences in soil moisture and the initial stem size of the trees in 2016 and 2019, respectively. Temperature and soil moisture were the main factors restricting daily SRG, but the growth of S. matsudana subjected to prolonged summer water stress would be less significantly limited by soil moisture in stage 3. These results showed that the seasonal pattern of SRG of S. matsudana was closely related to the dynamics of the soil and climatic factors. Therefore, seasonal variation in growth-limiting factors should be considered when forecasting tree growth in response to climate change in North China.