Increasing tree growth in subalpine forests of central China due to earlier onset of the thermal growing season

In recent decades, research into the influence of climatic warming on wood phenology, tree radial growth, and carbon uptake has focused largely on boreal and temperate Northern Hemisphere forests. Subtropical forests also play a vital role in sequestering carbon, however, warming influences on subtropical forests are still unclear. We collected Abies fargesii trees from the Shennongjia Mountains, subtropical China and measured basal area in-crements (BAI), the tree-ring width index (RWI) and oxygen isotope (delta 18O) spanning the period 1950-2017. BAI showed a rising trend which suggests enhanced tree growth over the past 40 years. We found that the positive influence of spring temperature on tree growth has weakened since late 1970s. Meanwhile, tree ring delta 18O records a strong signal of spring temperature since late 1970s, instead of summer precipitation prior to 1979. The increasingly early start of the thermal growing season (SOS), driven by springtime warming, amplifies the production and accumulation of photosynthates, thereby promoting tree radial growth and affecting growth- -climate relationships. Concurrently, earlier SOS might also increase the contribution of heavier delta 18O of springtime precipitation to cellulose synthesis, causing enhanced relationship between February-April temper-ature and tree-ring delta 18O. If so, springtime warming serves to modulate tree growth/isotope-climate relation-ships, potentially benefiting tree growth in the subtropical, subalpine fir forests of central China.

Automated summary

Research has predominantly focused on the impacts of climatic warming on forests in the boreal and temperate Northern Hemisphere, but the influence on subtropical forests remains unclear. This study examined Abies fargesii trees in subtropical China and found that tree growth has been enhanced over the past 40 years, while the positive effect of spring temperature on tree growth has weakened since the late 1970s. The study suggests that earlier start of the thermal growing season, driven by springtime warming, promotes tree radial growth and influences growth-climate relationships.