Density- and age- dependent influences of droughts and intrinsic water use efficiency on growth in temperate plantations

Droughts influenced the effects of increasing intrinsic water use efficiency (iWUE) on tree growth. Yet, litter is known on stand density- and age-related relationship among droughts, iWUE, and tree growth in plantations. We used tree-ring growth and delta 13C data from three dominated species (larch, spruce, and pine) to evaluate the density- and age-dependent relationship between droughts and iWUE, and their linkage to tree growth in temperate plantations. The results showed that low-density old-age stands were more sensitive to drought stress than high-density young-age stands. Drought-induced increased iWUE had translated into growth loss for lowdensity old-age stands. Whereas droughts showed weak influences on iWUE, and thus had no clear effects on tree growth in high-density young-age stands. High-density young-age trees had a more balance carbon uptake per water loss compared to low-density old-age trees, suggesting that competition to grow fast was of utmost important for high-density trees. Droughts reinforced stomatal closure to reduce carbon uptake in low-density old-age stands, while stomata were not sensitive to droughts in high-density stands. High-density young-age trees had higher resistance to drought, but lower recovery than low-density old-age trees. Our results suggested that maintaining high-density forests could reduce drought-induced growth loss if drought frequency is continuous increasing in the future. Considering the beneficial effects of thinning on growth could last for several years, proper thinning frequency and rotation period should be implemented in forest management to enhance trees' adaptation to droughts.

Automated summary

This study evaluated the relationship between drought, intrinsic water use efficiency (iWUE), and tree growth in temperate plantations using tree-ring growth and delta 13C data. The results showed that low-density old-age stands were more sensitive to drought stress, while high-density young-age stands were less affected. High-density trees had a more balanced carbon uptake per water loss, indicating the importance of competition for fast growth. Droughts had a weaker effect on iWUE and tree growth in high-density stands, but reinforced stomatal closure in low-density stands. High-density trees had higher resistance to drought but lower recovery than low-density trees. Maintaining high-density forests could reduce drought-induced growth loss, but proper thinning frequency and rotation period should be implemented for enhanced adaptation to droughts.