Season of drought affects growth, but not nonstructural carbohydrates dynamics, in Pinus taeda saplings.

Abstract

In temperate evergreen conifers, growth occurs mostly in summer but photosynthesis precedes year-round; thus, nonstructural carbohydrates (NSC) increase in winter but decrease in summer. Given that mild drought reduces growth but not photosynthesis, a drought in summer should increase NSCs more than one in winter. However, the active regulation hypothesis suggests that to increase future drought resilience, plants might downregulate growth to increase NSCs after a winter drought even if NSCs do not increase during the drought. To test if so, potted Pinus taeda saplings (age $< 1$ yr) were subjected to six-month droughts in a greenhouse with one treatment receiving drought during winter (Sep-Mar), and another during summer (Mar-Sep). Both treatments were compared to a control. To measure dry biomass and NSCs, we harvested plants monthly following each drought, while to assess changes in growth rates, we measured height and diameter monthly. While we observed seasonal variation and an overall increase during the study, we found no drought-related changes in NSC dynamics; however, drought did reduce growth. Furthermore, drought in winter did reduce growth during the following summer, but the reduction was less than for a drought in summer. We conclude that the effect of drought on NSCs was too small to detect in our plants. While better control of soil water would have reduced a major source of uncertainty, plants with larger NSC reserves or more intense stress would also yield easier-to-detect effects. Although not definitive, our results suggest that water stress does not lead to dramatic changes in seasonal NSC dynamics in its aftermath, despite what one might expect under the active regulation hypothesis.