PS 41-132 - What Limits Growth More, Carbon or Water?

Abstract: For the past 300 million years, light has been the greatest limitation to plant growth and survival. Some have hypothesized that global change type drought will cause a transition from light-limited to water-limited growth. Water is required for cell expansion and division in growth, reductions in the water supply is known to limit cell turgor and thus directly limit growth. However, direct comparisons between light-limited and water-limited growth remain rare. Our study tested the hypothesis that drought would limit growth more strongly than light limitation. We exposed 60 loblolly pine (Pinus taeda) saplings to a control, drought, shade, and shade + drought treatment (15 trees per treatment) for three weeks. We measured tree growth, photosynthesis, photochemistry, water potential, and sampled for leaf and twig NSC weekly, for 6 weeks, including 2 weeks post-treatment. We found significant declines in the photosynthetic capacity of shade trees, but not the drought trees. This is consistent with previous work showing that drought seldom effects the photosynthetic capacity of trees. Trees in the drought treatment grew less than shade trees, but closely followed the dynamics of shade + drought trees. Regardless, trees exposed only to drought recovered more rapidly after the treatment was removed. In contrast, trees in the shade and shade + drought treatment never recovered to their pre-experimental growth rates. Coupled with low photosynthesis and inhibited photosynthetic capacity, this suggests a direct correlation between carbon supply and growth. Although the recovery of light limited trees was inhibited by damage to the photosynthetic system, our results suggest that when drought is ongoing the decline in growth may be stronger than when trees are light limited. Finally, the rapid recovery of drought trees suggests high resilience of loblolly pine to drought-induced growth limitations.