COS 244-5 - The effects of fertilization on defoliation recovery in river birch and northern red oak

Abstract: Defoliation is a common stressor in forests that requires trees to remobilize nonstructural carbohydrate (NSC) reserves to regrow leaves. The refoliated canopy is often smaller than the original canopy, resulting in decreased growth, reduced carbon storage, and even death, however it is currently unclear what limits leaf reflush and longer-term recovery. While carbon (C) is often presumed to be limiting, incomplete NSC remobilization and its faster recovery relative to growth suggest other factors like nitrogen (N) availability may limit growth instead. But because N is a vital component of cell walls and Rubisco, if it does limit growth, it is unclear if N does so directly or indirectly by reducing leaf N concentrations and limiting carbon gain.

To better understand the impacts of defoliation, we explored how fertilization impacted the short- and long-term growth and NSC storage of Betula nigra (river birch) and Quercus rubra (northern red oak) after experimental defoliation. We sought to determine 1) if and how N availability limits reflush and growth after defoliation, and 2) if NSC remobilization and allocation are largely controlled by limited C demand. 1-yr old potted saplings from each species were divided into four treatments—control/low fertilizer; control/high fertilizer; defoliation/low fertilizer; defoliation/high fertilizer. Trees were harvested after three weeks (short-term) and four months (long-term). Final biomass, diameter change, stem and root NSC, and foliar N were measured; percent leaf mass recovery was calculated for defoliated trees.

Three weeks after defoliation, both species showed less than 100% leaf mass recovery, with fertilization significantly increasing reflush for birch but not oaks. For both species, fertilization increased foliar nitrogen concentration, potentially increasing photosynthetic capacity. Fertilization did not impact oak NSC concentrations after reflush but may have increased remobilization in birch. Four months after defoliation, there were no differences in foliar N concentrations, and fertilization had significantly increased leaf mass recovery in both oaks and birch. In this study, growth after defoliation was generally N-limited, but oak reflush may initially have been limited by their semi-determinate growth pattern or by NSC storage. Birch reflush may have been less C-limited due to their higher initial root starch concentrations. We discuss the long-term effects of defoliation on NSC and the potential for fertilization to increase growth at the expense of NSC storage, potentially reducing resilience to future disturbance and stressors.