COS 242-2 - Herbivore loss and invasive ants shift carbon allocation in East African ant-plants

Abstract: Trees have complex carbon allocation strategies which allow them to respond to their environment. Understanding how tree carbon allocation varies in different environmental contexts is thus necessary to predict both the dynamics of tree populations and resulting carbon storage in woody biomass, as well as terrestrial carbon fluxes. In central Kenyan savannas, the loss of vertebrate herbivores and the invasion by the ant Pheidole megacephala are disrupting mutualisms between the foundational tree Acacia (Vachellia) drepanolobium and its native ant defenders. Invaded trees suffer increased elephant damage, leading to lower aboveground biomass, but it is unclear how tree carbon allocation is affected. We piloted a carbon-13 (¹³C) pulse-labeling experiment to track carbon allocation to various carbon pools in trees from P. megacephala invaded and uninvaded sites, and inside and outside of an herbivore exclosure. Experimental trees were sealed inside of a clear plastic tent, and ¹³CO₂ was added incrementally. Following the ¹³C pulse-labeling, we sampled experimental and unlabeled control trees for leaves, stems, ants, and soil respiration.

Experimental trees withstood labeling tent conditions and took up sufficient labeled ¹³CO₂ for ¹³C to be detected in various carbon sinks, including ant mutualists. Differences were detected in amounts of newly assimilated carbon found in stems of experimental trees. In plots not invaded by P. megacephala, the tree exposed to herbivores allocated relatively more newly assimilated carbon to stem tissue compared to trees excluded from herbivores. This could be a result of trees investing in regrowth to replace aboveground biomass lost to browsers. However, the tree invaded by P. megacephala and exposed to herbivores invested relatively less newly assimilated carbon into stem tissue compared to the uninvaded tree exposed to herbivores. This suggests that investment in aboveground regrowth is not an optimal strategy when herbivore damage is so high, and the tree may allocate carbon elsewhere. Preliminary data of soil respiration shows no differences in ¹³C allocated belowground based on ant invasion or herbivore presence, so it is unlikely that allocation shifts belowground. However, carbon allocation may have shifted to greater investment in NSC storage or use in aboveground respiration. We conclude that carbon allocation of ant-defended plants changes when the mutualism is disrupted by herbivore loss and invasion, which may have implications for savanna carbon cycling.