COS 62-3 - When do herbivores change carbon sequestration in forest biomass?

Abstract: The challenge of meeting climate change objectives, and the financial opportunities of carbon credits, have incentivized forest management and conservation for carbon sequestration. In areas with high herbivore density, particularly where those herbivores are non-native, browsing may result in lower carbon sequestration in forest biomass, both directly through carbon consumption and indirectly through changes in species composition, understory density, and forest replacement and succession. However, understory plants make up a relatively small proportion of carbon sequestration in temperate forests, potentially weakening these effects. Understanding when and where high herbivore density may affect carbon sequestration in plant biomass is nonetheless essential to effective forest management and climate change mitigation. We conducted a literature review of herbivore exclusion studies and synthesized previous conceptual work to delineate the conditions under which herbivore management could significantly increase present or future carbon sequestration. We found that herbivore reduction does not have substantial impacts on forest carbon over the decadal scale, but study results suggest that over longer time scales changes in recruitment and succession could lead to larger effects. Most exclosure experiments are not yet long enough to demonstrate these carbon impacts, however. Herbivores are likely to have particularly important effects in systems where replacement of canopy trees is inhibited by heavy browsing, leading to a successional shift towards fast-growing ruderal species or slow-growing herbivore-resistant species. Non-native herbivores may have strong impacts where plants are less resistant or resilient to novel herbivore pressure. Similarly, systems where browsing by arboreal species leads to the death of adult trees would likely respond strongly to herbivore control. By contrast, in forests where browsing primarily removes understory species with low carbon density or fast turnover where carbon is not locked into plant tissues, herbivore control is likely to have minimal effects on forest-level carbon storage and herbivores may occasionally promote carbon uptake during plant regrowth. Herbivory may interact with other forms of disturbance, for example by resulting in higher mortality than would occur under one stressor alone. Disturbance that leads to succession and gap dynamics may accelerate the carbon storage effects of changes in plant composition that occur as a result of herbivory in the understory. Understanding the conditions under which herbivores likely decrease carbon storage and the magnitude of these effects can guide research and management efforts to prioritize control in areas with the greatest impact, and to realistically assess carbon sequestration in managed forests.