Effects of biotic disturbances on forest carbon cycling in theUnitedStates andCanada

Abstract

AbstractForest insects and pathogens are major disturbance agents that have affected millions of hectares inNorthAmerica in recent decades, implying significant impacts to the carbon (C) cycle. Here, we review and synthesize published studies of the effects of biotic disturbances on forestCcycling in theUnitedStates andCanada. Primary productivity in stands was reduced, sometimes considerably, immediately following insect or pathogen attack. After repeated growth reductions caused by some insects or pathogens or a single infestation by some bark beetle species, tree mortality occurred, altering productivity and decomposition. In the years following disturbance, primary productivity in some cases increased rapidly as a result of enhanced growth by surviving vegetation, and in other cases increased slowly because of lower forest regrowth. In the decades following tree mortality, decomposition increased as a result of the large amount of dead organic matter. Net ecosystem productivity decreased immediately following attack, with some studies reporting a switch to aCsource to the atmosphere, and increased afterward as the forest regrew and dead organic matter decomposed. Large variability inCcycle responses arose from several factors, including type of insect or pathogen, time since disturbance, number of trees affected, and capacity of remaining vegetation to increase growth rates following outbreak. We identified significant knowledge gaps, including limited understanding of carbon cycle impacts among different biotic disturbance types (particularly pathogens), their impacts at landscape and regional scales, and limited capacity to predict disturbance events and their consequences for carbon cycling. We conclude that biotic disturbances can have major impacts on forestCstocks and fluxes and can be large enough to affect regionalCcycling. However, additional research is needed to reduce the uncertainties associated with quantifying biotic disturbance effects on theNorthAmericanCbudget.