Decomposition of bark beetle-attacked trees after mortality varies across forests

Abstract

Bark beetle outbreaks result in massive amounts of deadwood which are a significant carbon reservoir that can impact forest ecosystems. Additionally, bark beetle-introduced symbiotic fungi in beetle-generated deadwood can increase termite presence and feeding. This could alter decomposition rates and saproxylic communities of bark beetle-generated deadwood. However, the fate of bark beetle-generated deadwood after tree mortality has received little attention despite the possibility bark beetles could alter decomposition processes and deadwood residence times at landscape scales following outbreaks. We tested the hypothesis that beetle-attacked trees decompose faster than unattacked trees. In Honduras, and Mississippi and Arizona, USA, we felled one recently bark beetle-attacked and one apparently healthy conspecific tree at each site that was cut into 120 experimental logs. Logs of each tree (attacked or unattacked) were assigned one of three metal mesh covering treatments: 1) fully covered to exclude all macroinvertebrates, 2) covered from above to exclude secondary bark beetle colonization, 3) no cover to allow all detrital food web organisms. Half of all logs at each site was collected after 1 and 2 years and the density loss, insect visual damage rating, and abundance of termites, ants, and beetles was measured. Bark beetle attack had the largest impacts at higher latitude and western forests: density loss between attacked and unattacked logs was consistently higher in Arizona (Flagstaff), initially slower (year 1) in Mississippi, but faster between years 1 and 2 resulting in similar density loss after 2 years, and did not differ in Honduras. Saproxylic insect wood damage rating accounted for 30% of the variation in decomposition across sites. Thus, decomposition rates of deadwood following bark beetle attack are highly variable across their geography and likely reflect important interactions among saproxylic organisms. This has implications for forest ecology and management including forest modeling with regard to carbon cycling and maintaining biodiversity of saproxylic organisms.