Since 2000, an epidemic of mountain pine beetle (Dendroctonus ponderosae) has infested over 86 million hectares of forest in the U.S.A. The resulting large-scale tree mortality has important carbon, water, and energy balance implications for forest systems. We compared 9-year records of gross primary productivity (GPP) and total ecosystem respiration fluxes (TER) in two high-elevation forests in Colorado, U.S.A., one impacted by the beetle (Fraser Experimental Forest) and one free of the outbreak (Niwot Ridge). In addition, we assessed carbon fluxes and biogeochemical pools in decade-long disturbance chronosequences in the same forests.
Results/Conclusions
We show that a dampening of the carbon cycle occurs in forests following mountain pine beetle outbreak due to a sustained suppression of both GPP and TER after tree mortality. Tree death leads to autotrophic respiration and rhizodeposition losses, ultimately decreasing soil respiration through substrate limitation. In the chronosequence plots we found decreases in soil microbial biomass and extractable pools of total organic carbon, dissolved nitrogen and phosphorus with transitory increases in inorganic nitrogen. A temporary recovery of these pools and fluxes occurred concurrently with the decomposition of dropped needle litter 5-6 years after mortality. Also during this time period, trees that survive beetle outbreak undergo significant competitive release, allocating more carbon to growth in response to interactions of enhanced light and moisture availability. The recovery of both GPP and respiration in the plot scale fluxes were also observable in the forest scale fluxes. In contrast to other disturbances such as fire or logging, the impact of tree mortality caused by the mountain pine beetle in Western North America is likely to have a subtle, long lasting impacts on the carbon cycle which require consideration of the mechanistic linkages between GPP and respiration.