Nitrogen Cycling Responses to Mountain Pine Beetle Disturbance in a High Elevation Whitebark Pine Ecosystem

Abstract

Ecological disturbances can significantly affect biogeochemical cycles in terrestrial ecosystems, but the biogeochemical consequences of the extensive mountain pine beetle outbreak in high elevation whitebark pine (WbP) (Pinus albicaulis) ecosystems of western North America have not been previously investigated. Mountain pine beetle attack has driven widespread WbP mortality, which could drive shifts in both the pools and fluxes of nitrogen (N) within these ecosystems. Because N availability can limit forest regrowth, understanding how beetle-induced mortality affects N cycling in WbP stands may be critical to understanding the trajectory of ecosystem recovery. Thus, we measured above- and belowground N pools and fluxes for trees representing three different times since beetle attack, including unattacked trees. Litterfall N inputs were more than ten times higher under recently attacked trees compared to unattacked trees. Soil inorganic N concentrations also increased following beetle attack, potentially driven by a more than two-fold increase in ammonium (NH4 +) concentrations in the surface soil organic horizon. However, there were no significant differences in mineral soil inorganic N or soil microbial biomass N concentrations between attacked and unattacked trees, implying that short-term changes in N cycling in response to the initial stages of WbP attack were restricted to the organic horizon. Our results suggest that while mountain pine beetle attack drives a pulse of N from the canopy to the forest floor, changes in litterfall quality and quantity do not have profound effects on soil biogeochemical cycling, at least in the short-term. However, continuous observation of these important ecosystems will be crucial to determining the long-term biogeochemical effects of mountain pine beetle outbreaks.

Disturbance Alters the Relative Importance of Topographic and Biogeochemical Controls on Microbial Activity in Temperate Montane Forests

“…In contrast to the burned plots, there is no change in pH in MPB plots ( Figure A3), which is consistent with several previous studies [32,36,91,92]. Unlike fire, which increases soil pH and potential for rapid leaching of soluble organic material, beetle-induced tree mortality reduces C and nutrient inputs to the soil from root exudates and litterfall in the short term, which is followed by increased C inputs from leaf, branch and root litter necromass [31,32].…”

“…Our study suggests that the environmental factors limiting microbial function shift depending on whether fire or pathogeninduced mortality is the disturbance agent. Our results are consistent with studies limited to single disturbances [31,32,40,50,92,94]. Soil biogeochemical processes can be represented through a variety of modeling approaches including a simple bulk substrate, moisture and Q10 response [105], a more complex treatment of soil substrate suitability [62,106], or even trait-based approaches [107,108].…”

Soil bacterial community structure remains stable over a 5-year chronosequence of insect-induced tree mortality

“…Given reported changes in soil chemistry following bark beetle infestations ( Morehouse et al, 2008 ; Clow et al, 2011 ; Xiong et al, 2011 ; Griffin and Turner, 2012 ; Keville et al, 2013 ), bark beetle-induced tree mortality might alter the structure of soil bacterial communities via environmental filtering. Alternatively, stronger relative influences of stochastic processes in the assembly of soil bacterial communities have been noted following disturbance ( Ferrenberg et al, 2013 ).…”

Consequences of mountain pine beetle outbreak on forest ecosystem services in western Canada

“…In another study, Clow et al (2011) observed no significant changes in stream-water NO 3 or DOC, however, total N and P increased. Interestingly, Griffin et al (2011) and Keville et al (2013) concluded that, although MPB outbreaks significantly altered the N cycling, the net effects were surprisingly minor given the extent of the beetle-caused mortality. Conversely, MPB outbreaks may also influence cation and aluminum fluxes as increased nitrification reduces the soil pH and leads to the exchange and loss of base cations (Ca 2+ , K + , Mg 2+ ); however further studies are required to confirm these changes.…”