Biogeochemistry of beetle-killed forests: Explaining a weak nitrate response

Abstract

A current pine beetle infestation has caused extensive mortality of lodgepole pine (Pinus contorta) in forests of Colorado and Wyoming; it is part of an unprecedented multispecies beetle outbreak extending from Mexico to Canada. In United States and European watersheds, where atmospheric deposition of inorganic N is moderate to low (<10 kg·ha·y), disturbance of forests by timber harvest or violent storms causes an increase in stream nitrate concentration that typically is close to 400% of predisturbance concentrations. In contrast, no significant increase in streamwater nitrate concentrations has occurred following extensive tree mortality caused by the mountain pine beetle in Colorado. A model of nitrate release from Colorado watersheds calibrated with field data indicates that stimulation of nitrate uptake by vegetation components unaffected by beetles accounts for significant nitrate retention in beetle-infested watersheds. The combination of low atmospheric N deposition (<10 kg·ha·y), tree mortality spread over multiple years, and high compensatory capacity associated with undisturbed residual vegetation and soils explains the ability of these beetle-infested watersheds to retain nitrate despite catastrophic mortality of the dominant canopy tree species.nitrogen biogeochemistry | streamwater chemistry | nitrate loss | watershed disturbance

Overlapping Bark Beetle Outbreaks, Salvage Logging and Wildfire Restructure a Lodgepole Pine Ecosystem

“…In spite of post-bark beetle increases in soil N, and unlike beetle outbreaks in parts of Europe that receive high atmospheric N deposition [68], Colorado beetle outbreaks have not threatened surface water with high N loading [69]. Research in Europe and the US highlights the role of nutrient demand and compensatory growth by recruiting and residual vegetation for intercepting surplus soil nutrients after tree mortality [11,12,68,69]. At Church's Park, post-fire IER-N levels and the risk of nitrate leaching will recede as understory plant cover increases.…”

“…New cohorts of lodgepole also establish readily after bark beetles (Dendroctonus ponderosae Hopkins) kill overstory pine [3,4]. The response of tree regeneration and understory plants following such disturbances determines forest vegetation dynamics and biodiversity [5][6][7][8][9] and has implications for ecosystem productivity and the biogeochemical processes that regulate soil nutrient retention and export [10][11][12]. For the 13,800 km 2 of forests infested by bark beetles since the early 2000s in Colorado, USA [13], the likelihood of overlapping disturbances increases with time as these forests are salvage logged or affected by wildfire [14].…”

Taking the pulse of a continent: expanding site‐based research infrastructure for regional‐ to continental‐scale ecology

“…For example, broad-scale variation in climate or disturbance events combined with variation in physical characteristics at the landscape scale can alter biogeochemical cycling and fluxes among levels of biological organization at the local scale that can be difficult or costly to measure with high accuracy (e.g., Rhoades et al 2013). Finescale biophysical factors interacting with broadscale climatic drivers can propagate spatially across spatial units to generate landscape-to regional-scale heterogeneity (e.g., Miller et al 2012).…”

Spatiotemporal index for analyzing controls on snow climatology: application in the Colorado Front Range

“…Landscape disturbance is already evident in the snow transition zone. The lodgepole pine forests that prevail throughout the transition zone have experienced an infestation of mountain pine beetle that began in 1996 and accelerated after 2004 (Rhoades et al, 2012). Warmer air temperatures (Mitton & Ferrenberg, 2012) and drought (Berryman, 1982) are both known to exasperate mountain pine beetle infestation.…”