Tracking the timing, persistence, and interdependence of stand structural and soil properties in mountain pine beetle-killed forests

Background/Question/Methods

Landscape-scale outbreaks of the mountain pine beetle (Dendroctonus ponderosae; MPB) have erupted across western North America, killing mature lodgepole pine (Pinus contorta) trees over millions of hectares.  MPB-induced mortality may influence the strength of interdependence among above (e.g., stand structure, pine litter chemistry, deposition, and decomposition) and belowground (e.g., soil moisture, nutrition, and total phenols) site properties resulting in cascading effects including altered seeding regeneration and successional trajectories.  Soil nutrient dynamic are influenced by the quantity, quality, and decomposition rate of litter inputs, with past work showing  that soil nitrate may rise (3 years post outbreak) and eventual fall (30 years post outbreak) according to the balance between these factors.  Yet, to our knowledge, no studies have measured early-stage (0-1 post outbreak) nutrient dynamics over two consecutive years; however, early-stage dynamics are of import to assessing the ecological viability of some forms of management intervention—such as seedling planting—within regions of recent and/or current outbreaks.  The goals of this research were to: 1) determine the strength and direction of relationships between above and belowground site properties and levels of MPB-induced pine mortality, 2) evaluate the temporal sensitivity of each to time since peak MPB outbreak.

Results/Conclusions

During the 2011 and 2012 growing seasons, we performed a survey of above  and belowground site properties associated with a gradient of MPB-induced tree mortality (0 – approx. 50 m²/ha basal area killed) in lodgepole pine (Pinus contorta) forests of northwestern Alberta, Canada.  Heavily attacked sites experienced peak canopy tree mortality one year prior (i.e., 2010) to our first survey.  We found that between one to two years following peak mortality, sites deposited significant quantities of needle litter, retained elevated concentrations of nitrogen and phosphorus in needles, and underwent enrichment of soil nitrate and moisture supply.  After 2-3 years however, sites show a pronounced decline in needlefall and soil nitrate, and a re-stabilization of moisture levels.  Our findings show that severe outbreaks induce mechanisms of change that drive ephemeral (e.g., litterfall) and persistent (e.g., soil nitrate) responses which result in physical and chemical legacies.  The persistence of such legacies may, in the context of forest management, translate into a temporal window within which opportunities for intervention (e.g., reforestation) may be either created or constrained.  Thus, characterizing the timing, persistence, and interdependence of these responses will inform the placement of temporal bounds around optimal management windows for beetle-killed forests.