Effect of the current major insect outbreaks on decadal phenological and LAI trends in southern Rocky Mountain forests

Abstract

Remote sensing is a valuable tool for monitoring the impact of landscape-scale disturbances on ecosystem structure and function. We explore the impact of the ongoing insect outbreaks (Dendroctonus ponderosae, D. rufipennis, and Ips spp.) on southern Rocky Mountain forests, with the goal of assessing the sensitivity of leaf area index (LAI) and phenology metrics to different disturbance severities. Specifically, we investigate the influence of the outbreaks on two important ecosystem metrics, LAI, and phenology (e.g. green-up date, green-up speed, amplitude, etc.). Both were assessed via MODIS: 1000 m LAI (MOD15A2) and 250 m NDVI (MYD13Q1) for the phenology assessment. Trends (2002–2010) in phenology metrics and LAI were compared to different cumulative severities and timing of tree mortality, as determined from aerial surveys. Trends in phenology were significantly correlated with disturbance severity but with very low predictive power. This seems likely due to yearly variations in the onset of snow-fall and snow-melt, which dominate the phenologic signal at the regional scale of this study. Trends in LAI were associated more strongly with both disturbance severity and timing, with landscapes disturbed early in the observation period showing recovery (e.g. a positive trend) in LAI. The LAI, which is related to various vital ecosystem properties like water use and gas exchange, seems to be fairly resilient to even heavy mortality. Further work determining the relative contribution of the various functional groups (trees, shrubs, and grasses) to the LAI recovery is needed to better understand the implications of this large-scale, pervasive disturbance on forest structure and function.