Persistent reduced ecosystem respiration after insect disturbance in high elevation forests

David J. P. Moore,Jose Negron,Nicole A. Trahan,Britton B. Stephens,Tristan Quaife,Phil Wilkes,Kelly Elder,Ankur R. Desai,Russell K. Monson,Josep Penuelas

doi:10.1111/ele.12097

Abstract

Amid a worldwide increase in tree mortality, mountain pine beetles (Dendroctonus ponderosae Hopkins) have led to the death of billions of trees from Mexico to Alaska since 2000. This is predicted to have important carbon, water and energy balance feedbacks on the Earth system. Counter to current projections, we show that on a decadal scale, tree mortality causes no increase in ecosystem respiration from scales of several square metres up to an 84 km2 valley. Rather, we found comparable declines in both gross primary productivity and respiration suggesting little change in net flux, with a transitory recovery of respiration 6–7 years after mortality associated with increased incorporation of leaf litter C into soil organic matter, followed by further decline in years 8–10. The mechanism of the impact of tree mortality caused by these biotic disturbances is consistent with reduced input rather than increased output of carbon.

Highlights

Forest disturbance is a fundamental driver of terrestrial carbon cycle dynamics (Adams et al 2010), but the long-term effects of different disturbance types are poorly characterised in land surface models (Running 2008)
We present a rare set of complimentary observations to characterise the response of both photosynthesis and respiration in a forest unaffected by the beetle outbreak (Niwot Ridge; NWT) and an 84 km2 forested valley that has been markedly impacted by the outbreak (Fraser Experimental Forest; FEF)
We found a sharp decline in gross primary productivity (GPP) at the infested FEF from 2006 onwards, corresponding to the peak of beetle infestation, but no corresponding decrease at the non-infested NWT (Fig. 1a), with the time series for the two sites diverging by 13–30%

Summary

Introduction

Forest disturbance is a fundamental driver of terrestrial carbon cycle dynamics (Adams et al 2010), but the long-term effects of different disturbance types are poorly characterised in land surface models (Running 2008). High altitude forests in recovery from historical logging and managed for some degree of fire suppression are responsible for the majority of carbon uptake and storage in the Western United States (Pacala et al 2001; Schimel & Braswell 2005). The majority of the damage originates from one species, the native Mountain Pine Beetle Dendroctonus ponderosae (Man 2010). This outbreak occurs within the context of a multi-decadal general increase in tree mortality across the Western United States and globally (van Mantgem et al 2009; Allen et al 2010); where climate and hydrological change coupled with changing and increasingly susceptible forest conditions are likely drivers

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Results

Conclusion