Mountain Pine Beetles, Climate and the Carbon Cycle
From the early 1990s, high levels of tree mortality have been observed across large areas of forest in North America infested by species of bark beetles. The majority of the damage originates from the native Mountain Pine Beetle Dendroctonus ponderosae. The beetles burrow underneath the bark of the trees to lay their eggs, disrupting water and nutrient flows within the sapwood during the processes. In addition they often carry blue stain fungus which further infects the phloem and sapwood. These effects lead to a slow death for the tree and infected areas are easily identified by the symptomatic red colouration in the forest canopy. Although the exact cause of the outbreak is unknown it appears to be in response to changing climatic conditions with warmer temperatures acting to increase the beetle’s natural range and to reduce levels of winter mortality which normally helps keep the population in check.
Impacts of the beetle infestation on human activity are numerous. A chief concern is damage to timber based economies, such as in British Columbia where forest product exports totalled $9.95 billion Canadian Dollars. There is also strong evidence relating beetle induced mortality to increased likelihood of wild fires: the dead trees remain standing for several years and dry out during the summer months leading to increased risk of combustion. In addition it has been hypothesised that the infestation could lead to a large source of carbon to the atmosphere, potentially acting a positive feedback to climate change. A paper published in Nature by Kurz et al. (2008) projected that between 2000–2020 there will be 270 megatonnes (Mt) carbon released to the atmosphere from infected forests in British Columbia alone. Whilst this is relatively small compared to annual emissions of carbon from global fossil fuel combustion this figure becomes more daunting when scaled up over the whole of North America. The Kurz et al. paper was sufficiently high profile to be reported in the New York Times.
But are these claims concerning large releases of carbon to the atmosphere really correct? A group of scientists, including myself, have been working to address this question for several years now. A key assumption in the calculations of post infestation carbon release is that respiration from soil micro-organisms (known as heterotrophic respiration) will continue unabated once the forest dies. This is the main route by which dead organic matter is returned to the atmosphere in a normal, undisturbed ecosystem. In other words heterotrophic respiration is a major term in the carbon balance of a mature forest. However, careful experiments carried out by Nicole Trahan as part of her PhD at the University of Colorado suggested that this may not be true for forests that have been attacked by the mountain pine beetle. By simulating beetle attacks on a small patch of healthy forest at Niwot Ridge, Colorado, and measuring the corresponding heterotrophic CO2 flux over a number of years, Nicole observed that respiration from soil microbes slowed down as the trees died. She then went on to show the same things happening in a nearby beetle infested forest in Fraser Valley. The explanation for this appears to be that as photosynthesis in a tree decreases, the amount of carbohydrates (such as glucose and sucrose) that are made available to the fungal mycorrhiza surrounding its root system also decreases. Without these priming agents the activity of the microbial community slows down.
More information on this component of the research can be seen in The Pine Beetle Project video on YouTube.
Will these field scale observations hold out over wider areas? This is an important question if we are to understand the impact of the beetle infestation on the carbon balance of North America and its potential to act as a positive feedback to climate change. Unfortunately both photosynthesis and heterotrophic respiration are notoriously difficult to measure on large scales. To address this we used two approaches: 1) satellite derived estimates of photosynthesis and 2) measurements of the night-time accumulation of CO2 at the bottom of Fraser valley as a proxy for heterotrophic respiration.
Satellite estimates of photosynthesis (or more specifically, Gross Primary Productivity, GPP) are now derived operationally for the whole global roughly every two weeks using sensors such as NASA’s Moderate Resolution Imaging Spectrometer (MODIS). However preliminary analysis of this data suggested that, for a number of reasons, it was not capturing the pattern of disturbance that we were observing on the ground. Instead we opted to calibrate a recently published model of satellite derived GPP against estimates of photosynthesis derived using eddy covariance trace gas flux measurement techniques at the Niwot Ridge site (which is a relatively healthy forest). This model was then applied over the infested Fraser Valley forest. The resulting estimates of annual GPP are shown in the figure below. The forest at Niwot Ridge shows no decline in photosynthesis for the period 2002-2011 whereas Fraser Valley exhibits the same temporal pattern as the Niwot Ridge forest from 2002 to 2006, but rapidly declines after 2006 (the year in which the beetle infestation started).
Starting in 2006 CO2 concentrations have been measured on the valley floor in Fraser. We used the averaged night-time accumulation as a proxy for heterotrophic respiration. During the night autotrophs (i.e. vegetation) stop respiring CO2 and hence the night-time accumulation is only produced by heterotrophs (primarily soil microbes). This assumes that the boundary layer is relatively stable so that CO2 is not being lost from the valley, and also that the valley is flushed out during the day. Meteorological observations were used to filter the data for nights where this was potentially a problem.
The plot above shows our results from the scaling up exercise and corroborates the field studies: as photosynthesis declines so does heterotrophic respiration. Consequently we can be more confident in our assertion that the Kurz et al. result is overstating the impact of the D. ponderosae in terms of the potential release of carbon to the atmosphere. Longer term the carbon in the dead wood will be released to the atmosphere, but the regrowth of new trees, which is a strong sink of carbon, is likely to offset much of this on similar time scales. This isn’t to say the beetles are not a problem, they most certainly are, but that the ecological puzzle is inevitably more complex than it first appears.
A manuscript is currently under consideration for Ecology Letters (Moore et al., submitted) that describes all of this in much more detail. I will update this post when that paper is accepted. Our next steps are two-fold: first, to try and scale our estimates up over even larger areas and second to incorporate our new knowledge into a prognostic model so that we can make better constrained projections of the interaction with climate.
To find out more:
Kurz W.A., Dymond C.C., Stinson G., Rampley G.J., Neilson E.T., Carroll A.L., Ebata T. and Safranyik L. (2008) Mountain pine beetle and forest carbon feedback to climate change. Nature 452, 987-990, doi:10.1038/nature06777
Moore D.J.P., Trahan N.A., Wilkes P., Quaife T., Desai A.R., Negron J.F., Stephens B.B., Elder K. and Monson R.K. (2013, submitted) Changes in carbon balance after insect disturbance in Western U.S. forests. Submitted to Ecology Letters.