Could increased boreal forest ecosystem productivity offset carbon losses from increased disturbances?

Abstract

To understand how boreal forest carbon (C) dynamics might respond to anticipated climatic changes, we must consider two important processes. First, projected climatic changes are expected to increase the frequency of fire and other natural disturbances that would change the forest age-class structure and reduce forest C stocks at the landscape level. Second, global change may result in increased net primary production (NPP). Could higher NPP offset anticipated C losses resulting from increased disturbances? We used the Carbon Budget Model of the Canadian Forest Sector to simulate rate changes in disturbance, growth and decomposition on a hypothetical boreal forest landscape and to explore the impacts of these changes on landscape-level forest C budgets. We found that significant increases in net ecosystem production (NEP) would be required to balance C losses from increased natural disturbance rates. Moreover, increases in NEP would have to be sustained over several decades and be widespread across the landscape. Increased NEP can only be realized when NPP is enhanced relative to heterotrophic respiration. This study indicates that boreal forest C stocks may decline as a result of climate change because it would be difficult for enhanced growth to offset C losses resulting from anticipated increases in disturbances.

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO 2 fertilization

“…We caution that neither mortality nor disturbance impacts are addressed here and that therefore extrapolation of future carbon storage in these forests is neither straightforward nor simple to achieve (3,6,8,42). Furthermore, the complex interplay of different biotic and abiotic drivers of boreal forest productivity, including unpredictable wildfires, insect population outbreaks, and other disturbances, may yield counterbalancing effects on the overall net carbon sequestration (6,42,43). Notably, insectinduced collapses in growth in western and eastern Canadian forests played a major role in defining growth trajectories at the turn of the 21st century (3,44,45), but their relative effects on growth are only partially, if at all, captured by this current NFI tree-ring analysis.…”

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…S6). We hypothesize that with ongoing global warming, growth decline events could increase substantially, given that the positive effect of CO 2 concentration on the growth of forests may not be strong enough to compensate for the loss of biomass to fires and climate change (Kurz et al, 2008), which could lead to the opening up of landscapes.…”

The Role of Mycorrhizas in Forest Soil Stability with Climate Change

“…This may limit growth of older trees or promote invasion of weedy plants that are less reliant on mycorrhizal fungi for meeting their resource needs. On balance, the present state of knowledge regarding climate change trajectories suggests that forest health will decline in the future and forest soils will become a net source of atmospheric CO 2 Pendall et al, 2008;Kliejunas et al, 2009;Kurz et al, 2008a). Changes at high latitudes, including thawing and warming of Arctic and boreal soils are especially at risk of strong positive CO 2 and CH 4 feedbacks to the atmosphere (Schuur et al, 2009), as is evidenced by the recent shift in Arctic soils from being a net carbon sink to a net carbon source (Apps et al, 2005).…”

“…However, climate change can upset the soil carbon balance, or its functional stability, by reducing carbon storage and causing a large positive feedback to atmospheric CO 2 levels. Indeed, the amount of CO 2 emissions being sequestered by terrestrial ecosystems is declining and they may become a source by the middle of the 21st century (Cox et al, 2000;Kurz et al, 2008a). When this happens, the atmospheric carbon trajectory will become less dependent on human activities and more so on the much larger carbon pools in terrestrial ecosystems and oceans (Cox et al, 2000).…”