Forests on thawing permafrost: fragmentation, edge effects, and net forest loss

Abstract

Much of the world's boreal forest occurs on permafrost (perennially cryotic ground). As such, changes in permafrost conditions have implications for forest function and, within the zone of discontinuous permafrost (30-80% permafrost in areal extent), distribution. Here, forested peat plateaus underlain by permafrost are elevated above the surrounding permafrost-free wetlands; as permafrost thaws, ground surface subsidence leads to waterlogging at forest margins. Within the North American subarctic, recent warming has produced rapid, widespread permafrost thaw and corresponding forest loss. Although permafrost thaw-induced forest loss provides a natural analogue to deforestation occurring in more southerly locations, we know little about how fragmentation relates to subsequent permafrost thaw and forest loss or the role of changing conditions at the edges of forested plateaus. We address these knowledge gaps by (i) examining the relationship of forest loss to the degree of fragmentation in a boreal peatland in the Northwest Territories, Canada; and (ii) quantifying associated biotic and abiotic changes occurring across forest-wetland transitions and extending into the forested plateaus (i.e., edge effects). We demonstrate that the rate of forest loss correlates positively with the degree of fragmentation as quantified by perimeter to area ratio of peat plateaus (edge : area). Changes in depth of seasonal thaw, soil moisture, and effective leaf area index (LAIe ) penetrated the plateau forests by 3-15 m. Water uptake by trees was sevenfold greater in the plateau interior than at the edges with direct implications for tree radial growth. A negative relationship existed between LAIe and soil moisture, suggesting that changes in vegetation physiological function may contribute to changing edge conditions while simultaneously being affected by these changes. Enhancing our understanding of mechanisms contributing to differential rates of permafrost thaw and associated forest loss is critical for predicting future interactions between the land surface processes and the climate system in high-latitude regions.

Climate change, community capitals, and food security: Building a more sustainable food system in a northern Canadian boreal community

“…Changes on the landscape due to permafrost thaw results in land subsidence, conversion of forest to muskeg, and changes in water resources, and have significantly affected the lands around the region (Baltzer, Veness, Chasmer, Sniderhan & Quinton, 2014;Coleman et al, 2015;Quinton, Hayashi & Chasmer, 2011;Williams, Quinton & Baltzer, 2013). This landscape change, and more importantly changes in water and ice conditions makes travel more difficult and less safe.…”

Recent climatic, cryospheric, and hydrological changes over the interior of western Canada: a review and synthesis

“…From a global perspective, permafrost-underlain watersheds, particularly those at or near 0 • C, are extremely sensitive to warming as changes in ground thermal status will alter all components of the hydrological cycle due to increases in subsurface storage for liquid water (Woo, 2012). This also impacts forest ecosystems and function, leading to thaw-induced collapse of forested peat plateaus and the loss of forest across vast parts of the boreal forest (Baltzer et al, 2014).…”

“…For instance, As ground temperatures have risen, there has also been an increase in the thaw depth of the seasonal active layer across northern Canada and local to widespread permafrost degradation in the discontinuous permafrost zone (Smith, 2011). In many areas, the melting of the near-surface ground ice has resulted in decreased stability and strength of the substrate, leading to surface subsidence and waterlogging of soils (both of which further amplify permafrost thaw), thermokarst development, and collapse of forested peat plateaus (Jorgenson et al, 2008;Smith et al, 2008;Baltzer et al, 2014). reported interannual variability of maximum seasonal thaw depths that ranged between 0.3 and 0.55 m, with a trend toward increasing thaw depths of 0.08 m over the period 1983-2008 in the outer Mackenzie Delta area.…”

“…This has the potential to substantially alter basin runoff production in the region (Connon et al, 2014). Further, it has been shown that the rates of permafrost and forested plateau loss have been accelerating, with the average rate of loss in seven different local areas of interest being over 3 times greater during the period -2010compared to 1977(Baltzer et al, 2014.…”

Surface CO2 Exchange Dynamics across a Climatic Gradient in McKenzie Valley: Effect of Landforms, Climate and Permafrost

“…Accelerated collapsing of the peat plateau would subject soil to waterlogging, thereby decreasing aerobic respiration. Furthermore, with receding of permafrost, the connectivity of collapsing features would drain the landscape [75,76] resulting in increased active layer depth, thereby making the previously frozen organic matter available for decomposition. However, an increase in the rate of surface assimilation by bryophytes is to be expected in the expanding collapsed areas.…”