Arctic and boreal ecosystems of western North America as components of the climate system

Abstract

Summary Synthesis of results from several Arctic and boreal research programmes provides evidence for the strong role of high‐latitude ecosystems in the climate system. Average surface air temperature has increased 0.3 °C per decade during the twentieth century in the western North American Arctic and boreal forest zones. Precipitation has also increased, but changes in soil moisture are uncertain. Disturbance rates have increased in the boreal forest; for example, there has been a doubling of the area burned in North America in the past 20 years. The disturbance regime in tundra may not have changed. Tundra has a 3–6‐fold higher winter albedo than boreal forest, but summer albedo and energy partitioning differ more strongly among ecosystems within either tundra or boreal forest than between these two biomes. This indicates a need to improve our understanding of vegetation dynamics within, as well as between, biomes. If regional surface warming were to continue, changes in albedo and energy absorption would likely act as a positive feedback to regional warming due to earlier melting of snow and, over the long term, the northward movement of treeline. Surface drying and a change in dominance from mosses to vascular plants would also enhance sensible heat flux and regional warming in tundra. In the boreal forest of western North America, deciduous forests have twice the albedo of conifer forests in both winter and summer, 50–80% higher evapotranspiration, and therefore only 30–50% of the sensible heat flux of conifers in summer. Therefore, a warming‐induced increase in fire frequency that increased the proportion of deciduous forests in the landscape, would act as a negative feedback to regional warming. Changes in thermokarst and the aerial extent of wetlands, lakes, and ponds would alter high‐latitude methane flux. There is currently a wide discrepancy among estimates of the size and direction of CO2 flux between high‐latitude ecosystems and the atmosphere. These discrepancies relate more strongly to the approach and assumptions for extrapolation than to inconsistencies in the underlying data. Inverse modelling from atmospheric CO2 concentrations suggests that high latitudes are neutral or net sinks for atmospheric CO2, whereas field measurements suggest that high latitudes are neutral or a net CO2 source. Both approaches rely on assumptions that are difficult to verify. The most parsimonious explanation of the available data is that drying in tundra and disturbance in boreal forest enhance CO2 efflux. Nevertheless, many areas of both tundra and boreal forests remain net sinks due to regional variation in climate and local variation in topographically determined soil moisture. Improved understanding of the role of high‐latitude ecosystems in the climate system requires a concerted research effort that focuses on geographical variation in the processes controlling land–atmosphere exchange, species composition, and ecosystem structure. Future studies must be conducted over a long enough time‐period to detect and qu...

Holocene carbon burial by lakes in SW Greenland

“…Lakes have tended to be ignored in regional C-models (Chapin et al, 2000) despite clear evidence that they can be net sources of CO 2 to the atmosphere (Kling et al, 1991(Kling et al, , 1992Sobek et al, 2005;Cole et al, 2007) Moreover, although plant biomass may increase with higher air temperatures and nutrient levels associated with microbial activity, there is growing evidence for soil C loss due to increased mineralization of soil C by bacterial activity at the same time as the landscape 'greens' (Mack et al, 2004). As the soil C pool declines with accelerated global warming and greater mineralization of soil C, the importance of the lake sediment component, which is essentially permanent may increase.…”

“…Future global change processes may impact the large C-pools in arctic soils (Oechel & Billings, 1992) with associated implications for both regional and global climate (Chapin et al, 2000). Lakes have tended to be ignored in regional C-models (Chapin et al, 2000) despite clear evidence that they can be net sources of CO 2 to the atmosphere (Kling et al, 1991(Kling et al, , 1992Sobek et al, 2005;Cole et al, 2007) Moreover, although plant biomass may increase with higher air temperatures and nutrient levels associated with microbial activity, there is growing evidence for soil C loss due to increased mineralization of soil C by bacterial activity at the same time as the landscape 'greens' (Mack et al, 2004).…”

Terrestrial biosphere models need better representation of vegetation phenology: results from the North American Carbon Program Site Synthesis

“…In all likelihood, however, these and similar models would also misrepresent other key feedbacks of terrestrial vegetation to the climate system during spring and autumn transition periods, e.g., through changing albedo, surface energy balance adjustment, and the changing partitioning of available energy to latent and sensible heat fluxes. This is of great importance because in addition to an influence on microclimate (e.g., ambient surface temperature, humidity, and radiative transfer through the canopy), phenology has effects on the planetary boundary layer, regional-to-global circulation patterns, and thus continental-scale climatic patterns (Hayden, 1998;Pielke et al, 1998;Chapin et al, 2000;Hogg et al, 2000;Fitzjarrald et al, 2001). Failure to represent phenology accurately in models that couple the land surface to the atmosphere could lead to large errors in the seasonal evolution of regional weather patterns, for example.…”

Uptake and recovery of soil nitrogen by bryophytes and vascular plants in an alpine meadow

“…Therefore, it is expected that a range of alternatively phenological patterns may be disturbed by the responses of plant phenology to future climates, but they are species specific (Woodward and Cramer 1996;Chapin et al 2000;Chmielewski and Rotzer 2001;Penuelas and Filella 2001;Badeck et al 2004;Cleland et al 2006;Sherry et al 2007). A longer growing season has been indicated at the community and biome levels across most areas of the Northern Hemisphere (Steltzer and Post 2009 (Kelly and Goulden 2008;Tylianakis et al 2008), which may induce difficulties to study intensively and precisely due to ecosystems' complexity.…”