The effects of nitrogen-phosphorus-potassium fertilization on primary production and plant biomass were determined for four tussock tundra sites in Alaska. The results generally supported conclusio...
The progress made in the Land–Atmosphere–Ice Interactions Flux Study over the past 4 yr to fully characterize the biophysical fluxes in the snow-free tundra ecosystem and their relationship to climate and climate change is described. This paper is the result of a synthesis effort to bring together the measurements of surface fluxes at various sites on the North Slope of Alaska in the snow-free period of 1995 with the results of modeling efforts for this region. It is found that methodological and site dissimilarities contribute to measurement differences at least as much as instrument and sampling error, even for closely collocated and similarly vegetated sites. The regional climate model employed in this study generally simulates fluxes that are within the range of measured fluxes, but tends to overestimate both net radiation and latent heat fluxes. The regional model also captures site to site variations quite well, which appear to be more sensitive to mesoscale meteorological conditions than on specifics of site characteristics. The active layer model employed in this study performs well in estimating ground heat flux but rather more poorly in simulating turbulent fluxes. The global climate model is unable to capture the broad-scale response of the land surface sensible heat flux and net radiation, although it performs rather better in the simulation of latent and ground heat fluxes. Finally, the intended purposes and applications of both data and model simulations have a strong impact on their applicability to other studies.
Polar ice-core records suggest that an arctic or boreal source was responsible for more than 30% of the large increase in global atmospheric methane (CH4) concentration during deglacial climate warming; however, specific sources of that CH4 are still debated. Here we present an estimate of past CH4 flux during deglaciation from bubbling from thermokarst (thaw) lakes. Based on high rates of CH4 bubbling from contemporary arctic thermokarst lakes, high CH4 production potentials of organic matter from Pleistocene-aged frozen sediments, and estimates of the changing extent of these deposits as thermokarst lakes developed during deglaciation, we find that CH4 bubbling from newly forming thermokarst lakes comprised 33 to 87% of the high-latitude increase in atmospheric methane concentration and, in turn, contributed to the climate warming at the Pleistocene-Holocene transition.
We agree with Corbera and Pascual that payments for ecosystem services (PES) should be deployed only if they improve overall human wellbeing. We also agree that the credibility and acceptability of PES schemes likely require that participants view both the process and the outcomes as fair. We do not
We successfully developed a series of models to explore the importance of species differences in phenologies of growth and nitrogen uptake to competitive interactions in upland tussock tundra. We developed growth models for 4 major tussock tundra species, based on observed growth rates and phenologies. We found that differences in phenology and nutrient use strategy could permit coexistence of some, but not all of the tundra plants modeled. The plant that was the best competitor, because of its rapid growth rate and superior ability to retranslocate nitrogen, may be naturally limited in its competitive ability by its tussock growth form. The mechanisms behind this limitation, and the contributions of patterns of mortality to observed production, will be explored in future modeling and experimental studies. In addition, our models point out that our understanding of the dynamics of nitrogen supply is still inadequate.
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 CO 2 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 CO 2 concentrations suggests that high latitudes are neutral or net sinks for atmospheric CO 2 , whereas field measurements suggest that high latitudes are neutral or a net CO 2 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 CO 2 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 quantify ecosystem feedbacks.
