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The Arctic climate is changing. Permafrost is warming, hydrological processes are changing and biological and social systems are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates convincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing point are being altered, hydrelogical and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to isolate due to compounding factors. In almost every discipline represented, we show how the biocomplexity of the Arctic system has highlighted and challenged a paucity of integrated scientific knowledge, the lack of sustained observational and experimental time series, and the technical and logistic constraints of researching the Arctic environment. This study supports ongoing efforts to strengthen the interdisciplinarity of arctic system science and improve the coupling of large scale experimental manipulation with sustained time series observations by incorporating and integrating novel technologies, remote sensing and modeling.

Complex interactions between processes contributing to feedbacks, variability over time and space in biological and physical processes have generated considerable uncertainties in estimating the net effects of climate change on terrestrial feedbacks to the climate system. The great potential importance of the feedback is necessary to analyze the uncertainties and recommend research and monitoring.

The key findings of the assessment of the climate change impact on tundra and polar desert ecosystems and extended summaries of its components are presented. An assessment of the impact of changes in climate and UV-B radiation on Arctic terrestrial ecosystems made within the Arctic Climate Impacts Assessment highlighted the profound implications of projected impacts for the future ecosystems.

Uncertainties arise from methodologies and conceptual framework, from unpredictable surprises and from lack of validation models. Recommendations to reduce the uncertainties are wide-ranging and relate to all disciplines within the assessment.

Assessments of the impact of changes in climate and UV-B radiation within four sub regions of the Arctic are determined by large-scale differences in weather and climate shaping factors. The geographical barriers to species relocation, particularly the distributions of landmasses and separation by seas would affect the northward shift in vegetation zones.

The likely impact of changes in climate and UV-B radiation on the functioning of Arctic ecosystems is assessed. Processes at the plot scales are highlighted and the impact of changes in climate and UV-B on processes at the landscape and regional scales are examined.

The impact of changes in climate and UV-B radiation on the structure of Arctic ecosystems would have consequences for the functioning of Arctic ecosystems in terms of flows of nutrients and carbon and exchanges between biosphere and atmosphere. It is important to relate the changes in ecosystem to changes in ecosystem function.

The existing changes recorded for the distribution, abundance and performance of Arctic species to project future changes of Arctic species related to a range of climate related factors are assessed. Species respond individualistically to environmental variables such as temperature but their responses are moderated by the way neighbors, competitors and facilitators respond to the same environmental change.

A holistic approach is made to assess impact of climate change on Arctic terrestrial ecosystem. The last glacial maximum of these regions were submerged beneath vast ice sheets wherein many of the biota comprising present Arctic ecosystems were found at lower latitudes.

The attributes of the existing Arctic species that are likely to constrain or facilitate the responses to a changing climate and UV-B regime are highlighted. Changes in the Arctic landscape processes and ecosystems in a future climatic and UV-B regime depends on the ability of the Arctic species to withstand or adapt to the new environment and their interaction with the immigrant species.

The findings of the report that focuses on terrestrial ecosystems of the Arctic, from the treeline ecozone to the polar deserts are presented. The assessment of the impact of changes on the terrestrial ecosystem is based on existing literature rather than on research or modeling activities within the Arctic Climate Impacts Assessment (ACIA).

Changes in the population structure of the lignotuberous resprouter Adenostoma fasciculatum were investigated by experimentally burning at various intensities an old stand in southern California chaparral. Survival after fire, time of resprouting, herbivory, and survival after resprouting were monitored for more than 1 year after the fire. Greater fire intensity increased plant mortality and the size of plants that died as a consequence of the fire. Time from the fire until resprouting increased with increasing fire intensity and was related to plant size: the larger the plants the earlier they resprouted. Post-resprouting mortality also increased with fire intensity and was related to time of resprouting. Herbivory increased with fire intensity and the size of plants affected by it changed with the intensity of the fire. Fire intensity had profound direct and indirect effects on the population structure of Adenostoma fasciculatum. Plant size strongly determined the direct and indirect lethal effects of fire.

The perennial foliage of the California coast live oak (Quercus agrifolia Nee) permits herbivores to feed on this oak species throughout the year. Patterns of herbivory for a two-year period on Q. agrifolia were observed in relation to seasonal and age-related changes in the nutritional and defensive characteristics of leaves. Nitrogen and phosphorus contents were higher in new leaves compared to mature foliage. Structural compounds (e.g., cellulose) in leaves rapidly increased with age. Concentrations of tatal phenolics (Folin-Denis) and astringency were higher in new foliage, and concentrations of condensed tannins gradually increased as the leaves matured. Peaks of herbivore damage were observed in June and in September-October, and were caused by outbreaks of the California oak moth (Phryganidia californica). P. californica, a bivoltine oak specialist, exhibited feeding preferences in June for old leaves over emerging leaves, and showed no preferences for leaf classes in September. These results suggest that P. californica is adapted to survive on nutritionally poor foliage and to circumvent "quantitative defenses" such as condensed tannins.

The effects of CO 2 enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C 3 ), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO 2 enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 μl/l for the CO 2 -enriched plants, compared to photosynthetic rates measured at 350 μl/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 μl/l were greater than photosynthetic rates of the non-enriched plants measured at 350 μl/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO 2 enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO 2 treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO 2 tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen.

A transportable system for field and laboratory measurement of plant CO 2 exchange and transpiration is described. A computer based data acquisition system which controls data-logging has been successfully proven in mediterranean zones of central Chile and southern California and in the taiga of central Alaska. Six Peltier-effect cooled acrylic plastic chambers are utilized in an open infrared gas analysis system for continuous monitoring of CO 2 exchange. Chamber air temperatures automatically track ambient air temperatures or can be preset to maintain a constant temperature. Use of Peltier-effect cooling modules and electric cartridge heaters result in light-weight chambers and heat exchange units with low power requirements. No refrigeration or liquid coolant is required under most operating conditions. A unique sampling method using natural illumination and six chambers allows highly replicated determinations of the temperature and light dependence of photosynthesis and/or respiration to be made over the course of a 6-d sampling period. Measurements are made continuously 24 h/day. Diurnal CO 2 exchange patterns are determined from one chamber that tracks the ambient air temperature. The other five chambers have air temperatures fixed at distinct levels over a 35° C range. Each day the chambers are changed to another temperature level according to a randomized sampling pattern. All the data are pooled at the end of the 6-d run to construct the light and temperature response surfaces of CO 2 exchange typical of the individuals sampled. Simultaneous determinations of conductance, leaf temperature, air temperature, and light are made.

Two concepts of vegetation function and organization are examined in this paper. The first concerns the possible relationship between energy acquisition and species dominance in a given growth form and vegetation type. The second questions whether homeostatic mechanisms operate at sufficient levels to yield similar and positive photosynthetic performance of evergreen shrubs across a marked environmental and geographical gradient in southern California. This gradient runs from the coast at 135 m to 1,435 m in the mountains and on to 1,150 m at the margin of the desert transition.Total photosynthesis in several species was estimated at each site from late spring through summer. Within a common growth form, species success correlated with photosynthetic capacity. The species with the greatest carbon uptake were best represented in the vegetation. This relationship would not be expected to hold either between widely differing growth forms or where other factors exclude or decrease the abundance or productivity of certain of the species.Across the gradient examined, species displayed a marked degree of photosynthetic homeostasis despite marked environmental changes. For example, photosynthetic uptake in Adenostoma fasciculatum varied by only 32% between the greatest and least productive areas despite large differences in precipitation and temperature. In most cases there was a strong correspondence between cover and photosynthesis across the transect studied.From the available data, the transitions from chaparral to coastal sage, montane forests, or desert transition vegetation do not appear to result from sharp suppression of photosynthetic uptake at these borders. Other factors such as those affecting germination and establishment may be responsible for these transitions. However, further investigations are required at the edges of species distribution to adequately describe the pattern that does exist and to ascertain the relationship between energy capture and species success in these areas.