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Measurements of the net CO2 flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO2 source–sink patterns, CO2 flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO2 (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model-data comparison. Flux measurements made over four vegetation types were used to calibrate the land-surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land-surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO2 uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). [ABSTRACT FROM AUTHOR]

Atmospheric measurements and land-based inventories imply that terrestrial ecosystems in the northern hemisphere are taking up significant amounts of anthropogenic cabon dioxide (CO2) emissions; however, there is considerable disagreement about the causes of this uptake, and its expected future trajectory. In this paper, we use the ecosystem demography (ED) model to quantify the contributions of disturbance history, CO2 fertilization and climate variability to the past, current, and future terrestrial carbon fluxes in the Eastern United States. The simulations indicate that forest regrowth following agricultural abandonment accounts for uptake of 0.11 Pg C yr−1 in the 1980s and 0.15 Pg C yr−1 in the 1990s, and regrowth following forest harvesting accounts for an additional 0.1 Pg C yr−1 of uptake during both these decades. The addition of CO2 fertilization into the model simulations increases carbon uptake rates to 0.38 Pg C yr−1 in the 1980s and 0.47 Pg C yr−1 in the 1990s. Comparisons of predicted aboveground carbon uptake to regional-scale forest inventory measurements indicate that the model's predictions in the absence of CO2 fertilization are 14% lower than observed, while in the presence of CO2 fertilization, predicted uptake rates are 28% larger than observed. Comparable results are obtained from comparisons of predicted total Net Ecosystem Productivity to the carbon fluxes observed at the Harvard Forest flux tower site and in model simulations free-air CO2 enrichment (FACE) experiments. These results imply that disturbance history is the principal mechanism responsible for current carbon uptake in the Eastern United States, and that conventional biogeochemical formulations of plant growth overestimate the response of plants to rising CO2 levels. Model projections out to 2100 imply that the carbon uptake arising from forest regrowth will increasingly be dominated by forest regrowth following harvesting. Consequently, actual carbon storage declines to near zero by the end of the 21st century as the forest regrowth that has occurred since agricultural abandonment comes into equilibrium with the landscape's new disturbance regime. Incorporating interannual climate variability into the model simulations gives rise to large interannual variation in regional carbon fluxes, indicating that long-term measurements are necessary to detect the signature of processes that give rise to long-term uptake and storage. [ABSTRACT FROM AUTHOR]

Abstract: Climate projections forecast a redistribution of seasonal precipitation for much of the globe into fewer, larger events spaced between longer dry periods, with negligible changes in seasonal rainfall totals. This intensification of the rainfall regime is expected to alter near‐surface water availability, which will affect plant performance and carbon uptake. This could be especially important in peatland systems, where large stores of carbon are tightly coupled to water surpluses limiting decomposition. Here, we examined the role of precipitation frequency on vegetation growth and carbon dioxide (CO2) balances for communities dominated by a Sphagnum moss, a sedge, and an ericaceous shrub in a cool temperate poor fen. Field plots and laboratory monoliths received one of three rainfall frequency treatments, ranging from one event every three days to one event every 14 days, while total rain delivered in a two‐week cycle and the entire season to each treatment remained the same. Separating incident rain into fewer but larger events increased vascular cover in all peatland communities: vascular plant cover increased 6× in the moss‐dominated plots, nearly doubled in the sedge plots, and tripled in the shrub plots in Low‐Frequency relative to High‐Frequency treatments. Gross ecosystem productivity was lowest in moss communities receiving low‐frequency rain, but higher in sedge and shrub communities under the same conditions. Net ecosystem exchange followed this pattern: fewer events with longer dry periods increased CO2 flux to the atmosphere from the moss while vascular plant‐dominated communities became more of a sink for CO2. Results of this study suggest that changes to rainfall frequency already occurring and predicted to continue will lead to increased vascular plant cover in peatlands and will impact their carbon‐sink function. [ABSTRACT FROM AUTHOR]

Boreal peatlands contain approximately 500 Pg carbon (C) in the soil, emit globally significant quantities of methane ( CH4), and are highly sensitive to climate change. Warming associated with global climate change is likely to increase the rate of the temperature-sensitive processes that decompose stored organic carbon and release carbon dioxide ( CO2) and CH4. Variation in the temperature sensitivity of CO2 and CH4 production and increased peat aerobicity due to enhanced growing-season evapotranspiration may alter the nature of peatland trace gas emission. As CH4 is a powerful greenhouse gas with 34 times the warming potential of CO2, it is critical to understand how factors associated with global change will influence surface CO2 and CH4 fluxes. Here, we leverage the Spruce and Peatland Responses Under Changing Environments ( SPRUCE) climate change manipulation experiment to understand the impact of a 0-9°C gradient in deep belowground warming ('Deep Peat Heat', DPH) on peat surface CO2 and CH4 fluxes. We find that DPH treatments increased both CO2 and CH4 emission. Methane production was more sensitive to warming than CO2 production, decreasing the C- CO2:C- CH4 of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ13C of CH4 suggesting an increasing contribution of acetoclastic methanogenesis to total CH4 production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH4 is <2%, CH4 represents >50% of seasonal C emissions in the highest-warming treatments when adjusted for CO2 equivalents on a 100-year timescale. These results suggest that warming in boreal regions may increase CH4 emissions from peatlands and result in a positive feedback to ongoing warming. [ABSTRACT FROM AUTHOR]

Determining whether the terrestrial biosphere will be a source or sink of carbon (C) under a future climate of elevated CO2 ( eCO2) and warming requires accurate quantification of gross primary production ( GPP), the largest flux of C in the global C cycle. We evaluated 6 years (2007-2012) of flux-derived GPP data from the Prairie Heating and CO2 Enrichment ( PHACE) experiment, situated in a grassland in Wyoming, USA. The GPP data were used to calibrate a light response model whose basic formulation has been successfully used in a variety of ecosystems. The model was extended by modeling maximum photosynthetic rate ( Amax) and light-use efficiency ( Q) as functions of soil water, air temperature, vapor pressure deficit, vegetation greenness, and nitrogen at current and antecedent (past) timescales. The model fits the observed GPP well ( R2 = 0.79), which was confirmed by other model performance checks that compared different variants of the model (e.g. with and without antecedent effects). Stimulation of cumulative 6-year GPP by warming (29%, P = 0.02) and eCO2 (26%, P = 0.07) was primarily driven by enhanced C uptake during spring (129%, P = 0.001) and fall (124%, P = 0.001), respectively, which was consistent across years. Antecedent air temperature (Tairant) and vapor pressure deficit ( VPDant) effects on Amax (over the past 3-4 days and 1-3 days, respectively) were the most significant predictors of temporal variability in GPP among most treatments. The importance of VPDant suggests that atmospheric drought is important for predicting GPP under current and future climate; we highlight the need for experimental studies to identify the mechanisms underlying such antecedent effects. Finally, posterior estimates of cumulative GPP under control and eCO2 treatments were tested as a benchmark against 12 terrestrial biosphere models ( TBMs). The narrow uncertainties of these data-driven GPP estimates suggest that they could be useful semi-independent data streams for validating TBMs. [ABSTRACT FROM AUTHOR]

Exponentially rising CO2 (currently ~400 μatm) is driving climate change and causing acidification of both marine and freshwater environments. Physiologists have long known that CO2 directly affects acid-base and ion regulation, respiratory function and aerobic performance in aquatic animals. More recently, many studies have demonstrated that elevated CO2 projected for end of this century (e.g. 800-1000 μatm) can also impact physiology, and have substantial effects on behaviours linked to sensory stimuli (smell, hearing and vision) both having negative implications for fitness and survival. In contrast, the aquaculture industry was farming aquatic animals at CO2 levels that far exceed end-of-century climate change projections (sometimes >10 000 μatm) long before the term 'ocean acidification' was coined, with limited detrimental effects reported. It is therefore vital to understand the reasons behind this apparent discrepancy. Potential explanations include 1) the use of 'control' CO2 levels in aquaculture studies that go beyond 2100 projections in an ocean acidification context; 2) the relatively benign environment in aquaculture (abundant food, disease protection, absence of predators) compared to the wild; 3) aquaculture species having been chosen due to their natural tolerance to the intensive conditions, including CO2 levels; or 4) the breeding of species within intensive aquaculture having further selected traits that confer tolerance to elevated CO2. We highlight this issue and outline the insights that climate change and aquaculture science can offer for both marine and freshwater settings. Integrating these two fields will stimulate discussion on the direction of future cross-disciplinary research. In doing so, this article aimed to optimize future research efforts and elucidate effective mitigation strategies for managing the negative impacts of elevated CO2 on future aquatic ecosystems and the sustainability of fish and shellfish aquaculture. [ABSTRACT FROM AUTHOR]

Frequency and severity of insect outbreaks in forest ecosystems are predicted to increase with climate change. How this will impact canopy leaf area in future climates is rarely tested. Here, we document function of insect outbreaks that fortuitously and rapidly occurred in an ecosystem under free-air CO2 enrichment. Over the first 2 years of CO2 fumigation of a naturally established mature Eucalyptus woodland, we continuously assessed population responses of three sap-feeding insect species of the psyllid genera Cardiaspina, Glycaspis and Spondyliaspis for up to ten consecutive generations. Concurrently, we quantified changes in the canopy leaf area index ( LAI). Large and rapid shifts in psyllid community composition were recorded between species with either flush ( Glycaspis) or senescence-inducing ( Cardiaspina, Spondyliaspis) feeding strategies. Within the second year, two psyllid species experienced significant and rapid population build-up resulting in two consecutive outbreaks: first, rainfall stimulated Eucalyptus leaf production increasing LAI, which supported population growth of flush-feeding Glycaspis without impacting LAI. Glycaspis numbers then crashed and were followed by the outbreak of senescence-feeding Cardiaspina fiscella that led to significant defoliation and reduction in LAI. For all three psyllid species, the abundance of lerps, protective coverings excreted by the sessile nymphs, decreased at e[ CO2]. Higher lerp weight at e[ CO2] for Glycaspis but not the other psyllid species provided evidence for compensatory feeding by the flush feeder but not the two senescence feeders. Our study demonstrates that rainfall drives leaf phenology, facilitating the rapid boom-and-bust succession of psyllid species, eventually leading to significant defoliation due to the second but not the first outbreaking psyllid species. In contrast, e[ CO2] may impact psyllid abundance and feeding behaviour, with psyllid species-specific outcomes for defoliation severity, nutrient transfer and trophic cascades. Psyllid populations feeding on Eucalyptus experience rapid boom-and-bust cycles depending on availability of suitable foliage driven by rainfall patterns and leaf phenology. [ABSTRACT FROM AUTHOR]

As Earth's atmosphere accumulates carbon dioxide ( CO2) and other greenhouse gases, Earth's climate is expected to warm and precipitation patterns will likely change. The manner in which terrestrial ecosystems respond to climatic changes will in turn affect the rate of climate change. Here we describe responses of an old-field herbaceous community to a factorial combination of four levels of warming (up to 4 °C) and three precipitation regimes (drought, ambient and rain addition) over 2 years. Warming suppressed total production, shoot production, and species richness, but only in the drought treatment. Root production did not respond to warming, but drought stimulated the growth of deeper (> 10 cm) roots by 121% in 1 year. Warming and precipitation treatments both affected functional group composition, with C4 grasses and other annual and biennial species entering the C3 perennial-dominated community in ambient rainfall and rain addition treatments as well as in warmed treatments. Our results suggest that, in this mesic system, expected changes in temperature or large changes in precipitation alone can alter functional composition, but they have little effect on total herbaceous plant growth. However, drought limits the capacity of the entire system to withstand warming. The relative insensitivity of our study system to climate suggests that the herbaceous component of old-field communities will not dramatically increase production in response to warming or precipitation change, and so it is unlikely to provide either substantial increases in forage production or a meaningful negative feedback to climate change later this century. [ABSTRACT FROM AUTHOR]

Current climate change exacerbates the environmental restrictions on temperate species inhabiting low latitude edges of their geographical ranges. We examined how temperature variations due to current and future climate change are likely to affect populations' persistence of stream-dwelling brown trout Salmo trutta at the vulnerable southern periphery of its range. Analysis of 33 years of air temperature data (1975-2007) by time-series models indicated a significant upward trend and a pronounced shift in air temperature around 1986-1987. This warming is associated with an ongoing population decline of brown trout, most likely caused by a loss of suitable thermal habitat in lower latitudes since the 1980s. Population decrease may not be attributed to physical habitat modification or angler pressure, as carrying capacity remained stable and populations were not overexploited. We developed regional temperature models, which predicted that unsuitable thermal habitat for brown trout increased by 93% when comparing climate conditions between 1975-1986 and 1993-2004. Predictions from climate envelope models showed that current climate change may be rendering unsuitable 12% of suitable thermal habitat each decade, resulting in an overall population decrease in the lower reaches of around 6% per year. Furthermore, brown trout catches markedly decreased 20% per year. Projections of thermal habitat loss under the ecologically friendly B2 SRES scenario showed that brown trout may lose half of their current suitable habitat within the study area by 2040 and become almost extinct by 2100. In parallel to the upstream movement of brown trout thermal habitat, warm water species are increasing their relative abundance in salmonid waters. Empirical evidence was provided of how current climate change threatens some of the most healthy native brown trout populations in Southern Europe and how forthcoming climate change is expected to further decrease the conservation status of the species. [ABSTRACT FROM AUTHOR]

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO concentration ( c) and climate in the Iberian Peninsula using annually resolved width and δC tree-ring chronologies since 1600. C discrimination (Δ≈ c/ c), leaf intercellular CO concentration ( c) and intrinsic water-use efficiency (iWUE) were inferred from δC values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c/ c ratio, leading to significant c and iWUE increases (active response to c); whereas a significant increase in c/ c resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c). A significant decrease in c/ c led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the δC series after removing the low-frequency trends due to the physiological responses to increasing c. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c changes modulated δC and masked the climate signal. Since 1970 higher δC values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO'fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a c rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth. [ABSTRACT FROM AUTHOR]

The rates of carbon bio-sequestration within silica phytoliths of the leaf litter of 10 economically important bamboo species indicates that (a) there is considerable variation in the content of carbon occluded within the phytoliths (PhytOC) of the leaves between different bamboo species, (b) this variation does not appear to be directly related to the quantity of silica in the plant but rather the efficiency of carbon encapsulation by the silica. The PhytOC content of the species under the experimental conditions ranged from 1.6% to 4% of the leaf silica weight. The potential phytolith carbon bio-sequestration rates in the leaf-litter component for the bamboos ranged up to 0.7 tonnes of carbon dioxide (CO2) equivalents (t-e-CO2) ha−1 yr−1 for these species. Assuming a median phytolith carbon bio-sequestration yield of 0.36 t-e-CO2 ha−1 yr−1, the global potential for bio-sequestration via phytolith carbon (from bamboo and/or other similar grass crops) is estimated to be ∼1.5 billion t-e-CO2 yr−1, equivalent to 11% of the current increase in atmospheric CO2. The data indicate that the management of vegetation such as bamboo forests to maximize the production of PhytOC has the potential to result in considerable quantities of securely bio-sequestered carbon. [ABSTRACT FROM AUTHOR]

Partitioning soil CO2 efflux into autotrophic ( RA) and heterotrophic ( RH) components is crucial for understanding their differential responses to climate change. We conducted a long-term experiment (2000–2005) to investigate effects of warming 2°C and yearly clipping on soil CO2 efflux and its components (i.e. RA and RH) in a tallgrass prairie ecosystem. Interannual variability of these fluxes was also examined. Deep collars (70 cm) were inserted into soil to measure RH. RA was quantified as the difference between soil CO2 efflux and RH. Warming treatment significantly stimulated soil CO2 efflux and its components (i.e. RA and RH) in most years. In contrast, yearly clipping significantly reduced soil CO2 efflux only in the last 2 years, although it decreased RH in every year of the study. Temperature sensitivity (i.e. apparent Q10 values) of soil CO2 efflux was slightly lower under warming ( P>0.05) and reduced considerably by clipping ( P<0.05) compared with that in the control. On average over the 4 years, RH accounted for approximately 65% of soil CO2 efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of RH to soil CO2 efflux reached a maximum in winter (∼90%) and a minimum in summer (∼35%). Annual soil CO2 efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO2 efflux may be mainly caused by precipitation distribution and summer severe drought. Our results suggest that the effects of warming and yearly clipping on soil CO2 efflux and its components did not result in significant changes in RH or RA contribution, and rainfall timing may be more important in determining interannual variability of soil CO2 efflux than the amount of annual precipitation. [ABSTRACT FROM AUTHOR]

Summer droughts are predicted to increase in severity and frequency in the United Kingdom, due to climate change. Few studies have addressed the impacts of drought on interactions between species, and the majority have focussed on increases in CO2 concentration and changes in temperature. Here, the effect of experimental summer drought on the strength of the plant-mediated interaction between leaf-mining Stephensia brunnichella larvae and root-chewing Agriotes larvae was investigated. Agriotes larvae reduced the abundance and performance of S. brunnichella feeding on a mutual host plant, Clinopodium vulgare, as well as the rate of parasitism of the leaf-miner. The interaction did not, however, occur on plants subjected to a severe drought treatment, which were reduced in size. Changes to summer rainfall, due to climate change, may therefore reduce the occurrence of plant-mediated interactions between insect herbivores. [ABSTRACT FROM AUTHOR]

Carbon cycling in lakes has been studied both in terms of the magnitude and global significance of carbon released to the atmosphere and carbon stored in the sediments. Interestingly, to the best of our knowledge there have been no studies examining whether instantaneous measures of carbon flux are directly related to how much carbon is being drawn down to the sediments. Here, we show that the saturation of CO2 in the water column is negatively related to the sedimentation rate. We, therefore, suggest that sedimentation should be explicitly considered as a factor directly controlling CO2 flux from freshwaters. Furthermore, we show that food webs and nutrients can alter CO2 flux by changing sedimentation. Oceanographers have long recognized the interaction between biota and sedimentation in the carbon dynamics of the ocean, a mechanism referred to as the biological pump. Our experiments rigorously test sedimentation and food web structure as factors controlling CO2 saturation and suggest a common framework for freshwater and marine systems. [ABSTRACT FROM AUTHOR]

SummaryNitrogen-stressed microcosms of the C3 grass Danthonia richardsonii gained nitrogen from the environment when grown under ambient or enriched (359, ‘amb’ or 719 μL L- 1 ‘enr’, respectively) atmospheric CO2 concentrations over a 4-y period. This gain was apparent at all rates of supplied mineral N (2.2, 6.7 or 19.8 g N m- 2 y- 1 – low-N, mid-N or high-N), although it was small at high-N. Small losses of N occurred from the microcosm as leachate, while gaseous losses of N were estimated to be between 10% and 25% of applied mineral N. Losses of applied mineral N were slightly lower under CO2 enrichment only at the highest rate of mineral N supply. Levels of 15N natural abundance in green leaf (δ15Ν) of - 2‰ (amb low-N) and of below - 4‰ (enr low- & mid-N) suggest that absorption of atmospheric NH3 may have been a source of some of the extra N in the low and mid-N treatments. Biological N2 fixation, of up to 2 g m- 2 y- 1 was hypothesized to form the remainder of the environmental N source. Microcosm C:N ratio was higher under CO2 enrichment. Nitrogen productivity of microcosm carbon gain (g C accumulated g- 1 leaf N day- 1) was increased (up to 100%) by CO2 enrichment at all rates of mineral N supply. Green leaf %N was reduced by CO2 enrichment, and there was less nitrogen in the green leaf pool under CO2 enrichment. Less, or the same amount of nitrogen was present in senesced leaf, surface litter and root under CO2 enrichment while more nitrogen was present in the soil in organic forms, and as NH4 + at the highest rate of mineral N supply. [ABSTRACT FROM AUTHOR]

AbstractLocal effects of climate change (CC) and elevated CO2 (2 × CO2, 660 μmol mol–1) on managed temperate grasslands were assessed by forcing a dynamic ecosystem model with weather scenarios. The aims of the study were to compare the relative importance of individual and combined effects of CC, 2 × CO2, and photosynthetic acclimation, and to assess the importance of local site conditions. The model was driven by hourly means for temperature (T), precipitation (P), global radiation (G), vapour pressure (VP), and wind speed (U). Local climate scenarios were derived by statistical downscaling techniques from a 2 × CO2 simulation with the General Circulation Model of the Canadian Climate Centre (CCC-GCMII). Simulations over 14 growing seasons to account for year-to-year variability of climate were carried out for a low, relatively dry site, and a high, more humid site. At both sites, shoot dry matter responded positively to 2 × CO2 with the site at low elevation being more sensitive than the higher site. The effect of assumed changes in climate was negative at the lower, but positive at the higher site. Shoot dry matter was more sensitive to the effects of 2 × CO2 than to CC. Both effects combined increased shoot dry matter by up to 20%. This was attributed to direct effects of 2 × CO2 and increased T, and indirect stimulation via increased soil N availability. Biomass partitioning to roots increased with 2 × CO2 but decreased with CC, while an intermediate response resulted from the combination. Leaf area index (LAI) increased under 2 × CO2, but not enough to compensate fully for a decrease in leaf conductance. Under the 2 × CO2 scenario evapotranspiration (ET) decreased, but increased under CC. Photosynthetic acclimation reduced the effect of 2 × CO2 on shoot growth, but had little effect on ET. The seasonal water use efficiency (WUE) was improved under 2 × CO2, and reduced under CC. With the combination of both factors, the change was small but still positive, especially at the high elevation site with more favourable soil water conditions. This reflects the stronger positive yield response in combination with a smaller increase in ET under cooler, more humid conditions. The results for the combination of factors suggest that except for shoot growth, effects of 2 × CO2 and CC tend to offset each other. While CC determines the sign of the ET response, the sign of the biomass response is determined by 2 × CO2. The results highlight the importance of a site-specific analysis of ecosystem responses by using a flexible approach based on a combination of state-of-the-art downscaling, spatially resolved data sets, and a mechanistic model to obtain quantitative and reproducible assessments of climate change impacts at the ecosystem level. [ABSTRACT FROM AUTHOR]

Abstract It is well recognized that photosynthesis of C3 plants is highly responsive to CO2 concentration. However, in natural ecosystems, plants are subject to a range of feed-back effects that can interact with increased photosynthetic carbon gain in different ways so that it is not clear to what extent increased photosynthesis will translate into increased growth. To assess the probable growth response of nutrient-limited forests to increasing CO2 concentration, we use a previously developed modelling framework and apply it under conditions where the supply of nutrients is affected by a range of different factors. Our analysis indicates that forest growth is likely to be highly stimulated by increasing CO2 concentration in forests with high fertility, in forests with nitrogen fixing plants, in those subject to fire or where nitrogen in wood is effectively removed from the biologically active cycle either through physical removal of stems in harvesting or through continued stem growth over long time periods. Forest growth is likely to be stimulated by CO2 concentration in both phosphorus- and sulphur-limited forests provided nutrients in heartwood of trees are removed from the active nutrient cycle. Without this removal from the cycling system, however, sulphur-limited forests should show little response to increasing CO2. In phosphorus-limited forests without phosphorus removal, the response to increasing CO2 depends further on the equilibration state of the large pool of unavailable secondary phosphorus. Considered over periods of centuries during which the secondary pool has equilibrated, growth of phosphorus-limited forests is likely to be only weakly stimulated by increasing CO2 concentration. However, over shorter periods, increasing CO2 concentration should lead to a substantial increase in productivity. In general, it can be concluded that systems that are more open... [ABSTRACT FROM AUTHOR]