Forest response to elevated CO2 is conserved across a broad range of productivity

Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO(2)] ("CO(2) fertilization"), thereby slowing the rate of increase in atmospheric [CO(2)]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO(2) fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO(2)] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO(2) ( approximately 550 ppm) in four free-air CO(2) enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO(2)] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 +/- 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO(2)] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.

Curbing the U.S. carbon deficit

The U.S. emitted approximately 1.58 petagrams (Pg) of fossil fuel carbon in 2001, approximately one-quarter of global CO(2) production. With climate change increasingly likely, strategies to reduce carbon emissions and stabilize climate are needed, including greater energy efficiency, renewable energy sources, geoengineering, decarbonization, and geological and biological sequestration. Two of the most commonly proposed biological strategies are restoring organic carbon in agricultural soils and using plantations to sequester carbon in soils and wood. Here, we compare scenarios of land-based sequestration to emissions reductions arising from increased fuel efficiency in transportation, targeting ways to reduce net U.S. emissions by 10% ( approximately 0.16 Pg of carbon per year). Based on mean sequestration rates, converting all U.S. croplands to no-till agriculture or retiring them completely could sequester approximately 0.059 Pg of carbon per year for several decades. Summary data across a range of plantations reveal an average rate of carbon storage an order of magnitude larger than in agricultural soils; in consequence, one-third of U.S. croplands or 44 million hectares would be needed for plantations to reach the target of approximately 0.16 Pg of carbon per year. For fossil fuel reductions, cars and light trucks generated approximately 0.31 Pg of carbon in U.S. emissions in 2001. To reduce net emissions by 0.16 Pg of carbon per year, a doubling of fuel efficiency for cars and light trucks is needed, a change feasible with current technology. Issues of permanence, leakage, and economic potentials are discussed briefly, as is the recognition that such scenarios are only a first step in addressing total U.S. emissions.

Mineral dynamics in Spanish moss, Tillandsia usneoides L. (Bromeliaceae), from Central Florida, USA

Epiphytes absorb water and nutrients from the atmosphere through precipitation and dry deposition and from their hosts through stemflow and throughfall. These commensals have been used as biological indicators or monitors of air quality. To measure temporal changes in Spanish moss (Tillandsia usneoides) mineral concentrations, we revisited sites in Central Florida where this epiphyte was collected and analyzed in 1973/1974. After 24-25 years, using comparable methods, concentrations of Ca, Mg, K and Cu decreased in the tissue samples while Fe increased. These declines in base cations corresponded to global atmospheric decreases. In the earlier study, patterns of elemental concentrations in Spanish moss corresponded to the host tree categories primarily reflecting a P gradient that increased from pine (Pinus spp.) to cypress (Taxodium spp.) to hardwood (e.g. Quercus spp.) hosts. Such host-specific associations were mostly absent from the recent study, suggesting that epiphytic preferences based on the chemistry of phorophyte leachates have become less important in this region, perhaps, resulting from local (suburbanization) or regional (atmospheric composition) changes.

Impacts of Fine Root Turnover on Forest NPP and Soil C Sequestration Potential

Estimates of forest net primary production (NPP) demand accurate estimates of root production and turnover. We assessed root turnover with the use of an isotope tracer in two forest free-air carbon dioxide enrichment experiments. Growth at elevated carbon dioxide did not accelerate root turnover in either the pine or the hardwood forest. Turnover of fine root carbon varied from 1.2 to 9 years, depending on root diameter and dominant tree species. These long turnover times suggest that root production and turnover in forests have been overestimated and that sequestration of anthropogenic atmospheric carbon in forest soils may be lower than currently estimated.

The nitrogen budget of a pine forest under free air CO2 enrichment

Elevated concentrations of atmospheric CO₂ increase plant biomass, net primary production (NPP) and plant demand for nitrogen (N). The demand for N set by rapid plant growth under elevated CO₂ could be met by increasing soil N availability or by greater efficiency of N uptake. Alternatively, plants could increase their nitrogen-use efficiency (NUE), thereby maintaining high rates of growth and NPP in the face of nutrient limitation. We quantified dry matter and N budgets for a young pine forest exposed to 4 years of elevated CO₂ using free-air CO₂ enrichment technology. We addressed three questions: Does elevated CO₂ increase forest NPP and the demand for N by vegetation? Is demand for N met by greater uptake from soils, a shift in the distribution of N between plants, microbes, and soils, or increases in NUE under elevated CO₂? Will soil N availability constrain the NPP response of this forest as CO₂ fumigation continues? A step-function increase in atmospheric CO₂ significantly increased NPP during the first 4 years of this study. Significant increases in NUE under elevated CO₂ modulated the average annual requirement for N by vegetation in the first and third growing seasons under elevated CO₂; the average stimulation of NPP in these years was 21% whereas the average annual stimulation of the N requirement was only 6%. In the second and fourth growing seasons, increases in NPP increased the annual requirement for N by 27-33%. Increases in the annual requirement for N were largely met by increases in N uptake from soils. Retranslocation of nutrients prior to senescence played only a minor role in supplying the additional N required by trees growing under elevated CO₂. NPP was highly correlated with between-plot variation in the annual rate of net N mineralization and CO₂ treatment. This demonstrates that NPP is co-limited by C availability, as CO₂ from the atmosphere, and N availability from soils. There is no evidence that soil N mineralization rates have increased under elevated CO₂. The correlation between NPP and N mineralization rates and the increase in the annual requirement for N in certain years imply that soil N availability may control the long-term productivity response of this ecosystem to elevated CO₂. Although we have no evidence suggesting that NPP is declining in response to >4 years of CO₂ fumigation, if the annual requirement of N continues to be stimulated by elevated CO₂, we predict that the productivity response of this forest ecosystem will decline over time.

Ecological forecasts: an emerging imperative

Planning and decision-making can be improved by access to reliable forecasts of ecosystem state, ecosystem services, and natural capital. Availability of new data sets, together with progress in computation and statistics, will increase our ability to forecast ecosystem change. An agenda that would lead toward a capacity to produce, evaluate, and communicate forecasts of critical ecosystem services requires a process that engages scientists and decision-makers. Interdisciplinary linkages are necessary because of the climate and societal controls on ecosystems, the feedbacks involving social change, and the decision-making relevance of forecasts.

Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2

The current rise in atmospheric CO2 concentration is thought to be mitigated in part by carbon sequestration within forest ecosystems, where carbon can be stored in vegetation or soils. The storage of carbon in soils is determined by the fraction that is sequestered in persistent organic materials, such as humus. In experimental forest plots of loblolly pine (Pinus taeda) exposed to high CO2 concentrations, nearly half of the carbon uptake is allocated to short-lived tissues, largely foliage. These tissues fall to the ground and decompose, normally contributing only a small portion of their carbon content to refractory soil humic materials. Such findings call into question the role of soils as long-term carbon sinks, and show the need for a better understanding of carbon cycling in forest soils. Here we report a significant accumulation of carbon in the litter layer of experimental forest plots after three years of growth at increased CO2 concentrations (565 microl l(-1)). But fast turnover times of organic carbon in the litter layer (of about three years) appear to constrain the potential size of this carbon sink. Given the observation that carbon accumulation in the deeper mineral soil layers was absent, we suggest that significant, long-term net carbon sequestration in forest soils is unlikely.

Forecasting agriculturally driven global environmental change

During the next 50 years, which is likely to be the final period of rapid agricultural expansion, demand for food by a wealthier and 50% larger global population will be a major driver of global environmental change. Should past dependences of the global environmental impacts of agriculture on human population and consumption continue, 10(9) hectares of natural ecosystems would be converted to agriculture by 2050. This would be accompanied by 2.4- to 2.7-fold increases in nitrogen- and phosphorus-driven eutrophication of terrestrial, freshwater, and near-shore marine ecosystems, and comparable increases in pesticide use. This eutrophication and habitat destruction would cause unprecedented ecosystem simplification, loss of ecosystem services, and species extinctions. Significant scientific advances and regulatory, technological, and policy changes are needed to control the environmental impacts of agricultural expansion.

Net primary production of a forest ecosystem with experimental CO2 enrichment

The concentration of atmospheric carbon dioxide was increased by 200 microliters per liter in a forest plantation, where competition between organisms, resource limitations, and environmental stresses may modulate biotic responses. After 2 years the growth rate of the dominant pine trees increased by about 26 percent relative to trees under ambient conditions. Carbon dioxide enrichment also increased litterfall and fine-root increment. These changes increased the total net primary production by 25 percent. Such an increase in forest net primary production globally would fix about 50 percent of the anthropogenic carbon dioxide projected to be released into the atmosphere in the year 2050. The response of this young, rapidly growing forest to carbon dioxide may represent the upper limit for forest carbon sequestration.

Offsetting changes in biomass allocation and photosynthesis in ponderosa pine (Pinus ponderosa) in response to climate change

We examined the effect of climate on aboveground biomass allocation of ponderosa pine (Pinus ponderosa) by measuring trees in disjunct forest stands growing on the same substrate at high-elevation montane sites and low-elevation desert sites. Climatic differences between the sites were comparable to the difference between present and future climates of interior North America that is expected to result from a doubling of atmospheric CO(2) concentration. Relative to the montane populations, the desert populations allocated a greater proportion of biomass to sapwood (functional xylem) at the expense of foliage. The leaf/sapwood area ratio and percent of aboveground biomass in sapwood for trees of the same height were 0.201 m(2) cm(-2) and 58% for montane trees and 0.104 m(2) cm(-2) and 71% for desert trees. In a phytotron experiment, increases in net photosynthesis and net assimilation rate for seedlings grown under future conditions of high CO(2) and temperature were offset by a decrease in leaf area ratio. As was observed for large trees at different elevations, increased temperatures caused an increase in biomass allocation to stem in the phytotron seedlings. Thus, CO(2)- and temperature-driven shifts in biomass allocation negated the effect on growth of the CO(2)-driven increase in carbon assimilation rate. Our data from the controlled growth chamber and field experiments suggest that future climate conditions, including elevated atmospheric CO(2), may not stimulate growth and productivity of ponderosa pine.