The ongoing increase in atmospheric CO2 concentration ([CO2]) can potentially alter litter decomposition rates by changing: (i) the litter quality of individual species, (ii) allocation patterns of individual species, (iii) the species composition of ecosystems (which could alter ecosystem-level litter quality and allocation), (iv) patterns of soil moisture, and (v) the composition and size of microbial communities. To determine the relative importance of these mechanisms in a California annual grassland, we created four mixtures of litter that differed in species composition (the annual legume Lotus wrangelianus Fischer & C. Meyer comprised either 10% or 40% of the initial mass) and atmospheric [CO2] during growth (ambient or double-ambient). These mixtures decomposed for 33 weeks at three positions (above, on, and below the soil surface) in four types of grassland microcosms (fertilized and unfertilized microcosms exposed to elevated or ambient [CO2]) and at a common field site. Initially, legume-rich litter mixtures had higher nitrogen concentrations ([N]) than legume-poor mixtures. In most positions and environments, the different litter mixtures decomposed at approximately the same rate. Fertilization and CO2 enrichment of microcosms had no effect on mass loss of litter within them. However, mass loss was strongly related to litter position in both microcosms and the field. Nitrogen dynamics of litter were significantly related to the initial [N] of litter on the soil surface, but not in other positions. We conclude that changes in allocation patterns and species composition are likely to be the dominant mechanisms through which ecosystem-level decomposition rates respond to increasing atmospheric [CO2].

In a microcosm experiment, I tested how species composition, species, richness. and community age affect the Susceptibility of grassland communities to invasion by a noxious weed (Centaurea solstitialis L.). I also examined how these factors influenced Centaurea's impact on the rest of the plant community.

Although the impacts of exotic plant invasions on community structure and ecosystem processes are well appreciated, the pathways or mechanisms that underlie these impacts are poorly understood. Better exploration of these processes is essential to understanding why exotic plants impact only certain systems, and why only some invaders have large impacts. Here, we review over 150 studies to evaluate the mechanisms underlying the impacts of exotic plant invasions on plant and animal community structure, nutrient cycling, hydrology and fire regimes. We find that, while numerous studies have examined the impacts of invasions on plant diversity and composition, less than 5% test whether these effects arise through competition, allelopathy, alteration of ecosystem variables or other processes. Nonetheless, competition was often hypothesized, and nearly all studies competing native and alien plants against each other found strong competitive effects of exotic species. In contrast to studies of the impacts on plant community structure and higher trophic, levels, research examining impacts on nitrogen cycling, hydrology and fire regimes is generally highly mechanistic, often motivated by specific invader traits. We encourage future studies that link impacts on community structure to ecosystem processes, and relate the controls over invasibility to the controls over impact.

Fossil fuels developed from ancient deposits of organic material, and thus can be thought of as a vast store of solar energy from which society meets >80% of its current energy needs. Here, using published biological, geochemical, and industrial data, I estimate the amount of photosynthetically fixed and stored carbon that was required to form the coal, oil, and gas that we are burning today. Today's average U. S. Gallon (3.8 L) of gasoline required approximately 90 metric tons of ancient plant matter as precursor material. The fossil fuels burned in 1997 were created from organic matter containing 44x10(18) g C, which is >400 times the net primary productivity (NPP) of the planet's current biota. As stores of ancient solar energy decline, humans are likely to use an increasing share of modern solar resources. I conservatively estimate that replacing the energy humans derive from fossil fuels with energy from modern biomass would require 22% of terrestrial NPP, increasing the human appropriation of this resource by similar to50%.

A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.

Question: What are the physical and chemical effects of plant litter on annual grassland community composition, above-ground net primary production (ANPP), and density?

Many serious ecosystem consequences of climate change will take decades or even centuries to emerge. Long-term ecological responses to global change are strongly regulated by slow processes, such as changes in species composition, carbon dynamics in soil and by long-lived plants, and accumulation of nutrient capitals. Understanding and predicting these processes require experiments on decadal time scales. But decadal experiments by themselves may not be adequate because many of the slow processes have characteristic time scales much longer than experiments can be maintained. This article promotes a coordinated approach that combines long-term, large-scale global change experiments with process studies and modeling. Long-term global change manipulative experiments, especially in high-priority ecosystems such as tropical forests and high-latitude regions, are essential to maximize information gain concerning future states of the earth system. The long-term experiments should be conducted in tandem with complementary process studies, such as those using model ecosystems, species replacements, laboratory incubations, isotope tracers, and greenhouse facilities. Models are essential to assimilate data from long-term experiments and process studies together with information from long-term observations, surveys, and space-for-time studies along environmental and biological gradients. Future research programs with coordinated long-term experiments, process studies, and modeling have the potential to be the most effective strategy to gain the best information on long-term ecosystem dynamics in response to global change.

Global environmental changes are altering interactions among plant species, sometimes favoring invasive species. Here, we examine how a suite of five environmental factors, singly and in combination, can affect the success of a highly invasive plant. We introduced Centaurea solstitialis L. (yellow starthistle), which is considered by many to be California's most troublesome wildland weed, to grassland plots in the San Francisco Bay Area. These plots experienced ambient or elevated levels of warming, atmospheric CO2, precipitation, and nitrate deposition, and an accidental fire in the previous year created an additional treatment. Centaurea grew more than six times larger in response to elevated CO2, and, outside of the burned area, grew more than three times larger in response to nitrate deposition. In contrast, resident plants in the community responded less strongly (or did not respond) to these treatments. Interactive effects among treatments were rarely significant. Results from a parallel mesocosm experiment, while less dramatic, supported the pattern of results observed in the field. Taken together, our results suggest that ongoing environmental changes may dramatically increase Centaurea's prevalence in western North America.