Contributions of land-use history to carbon accumulation in U.S. forests.

Carbon accumulation in forests has been attributed to historical changes in land use and the enhancement of tree growth by CO2 fertilization, N deposition, and climate change. The relative contribution of land use and growth enhancement is estimated by using inventory data from five states spanning a latitudinal gradient in the eastern United States. Land use is the dominant factor governing the rate of carbon accumulation in these states, with growth enhancement contributing far less than previously reported. The estimated fraction of aboveground net ecosystem production due to growth enhancement is 2.0 +/- 4.4%, with the remainder due to land use.

Consistent land- and atmosphere-based U.S. carbon sink estimates.

For the period 1980-89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 10(15) grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980-89 are similar to those for 1985-89 and 1990-94, indicating a relatively stable U.S. sink throughout the period.

Recent advances in ecosystem-atmosphere interactions: an ecological perspective.

The atmosphere and terrestrial ecosystems are fundamentally coupled on a variety of time-scales. On short time-scales, this bi-directional interaction is dominated by the rapid exchange of CO(2), water and energy between the atmosphere and the land surface; on long time-scales, the interaction involves changes in ecosystem structure and composition in response to changes in climate that feed back through biophysical and biogeochemical mechanisms to influence climate over decades and centuries. After briefly describing some early pioneering work, I focus this review on recent advances in understanding long-term ecosystem-atmosphere interactions through a discussion of three case studies. I then examine how efforts to assess the stability and resilience of ecosystem-atmosphere interactions over these long time-scales using Dynamic Global Vegetation Models are hampered by the presence of important functional diversity and heterogeneity within plant communities. Recent work illustrates how this issue can be addressed through the use of Structured Ecosystem Models that more accurately scale between the short-term physiological responses of individual plants and the long-term, large-scale dynamics of heterogeneous, functionally diverse ecosystems.

Parasite-associated polymorphism in a cyclic ungulate population.

An unmanaged population of Soay sheep living on Hirta, St Kilda, Scotland is persistently unstable, fluctuating between about 600 and 1600 individuals. Population crashes occurring approximately every 3 years are primarily due to winter food shortage. In this paper we show that sheep experimentally relieved of their gastrointestinal nematodes (predominantly Teladorsagia spp.) survived a crash better than matched controls, showing that nematode parasites contribute to the probability that a sheep dies in a crash. We also show that over three successive crashes mortality was significantly different between individuals of the three different genotypes at the diallelic adenosine deaminase locus (Ada). FF animals were most likely to die, SS animals had an intermediate probability of dying, and FS animals were least likely to die. Finally, three independent lines of evidence suggest that nematode burdens differ between the three Ada genotypes. First, in August, heterozygous females are less likely to have nematode eggs in their faeces than homozygous females. Second, at lambing, the periparturient rise in faecal egg count was highest in homozygous FF individuals. Finally, during the Autumn mating season, heterozygous males has lower faecal egg counts than homozygotes, although this relation was complicated by interactions with year and age of male. These results are consistent with the idea that Ada allele frequencies are maintained in the sheep population by parasite-associated selection.

Density-dependent selection in a fluctuating ungulate population.

Despite considerable theoretical interest no direct examples of density-dependent natural selection acting on simple polymorphic variation have been documented in a natural population. Here we show that the magnitude of selective differences in survival between phenotypes in two conspicuous polymorphisms of coat colour and horn type in Soay sheep Ovis aries living on St Kilda, Scotland are associated with marked changes in population density. Selection is strongest in years of high density but weak in years of low density. In addition to direct observations of density-dependent 'soft' selection in a natural population, the analysis revealed that the level of overcompensatory mortality (responsible for promoting population instability) was higher after accounting for genetic variation in the coat and horn morph traits. The results emphasize the importance of understanding the interaction between selection and population demography for both genetic and ecological studies of natural populations.

Potential role of natural enemies during tree range expansions following climate change.

Recent investigations have shown how chance, long-range dispersal events can allow tree populations to migrate rapidly in response to changes in climate. However, this apparent solution to Reid's paradox applies solely within the context of single species models, while the rapid migration rates seen in pollen records occurred within multispecies communities. Ecologists are therefore presented with a new challenge: reconciling the macroscopic dynamics of spread seen in the pollen record with the rules and interactions governing plant community assembly. A case that highlights this issue is the rapid spread of Beech during the Holocene into a landscape already dominated by a close competitor, Hemlock. In this study, we analyse a simple model of plant community assembly incorporating competition for space and dispersal dynamics, showing how, even when a species is capable of rapid migration into an empty landscape, the presence of an ecologically similar competitor causes Reid's paradox to re-emerge because of the dramatic slowing effect of competitive interactions on a species' rate of spread. We then show how the answer to the question of how tree species dispersed rapidly into occupied landscapes may lie in secondary interactions with host-specific pathogens and parasites. Inclusion of host-specific pathogens into the simple community assembly model illustrates how tree species undergoing range expansions can temporarily outstrip specialist predators, giving rise to a transient Jansen-Connell effect, in which the invader acts as temporary 'super-species' that spreads rapidly into communities already occupied by competitors at rates consistent with those observed in the paleo-record.

ESS analysis of mechanistic models for territoriality: the value of scent marks in spatial resource partitioning.

In this paper elements of game theory are used to analyse a spatially explicit home range model for interacting wolf packs. The model consists of a system of nonlinear partial differential equations whose parameters reflect the movement behavior of individuals within each pack and whose solutions describe the patterns of space-use by each pack. By modifying the behavioral parameters, packs adjust their patterns of movement so as to maximize their reproductive output. This involves a tradeoff between maximizing prey intake and minimizing conflict with neighbors. Evolutionarily stable choices of the behavioral parameters yields territories that are immune to invasion by groups with alternate behaviors.

Projecting the future of the U.S. carbon sink.

Atmospheric and ground-based methods agree on the presence of a carbon sink in the coterminous United States (the United States minus Alaska and Hawaii), and the primary causes for the sink recently have been identified. Projecting the future behavior of the sink is necessary for projecting future net emissions. Here we use two models, the Ecosystem Demography model and a second simpler empirically based model (Miami Land Use History), to estimate the spatio-temporal patterns of ecosystem carbon stocks and fluxes resulting from land-use changes and fire suppression from 1700 to 2100. Our results are compared with other historical reconstructions of ecosystem carbon fluxes and to a detailed carbon budget for the 1980s. Our projections indicate that the ecosystem recovery processes that are primarily responsible for the contemporary U.S. carbon sink will slow over the next century, resulting in a significant reduction of the sink. The projected rate of decrease depends strongly on scenarios of future land use and the long-term effectiveness of fire suppression.