B34A-01 INVITED
Adapting to Climate Change: Reconsidering the Role of Protected Areas and Protected Organisms in Western North America
With the recent publication of the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), little doubt remains among scientists that the global climate system is changing due to human influence and that climate change will have far-reaching and fundamental impacts on ecosystems and biodiversity. Arguably the best-documented evidence linking 20th Century warming trends to changes in physical and biological systems comes from the mountains of western North America (e.g., Figure SPM1 in Summary of Working Group 11 Report). In the West, ecosystem impacts include changes in the distribution of species as well as changing functional linkages between species such as the synchrony between flower emergence and pollinating insects. These climate impacts, when combined with other environmental stressors (e.g., altered disturbance regimes, land-use change and habitat fragmentation) portend an amplification of species extinction rates. One of the great challenges in adapting to climate change is developing and implementing policies that enhance ecological resilience in the face of these change. Clearly, the current system of nature reserves in Western North America is a fundamental asset for maintaining biodiversity and ecosystem services. However, the fixed- boundary nature of these protected areas presents a problem as species' ranges shift with future climate change. The loss of species whose ranges move outside of fixed park boundaries and the arrival of other species that move into protected areas could lead to significant turnover of species diversity, new species assemblages, and altered functionality. In short, reserves that were designed to protect particular species or communities may no longer serve their intended purpose under a changing climate. In this talk, we use case studies from the Greater Yellowstone Ecosystem and the Sonoran Desert Ecosystem to define strategies for enhancing ecological resilience to climate change at regional scales, taking into account the need for creating ecological connectivity between protected areas. We are particularly interested in defining opportunities in which traditional "working landscapes", such as large ranches in western North America, play a functional role in enhancing connectivity in the near-term as well as into the future. Based on our own work and that of others, we define the scientific roadmap for identifying and selecting corridors that are robust to climate change and other stressors and that are politically and socially viable as an adaptation strategy.
B34A-02
From Fireproof Desert to Flammable Grassland: Buffelgrass Invasion in the Sonoran Desert
Only a few decades ago, the Sonoran Desert of northwestern Mexico and southern Arizona was considered mostly fireproof, a case of not enough fine fuel to connect the dominant shrubs and cacti. This has changed with invasions by non-native, winter annual and summer-flower perennial grasses that are rapidly transforming fireproof desert into flammable grassland. Of particular concern is buffelgrass, Pennisetum ciliare, a fire-prone and invasive African perennial grass that has already converted millions of hectares across Sonora since the mid-1960s and has made quick headway in southern and central Arizona beginning in the 1980s. Near Tucson and Phoenix, AZ, buffelgrass invasion is proceeding exponentially, with population expansion (and the costs of mitigation) more than doubling every year. As this conversion progresses, there will be increased fire risks, lost tourist revenue, diminished property values, insurmountable setbacks to conservation efforts, and the threat of large ignition fronts in desert valleys routinely spreading into the mountains. Although somewhat belated, an integrated, multi-jurisdictional effort is being organized to reduce ecological and economic impacts. My presentation will summarize the history and context of buffelgrass introduction and invasion, the disconnect in attitudes and policies across state and international boundaries, ongoing management efforts, the role of science and responsibilities of scientists, accelerated spread with changing climate, and impacts to regional ecosystems and economies. This narrative may serve as a template for other semi-arid lands where buffelgrass and similar grasses have become invasive, including Australia, South America, and many islands in the Pacific Ocean (including Hawaii), Indian Ocean, and Caribbean Sea. http://www.buffelgrass.org
B34A-03
Interactions Between Climate Change, Fire and Invasive Plants in California Ecosystems
Changes in fire regimes are predicted in many climate change scenarios. The types of changes are greatly affected by the fuel structure of the ecosystem and different trajectories of change are expected in surface fire regimes than in crown fire regimes. However, in the multi-factorial world of natural ecosystems, climate is only one of the drivers of future change and some of the known threats to ecosystem stability are expected to push the system over particular thresholds of tolerance very rapidly. Invasive species have been widely recognized as drivers of ecosystem change, often generating feedback effects that alter fuel structure and future fire behavior. Human demographic changes, and the concomitant changes in anthropogenic fire ignitions are an additional threat to future ecosystem stability. I will address how these factors might interact with climate change in ecosystems of very different fuel structure, including surface fire ponderosa pine forests and crown fire chaparral shrublands in California. http://www.werc.usgs.gov/seki/keeley.asp
B34A-04 INVITED
Beyond CO2: Changes in Limiting Resources in California Oak Woodland
As atmospheric CO2 continues to increase, other resources become even more limiting to plants and the wildland ecosystems they support. Traditionally, California Mediterranean-type ecosystems are limited by water, then N. In these ecosystems, CO2 enrichment causes a minor increase in production associated with enhanced water-use efficiency, but N rapidly becomes the limiting factor to both production and to soil organism dynamics. In urbanizing areas, such as southern California, strong gradients in NOx deposition are also created by vehicular pollution. We have studied the regulation of N uptake by mycorrhizae in Coast Live Oak (Quercus agrifolia) using information with natural abundance from the early 1900s, current plants and fungi, and modeling change. Contrasts were made from a high NOx deposition site, a low deposition site, and a site where NOx deposition is rapidly increasing. We examined natural abundance δ15 N of current and past plant material (leaves, wood), mycorrhizal and saprobic fungal fruiting bodies, and soil. We modeled relative N uptake, fractionation, and transport between soil, fungus and plant. Our data show complex interactions between increasing NOx deposition and increasing atmospheric CO2 on mycorrhizal-plant interactions. There is a significant shift in N sources and reduction upon mycorrhizae with NOx deposition. However, the elevated CO2 appears to also have created a greater N demand on the trees, increasing dependence on mycorrhizae and the ability of the fungi to acquire organic N and NH4. The individual fungal species differ among sites, but complex trends between fungal genera and trees can be seen. Projections of increasing atmospheric CO2 and regional NOx deposition suggest strong but complex gradients in fungal-oak interactions with decreasing dependence on mycorrhizae near urbanizing areas, mediated by the rate of increasing CO2 and inorganic NOx deposition, and paradoxically, increasing dependency on mycorrhizae and organic N acquisition in lower N deposition areas.
B34A-05
Modelling Plant and Soil Nitrogen Feedbacks Affecting Forest Carbon Gain at High CO2
Short-term, direct effects of elevated atmospheric CO2 concentrations on plant carbon gain are relatively well understood. There is considerable uncertainty, however, about longer-term effects, which are influenced by various plant and ecosystem feedbacks. A key feedback in terrestrial ecosystems occurs through changes in plant carbon (C) allocation patterns. For instance, if high CO2 were to increase C allocation to roots, then plants may experience positive feedback through improved plant nutrition. A second type of feedback, associated with decomposition of soil-organic matter, may reduce soil-nutrient availability at high CO2. This paper will consider mechanistic models of both feedbacks. Effects of high CO2 on plant C allocation will be investigated using a simple model of forest net primary production (NPP) that incorporates the primary mechanisms of plant carbon and nitrogen (N) balance. The model called MATE (Model Any Terrestrial Ecosystem) includes an equation for annual C balance that depends on light- saturated photosynthetic rate and therefore on [CO2], and an equation for N balance incorporating an expression for N uptake as a function of root mass. The C-N model is applied to a Free Air CO2 Exchange (FACE) experiment at Oak Ridge National Laboratory (ORNL) in Tennessee, USA, where closed-canopy, monoculture stands of the deciduous hardwood sweetgum ( Liquidambar styraciflua) have been growing at [CO2] of 375 and 550 ppm for ten years. Features of this experiment are that the annual NPP response to elevated CO2 has averaged approximately 25% over seven years, but that annual fine-root production has almost doubled on average, with especially large increases in later years of the experiment (Norby et al. 2006). The model provides a simple graphical approach for analysing effects of elevated CO2 and N supply on leaf/root/wood C allocation and productivity. It simulates increases in NPP and fine-root production at the ORNL FACE site that are consistent with experimental measurements. Increased below-ground C allocation has been observed at other forest high-CO2 experiments including the Duke FACE and Flakaliden experiments in North Carolina, USA, and Sweden, respectively. This result is predicted by MATE, but not by biogeochemical-cycling models such as our plant-soil model G'DAY (Generic Decomposition And Yield), which predicts instead that a positive CO2 response will enhance litter quantity, and hence will increase soil N immobilisation and reduce the pool of N available for plant uptake (Pepper et al. 2007). We will use G'DAY and MATE to determine the sensitivity of the modelled CO2 response to key model parameters on contrasting timescales. One conclusion is that models are required that simulate both increased N uptake at high CO2 as a consequence of increased root production and soil N-cycling feedbacks. References: Norby RJ, Wullschleger SD, Hanson PJ, Gunderson CA, Tschaplinski TJ, Jastrow JD (2006) CO2 enrichment of a deciduous forest: the Oak Ridge FACE experiment. Ecological Studies 187: 231-251 (Springer-Verlag, Berlin). Pepper DA, Eliasson PE, McMurtrie RE, Corbeels M, Ågren GI, Strömgren M, Linder S (2007) Simulated mechanisms of soil N feedback on the forest CO2 response. Global Change Biology 13: 1265-1281.
B34A-06
Using Tree-Ring Width Data From 1000 Sites to Predict how American Forests Will Respond to Climate Change
Beginning in the early 1900s, tree-ring scientists began analyzing the relative widths of annual growth rings preserved in the cross-sections of trees. Over the years, many ring-width index chronologies, each representing a specific site and species, have been developed and analyzed to infer details regarding past climate, growth response to environmental fluctuation, fire activity, logging practices by past societies, and more. Of the many ring-width chronologies constructed, 1035 represent sites within the continental United States and have been published online within The International Tree-Ring Data Bank as of September 2007 (ITRDB, http://www.ncdc.noaa.gov/paleo/treering.html). Approximately 85% of these sites are located west of the Mississippi River. Here we present results from a three-step study, using this large reserve of tree-growth data to determine how various tree species in various regions have responded to climate fluctuations in the past and how they can be expected to respond to future change. In the first step, we used linear regression to compare each time series of ring-width index values to a suite of local monthly climate variables that may influence tree growth, such as rainfall, temperature, and drought severity (PDSI). We identified the range of months (of a 24- month period) during which each climate parameter most strongly affects growth by comparing Pearson correlation coefficients. In the second step, we identified all sites where at least one climate parameter, during some rage of months, correlates significantly (95% confidence) with ring-width index values. For each of these sites, we constructed a growth model that uses each significantly correlating climate parameter as a growth predictor. In the third step, we applied the growth model to predict the next 100 years of growth response to a monthly climate forecast created by the Hadley Centre for Climate Prediction and Research. This forecast (HadCM3 IS92a) assumes a business as usual scenario with no measures to reduce greenhouse-gas emissions. By comparing predictions of future growth at each site to records of past growth, and to predications of future growth at nearby sites, we identify stands of trees and regional forests where we expect significantly increased growth, decreased growth, and/or potential directional changes in species composition to occur during the next century in response to climate change.
B34A-07
Nitrogen Deposition, Nitrogen Saturation and Carbon Interactions in Mid-Atlantic Forest Ecosystems
Scientists have long debated whether anthropogenic inputs of nitrogen deposition over the decades have caused N saturation in northeastern forests. Although regulations for NOx emissions reductions have been implemented, N deposition showed limited change during the 1990s for much of the eastern United States until recent trends showing steady reduction of N deposition. In this study, we investigated how forest ecosystems respond to long term N deposition in terms of changing productivity and function for retaining inputs of nitrogen deposition. We applied our GIS variant of the PnET-CN model for this analysis. Besides modeling forest productivity, N retention and losses associated with N deposition trends over last few decades, we also predicted and evaluated responses of forest ecosystems into 2050 under five future nitrogen deposition scenarios: N deposition before regulations, holding emissions steady at 1998 levels, and three scenarios that extend the national EPA nitrogen deposition trends data from 1995-2005 to simulate possible trajectories of nitrogen reduction levels. The model results show that the increased N deposition has increased forest productivity in the region by 18%, but may reduce carbon accumulation in growing stock as forests saturated with N inputs. The predictions for the future N deposition scenarios up to 2050 attested that forests have approached N saturation under current N deposition level. The model estimates indicate insignificant increases in forest NPP with additional N inputs above 2000 levels, but show different forest N losses under different N deposition scenarios. Additional N inputs will only cause more N exports from forests to aquatic systems in the region, which could range from double to more than tenfold dependent on regulation strength of N deposition.
B34A-08
Response of the Terrestrial Carbon Cycle to the El Nino-Southern Oscillation
Interannual variability of atmospheric CO2 growth rate is strongly associated with the El Niño Southern Oscillation (ENSO). Recent studies have emphasized land as the dominant contributor to the interannual variability of global carbon fluxes. In this study, we used a dynamic terrestrial carbon cycle model, VEGAS (VEgetation-Global-Atmosphere-Soil), to identify and isolate the responses of terrestrial carbon cycle to the physical climate variations associated with ENSO. The veracity of the simulated terrestrial carbon flux (Fta) of VEGAS is verified by evaluating it against observations on interannual time scales. The simulated global total land-atmosphere flux agrees well with the observationsbased on inversion modeling studies on ENSO timescales and both of them are consistent with the atmospheric CO2 growth rate variation at the Mauna Loa station. Correlation of the model based Leaf Area Index (LAI) with satellite derived NDVIproduces values values significant at the 99% level. Regional and Global comparison indicates the tropics as the dominant contributor to the global carbon flux. Composite analysis of the variations of terrestrial responses and climate factors during El Niño and La Niña has identified that tropical carbon flux anomaly lags ENSO 5-6 months, similar to the atmospheric CO2 growth rate. This Fta anomaly in the tropics during ENSO period is suggested to be related to physical climate fields (e. g. temperature, precipitation) in regulating the variation of vegetation activity and soil decomposition. Carefully designed sensitivity simulations of VEGAS were conducted to isolate and quantify the effects of the individual climate fields. It was estimated that precipitation variation during ENSO contributes 56% of this Fta anomaly mainly through photosynthesis. Temperature variation accounts for the remaining 44%, which includes 25% from its direct effect on the temperature-dependent soil decomposition, 7% from its direct effect on the photosynthesis, and 12% from its indirect effect on the photosynthesis through soil wetness. Such a decomposition of the direct and indirect effects of climaticfactors also emphasizes the importance of the poorly studied factor of the global terrestrial carbon flux: soil moisture