S. P. Harrison

University of Reading, Reading, England, United Kingdom

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Publications (203)888.86 Total impact

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    Climate of the Past 12/2014; 10(6):2237-2252. · 3.56 Impact Factor
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    ABSTRACT: Much evidence suggests that plant communities on infertile soils are relatively insensitive to increased water deficit caused by increasing temperature and/or decreasing precipitation. However, a multi-decadal study of community change in the western USA does not support this conclusion. This paper tests explanations related to macroclimatic differences, overstorey effects on microclimate, variation in soil texture and plant functional traits. A re-analysis was undertaken of the changes in the multi-decadal study, which concerned forest understorey communities on infertile (serpentine) and fertile soils in an aridifying climate (southern Oregan) from 1949-1951 to 2007-2008. Macroclimatic variables, overstorey cover and soil texture were used as new covariates. As an alternative measure of climate-related change, the community mean value of specific leaf area was used, a functional trait measuring drought tolerance. We investigated whether these revised analyses supported the prediction of lesser sensitivity to climate change in understorey communities on infertile serpentine soils. Overstorey cover, but not macroclimate or soil texture, was a significant covariate of community change over time. It strongly buffered understorey temperatures, was correlated with less change and averaged >50 % lower on serpentine soils, thereby counteracting the lower climate sensitivity of understorey herbs on these soils. Community mean specific leaf area showed the predicted pattern of less change over time in serpentine than non-serpentine communities. Based on the current balance of evidence, plant communities on infertile serpentine soils are less sensitive to changes in the climatic water balance than communities on more fertile soils. However, this advantage may in some cases be lessened by their sparser overstorey cover. © The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.
    Annals of Botany 11/2014; · 3.45 Impact Factor
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  • D I Kelley, S P Harrison
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    ABSTRACT: Climate projections show Australia becoming significantly warmer during the 21st century, and precipitation decreasing over much of the continent. Such changes are conventionally considered to increase wildfire risk. Nevertheless, we show that burnt area increases in southern Australia, but decreases in northern Australia. Overall the projected increase in fire is small (0.72–1.31% of land area, depending on the climate scenario used), and does not cause a decrease in carbon storage. In fact, carbon storage increases by 3.7–5.6 Pg C (depending on the climate scenario used). Using a process-based model of vegetation dynamics, vegetation–fire interactions and carbon cycling, we show increased fire promotes a shift to more fire-adapted trees in wooded areas and their encroachment into grasslands, with an overall increase in forested area of 3.9–11.9%. Both changes increase carbon uptake and storage. The increase in woody vegetation increases the amount of coarse litter, which decays more slowly than fine litter hence leading to a relative reduction in overall heterotrophic respiration, further reducing carbon losses. Direct CO2 effects increase woody cover, water-use efficiency and productivity, such that carbon storage is increased by 8.5–14.8 Pg C compared to simulations in which CO2 is held constant at modern values. CO2 effects tend to increase burnt area, fire fluxes and therefore carbon losses in arid areas, but increase vegetation density and reduce burnt area in wooded areas.
    Environmental Research Letters 10/2014; 9(10):104015. · 3.58 Impact Factor
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    Risto Virtanen, Anu Eskelinen, Susan Harrison
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    ABSTRACT: Bryophyte communities of semi-arid grassland ecosystems are little studied, and it is poorly known how they vary in relation to soil quality, disturbance and the invasion status of vascular plants. By analogy with short-statured, competitively inferior native vascular plants, we hypothesized that bryophytes would show high beta diversity and increasing alpha diversity along a gradient from productive non-serpentine grasslands dominated by exotics to low productivity serpentine grasslands dominated by native vascular plants. We also predicted that disturbance by gophers would benefit bryophyte alpha and beta diversity, especially at the productive end of the soil fertility gradient. We sampled bryophytes in 48 sites in a 3,000 ha landscape in the Inner Coast Range of California, USA. We used several multivariate (non-metric multidimensional scaling, PERMDISP) and univariate analysis methods (generalized linear models). As predicted, we found high beta diversity of bryophytes along the soil gradient, and higher cover and richness in unproductive rocky serpentine grasslands than productive and exotic-dominated non-serpentine grasslands. Gopher burrowing had a unimodal influence on bryophyte alpha diversity. Our results show that bryophyte species composition and diversity respond strongly to the same soil gradient that controls vascular grassland species. Bryophytes are likely imperiled by the continued spread of exotic vascular plants, but promoted by moderate disturbances such as gopher burrowing.
    Biodiversity and Conservation 09/2014; · 2.26 Impact Factor
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    ABSTRACT: Global controls on month-by-month fractional burnt area (2000–2005) were investigated by fitting a generalised linear model (GLM) to Global Fire Emissions Database (GFED) data, with 11 predictor variables representing vegetation, climate, land use and potential ignition sources. Burnt area is shown to increase with annual net primary production (NPP), number of dry days, maximum temperature, grazing-land area, grass/shrub cover and diurnal temperature range, and to decrease with soil moisture, cropland area and population density. Lightning showed an apparent (weak) negative influence, but this disappeared when pure seasonal-cycle effects were taken into account. The model predicts observed geographic and seasonal patterns, as well as the emergent relationships seen when burnt area is plotted against each variable separately. Unimodal relationships with mean annual temperature and precipitation, population density and gross domestic product (GDP) are reproduced too, and are thus shown to be secondary consequences of correlations between different controls (e.g. high NPP with high precipitation; low NPP with low population density and GDP). These findings have major implications for the design of global fire models, as several assumptions in current models – most notably, the widely assumed dependence of fire frequency on ignition rates – are evidently incorrect.
    Biogeosciences 09/2014; 11:5087-5101. · 3.75 Impact Factor
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    ABSTRACT: Background/Question/Methods Does environmental change affect ecosystem function consistently across landscapes, or do impacts vary with site properties? We addressed this question for functions mediated by soil microbes at a site where three very different soil types (coarse serpentine, serpentine clay and non-serpentine loam) occur in close proximity. The grasslands on these soils (hereafter, “habitats”) differ considerably in composition and productivity. Since 2010, replicate plots in each habitat have received late spring precipitation addition and NPK fertilization in a 2 x 2 factorial design. To understand whether these treatments affect potential microbial decomposition of organic matter differently across habitats, we assayed potential activities of five soil enzymes in spring 2013. Fluorometric assays measured β-glucosidase (BG), β-N-acetylglucosaminidase (NAG), and acid phosphatase (AP), and colorimetric assays measured polyphenol oxidase (PPO) and peroxidase (PER). To understand impacts on soil bacterial and archaeal community composition, we sequenced the V4 region of the 16S rRNA gene. Results/Conclusions Watering affected potential enzyme activity differently across habitats; it increased potential activity of NAG, PPO, and PER on non-serpentine loam and of NAG and PPO on serpentine clay, but had no effects on coarse serpentine, which has lower water holding capacity. In contrast, fertilization affected potential enzyme activity consistently across habitats; it modestly increased activity of the hydrolytic enzymes involved in C and C+N acquisition (BG, NAG), whereas activities of the oxidative enzymes (PPO, PER) and of AP were unaffected. Bacterial and archaeal composition was significantly affected by both treatments. Fertilization affected composition in a strongly habitat-dependent manner, while the effects of watering on composition were more weakly dependent on habitat. In sum, habitat-dependence varied with the type of environmental change and the metric considered, but the frequency of habitat-dependence in our results suggests that soil and associated plant community types - not just climatic factors - should be included when predicting how ecosystem functions will change across landscapes.
    99th ESA Annual Convention 2014; 08/2014
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    ABSTRACT: Background/Question/Methods The composition of plant communities can change over time to favor species from warmer climates, in response to increases in regional temperature. This “thermophilization” can be slowed by increases in forest cover, which can have a strong cooling effect on understory microclimate. We explore the corollary of this prediction, and ask whether increases in disturbance severity contribute to increased local thermophilization of understory plant communities. We sampled understory forest vegetation at twelve forested sites in California, where each site had a gradient of disturbance severity due to forest thinning, low-severity wildfire, and high-severity wildfire. To measure thermophilization, we assigned species to one of two biogeographic affinities in western North America: North-temperate species, which diversified during the mild, wet Eocene, and Southern-xeric or “thermophilic” species, which diversified during the late Tertiary and Quaternary during a period of regional drying. We compared the relative abundance of these two groups, and the distribution of individual species’ functional traits, among different disturbance severities and across a precipitation gradient. This allowed us to investigate the importance of disturbance and associated higher climatic water deficit in filtering the regional species pool into local plant communities. Results/Conclusions We found that the ratio of North-temperate to thermophilic species decreased as forest disturbance severity increased. This pattern held across the strong precipitation gradient in this study, however the degree of thermophilization following disturbance was greater in forests with higher precipitation and thus greater pre-disturbance canopy cover. Variation in climatic water deficit appears to drive thermophilization: among high-severity wildfire sites with extremely low canopy cover, there was still a trend towards more thermophilic species at low precipitation sites. Furthermore, despite the strong negative correlation between leaf carbon-nitrogen ratios (C:N) and specific leaf area (SLA) at the species-level, high-severity wildfire stands had lower plot-level C:N without increased SLA, suggesting that water deficit is constraining SLA in these highly disturbed stands. Collectively, these results indicate that small-scale decreases in forest canopy cover due to different disturbance regimes leads to strong, predictable shifts in understory plant communities. Thermophilization of understory plant communities can be attributed to increased understory temperature and climatic water deficit, and can occur rapidly in response to disturbance without any accompanying change in regional climate.
    99th ESA Annual Convention 2014; 08/2014
  • Anu Eskelinen, Susan P. Harrison
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    ABSTRACT: Background/Question/Methods Human impacts are often thought of as homogenizing natural communities, but it is unclear how interacting impacts alter the beta diversity (spatial variability) of plant communities. In a grassland with high beta diversity along a soil fertility gradient, we asked which combinations of nutrient enrichment, precipitation enhancement, and disturbance would allow species to expand their distributions across the gradient; what plant traits would mediate species responses; and how beta diversity would change as a result. To alleviate dispersal limitation as a constraint on treatment responses, we added seeds of 15 species originating from different parts of the soil fertility gradient to our 132 plots. Results/Conclusions Fertilization and water addition permitted species from fertile soils to invade infertile soils, an effect that strengthened with time. Disturbance allowed species from infertile soils to transiently invade fertile soils. Plant height and exotic/native status best predicted the abilities of species to invade harsh habitats under ameliorated conditions. Among-habitat beta diversity of seeded species was reduced by fertilization and the joint treatment of fertilization and watering, although not by watering alone. Our findings are a novel demonstration of how interacting global change factors affect the biotic and abiotic resistances that control species’ invasions along a soil fertility gradient and thereby maintain beta diversity.
    99th ESA Annual Convention 2014; 08/2014
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    ABSTRACT: Background/Question/Methods Plant species richness can be influenced by multiple factors including local environment, landscape connectivity, and disturbance history, but understanding the relative importance of these factors for plant communities remains a challenge for ecologists. Previous studies of the effects of habitat spatial structure on plant communities have reached inconsistent conclusions, but often do not (1) control for additional factors influencing communities, such as local environmental conditions, or (2) consider differential responses of unique plant guilds, such as habitat specialists. Here, we ask what factors drive total species richness and richness of specialist species in Ozark dolomite glades—fire-adapted xeric grasslands that vary significantly in spatial structure and environmental characteristics. We hypothesized that total richness is driven primarily by local environment, and that landscape connectivity has a greater effect on specialist richness than total richness. To test these hypotheses, we sampled herbaceous plant communities at 56 glades across the Missouri Ozarks that are managed with frequent, low-intensity fire, and used structural equation modeling (SEM) to analyze the effects of local soil characteristics, topographic slope, landscape connectivity (measured using the proximity index metric), and fire history on plant community response variables. Results/Conclusions We found that both local and regional drivers contribute to local species richness. Local soil fertility was the strongest predictor of total species richness in glades, and landscape connectivity and time since fire also had significant effects. As we hypothesized, landscape connectivity had a stronger influence on glade specialist richness than it did on total species richness, although we were able to explain less of the overall variation in specialist richness than total species richness. Topographic slope and soil were also significant predictors of glade specialist richness, but fire history did not have a significant influence on specialists. Our results underscore the importance of considering multiple predictors and indirect effects in studies of plant diversity; our SEM indicated significant relationships that were not apparent in bivariate analyses. Our results suggest that local environmental conditions may set an upper limit for restoration of plant species richness in glades but that landscape connectivity may be especially important for the conservation of specialist species.
    99th ESA Annual Convention 2014; 08/2014
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    ABSTRACT: We present and examine a~multi-sensor global compilation of Mid-Holocene (MH) sea surface temperatures (SSTs), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations.
    03/2014; 10(2).
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    ABSTRACT: One important but largely unanswered question about floristic responses to climate change is how interactions such as competition, facilitation and plant–soil feedbacks will influence the ability of species to track shifting climates. In a rugged and moisture-limited region that has recently warmed by 2° (Siskiyou Mountains, OR, USA), we planted three species into cooler aspects and elevations than those they currently inhabit, with and without removal of neighbouring plants, and tracked them over 2 years. Two species had higher success in cooler topographic locations, and this success was enhanced by neighbouring plants, which appeared to modulate minimum growing season temperatures. One species' success was also facilitated by the higher soil organic matter found in cooler sites. These results are a novel experimental demonstration of two important factors that may buffer climate change impacts on plants: rugged topography and plant–plant facilitation.
    Ecology Letters 03/2014; · 17.95 Impact Factor
  • Anu Eskelinen, Susan Harrison
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    ABSTRACT: To predict the net impact of climate change on invasions, it is critical to understand how its effects interact with environmental and biotic context. In a factorial field experiment, we examined how increased late-season rainfall influences the growth and reproductive success of two widespread invasive species (Centaurea solstitialis and Aegilops triuncialis) in heterogeneous Californian grasslands, and, in particular, how its impact depends on habitat type, nutrient addition, and competition with resident species. Rainfall enhancement alone exhibited only weak effects, especially in naturally infertile and relatively uninvaded grasslands. In contrast, watering and fertilization together exhibited highly synergistic effects on both invasive species. However, the benefits of the combined treatment were greatly reduced or offset by the presence of surrounding competitors. Our results highlight the roles of nutrient limitation and biotic resistance by resident competitors in constraining the responses of invasive species to changes in rainfall. In systems with strong environmental control by precipitation, enhanced rainfall may promote invasions mainly under nutrient-rich and disturbed conditions, while having lesser effects on nutrient-poor, native "refuges".
    Ecology 03/2014; 95(3):682-92. · 5.18 Impact Factor
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    ABSTRACT: Global controls on month-by-month fractional burnt area (2000-2005) were investigated by fitting a generalized linear model to Global Fire Emissions Database data with 11 predictor variables representing vegetation, climate, land use and potential ignition sources. Burnt area is shown to increase with annual net primary production (NPP), number of dry days, maximum temperature, grazing-land area, grass/shrub cover and diurnal temperature range, and to decrease with soil moisture, cropland area and population density. Lightning showed an apparent (weak) negative influence, but this disappeared when pure seasonal-cycle effects were taken into account. The model predicts observed geographic and seasonal patterns, and the emergent relationships seen when burnt area is plotted against each variable separately. Unimodal relationships to mean annual temperature and precipitation, population density and gross domestic product (GDP) are reproduced too, and thus shown to be secondary consequences of correlations among different controls (e.g. high NPP with high precipitation; low NPP with low population density and GDP). These findings have major implications for the design of global fire models, as several assumptions in current models - most notably, the widely assumed dependence of fire frequency on ignition rates - are evidently incorrect.
    Biogeosciences Discussions 01/2014; 11:3865–3892.
  • D. I. Kelley, S. P. Harrison, I. C. Prentice
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    ABSTRACT: The Land surface Processes and eXchanges (LPX) model is a fire-enabled dynamic global vegetation model that performs well globally but has problems representing fire regimes and vegetative mix in savannas. Here we focus on improving the fire module. To improve the representation of ignitions, we introduced a treatment of lightning that allows the fraction of ground strikes to vary spatially and seasonally, realistically partitions strike distribution between wet and dry days, and varies the number of dry-days with strikes. Fuel availability and moisture content were improved by implementing decomposition rates specific to individual plant functional types and litter classes, and litter drying rates driven by atmospheric water content. To improve water extraction by grasses, we use realistic plant-specific treatments of deep roots. To improve fire responses, we introduced adaptive bark thickness and post-fire resprouting for tropical and temperate broadleaf trees. All improvements are based on extensive analyses of relevant observational data sets. We test model performance for Australia, first evaluating parameterisations separately and then measuring overall behaviour against standard benchmarks. Changes to the lightning parameterisation produce a more realistic simulation of fires in southeastern and central Australia. Implementation of PFT-specific decomposition rates enhances performance in central Australia. Changes in fuel drying improve fire in northern Australia, while changes in rooting depth produce a more realistic simulation of fuel availability and structure in central and northern Australia. The introduction of adaptive bark thickness and resprouting produces more realistic fire regimes in savannas, including simulating biomass recovery rates consistent with observations. The new model (LPX-Mv1) improves Australian vegetation composition by 33% and burnt area by 19% compared to LPX.
    12/2013; 7(1).
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    ABSTRACT: Earth system models are increasing in complexity and incorporating more processes than their predecessors, making them important tools for studying the global carbon cycle. However, their coupled behaviour has only recently been examined in any de-tail, and has yielded a very wide range of outcomes, with coupled climate-carbon cycle 5 models that represent land-use change simulating total land carbon stores by 2100 that vary by as much as 600 Pg C given the same emissions scenario. This large un-certainty is associated with differences in how key processes are simulated in different models, and illustrates the necessity of determining which models are most realistic us-ing rigorous model evaluation methodologies. Here we assess the state-of-the-art with 10 respect to evaluation of Earth system models, with a particular emphasis on the sim-ulation of the carbon cycle and associated biospheric processes. We examine some of the new advances and remaining uncertainties relating to (i) modern and palaeo data and (ii) metrics for evaluation, and discuss a range of strategies, such as the in-clusion of pre-calibration, combined process-and system-level evaluation, and the use 15 of emergent constraints, that can contribute towards the development of more robust evaluation schemes. An increasingly data-rich environment offers more opportunities for model evaluation, but it is also a challenge, as more knowledge about data uncer-tainties is required in order to determine robust evaluation methodologies that move the field of ESM evaluation from "beauty contest" toward the development of useful 20 constraints on model behaviour.
    Biogeosciences 12/2013; 10:8305-8328. · 3.75 Impact Factor
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    ABSTRACT: 1.The physiological tolerance hypothesis proposes that plant species richness is highest in warm and/or wet climates because a wider range of functional strategies can persist under such conditions. Functional diversity metrics, combined with statistical modeling, offer new ways to test whether diversity-environment relationships are consistent with this hypothesis. 2.In a classic study by R. H. Whittaker (1960), herb species richness declined from mesic (cool, moist, northerly) slopes to xeric (hot, dry, southerly) slopes. Building on this dataset, we measured four plant functional traits (plant height, specific leaf area, leaf water content and foliar C:N) and used them to calculate three functional diversity metrics (functional richness, evenness, and dispersion). We then used a structural equation model to ask if ‘functional diversity’ (modeled as the joint responses of richness, evenness, and dispersion) could explain the observed relationship of topographic climate gradients to species richness. We then repeated our model examining the functional diversity of each of the four traits individually. 3.Consistent with the physiological tolerance hypothesis, we found that functional diversity was higher in more favorable climatic conditions (mesic slopes), and that multivariate functional diversity mediated the relationship of the topographic climate gradient to plant species richness. We found similar patterns for models focusing on individual trait functional diversity of leaf water content and foliar C:N. 4.Synthesis. Our results provide trait-based support for the physiological tolerance hypothesis, suggesting that benign climates support more species because they allow for a wider range of functional strategies. This article is protected by copyright. All rights reserved.
    Journal of Ecology 12/2013; · 5.69 Impact Factor
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    ABSTRACT: Spatially distinct communities can arise through interactions and feedbacks between abiotic and biotic factors. We suggest that, for plants, patches of infertile soils such as serpentine may support more distinct communities from those in the surrounding non-serpentine matrix in regions where the climate is more productive (i.e., warmer and/or wetter). Where both soil fertility and climatic productivity are high, communities may be dominated by plants with fast-growing functional traits, whereas where either soils or climate impose low productivity, species with stress-tolerant functional traits may predominate. As a result, both species and functional composition may show higher dissimilarity between patch and matrix in productive climates. This pattern may be reinforced by positive feedbacks, in which higher plant growth under favorable climate and soil conditions leads to higher soil fertility, further enhancing plant growth. For 96 pairs of sites across a 200-km latitudinal gradient in California, we found that the species and functional dissimilarities between communities on infertile serpentine and fertile non-serpentine soils were higher in more productive (wetter) regions. Woody species had more stress-tolerant functional traits on serpentine than non-serpentine soil, and as rainfall increased, woody species functional composition changed toward fast-growing traits on non-serpentine, but not on serpentine soils. Soil organic matter increased with rainfall, but only on non-serpentine soils, and the difference in organic matter between soils was positively correlated with plant community dissimilarity. These results illustrate a novel mechanism wherein climatic productivity is associated with higher species, functional, and landscape-level dissimilarity (beta diversity).
    Ecology 09/2013; 94(9):2007-18. · 5.18 Impact Factor
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    ABSTRACT: Past climates provide a test of models' ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.
    Climate Dynamics 08/2013; · 4.23 Impact Factor
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Publication Stats

7k Citations
888.86 Total Impact Points


  • 2013–2014
    • University of Reading
      Reading, England, United Kingdom
  • 2010–2014
    • Macquarie University
      • Department of Biological Sciences
      Sydney, New South Wales, Australia
    • Université Bordeaux 1
      Talence, Aquitaine, France
    • Washington University in St. Louis
      • Department of Biology
      Saint Louis, MO, United States
  • 2–2014
    • University of California, Davis
      • Department of Environmental Science and Policy
      Davis, California, United States
  • 2005–2013
    • University of Bristol
      • • School of Earth Sciences
      • • School of Geographical Sciences
      Bristol, England, United Kingdom
  • 2011
    • University of British Columbia - Vancouver
      • Department of Geography
      Vancouver, British Columbia, Canada
    • USGS National Wetlands Research Center
      Lafayette, Louisiana, United States
  • 2003–2011
    • University of Wisconsin, Madison
      • • Department of Zoology
      • • Center for Climatic Research
      Madison, MS, United States
  • 2000–2009
    • CSU Mentor
      Long Beach, California, United States
  • 1998–2008
    • Max Planck Institute for Biogeochemistry Jena
      Jena, Thuringia, Germany
  • 2007
    • Claude Bernard University Lyon 1
      Villeurbanne, Rhône-Alpes, France
  • 2002
    • Friedrich-Schiller-University Jena
      • Department of Geosciences
      Jena, Thuringia, Germany
  • 1993–1998
    • Lund University
      • Department of Earth and Ecosystem Sciences
      Lund, Skåne, Sweden