How can model comparison help improving species distribution models?

Today, more than ever, robust projections of potential species range shifts are needed to anticipate and mitigate the impacts of climate change on biodiversity and ecosystem services. Such projections are so far provided almost exclusively by correlative species distribution models (correlative SDMs). However, concerns regarding the reliability of their predictive power are growing and several authors call for the development of process-based SDMs. Still, each of these methods presents strengths and weakness which have to be estimated if they are to be reliably used by decision makers. In this study we compare projections of three different SDMs (STASH, LPJ and PHENOFIT) that lie in the continuum between correlative models and process-based models for the current distribution of three major European tree species, Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L. We compare the consistency of the model simulations using an innovative comparison map profile method, integrating local and multi-scale comparisons. The three models simulate relatively accurately the current distribution of the three species. The process-based model performs almost as well as the correlative model, although parameters of the former are not fitted to the observed species distributions. According to our simulations, species range limits are triggered, at the European scale, by establishment and survival through processes primarily related to phenology and resistance to abiotic stress rather than to growth efficiency. The accuracy of projections of the hybrid and process-based model could however be improved by integrating a more realistic representation of the species resistance to water stress for instance, advocating for pursuing efforts to understand and formulate explicitly the impact of climatic conditions and variations on these processes

Global Changes and European Terrestrial Ecosystems

Global environmental changes are topics of important societal concern and current scientific interest. These changes are driven by many different forcing and feedbacks occurring at various time and geographic scales and are largely influencing the various Earth systems. Despite the fact that the natural component of these changes has always occurred, today it's the anthropogenic component and its magnitude which is worrying. At the European scale, such changes are likely to profoundly affect terrestrial ecosystems in term of physiology, phenology, distribution and evolution. In order to improve our understanding of the effect of global changes on ecosystems, this thesis describes a range of diverse analyses into the different forcing factors for different periods of time. In the first study, palaeorecords show that climatic conditions similar to the ones predicted to occur during the coming century already occurred. European terrestrial ecosystems however were different in structure and composition. Future predictions of climatic conditions associated to plant species specific tolerances ranges have been used in the second and third studies to quantify geographically the risk for such species in being able to find suitable climate space by the end of the century. In general, northern species will face a drastic reduction of their suitable climatic space as southern species are likely to face an expansion in climate space. However, to reach a suitable climate species may have to travel great distances and overcome natural and human barriers. Finally, the last two studies explored with the help of the state of the art dynamic vegetation model LPJ-GUESS, the different interactions between climatic changes, disturbances rates and exotic plant invasion and Mediterranean European ecosystems compositions and functions. Disturbance has been shown to be the main driving factor of these ecosystems characteristics, and influenced largely the likely success of invasions in some ecosystems. However, at the local scale, climatic conditions and the degree of invasion, were likely to influence ecosystems composition, distribution and functions but in a less radical ways. Such approaches, allowed us to explore the relevance of the different forcing factors on ecosystems changes. Future development of these analytic methods by adopting more integrating methods should aim to support societal decisions and actions

Vulnerability of Mediterranean Basin ecosystems to climate change and invasion by exotic plant species

Aim To assess at a broad scale the vulnerability of Mediterranean vegetation to alien plant invasion under different climatic and disturbance scenarios. Location We simulated the vegetation biogeography and dynamics on five of the main islands of the Mediterranean Basin: Mallorca, Corsica, Sardinia, Crete and Lesvos. Methods We used LPJ-GUESS, a generalized ecosystem model based on dynamic processes describing establishment, competition, mortality and ecosystem biogeochemistry. We simulated the vegetation distribution and dynamics using a set of plant functional types (PFTs) based on bioclimatic and physiological parameters, which included tree and shrub PFTs defined especially for the Mediterranean. Additionally, two invasive PFTs, an invasive tree type and an invasive herb type, were defined and used to estimate the vulnerability to invasion of a range of different ecosystems. The model was used to simulate climate changes and associated changes in atmospheric [CO2] to 2050 according to two Special Report on Emissions Scenarios climate scenarios (A1Fi and B1) combined with mean disturbance intervals of 3 and 40 years. Results The simulations and scenarios showed that the effect of climate change alone is likely to be negligible in many of the simulated ecosystems, although not all. The simulated progression of an invasion was highly dependent on the initial ecosystem composition and local environmental conditions, with a particular contrast between drier and wetter parts of the Mediterranean, and between mountain and coastal areas. The rate of ecosystem disturbance was the main factor controlling susceptibility to invasion, strongly influencing vegetation development on the shorter time scale. Main conclusions Further invasion into Mediterranean island ecosystems is likely to be an increasing problem: our simulations predict that, in the longer term, almost all the ecosystems will be dominated by exotic plants irrespective of disturbance rates

Estimating consensus and associated uncertainty between inherently different species distribution models

1. Forecasting shifts in biome and species distribution is crucially needed in the current context of global change. So far, most projections of vegetation distribution rely on correlative species distribution models (SDMs). Yet, process-based or hybrid models based on explicit physiological description may be more robust to extrapolation under future climatic conditions. Differences between model projections may be wide, leading to scepticism among environmental stakeholders. 2. Here, we propose to combine outputs of several distributionmodels based on physiological responses, to produce both consensual maps of occurrences and maps of associated uncertainty. The consensus map relies on the conditional projections of each SDM. Because the models used are based on processes, their errors are likely to vary consistently with climate as some processes not implemented in a model might be important under a given set of climatic conditions. Uncertainty of the consensus model is thus assessed through multimodel regression of deviance maps with respect to current climatic conditions, and can be extrapolated to forecast climates. 3. We illustrate this approach using three SDMs, on three widely distributed European trees (Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.), and project their distributions under two scenarios. The conditional consensus outperforms classical methods of model consensus (i.e. to use the mean, the median or a weighted average of individualSDMoutputs) in projecting current occurrences. 4. Consistently, with the results of individual SDMs, the conditional consensus projects that the suitable areas for F. sylvatica and Q. robur will expand towards north-eastern Europe, while that of P. sylvestris will contract. Projections of future occurrence are most uncertain towards themargins of the distribution (particularly the trailing edge). 5. Our approach can help modellers identify the limitations of each SDMand stakeholders pinpoint the regions of models agreement and highest certainty

Predicting potential shifts in Fagus sylvatica range in response to climate change: comparing and integrating multiple models based on different structural concepts to reduce predictions uncertainty

International audienceClimate change effects on tree function, structure and growth have been observed in various forests throughout Europe and France in the last decades. Forest productivity and survival are limited by a variety of environmental abiotic factors such as soil nutrients, temperature, atmospheric CO2 concentration, and water balance. Differential impacts of global climatic changes on forests, as well as complex interactions with the carbon and water balance, and vegetation feedbacks on climate makes it hard to estimate future vegetation patterns. Obtaining reliable predictions of species range shifts under climate change is a crucial challenge for ecologists and stakeholders. However, it is evident that no single modelling procedure can provide appropriate predictions in the future. Model comparison is a powerful tool to evaluate models in a more conceptual way by comparing and interpreting predictions on the basis of the underlying model approaches and assumptions. This work aims at understanding future spatial distribution, physiological response and potential adaptation of 7 species in France under future climate, by using and comparing simulations stemming from different model concepts. Here, we present only the results for Temperate broadleaf species (Fagus sylvatica and Quercus robur) and compare predictions of range shifts under climate change scenarios for 2050 derived from three niche-based models (BIOMOD, NBM and STASH) with those derived from a mechanistic tree growth process-based model (CASTANEA), a phenology-based model (PHENOFIT) and three dynamic global vegetation model DGVM (LPJ, ORCHIDEE and IBIS). A contrasted pattern emerged from our comparisons: although all models project significant losses ofT emperate broadleaf species range in France in 2050, significant divergences between models appear: niche-based models tend to predict a stronger level of range loss in the plains across France than the other models, and they project less range loss in mountains than in lowlands. LPJ projects the smallest loss of range in plains and increased presence of beech in mountains. In general, LPJ, Orchidee, Castanea and PhenoFit are more conservative in the North, East and North-East of France than in the South, West and South West. This result likely arises because niche-based models do not take complex interactions between physiological processes and climate, phenotypic plasticity, and local adaptation into account. Nevertheless, there is much greater uncertainty in projected tree response to climate change than previous studies have suggested. This means that forest management must focus on dealing with an uncertain future, i.e., by increasing the resilience of forest ecosystems

Climate or migration: what limited European beech post-glacial colonization?

International audienceAimDespite the recent improvements made in species distribution models (SDMs), assessing species' ability to migrate fast enough to track their climate optimum remains a challenge. This study achieves this goal and demonstrates the reliability of a process‐based SDM to provide accurate projections by simulating the post‐glacial colonization of European beech.LocationEurope.MethodsWe simulated the post‐glacial colonization of European beech over the last 12,000 years by coupling a process‐based SDM (PHENOFIT) and a new migration model based on Gibbs point processes, both parameterized with modern ecological data. Simulations were compared with palaeoarchives and phylogeographic data on European beech.ResultsModel predictions are consistent with palaeoarchives and phylogeographic data over the Holocene. The results suggest that post‐glacial expansion of European beech was limited by climate on its north‐eastern leading edge, while limited by its migration abilities on its north‐western leading edge. The results show a mean migration rate of beech varying from 270 m yr −1 to 280 m yr−1 and a maximum migration rate varying from 560 m yr−1 to 630 m yr−1, when limited and not limited by climate, respectively. They also highlight the relative contribution of known and suspected glacial refugia in present beech distribution and confirm the results of phylogeographic studies.Main conclusionsFor the first time, we were able to reproduce accurately the colonization dynamics of European beech during the last 12 kyr using a process‐based SDM and a migration model, both parameterized with modern ecological data. Our methodology has allowed us to identify the different factors that affected European beech migration during its post‐glaciation expansion in different parts of its range. This method shows great potential to help palaeobotanists and phylogeographers locate putative glacial refugia, and to provide accurate projections of beech distribution change in the future

Average cross-correlation between models over the 20 monoscales (columns: LPJ-STASH; LPJ-PHENOFIT; lines: <i>F</i><i>. sylvatica</i>; <i>Q</i><i>. robur</i>; <i>P</i><i>. sylvestris</i>).

<p>Average cross-correlation between models over the 20 monoscales (columns: LPJ-STASH; LPJ-PHENOFIT; lines: <i>F</i><i>. sylvatica</i>; <i>Q</i><i>. robur</i>; <i>P</i><i>. sylvestris</i>).</p

Mean Kappa of the 3 models projections for species present distributions ((a) circle: <i>F</i><i>. sylvatica</i>; square: <i>Q</i><i>. robur</i>; triangle: <i>P</i><i>. sylvestris</i>) and relative anomalies (×: LPJ; +: STASH; *: PHENOFIT) for (b) <i>F</i><i>. sylvatica</i>; (c): <i>Q</i><i>. robur</i>; (d): <i>P</i><i>. sylvestris</i>.

<p>Mean Kappa of the 3 models projections for species present distributions ((a) circle: <i>F</i><i>. sylvatica</i>; square: <i>Q</i><i>. robur</i>; triangle: <i>P</i><i>. sylvestris</i>) and relative anomalies (×: LPJ; +: STASH; *: PHENOFIT) for (b) <i>F</i><i>. sylvatica</i>; (c): <i>Q</i><i>. robur</i>; (d): <i>P</i><i>. sylvestris</i>.</p