Lluís

Our aim was to map the climate dependence of tree species distributions (probability of occurrence) and forest growth (net primary productivity) by comparing the congruence and incongruence between correlative and process-based modelling approaches.

Iberian Peninsula, south-western Europe.

We used forest inventory data for three widespread tree species (Quercus ilex, Pinus halepensis and Pinus sylvestris) to model climatic suitability with an ensemble of seven correlative species distribution models (using biomod). We then simulated forest net primary productivity (NPP) as a surrogate of forest growth for forests of each species using an ecophysiological process-based model (gotilwa+) along a gradient of climatic suitability. The spatial distribution of the growth estimates was then compared with that of the suitability estimates, and robust regression was used to classify regions in terms of model congruence.

Quercus ilex and P. sylvestris both showed a positive relationship between forest NPP and climatic suitability. The main discrepancies were found in the north of the peninsula, where there was high potential forest growth but low climate suitability. Low forest-growth estimates in areas of high suitability only appeared for P. sylvestris in southern montane regions. Pinus halepensis always showed a negative relationship between estimated growth and climatic suitability. The analysis of other ecophysiological parameters (mean leaf life and leaf area index) suggests that this tree species has different physiological strategies that allow differential growth rates in areas of low suitability.

We found that the relationship between estimated growth and distribution varies strongly in different areas and species. Mapping the incongruences between the predicted climatic suitability and growth allowed us to identify regions where other factors (e.g. biotic interactions) may be more significant than the physiological limits on growth. We show that new insights into species distributions can be gained from mapping the differences between correlative and process-based models.

In response to environmental changes and to avoid extinction, species may either track suitable environmental conditions or adapt to the modified environment. However, whether and how species adapt to environmental changes remains unclear. By focusing on the realized niche (i.e. the actual space that a species inhabits and the resources it can access as a result of limiting biotic factors present in its habitat), we here examine shifts in the realized-niche width (i.e. ecological amplitude) and position (i.e. ecological optimum) of 26 common and widespread forest understorey plants across their distributional ranges.

Temperate forests along a ca. 1800-km-long latitudinal gradient from northern France to central Sweden and Estonia.

We derived species' realized-niche width from a β-diversity metric, which increases if the focal species co-occurs with more species. Based on the concept that species' scores in a detrended correspondence analysis (DCA) represent the locations of their realized-niche positions, we developed a novel approach to run species-specific DCAs allowing the focal species to shift its realized-niche position along the studied latitudinal gradient while the realized-niche positions of other species were held constant.

None of the 26 species maintained both their realized-niche width and position along the latitudinal gradient. Few species (9 of 26: 35%) shifted their realized-niche width, but all shifted their realized-niche position. With increasing latitude, most species (22 of 26: 85%) shifted their realized-niche position for soil nutrients and pH towards nutrient-poorer and more acidic soils.

Forest understorey plants shifted their realized niche along the latitudinal gradient, suggesting local adaptation and/or plasticity. This macroecological pattern casts doubt on the idea that the realized niche is stable in space and time, which is a key assumption of species distribution models used to predict the future of biodiversity, hence raising concern about predicted extinction rates.

Theory predicts that fragmentation aggravates habitat loss, increasing the extinction threshold of habitat specialists. However, contradictory empirical results have fuelled claims that fragmentation has been overemphasized, and more attention should be given to habitat loss for preserving species. We assess variation in species sensitivity to forest amount and fragmentation and evaluate if fragmentation is related to extinction thresholds in seven forest bird species.

Europe.

We use the percentage of forest cover and the proportion of cover occurring in the largest patch to partition effects of forest amount versus fragmentation, and apply logistic regression to model the presence–absence of 17 forest bird species. For seven species showing robust models, we define two fragmentation scenarios, low and maximum, across the forest cover gradient and quantify species' sensitivity to forest contraction with no fragmentation, and to fragmentation under constant forest cover. Finally, we develop two tests of the extinction threshold hypothesis by comparing the occurrence probability of each species under the two fragmentation scenarios at different forest covers.

As expected, forest contraction had negative impacts on the occurrence probability of all seven species modelled, but – in line with theory – fragmentation also led to a higher extinction threshold for three (Western capercaillie, Hazel grouse and Eurasian pygmy-owl). One species (Black woodpecker) exhibited the opposite pattern indicating that it probably benefits from fragmentation. Differences among species responses may reflect dispersal abilities, specializations in resources/habitat characteristics and/or sensitivity to potential modifications of interspecific interactions.

Although forest amount is of primary importance for the persistence of forest specialist birds, fragmentation is also relevant for some, and neglecting forest fragmentation would be a mistake for these species. Species-specific traits can be helpful for interpreting species' reactions to fragmentation, and it should not be assumed that it always, or never, matters.

Conservation strategies need predictions that capture spatial community composition and structure. Currently, the methods used to generate these predictive maps generally focus on deterministic processes and omit stochasticity and other uncertainty in model outputs. Here we present a novel approach to model the means and variance of assemblage properties.

The western Swiss Alps.

We propose a new approach to processing probabilistic predictions derived from stacked species distribution models (S-SDMs) in order to predict and assess the uncertainty in predictions of community properties. We compare the utility of our novel approach with that of a traditional threshold-based approach. We used data sampled in 2009 and 2010 from 192 sites in total for mountain butterfly communities spanning a large elevational gradient as a case study and evaluated the ability of our approach to model the species richness and phylogenetic diversity of communities within an ensemble forecasting framework.

Our approach allowed mapping of the variability in species richness and phylogenetic diversity projections, in addition to the mean, for 78 butterfly species. S-SDMs reproduced the observed decrease in phylogenetic diversity and species richness with elevation, a consequence of environmental filtering. The prediction accuracy of community properties varied along environmental gradients: at low elevations, variability was higher for predictions of species richness while it was the opposite for phylogenetic diversity.

The use of our probabilistic approach to process species distribution model outputs in order to reconstruct communities provides an improved picture of the range of possible assemblage realizations under similar environmental conditions given modelling uncertainty, and helps to inform managers of the usefulness of modelling results.

Pyrogeographical theory suggests that fire is controlled by spatial gradients in resources to burn (fuel amount) and climatic conditions promoting combustion (fuel moisture). Examining trade-offs among these environmental constraints is critical to understanding future fire activity. We evaluate constraints on fire frequency in modern fire records over the entire Mediterranean biome and identify potential shifts in fire activity under an ensemble of global climate projections.

 The biome encompassing the Mediterranean-type ecosystems (MTEs).

We evaluate potential changes in fire over the 21st century in MTEs based on a standardized global framework. Future fire predictions are generated from statistical fire−climate models driven by ensembles of climate projections under the IPCC A2 emissions scenario depicting warmer–drier and warmer–wetter syndromes. We test the hypothesis that MTEs lie in the transition zone discriminating fuel moisture versus fuel amount as the dominant constraint on fire activity.

Fire increases reported in MTEs in recent decades may not continue throughout the century. MTEs occupy a sensitive portion of global fire−climate relationships, especially for precipitation-related variables, leading to highly divergent fire predictions under drier versus wetter syndromes. Warmer–drier conditions could result in decreased fire activity over more than half the Mediterranean biome by 2070–2099, and the opposite is predicted under a warmer–wetter future. MTEs encompass, however, a climate space broad and complex enough to include spatially varied fire responses and potential conversions to non-MTE biomes.

 Our results strongly support the existence of both fuel amount and fuel moisture constraints on fire activity and show their geographically variable influence throughout MTEs. Climatic controls on fire occurrence in MTEs lie close to ‘tipping points’, where relatively small changes in future climates could translate into drastic and divergent shifts in fire activity over the Mediterranean biome, mediated by productivity alterations.