Jonathan Lenoir

A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century

Spatial fingerprints of climate change on biotic communities are usually associated with changes in the distribution of species at their latitudinal or altitudinal extremes. By comparing the altitudinal distribution of 171 forest plant species between 1905 and 1985 and 1986 and 2005 along the entire elevation range (0 to 2600 meters above sea level) in west Europe, we show that climate warming has resulted in a significant upward shift in species optimum elevation averaging 29 meters per decade. The shift is larger for species restricted to mountain habitats and for grassy species, which are characterized by faster population turnover. Our study shows that climate change affects the spatial core of the distributional range of plant species, in addition to their distributional margins, as previously reported.

Changes in plant community composition lag behind climate warming in lowland forests.

Climate change is driving latitudinal and altitudinal shifts in species distribution worldwide, leading to novel species assemblages. Lags between these biotic responses and contemporary climate changes have been reported for plants and animals. Theoretically, the magnitude of these lags should be greatest in lowland areas, where the velocity of climate change is expected to be much greater than that in highland areas. We compared temperature trends to temperatures reconstructed from plant assemblages (observed in 76,634 surveys) over a 44-year period in France (1965–2008). Here we report that forest plant communities had responded to 0.54 °C of the effective increase of 1.07 °C in highland areas (500–2,600 m above sea level), while they had responded to only 0.02 °C of the 1.11 °C warming trend in lowland areas. There was a larger temperature lag (by 3.1 times) between the climate and plant community composition in lowland forests than in highland forests. The explanation of such disparity lies in the following properties of lowland, as compared to highland, forests: the higher proportion of species with greater ability for local persistence as the climate warms, the reduced opportunity for short-distance escapes, and the greater habitat fragmentation. Although mountains are currently considered to be among the ecosystems most threatened by climate change (owing to mountaintop extinction), the current inertia of plant communities in lowland forests should also be noted, as it could lead to lowland biotic attrition.

Cross-Scale Analysis of the Region Effect on Vascular Plant Species Diversity in Southern and Northern European Mountain Ranges

Background The divergent glacial histories of southern and northern Europe affect present-day species diversity at coarse-grained scales in these two regions, but do these effects also penetrate to the more fine-grained scales of local communities? Methodology/Principal Findings We carried out a cross-scale analysis to address this question for vascular plants in two mountain regions, the Alps in southern Europe and the Scandes in northern Europe, using environmentally paired vegetation plots in the two regions (n = 403 in each region) to quantify four diversity components: (i) total number of species occurring in a region (total γ-diversity), (ii) number of species that could occur in a target plot after environmental filtering (habitat-specific γ-diversity), (iii) pair-wise species compositional turnover between plots (plot-to-plot β-diversity) and (iv) number of species present per plot (plot α-diversity). We found strong region effects on total γ-diversity, habitat-specific γ-diversity and plot-to-plot β-diversity, with a greater diversity in the Alps even towards distances smaller than 50 m between plots. In contrast, there was a slightly greater plot α-diversity in the Scandes, but with a tendency towards contrasting region effects on high and low soil-acidity plots. Conclusions/Significance We conclude that there are strong regional differences between coarse-grained (landscape- to regional-scale) diversity components of the flora in the Alps and the Scandes mountain ranges, but that these differences do not necessarily penetrate to the finest-grained (plot-scale) diversity component, at least not on acidic soils. Our findings are consistent with the contrasting regional Quaternary histories, but we also consider alternative explanatory models. Notably, ecological sorting and habitat connectivity may play a role in the unexpected limited or reversed region effect on plot α-diversity, and may also affect the larger-scale diversity components. For instance, plot connectivity and/or selection for high dispersal ability may increase plot α-diversity and compensate for low total γ-diversity.

Big moving day for biodiversity? A macroecological assessment of the scope for assisted colonization as a conservation strategy under global warming

Future climate change constitutes a major threat to Earths biodiversity. If anthropogenic greenhouse gas emissions continue unabated, 21st century climate change is likely to exceed the natural adaptive capacity of many natural ecosystems and a large proportion of species may risk extinction. A recurrent finding is that the degree of negative impact depends strongly on the dispersal potential of the species. However, there is a growing realization that many, if not most species would be unlikely to disperse as fast and far as required. As a consequence, it has been proposed that species at risk should be actively translocated into unoccupied, but environmentally suitable areas that are likely to stay suitable over the next 100 or more years (assisted colonization or assisted migration). This solution is controversial, though, reflecting negative experiences with introduced exotics and probably also the traditional emphasis in conservation management on preserving a certain local, often historical situation with a static species composition, and a tendency among ecologists to think of biological communities as generally saturated with species. Using the European flora as a case study, we here estimate the main environmental controls of plant species richness, assess how the maximum observed species richness depends on these environmental controls, and based here on estimate how many species could at least be added to an area before further species additions would perhaps inevitably lead to corresponding losses locally. Our results suggest that there is substantial room for additional plant species across most areas of Europe, indicating that there is considerable scope for implementing assisted colonization as a proactive conservation strategy under global warming without necessarily implicating negative effects on the native flora in the areas targeted for establishment of translocated populations. Notably, our results suggest that 50% of the cells in Northern Europe, the likely target area for many translocations, could harbor at least 1/3 as many additional species as they have native species. However, we also emphasize that other, more traditional conservation strategies should also be strengthened, notably providing more space for nature and reducing nitrogen deposition to increase population resilience and facilitate unassisted colonization. Furthermore, any implementation of assisted colonization should be done cautiously, with a careful analysis on a species-by-species case.

Un cas de forêt linéaire ancienne dans les paysages ruraux : de la difficile reconnaissance des haies anciennes à l’étude de leur diversité végétale

La haie est incontestablement un élément des paysages ruraux, qui atteint son plus fort développement dans les systèmes bocagers, où elle revêt souvent une valeur de marqueur identitaire pour les habitants. En de maints lieux pourtant, la révolution « verte » des années 1960 et les opérations de remembrement qui ont suivi, ont éliminé la haie du paysage et celle-ci est souvent perçue comme une relique d’un système économique paysan désuet. Pourtant, l’émergence de préoccupations environnementales et, récemment, la politique des « trames vertes » (loi Grenelle 2) amènent à un nouveau regard sur ces linéaires boisés, qui acquièrent le statut de possible « corridor écologique » pour la flore et la faune forestières dans des paysages fortement anthropisés où la surface forestière est généralement très fragmentée, corridors susceptibles d’aider nombre d’espèces forestières à faire face aux migrations induites par le réchauffement climatique, mais aussi de délivrer aux sociétés un grand nombre de « services écosystémiques ». Ainsi, les haies sont de plus en plus perçues comme des habitats « naturels », alors qu’elles sont, dans la plupart des cas, des plantations artificielles, des créations humaines qui trouvent leur apogée dans les paysages bocagers, archétypes d’une nature anthropisée. Certes, les traces de l’artificialisation sont parfois devenues indiscernables quand les haies sont laissées en évolution libre et se sont ensauvagées : plus une haie est vieille, plus elle accumule d’espèces, conformément à la relation temps-espèces bien connue des écologues (Rosenzweig, 1995). Les haies peuvent ainsi être considérées comme des « objets hybrides », à la fois naturels et culturels, des objets fabriqués au devenir naturel que l’on ne maîtrise pas complètement, manifestation à part entière de la « technonature » (Larrère et Larrère, 2009).