From broadleaves to conifers: The effect of tree composition and density on understory microclimate across latitudes

Forest canopies buffer the macroclimate and thus play an important role in mitigating climate-warming impacts on forest ecosystems. Despite the importance of the tree layer for understory microclimate buffering, our knowledge about the effects of forest structure, composition and their interactions with macroclimate is limited, especially in mixtures of conifers and broadleaves. Here we studied five mixed forest stands along a 1800 km latitudinal gradient covering a 7 degrees C span in mean annual temperature. In each of these forests we established 40 plots (200 in total), in which air and soil temperatures were measured continuously for at least one year. The plots were located across gradients of forest density and broadleaved proportions (i.e. from open to closed canopies, and from 100% conifer to 100% broadleaved tree dominance). Air minimum, mean and maximum temperature offsets (i.e. difference between macroclimate and microclimate) and soil mean temperature offsets were calculated for the coldest and warmest months. Forest structure, and especially forest density, was the key determinant of understory temperatures. However, the absolute and relative importance of the proportion of broadleaves and forest density differed largely between response variables. Forest density ranged from being independent of, to interacting with, tree species composition. The effect of these two variables was independent of the macroclimate along our latitudinal gradient. Temperature, precipitation, snow depth and wind outside forests affected understory temperature buffering. Finally, we found that the scale at which the overstory affects soil microclimate approximated 6-7 m, whereas for air microclimate this was at least 10 m. These findings have implications for biodiversity conservation and forest management in a changing climate, as they facilitate the projection of understory temperatures in scenarios where both forest structure and macroclimate are dynamic. This is especially relevant given the global importance of ongoing forest conversion from conifers to broadleaves, and vice versa

Interactions between local and global drivers determine long-term trends in boreal forest understorey vegetation

Aim Global change effects on forest ecosystems are increasingly claimed to be context dependent, indicated by interactions between global and local environmental drivers. Most examples of such context dependencies originate from temperate systems, while limited research comes from the boreal biome. Here we set out to test if interactions between climate warming, nitrogen deposition, land-use change resulting in increasing forest density, and soil pH drive long-term changes in understorey vegetation in boreal forests. Location Sweden. Time period 1953-2012. Major taxa studied Vascular plants. Methods We used long-term (50 years) National Forest Inventory data on forest understorey vegetation in Sweden to model the combined effects of climate warming, nitrogen deposition, increase in forest density (tree basal area), and soil pH. Results Our results identify increasing temperature, nitrogen deposition and denser, shadier forest conditions as the main drivers of understorey vegetation changes during this time period. More importantly, we found that these effects varied with local conditions, that is, that the change towards a more nitrophilic understorey vegetation was more pronounced at low than high soil pH. Forest density was an important modulator of nitrogen deposition and temperature increase, with effects generally decreasing with density. Decreased cover of ericaceous dwarf shrubs was driven by both forest density and nitrogen deposition, with a stronger effect at low than at high pH. Main conclusions Our results highlight that to understand forest ecosystems' response to global change, and to make adequate management decisions to mitigate the effects of global change, we need to understand how changes in local environmental factors (forest density and soil pH) interact with global-scale drivers (nitrogen deposition and climate warming). Neglecting such interactions will lead to incorrect estimations of effects. In our case, we would for example, have underestimated the eutrophication effects on acid soils, which constitute a considerable part of the boreal biome

A spatial pattern analysis of the halophytic species distribution in an arid coastal environment

status: publishe

From broadleaves to conifers: The effect of tree composition and density on understory microclimate across latitudes

Forest canopies buffer the macroclimate and thus play an important role in mitigating climate-warming impacts on forest ecosystems. Despite the importance of the tree layer for understory microclimate buffering, our knowledge about the effects of forest structure, composition and their interactions with macroclimate is limited, especially in mixtures of conifers and broadleaves. Here we studied five mixed forest stands along a 1800 km latitudinal gradient covering a 7°C span in mean annual temperature. In each of these forests we established 40 plots (200 in total), in which air and soil temperatures were measured continuously for at least one year. The plots were located across gradients of forest density and broadleaved proportions (i.e. from open to closed canopies, and from 100% conifer to 100% broadleaved tree dominance). Air minimum, mean and maximum temperature offsets (i.e. difference between macroclimate and microclimate) and soil mean temperature offsets were calculated for the coldest and warmest months. Forest structure, and especially forest density, was the key determinant of understory temperatures. However, the absolute and relative importance of the proportion of broadleaves and forest density differed largely between response variables. Forest density ranged from being independent of, to interacting with, tree species composition. The effect of these two variables was independent of the macroclimate along our latitudinal gradient. Temperature, precipitation, snow depth and wind outside forests affected understory temperature buffering. Finally, we found that the scale at which the overstory affects soil microclimate approximated 6-7 m, whereas for air microclimate this was at least 10 m. These findings have implications for biodiversity conservation and forest management in a changing climate, as they facilitate the projection of understory temperatures in scenarios where both forest structure and macroclimate are dynamic. This is especially relevant given the global importance of ongoing forest conversion from conifers to broadleaves, and vice versa

Using warming tolerances to predict understory plant responses to climate change

Climate change is pushing species towards and potentially beyond their critical thermal limits. The extent to which species can cope with temperatures exceeding their critical thermal limits is still uncertain. To better assess species' responses to warming, we compute the warming tolerance (ΔTniche) as a thermal vulnerability index, using species' upper thermal limits (the temperature at the warm limit of their distribution range) minus the local habitat temperature actually experienced at a given location. This metric is useful to predict how much more warming species can tolerate before negative impacts are expected to occur. Here we set up a cross-continental transplant experiment involving five regions distributed along a latitudinal gradient across Europe (43° N–61° N). Transplant sites were located in dense and open forests stands, and at forest edges and in interiors. We estimated the warming tolerance for 12 understory plant species common in European temperate forests. During 3 years, we examined the effects of the warming tolerance of each species across all transplanted locations on local plant performance, in terms of survival, height, ground cover, flowering probabilities and flower number. We found that the warming tolerance (ΔTniche) of the 12 studied understory species was significantly different across Europe and varied by up to 8°C. In general, ΔTniche were smaller (less positive) towards the forest edge and in open stands. Plant performance (growth and reproduction) increased with increasing ΔTniche across all 12 species. Our study demonstrated that ΔTniche of understory plant species varied with macroclimatic differences among regions across Europe, as well as in response to forest microclimates, albeit to a lesser extent. Our findings support the hypothesis that plant performance across species decreases in terms of growth and reproduction as local temperature conditions reach or exceed the warm limit of the focal species

Forest understorey communities respond strongly to light in interaction with forest structure, but not to microclimate warming

International audienceForests harbour large spatiotemporal heterogeneity in canopy structure. This variation drives the microclimate and light availability at the forest floor. So far, we do not know how light availability and sub-canopy temperature interactively mediate the impact of macroclimate warming on understorey communities. We therefore assessed the functional response of understorey plant communities to warming and light addition in a full factorial experiment installed in temperate deciduous forests across Europe along natural microclimate, light and macroclimate gradients. Furthermore, we related these functional responses to the species' life-history syndromes and thermal niches. We found no significant community responses to the warming treatment. The light treatment, however, had a stronger impact on communities, mainly due to responses by fast-colonizing generalists and not by slow-colonizing forest specialists. The forest structure strongly mediated the response to light addition and also had a clear impact on functional traits and total plant cover. The effects of short-term experimental warming were small and suggest a time-lag in the response of understorey species to climate change. Canopy disturbance, for instance due to drought, pests or logging, has a strong and immediate impact and particularly favours generalists in the understorey in structurally complex forests

Microclimate and forest density drive plant population dynamics under climate change

Peer reviewe