The positive impact of grazing herbivores on plant diversity has been frequently reported in fertile grasslands. Grazing-induced heterogeneity is often evoked as an explanation for this influence. However, the relative importance of plant community heterogeneity induced by herbivores compared to other mechanisms linked to grazing remains unclear. We investigated this question by looking at 3 plant communities found in wet grasslands situated along the French Atlantic coast and traditionally grazed by horses and cattle. An experimental design set up in 1995 allowed us to compare the consequences of cattle-grazing, horse-grazing, and grazing abandonment on plant community diversity and heterogeneity. Floristic measurements made in 2007 showed that cattle and horses both had a positive impact on species richness and Shannon diversity index and that patchiness only occurred in grazed situations. The relative importance of grazing-induced patchiness within the overall positive effect of grazing on plant diversity was assessed by partitioning the diversity gain due to grazing into its additive within- and among-patch components. Grazing-induced patchiness entirely explained the increase in plant richness, whereas it accounted for only a small part of Shannon diversity. Grazing-related processes operating at the patch scale explained the main part of the increase in Shannon diversity. These processes make only a limited contribution to species recruitment, but they result in a more even species abundance distribution.

Estimates of historical variability in river ecosystems are often lacking, but long-lived freshwater mussels could provide unique opportunities to understand past conditions in these environments. We applied dendrochronology techniques to quantify historical variability in growth-increment widths in valves (shells) of western pearlshell freshwater mussels (Margaritifera falcata). A total of 3 growth-increment chronologies, spanning 19 to 26 y in length, were developed. Growth was highly synchronous among individuals within each site, and to a lesser extent, chronologies were synchronous among sites. All 3 chronologies negatively related to instrumental records of stream discharge, while correlations with measures of water temperature were consistently positive but weaker. A reconstruction of stream discharge was performed using linear regressions based on a mussel growth chronology and the regional Palmer Drought Severity Index (PDSI). Models based on mussel growth and PDSI yielded similar coefficients of prediction (R2Pred) of 0.73 and 0.77, respectively, for predicting out-ofsample observations. From an ecological perspective, we found that mussel chronologies provided a rich source of information for understanding climate impacts. Responses of mussels to changes in climate and stream ecosystems can be very site- and process-specific, underscoring the complex nature of biotic responses to climate change and the need to understand both regional and local processes in projecting climate impacts on freshwater species.

To understand changes in habitat selection in response to timber harvesting, we used radio-telemetry data from 82 adult wood frogs (Lithobates sylvaticus, formerly Rana sylvatica) and logistic regression modelling to assess habitat selection in response to an unharvested control and 3 forest management techniques: clearcutting (with removal of all merchantable timber > 10 cm diameter), clearcutting with coarse woody debris (CWD) retention, and partial harvesting with retention of ∼50% canopy cover. At the home range scale, frogs selected the partially harvested treatment in spring 2005 and avoided the CWD-retained treatment in fall 2006. Frogs spent 5 ± 2 d (mean ± SE) longer in forested treatments than in both clearcut treatments, but certain individuals were able to specialize on the clearcut treatments. At the weekly activity centre scale, the best-supported models indicated that frogs were more likely to occupy locations with more complex ground structure, especially coarse woody debris, warmer temperatures, moister substrates, and greater canopy cover than random. Resource use among frogs overlapped substantially at both the weekly activity centre and daily microhabitat scales. Frogs selected daily microhabitats with more complex ground structure, greater canopy cover, and moister substrates than random. Selection at coarser scales may be mechanistically linked to finer scale resource selection by the physiological processes of thermoand hydro-regulation. Our results support recommendations for minimizing the impact of logging by retaining coarse woody debris in clearcuts and partial harvesting with retention of ∼50% canopy cover.

Several keystone herbivores in boreal forests feed on both freshwater and terrestrial plants. Quantifying the amount of freshwater and terrestrial plant biomass in the diets of these species is paramount to understanding their dietary ecology and role in mediating trophic linkages across these ecosystems. We examined the potential to distinguish freshwater and terrestrial plant sources with δ¹³C and δ¹⁵N stable isotope analysis. We collected freshwater and terrestrial vascular plants that are consumed by vertebrate herbivores from 2 distinct and geographically distant subarctic regions in Canada. We consistently observed more positive isotopic values in freshwater than terrestrial vascular plants ( 1.8 to 10.4‰ for δ¹³C and up to 3.2%o for δ¹⁵N) whether the aquatic plants grew in lakes or rivers. We could correctly classify 71 to 100% of plant isotope values into freshwater and terrestrial groups at a locality with cluster analyses of both δ¹³C and δ¹⁵N values. Submergent freshwater and terrestrial plants were the most isotopically differentiated, whereas emergent freshwater plants growing near shorelines contributed to some isotopic overlap between freshwater and terrestrial categories. Collectively, these results establish that site-specific δ¹³C and δ¹⁵N separation between freshwater and terrestrial vascular plants can be used to quantify their respective contribution to herbivore diets and document trophic linkages across freshwater-terrestrial interfaces.

We analyzed the relationship between organic C and N in the upper soil and the quantity (mass) and quality (N, soluble phenolic, and lignin concentrations) of plant aboveground litter and upper fine roots at contrasting microsites (plant patch and bare soil) in 15 study sites across an aridity gradient in Patagonia. At each site, we estimated the total, grass, and shrub cover and randomly selected 10 plant patches of modal size (height and crown diameter) and species composition. We extracted an upper soil core (0–10 cm depth) and collected the aboveground litter underneath each plant patch canopy and at the contiguous bare soil. We separated the fine roots (< 2 mm in diameter) from the soil and assessed the biomass and the concentrations of organic C and N in soil and the lignin, soluble phenolic, and N in shrub and grass components of aboveground litter and in fine roots. Total plant and grass cover decreased with increasing aridity. Total litter mass did not vary across the aridity gradient, but the proportion of shrub components in litter increased and litter quality decreased with increasing aridity. The mass of fine roots was positively correlated to soil organic C and N and decreased with increasing aridity. All these trends were consistent at plant patch and bare soil microsites. We conclude that the decrease in soil organic C and N across the aridity gradient was the outcome of different above- and belowground controls resulting from the replacement of grasses by shrubs. Accordingly, the main controls of soil organic C and N could be the quality of aboveground litter and the quantity of the organic matter input to soil through fine-root turnover from belowground.

Arctic fauna is undergoing significant alteration in response to global climate change, yet we know little regarding the factors that determine species assemblages at northern latitudes. We used a latitudinal transect to assess environmental determinants of ground-dwelling spider assemblages across the boreal forest-tundra transition at a regional scale. Using multivariate techniques, we tested 3 complementary hypotheses regarding the factors that best explain patterns of assemblage structure. We predicted that spider assemblages would respond most strongly to vegetation composition and structure and that climate and spatial variables would explain less of the variation in the data. We sampled ground dwelling spiders using pitfall traps placed at 36 sites along the latitudinal transect. We constructed 3 separate matrices of spatial, climate, and vegetation variables, with each matrix representing a hypothesis. We used redundancy analysis with variation partitioning to determine which matrix of environmental variables best explained patterns in a matrix of spider abundances. We then used a separate redundancy analysis to determine which environmental variables best explained the variation in measures of species richness and activity density. We collected a total of 2890 individual spiders representing 103 species, 58 genera, and 13 families. Our analysis supports the hypothesis that vegetation composition and its related structure best explain patterns in northern spider assemblages at a regional scale.

Fire is a key ecological force in pine forests worldwide, and faunal responses to this disturbance have been a major topic of ecology, yet little is known for oceanic island environments. Using line transects we surveyed the bird community of a natural Pinus canariensis forest burned in the summer of 2007 on Tenerife, Canary Islands, Spain. Our goal was to identify important environmental variables and the thresholds that influenced the pattern of bird abundance at the community, foraging guild, and species-specific level. Models were constructed by means of regression trees and cross validation applying the 1-SE rule. Mixed results were observed and only 2 species were clearly affected by fire severity. Overall, total bird abundance, total species richness, and total bird diversity were positively influenced by low, very low, and moderate canopy fire severity, respectively, and the presence of either Erica or Myrica shrubs. Consequently, high canopy fire severity had an overall negative effect on bird community characteristics. Abundance of Myrica shrubs affected positively general ground foragers such as common blackbird (Turdus merula cabrerae) and Canary Islands chiffchaff (Phylloscopus canariensis), a canopy forager. The number of thin trees was important for canopy foragers such as goldcrest (Regulus regulus teneriffae) and African blue tit (Cyanistes teneriffae teneriffae), but also for a bark forager, the great spotted woodpecker (Dendrocopos major canariensis). European robin (Erithacus rubecula superbus) and the ubiquitous endemic blue chaffinch (Friugilla teydea teydea) were influenced by low and moderate canopy fire severity, respectively. We recommend that land managers incorporate these results into future post-fire management plans. Priority post-fire actions should be directed toward the protection and conservation of the endemic blue chaffinch, a species of conservation concern on the nearby island of Gran Canaria.

This study examined the climate sensitivity of the growth of riparian Alnus incana ssp. tenuifolia (thinleaf alder), a keystone nitrogen-fixer, on the Tanana River floodplain of interior Alaska. We investigated correlations between alder radial growth and inter-annual variation in monthly meteorology and hydrology, spatial patterns of alder climate sensitivity in relationship to depth of subsurface water, and long-term climatic trends. Annual radial growth of alder was positively correlated with June precipitation, river discharge, and Palmer Drought Severity Index values and was negatively correlated with June temperature, suggesting a susceptibility of growth to early-season moisture limitation, due to the co-occurrence of dry meteorological conditions and low levels of hyporheic flow. Alder radial growth was positively correlated with August discharge and July temperature, suggesting that moisture may also be limiting in August, but not in July. The sensitivity of alder growth to variation in temperature and precipitation was greater on higher terraces where depth to water table was greater, while the sensitivity to variation in river level was greater on lower terraces, suggesting that subsurface water more strongly influences moisture availability at these sites. Long-term climatic trends in this region suggest increasing drought conditions; however, the overall influence on alder growth and associated rates of nitrogen fixation are uncertain due to the contrasting relationship of growth with climate in June/August versus July.

Lichens are an important winter forage for large, migratory herds of caribou (Rangifer tarandus granti) that can influence population dynamics through effects on body condition and in turn calf recruitment and survival. We investigated the vegetative and physiographic characteristics of winter range of the Western Arctic Herd in northwest Alaska, one of the largest caribou herds in North America. We made 3 broad comparisons: habitats used by caribou versus random locations, burned versus unburned habitats, and habitats within the current winter range versus those in the historic winter range and potential winter ranges. We found that lichen abundance was more than 3 times greater at locations used by caribou than found at random. The current winter range does not appear to be overgrazed as a whole, but continued high grazing pressure and consequences of climate change on plant community structure might degrade its condition. Within the current winter range, lichen abundance was more than 4 times greater at unburned locations than at recently (< 58 y) burned locations. Other than lichen abundance, there were few vegetative differences between burned (mean = 37 ± 1.7 y) and unburned locations. The historic winter range has low lichen abundance, likely due to sustained grazing pressure exerted by the herd, which suggests that range deterioration can lead to range shifts. Recovery of this range may be slowed by continued grazing and trampling during migration of caribou to and from their current winter range, as well as by high wildfire frequency and other consequences of climate change. The area identified as potential winter range is unlikely to be utilized regularly by large numbers of caribou primarily due to low lichen abundance associated with extensive deciduous stands, large areas of riparian habitat, high moose (Alces alces) densities, and greater prevalence of wildfire. Our results suggest that lichens are important in the overwintering ecology of caribou that face the energetic costs of predator avoidance and migration.

The effects of logging on wildlife have been evaluated mainly in landscapes dominated by even-aged forest stands, leaving the impacts of harvesting on wildlife in irregularly structured stands less documented. We assessed the response of small mammals and birds to 4 silvicultural treatments with different levels of tree retention (from < 10% to 73%) in old-growth boreal forests of eastern Canada, where approximately 70% of stands have irregular structure. The experimental design controlled for local variation in the abundance of forest birds and small mammals by pairing each experimental harvested stand with an uncut stand. We found that species richness of small mammals and birds did not vary according to tree retention. Moss cover and vertical cover influenced small mammal community composition, whereas forest bird assemblages responded to conifer basal area, vertical cover, and snag availability. Species associated with silvicultural practices entailing retention levels > 55% included those that prefer closed canopy, such as the brown creeper (Certhia americana), ruby-crowned kinglet (Regulus calendula), and red-backed vole (Myodes gapperi). Our study demonstrates that silvicultural practices with retention levels > 55% are the most suitable for mitigating the impact of logging on animal communities in old-growth irregularly structured stands.