Despite an increasing number of studies highlighting the impacts of climate change on boreal species, the main factors that will drive changes in species assemblages remain ambiguous. We quantify two climate-induced pathways based on direct and indirect effects on species occupancy and assemblage dissimilarity under different harvest management scenarios. The direct climate effects illustrate immediate impact of climate variables while the indirect effects are reflected through the changes in land cover composition. To understand the main causes in assemblage dissimilarity, we analyze the regional and the latitudinal species assemblage dissimilarity by decomposing it into balanced variation in species occupancy and occurrence and occupancy and occurrence gradient. We develop empirical models to predict the distribution of more than 100 bird and beetle species in the Côte-Nord region of Québec over the next century. Our results show the two pathways that are based on immediate and lagged climate change effects are complementary and alter biodiversity, mainly caused by balanced variation in species occupancy and occurrence. At the regional scale, both effects have an impact on decreasing the number of winning species. Yet, responses are much larger in magnitude under mixed climate effects (a mixture of direct and indirect effects). Regional assemblage dissimilarity reached 0.77 and 0.69 under mixed effects versus 0.09 and 0.10 under indirect effects for beetles and birds, respectively, between RCP 8.5 and baseline climate scenarios when considering harvest. Therefore, inclusion of climatic variables considers aspects other than just those related to forest landscapes, such as life-cycle of animal species. Latitudinally, assemblage dissimilarity increased following the climate conditions pattern. Our analysis contributes to the understanding of how climate change alters biodiversity by reshaping community composition and highlights the importance of climate variables in biodiversity prediction.
Logging is the main human disturbance impacting biodiversity in forest ecosystems. However, the impact of forest harvesting on biodiversity is modulated by abiotic conditions through complex relationships that remain poorly documented. Therefore, the interplay between forest management and climate change can no longer be ignored. Our aim was to study the expected long-term variations in the assemblage of bird and beetle communities following modifications in forest management under different climate change scenarios. We developed species distribution models to predict the occurrence of 88 species of birds and beetles in eastern Canadian boreal forests over the next century. We simulated three climate scenarios (baseline, RCP4.5 and RCP8.5) under which we varied the level of harvesting. We also analyzed the regional assemblage dissimilarity by decomposing it into balanced variations in species occupancy and occupancy gradient. We predict that forest harvesting will alter the diversity by increasing assemblage dissimilarity under all the studied climate scenarios, mainly due to species turnover. Species turnover intensity was greater for ground-dwelling beetles, probably because they have lower dispersal capacity than flying beetles or birds. A good dispersal capacity allows species to travel more easily between ecosystems across the landscape when they search for suitable habitats after a disturbance. Regionally, an overall increase in the probability of occupancy is projected for bird species, whereas a decrease is predicted for beetles, a variation that could reflect differences in ecological traits between taxa. Our results further predict a decrease in the number of species that increase their occupancy after harvest under the most severe climatic scenario for both taxa. We anticipate that under severe climate change, increasing forest disturbance will be detrimental to beetles associated with old forests but also with young forests after disturbances.
Annual estimates of defoliation are important tools for managing forest insect defoliators such as the hemlock looper, which feeds on conifer needles of all age classes. We tested the accuracy of defoliation classes obtained from aerial surveys by comparing them with ground-based estimates during a recent outbreak of this insect. We used an approach derived from the Fettes method to estimate defoliation on the current-year shoots as well as on the shoots of the four previous years. Defoliation on the current-year shoots provided accurate estimates of the overall defoliation and the strength of the relationship gradually decreasing for one-year-old to four-year-old foliage. The aerial survey provided accurate estimates of light and moderate defoliation during the first year of the outbreak, but accuracy was lower for both ends of the defoliation gradient and was much less reliable after the second year of the outbreak. All levels of defoliation were then observed in stands where defoliation had not been detected by an aerial survey. Cumulative defoliation on all age classes of foliage brings a new challenge to crews assigned to aerial survey programs. Ground-level defoliation estimates on the current-year shoots can help appraise the risk of tree mortality in the following year.
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