Not all studies have empirically supported the model that predicts a positive relationship between habitat heterogeneity and biodiversity. We hypothesized that these different results stem from the methods used to assess habitat heterogeneity; many studies used variables that are somewhat correlated in nature and measure 2 different features of the environment: a) the number of structure types (habitat heterogeneity) and b) the number of structures, disregarding their types (habitat amount). We tested this hypothesis with a single experiment that assigned orthogonal values of habitat heterogeneity and habitat amount to artificial environments located on the floor of a forest remnant. We statistically controlled the number of individuals in each environment to prevent a random sample effect. We used the number of arthropod morphospecies present in the environments after 60 d as our dependent variable. The results indicate that habitat heterogeneity had no significant effect on species richness, while habitat amount showed a positive effect when the number of individuals was not controlled. Neither habitat heterogeneity nor habitat amount affected species richness when the number of individuals was controlled. We conclude that conflicting results in previous tests of the heterogeneity model could stem from conceptual and methodological problems in experimental conception. We suggest that further studies distinguish between heterogeneity and area effects, design proper controls for different effects, and consider the spatial scale of the ecological processes that influence species diversity.

The exotic disease white pine blister rust (caused by Cronartium ribicola) damages and kills whitebark pine (Pinus albicaulis), even in the extreme environments of alpine treeline communities. We surveyed P. albicaulis trees and tree islands for blister rust in 2 distinct alpine treeline communities in Montana, USA, and examined meso- and microtopographic factors potentially related to the climatic requirements for blister rust infection. For each of 60 sampling plots, we created high-resolution digital elevation models, derived microtopography variables, and compared these and distance to water feature variables with blister rust occurrence and intensity (number of cankers per infected tree) for every sampled P. albicaulis tree. Infection rates were 19% (of 328 sampled trees) and 24% (of 585 sampled trees) at the 2 sites. Tree island P. albicaulis had higher infection percentages than solitary trees. Using Bayesian analysis and a zero-inflated Poisson regression model, we determined that solar radiation and moisture-related variables correlated with both presence and number of blister rust cankers on P. albicaulis. Site factors that influence moisture, such as local topography, hydrology, and climate, differed between the 2 treeline study areas, which may account for the model variability.

Plant secondary metabolites are important traits that can benefit an invasive plant in its new environment. In the case of rapid evolution in the invaded area, the chemical weapons of introduced plants may diversify, and novel combinations or extreme concentrations of these secondary metabolites may be expressed. The invasive Fallopia species complex (F. japonica, F. sachalinensis, and the F. × bohemica interspecific hybrids) is a good model to assess how chemical traits can vary during post-introduction evolution. We analyzed and compared the composition of secondary metabolite extracts in F. × bohemica hybrids and in the parental species grown in the introduced area. HPLC-DAD profiles were obtained for each Fallopia species, and the main peaks of the HPLC chromatograms represent phenolic compounds. Analyses based on secondary metabolite profiles showed that F. × bohemica hybrids are closer to F. japonica. The F. × bohemica hybrids expressed the compound families described in F. japonica and F. sachalinensis, with quantitative variations between them. Hybrid chemical cocktails showed a diversification of chemical weapons. Furthermore, transgressive segregation was observed. Three dianthrones were identified for the first time in a Fallopia species and were more highly expressed in F. japonica and F. × bohemica hybrids. These results suggest an evolution in the chemical traits of Fallopia taxa in invaded areas such that certain genotypes may well have acquired new chemical cocktails resulting from post-introduction hybridizations.

Attempts to understand how communities assemble following a disturbance are challenged by the difficulty of determining the relative importance of stochastic and deterministic processes. Biological legacies, which result from organisms that survive a disturbance, can favour deterministic processes in community assembly and improve predictions of successional trajectories. Recently disturbed ecosystems are often so rapidly colonized by propagules that the role of biological legacies is obscured. We studied biological legacies on a remote volcanic island in Alaska following a devastating eruption where the role of colonization from adjacent communities was minimized. The role of biological legacies in the near shore environment was not clear, because although some kelp survived, they were presumably overwhelmed by the many vagile propagules in a marine environment. The legacy concept was most applicable to terrestrial invertebrates and plants that survived in remnants of buried soil that were exposed by post-eruption erosion. If the legacy concept is extended to include ex situ survival by transient organisms, then it was also applicable to the island's thousands of seabirds, because the seabirds survived the eruption by leaving the island and have begun to return and rebuild their nests as local conditions improve. Our multi-trophic examination of biological legacies in a successional context suggests that the relative importance of biological legacies varies with the degree of destruction, the availability of colonizing propagules, the spatial and temporal scales under consideration, and species interactions. Understanding the role of biological legacies in community assembly following disturbances can help elucidate the relative importance of colonists versus survivors, the role of priority effects among the colonists, convergence versus divergence of successional trajectories, the influence of spatial heterogeneity, and the role of island biogeographical concepts.

Alternatives are being sought to the widespread use of clear-cut logging in boreal forests. Group retention harvesting is a silvicultural treatment in which well-distributed but relatively small residual forest patches (ca 10 m wide) are left inside cutover sites. The objective of this study was to compare vascular plant communities, tree species regeneration, and dead wood retention in tree retention groups and adjacent clear-cuts with soil protection. Our results indicate that plant diversity is relatively similar inside tree retention groups and the adjacent clear-cut area. This result may be explained by the important spatial variability observed among the stands, which were located in different geographical locations, the fact that soils were little disturbed during harvesting in clear-cuts and few opportunities were present for the establishment of pioneer species, and the relatively short time span since harvesting. Using a functional trait approach, we found that shade tolerance still plays a significant but relatively minor role in explaining species abundance between the 2 environments. Tree retention groups also retain a greater quantity and greater variability of dead wood materials compared with clear-cuts, and a higher abundance of regenerating trees, which is likely explained in part by the direct damage caused by the harvesting operations outside tree retention groups. Overall, retention groups do not appear to confer superior protection for late-successional plants compared with traditional clear-cut logging with soil protection. Their ecological value mostly consists in ensuring a minimal input of dead woody materials for saproxylic species.

Trembling aspen (Populus tremuloides) is widely distributed in North American forests. Increased stand productivity with resource availability has been reported, but the relationship between soil microbial community structure and stand productivity remains unclear. To examine soil microbial composition of 4 aspen stand productivity classes, we assessed soil properties, microbial biomass and respiration, and bacterial and ectomycorrhizal diversity. Most variables showed no significant differences between productivity classes. However, mean values for basal respiration (0.05 to 27.99 µg CO₂-C·g^-1 soil·h^-1), bacterial biomass, and metabolic quotient (0.08 to 5.22 CO₂-C·mg^-1 C_mic·h^-1) were lowest in low productivity (Class 1) sites. Bacteria to fungi ratios were significantly lower (P = 0.05) in Class 1 compared to other classes. Microbial biomass ranged from 1.39 to 8.11 mg C_mic·g^-1 soil. Thirty-seven distinct aspen ectomycorrhizas (ECM) were characterized, 21 were considered rare (from ≤3 trees). ECM richness did not differ significantly between classes, although relative abundance for some types did. Canonical correspondence analysis showed productivity class explained most microbial community variation, e.g., ECM fungi (80% explained) and soil bacteria (46%). Despite some differences, we could not identify statistically significant bacterial or ECM assemblages linked to stand productivity. Results may reflect a strong association between microbial processes and the dominant host, aspen. Aspen associated with widely distributed fungi common to all classes, possibly facilitating its survival and growth, including on sites exhibiting low pH and low soil fertility.

Interspecific interactions often determine the structure and stability of biotic communities. In low-productivity and highly seasonal environments such as the alpine tundra, most interactions occur during a short, snow-free period. The strength and direction of these interactions are likely to be determined by the availability of resources, particularly among species of the same ecological guild. Understanding how species interact in such environments can provide insights into the conditions that facilitate their coexistence. We determined the potential for interspecific interactions among 3 resident medium-sized mammalian herbivores inhabiting the alpine tundra and investigated how they share available space and resources. Overlap in their respective activity areas indicated that these species were aggregated at a landscape scale, but other mechanisms allowed their coexistence at a finer scale. Their distributions were primarily associated with shorter distances to heterospecifics and, secondly, with habitat features related to shelter and escape from predation. Our results suggest that these species can (and do) coexist by partitioning their ecological niches. Competition is likely not a major factor in structuring these communities; in turn, facilitative mechanisms may allow co-occurrence of these sympatric herbivores in seasonal, low productivity environments.

Many high-altitude summits across the boreal forest zone of Quebec are colonized by tundra vegetation. In this study, the origin and plant composition of these remote, isolated tundra summits have been documented to link their nature and floristic diversity to several potential causal factors. Analysis of spruce macrocharcoal pieces distributed at the soil surface indicates that wildfire is the chief factor behind the creation of the tundra summits across the boreal forest zone. All of the summits were deforested by fire during 2 main periods of the Holocene, around 100–500 cal y BP and 1150–1600 cal y BP. However, fire activity seems to have little influence on vegetation composition and diversity of post-fire tundra summits; latitudinal position and surface area are the main driving factors influencing floristic diversity. Given the remoteness of the sources of the arctic—alpine flora and young age of tundra summits associated with recent deforestation, only a small number of arctic—alpine species are colonizing the sites. Because of the regional dominance of boreal flora composed of common and widespread species adapted to nutrient-poor soils, it is probable that arctic—alpine species located on the tundra summits will go extinct in a warmer world promoting forest recovery.

Climate-induced changes in the tundra fire regime are expected to alter shrub abundance and distribution across the Arctic. However, little is known about how fire may indirectly impact shrub performance by altering mycorrhizal symbionts. We used molecular tools, including ARISA and fungal ITS sequencing, to characterize the mycorrhizal communities on resprouting Betula nana shrubs across a fire-severity gradient after the largest tundra fire recorded in the Alaskan Arctic (July—October 2007). Fire effects on the components of fungal composition were dependant on the scale of taxonomic resolution. Variation in fungal community composition was correlated with fire severity. Fungal richness and relative abundance of dominant taxa declined with increased fire severity. Yet, in contrast to temperate and boreal regions with frequent wildfires, mycorrhizal fungi on resprouting shrubs in tundra were not strongly differentiated into fire-specialists and fire-sensitive fungi. Instead, dominant fungi, including taxa characteristic of late successional stages, were present regardless of fire severity. It is likely that the resprouting life history strategy of tundra shrubs confers resilience of dominant mycorrhizal fungi to fire disturbance by maintaining an inoculum source on the landscape after fire. Based on these results, we suggest that resprouting shrubs may facilitate post-fire vegetation regeneration and potentially the expansion of trees and shrubs under predicted scenarios of increased warming and fire disturbance in Arctic tundra.

Browsing by cervids plays a key role in structuring forest ecosystems and dynamics. Many boreal forest systems are managed for timber resources, and at the same time the wild cervid populations are also harvested. Thus, the determination of sustainable densities of cervids for the purpose of forest and game management is challenging. In this study we report on a red deer (Cervus elaphus) exclosure experiment in the mature forests of Western Norway. Ten pairs of exclosures and browsed plots were initiated in 2008. The rate of browsing and height growth of marked individuals was recorded annually, and the total densities of all tree species assessed over the following 4 y. We found that height growth of rowan (Sorbus aucuparia) saplings (1 m tall), the most numerous tree species at the site, was prevented when 20% of the shoots were browsed. Outside of the exclosures, net height growth of rowan saplings tended to be positive when trees were below 40 cm in height, but growth was constrained in rowan saplings over this height. The density of rowan also increased in both treatments, showing that recruitment was occurring, but the increase was greater where browsed than in the exclosure. The increase in density of rowan, combined with the curtailment of height growth in the presence of red deer, serves to create a carpet of short stature rowan saplings. This has parallels with the browsing lawn concept, but it seems to occur in interaction with snow depth; individuals protruding above the snow layer are likely to be browsed during the winter, whilst smaller individuals are protected during this season, when browsing is at its peak.