We used multivariate analysis to model boreal forest stand structure and dynamics at Riding Mountain National Park, Manitoba based on data from 202 sampled stands. Eight forest stand-types were recognized based on canopy composition: black spruce on peat substrates, jack pine - black spruce, bur oak, eastern deciduous (green ash - American elm - Manitoba maple), balsam fir, trembling aspen - paper birch - mountain maple, trembling aspen - balsam poplar, and white spruce. The first four stand-types occur in edaphically distinct environments, while the four remaining boreal mixedwood stand-types occur in edaphically similar environments. We found that the composition and abundance of advance regeneration were best predicted by current canopy composition (redundancy = 54.4%); this reflects both the limited dispersal of conifer seeds and the strong vegetative reproductive capacity of hardwoods. Biotically-controlled site factors such as bareground, herb and shrub cover, ungulate browsing intensity, and stand age were also reasonably good predictors f advance regeneration (redundancy = 31.7%). Edaphic variables such as soil pH, conductivity, particle size, organic horizon depth and slope proved to be poor predictors of advance regeneration, however (redundancy = 18.1%). Size-class ordination indicated that many stand-types have relatively short successional trajectories, suggesting limited change in forest canopy composition over time. There are two exceptions: in the jack pine - black spruce stand-type, black spruce will increase over time, and in the trembling aspen - paper birch - mountain maple stand-type, eastern deciduous species (green ash, American elm, Manitoba maple, and bur oak) are forecast to become increasingly dominant. We also describe a synoptic model of mixedwood boreal forest stand dynamics for the Riding Mountain area. The model includes a number of factors that we consider to be critical determinants of forest dynamics, such as seed source availability, small and large-scale disturbances, species life-history characteristics, and environmental gradients. Our succession model is more similar those described for eastern than western Canada, which may reflect the lower frequency of catastrophic fires in the Riding Mountain area compared to boreal forests further west. Our model emphasizes that successional trajectories do not converge towards a single self-perpetuating "climax". Instead, successional vectors may diverge, converge or remain cyclical, and multiple potential pathways are possible for each stand-type. Our results also illustrate that species assemblages, and the propensity for canopy change in the absence of fire, are governed by the cumulative and synergistic effects of climate, topography, disturbance frequency, size and intensity, edaphic conditions, and the proximity of parental seed sources. Fire suppression in the southern boreal forest has resulted in a paradigm shift in disturbance regime, from large, synchronous catastrophic fires to small-scale, asynchronous gap formation. A major challenge for boreal forest ecologists is to determine the long-term consequences of this paradigm shift on the composition, structure and health of boreal forest stands and landscapes.
Authors:S. Ibanez, M. Bison, S. Lavorel, and M. Moretti
Herbivory is a major driver of plant communities. Most herbivores preferentially consume dominant species and slow down plant succession, but it remains unclear in which ways different herbivore species have contrasting effects on plant communities. In this study, we investigated the extent to which closely related insect herbivores with slight differences in their feeding behaviour induce contrasting reactions in plant communities. We studied the impact of herbivory by two Gomphocerinae grasshopper species, Chorthippus scalaris and Euthystira brachyptera, on the outcome of competition between the plant species Dactylis glomerata and Festuca paniculata. Under a controlled choice experiment, C. scalaris preferentially consumed D. glomerata while E. brachyptera preferred F. paniculata, but in an experimental plant community (mesocosm) both species consumed the plant species proportionally to their abundance and the amount of herbivory marks detected on the leaves did not depend on the specific grasshopper species. The herbivory pressure of both grasshopper species significantly reduced the vegetative height and the number of tillers of F. paniculata, with C. scalaris showing a stronger effect. As a consequence, herbivory by E. brachyptera did not significantly affect interspecific plant competition, whereas C. scalaris enhanced the dominance of D. glomerata. Our study shows that closely related herbivores that only slightly differ in their feeding behaviour can induce contrasting effects on interspecific plant competition, and that the dominant species D. glomerata is more tolerant to herbivory than F. paniculata. The specific plant and herbivore traits responsible for contrasting herbivory effects on plant competition remains to be explored.
The present study uses transition matrices to compare successional processes (colonization, disturbance, persistence and replacement) of fouling communities submitted to different light effects on Cabo Frio Island, a seasonal upwelling region. Twelve functional groups were identified, and differences in the transition probabilities shown by the matrices suggest a preference for the replacement property of functional groups, which indicates the facilitation successional mechanism. The probability of colonization of these groups differed according to the direction of the substrate, which caused a negative effect of light reduction on algae with a greater probability of disturbance (sensu species replacement), which is typical of a more stressful environment. Species of the same functional group replace each other through competition and herbivory, which promotes the distinction between earlier and later groups on the successional process. Successional trajectories evaluated through global transition matrices change at each time step because they depend on the species turnover rate, and therefore, they are informative of the changing processes of the community. The probabilistic rate of changes related to successional processes may be used to evaluate future conditions of fouling communities, and the deterministic components and stochastic elements will render these communities self-organizable.
Authors:S. Burrascano, R. Copiz, E. Del Vico, S. Fagiani, E. Giarrizzo, M. Mei, A. Mortelliti, F. M. Sabatini, and C. Blasi
In recent decades, the European populations of wild boar have grown substantially, as has the impact of this species, owing above all to its rooting activity. Our aim was to investigate the relationships between vascular plant understorey and wild boar rooting intensity. The questions we addressed are: does rooting intensity influence understorey species composition and diversity? Which functional traits are associated with different levels of rooting? We performed a comparative analysis of plant communities in areas with contrasting levels of rooting intensity within a Mediterranean deciduous lowland forest in central Italy. Besides comparing species composition and diversity, we tested the association between species traits and rooting levels through fourth-corner analysis. We found that contrasting levels of rooting were associated to different understorey species composition and evenness, while we observed no significant difference in species richness. In contrast with our expectations, sites with lower rooting returned i) lower evenness values and ii) a higher proportion of species characterized by traits related to resistance or response to herbivory, i.e., spinescence, clonality, endozoochory, underground storage organs, and low height values. Our findings suggest that current vegetation patterns partly depend on the legacy effect of past rooting disturbance, since the areas currently subjected to low rooting intensity were likely to be intensely rooted in the past. These areas may have developed a marked dominance of clonal thorny species that, in turn, inhibited further feeding activities by wild boar.
The study of invasion ecology usually focuses on the negative impacts of alien species, while potential positive impacts are often overlooked. Understanding of biotic interactions may thus be skewed towards the negative, which could have important implications for ecological management and conservation. This article provides a comprehensive review of all types of impacts, both beneficial and detrimental, that can result from species translocation. An extensive review of literature on species introductions to terrestrial, freshwater and marine ecosystems and involving a wide range of taxa (including microorganisms, parasites, plants, insects, amphibians, reptiles, birds, mammals, fish and Crustacea) showed that, despite limited research into facilitative alien-native interactions, such interactions occur surprisingly frequently. Examples were found of introduced species acting as hosts, food sources, pollinators or seed dispersers for native species, as well as providing herbivory, predatory or parasite release. However, research showed that numerous negative interactions also occurred and combination impacts (when an alien benefits some natives but disadvantages others) were common. In many cases, the traditional view that biological invasions constitute a significant threat to native biota is both accurate and appropriate. Efforts to prevent translocation and control non-native species can be vital. However, the “native good, alien bad” maxim does not convey the complexity of invasion ecology: alien species do not axiomatically pose a threat to native biota. In order to move understanding of invasion ecology forward and to develop maximally-effective management strategies, facilitative alien-native interactions need to be added into the alien species debate.
Testing the full impact of neighbourhood crowding within natural vegetation requires more than just effects incurred by established plants. It must also include measurements that take into account suppressive effects on the earliest plant life stages of resident individuals — seeds, their germination (emergence of radicles and cotyledons), and very young rooted seedlings. In this study, we explored the potential for these effects in a field experiment spanning three years, using a novel design for controlling granivory and small mammal herbivory. This allowed us to assess the limitations of natural crowding on seed recruitment success for non-resident species introduced into both natural and denuded neighbourhood plots within a temperate mesic old field meadow in eastern Ontario, Canada. Our results show that crowding by standing vegetation of resident species caused an overall reduction of seed recruitment success by more than 90%. These data provide strong inference that suppression resulting directly from near neighbour effects are likely to impose routinely intense natural selection within temperate mesic old field habitats like our study site. The consequences of this selection, in terms of traits promoting plant fitness under competition, are traditionally interpreted in terms of superior resource depletion/uptake, typically associated with greater growth accumulation and larger potential body size. We suggest, however, that these consequences are rare. Individuals of any species approach maximum potential body size only when near neighbour effects are relatively weak — not within crowded neighbourhoods. Recent studies suggest that severe neighbourhood crowding (where virtually all resident plants are forced to remain relatively small) selects instead for ‘reproductive economy’ — i.e., capacity to produce at least a few (or even at least one) offspring despite severe body size suppression, involving a relatively small minimum reproductive threshold size. Potential for additional component traits of reproductive economy are also suggested for investigation in future research.