Multiple successional pathways of boreal forest stands in central Canada

Predicting forest composition change through time is a key challenge in forest management. While multiple successional pathways are theorized for boreal forests, empirical evidence is lacking, largely because succession has been inferred from chronosequence and dendrochronological methods. We tested the hypotheses that stands of compositionally similar overstory may follow multiple successional pathways depending on time since last stand‐replacing fire (TSF), edaphic conditions, and presence of intermediate disturbances. We used repeated measurements from combining sequential aerial photography and ground surveys for 361 boreal stands in central Canada. Stands were measured in 8–15 yr intervals over a ~ 60 yr period, covering a wide range of initial stand conditions. Multinomial logistic regression was used to analyze stand type transitions. With increasing TSF, stands dominated by shade‐intolerant Pinus banksiana, Populus sp., and Betula papyrifera demonstrated multiple pathways to stands dominated by shade‐tolerant Picea sp., Abies balsamea, and Thuja occidentalis. Their pathways seemed largely explained by neighborhood effects. Succession of stands dominated by shade‐tolerant species, with an exception of stands dominated by Picea sp., was not related to TSF, but rather dependent on edaphic conditions and presence of intermediate disturbances. Varying edaphic conditions caused divergent pathways with resource limited sites being dominated by nutrient‐poor tolerant species, and richer sites permitting invasion of early successional species and promoting species mixtures during succession. Intermediate disturbances promoted deciduous persistence and species diversity in A. balsamea and mixed‐conifer stands, but no evidence was detected to support “disturbance accelerated succession”. Our results demonstrate that in the prolonged absence of stand‐replacing disturbance boreal forest stands undergo multiple succession pathways. These pathways are regulated by neighborhood effects, resource availability, and presence of intermediate disturbance, but the relative importance of these regulators depends on initial stand type. The observed divergence of successional pathways supports the resource‐ratio hypothesis of plant succession.     

Fifty‐seven years of composition change in the eastern boreal forest of Canada

Question: In the boreal forest of eastern Canada, how does forest vegetation change in the sustained absence of fire? Location: Eastern boreal forest in Quebec's North Shore region, Canada (49°30′–50°00′N; 67°30′–68°35′W). Methods: Aerial photos from three different periods (1930, 1965 and 1987) were used to characterize changes in vegetation composition in 23 scenes of 200 ha. Time since fire, presence of secondary disturbances and data on soil and topographic variables were obtained. Ordination and clustering techniques were used to define compositional trajectories of change over the 57‐yr period. These trajectories were further grouped into pathways based on compositional changes, time since fire and preferential deposit‐drainage types. Results: Among the 26 compositional trajectories, three successional pathways were distinguished. Two start post‐fire succession with a dominance of intolerant hardwood. In one of these, this is followed by an increase in Abies balsamea, while in the second the importance of Picea mariana increases with time. In the third pathway P. mariana is an important component from the outset. In this pathway, we observed modest fluctuation in the relative dominance of P. mariana and A. balsamea and variation in stand structure. Conclusion: The boreal forest vegetation of Eastern Canada is diverse and dynamic even in the absence of fire, notably under the influence of partial disturbances. Such disturbances can be associated with changes in composition or stand structure. The development of management strategies aimed at maintaining stand diversity by emulating a broader variety of partial and secondary disturbances should be encouraged.     

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed‐wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance‐caused mortality classes, canopy gap, moderate‐severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade‐tolerant, late‐successional conifer species remained dominant after canopy gap and moderate‐severity disturbances, whereas shade‐intolerant, early‐successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid‐tolerant and shade‐intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate‐severity disturbances. For the sub‐canopy tree and shrub community, relative species abundances were most similar after moderate‐severity disturbances and catastrophic fire. Vegetation responses to moderate‐severity disturbances thus had commonalities with both extremes of the disturbance‐caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio‐temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi‐cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.     

Using biodiversity deconstruction to disentangle assembly and diversity dynamics of understorey plants along post‐fire succession in boreal forest

Aim The study aims to decipher the co‐occurrence of understorey plant assemblages and, accordingly, to identify a set of species groups (diversity deconstruction) to better understand the multiple causal processes underlying post‐fire succession and diversity patterns in boreal forest. Location North‐eastern Canadian boreal forest (49°07′–51°44′ N; 70°13′–65°15′ W). Methods Data on understorey plant communities and habitat factors were collected from 1097 plots. Species co‐occurrence was analysed using null model analysis. We derive species groups (i.e. biodiversity deconstruction) using the strength of pairwise species co‐occurrences after accounting for random expectation under a null model and cluster analyses. We examine the influence of a set of spatiotemporal environmental variables (overstorey composition, time‐since‐fire, spatial location and topography) on richness of species groups using Bayesian model averaging, and their relative influence through hierarchical partitioning of variance. Results Understorey plant assemblages were highly structured, with co‐occurrence‐based classification providing species groups that were coherently aggregated within, but variably segregated between, species groups. Group richness models indicate both common and distinct responses to factors affecting plant succession. For example, Group 2 (e.g. Rhododendron groenlandicum and Cladina rangiferina) showed concurrent contrasting responses to overstorey composition and was strongly segregated from Groups 3 (e.g. Clintonia borealis and Maianthenum canadense) and 4 (e.g. Epilobium angustifolium and Alnus rugosa). Groups 3 and 4 showed partial similarity, but they differed in their response to time‐since‐fire, drainage and latitude, which were more important for Group 1 (e.g. Ptilium crista‐castrensis and Empetrum nigrum). A single successional model based on total richness masked crucial group‐level relationships with factors that we examined, such as latitude. Main conclusions By demonstrating the co‐occurrence structure and linking to causal factors, the results from this study characterize both common and distinct responses of understorey plants to biophysical attributes of sites, and potential interspecific interactions, behind non‐random assemblage structure during post‐fire succession. A biodiversity deconstruction approach could offer a concise and explicit framework to gain a better understanding of the complex assembly of ecological communities during succession.     

Long‐term persistence and spatial assortment of nonnative plant species in second‐growth forests

Long‐term conservation of forested areas requires an estimate of nonnative species’ impact on a scale of centuries. However, long‐term changes in forest structure have not been included in previous succession studies, which have typically covered only 20–50 yr. To estimate persistence of nonnative plant species on a longer time scale, and to examine the development of spatial structure in their distributions, we selected thirty‐seven second‐growth forest sites in southeastern Ohio, USA, to form a chronosequence spanning 160 yr. At each site nonnative species were surveyed in 100 contiguous plots forming a 50‐m belt transect. Nineteen nonnative species were encountered, largely consisting of shade‐intolerant herbs typical of abandoned agricultural land. Nonnative species richness, abundance, and frequency declined through the chronosequence with most species dropping out after the first twenty years, although three persisted 120–140 yr. Most species appeared to be residual populations from the open‐habitat stage. Presence of Rosa multiflora and Polygonum hydropiper in stands older than their date of arrival in the region indicate their capacity to colonize long‐established forest. Distributions of four species showed dependence on micro‐environmental gradients. All four were positively autocorrelated at scales up to 2–8 m, but pattern did not vary among age classes. We conclude that pattern in nonnative species’ distributions arises very early, possibly in the open‐habitat stage. Most species encountered in these sites are not likely to become long‐term components of the forest community and do not require active management. Small‐gap disturbances have not allowed colonization of long‐established forest stands, nor released pre‐existing populations from competitive suppression. Management should focus on the small number of shade‐tolerant species able to persist in long‐established forest.     

Long‐term post‐fire changes in the northeastern boreal forest of Quebec

Abstract. Natural dynamics in the boreal forest is influenced by disturbances. Fire recurrence affects community development and landscape diversity. Forest development was studied in the northeastern boreal forest of Quebec. The objective was to describe succession following fire and to assess the factors related to the changes in forest composition and structure. The study area is located in northeastern Quebec, 50 km north of Baie‐Comeau. We used the forest inventory data gathered by the Ministère des Ressources naturelles du Québec (MRNQ). In circular plots of 400 m², the diameter at breast height (DBH) of all stems of tree species greater than 10 cm was recorded and in 40 m² subplots, stems smaller than 10 cm were measured. A total of 380 plots were sampled in an area of 6000 km². The fire history reconstruction was done based on historical maps, old aerial photographs and field sampling. A time‐since‐fire class, a deposit type, slope, slope aspect and altitude were attributed to each plot. Each plot was also described according to species richness and size structure characteristics. Traces of recent disturbance were also recorded in each plot. Changes in forest composition were described using ordination analyses (NMDS and CCA) and correlated with the explanatory variables. Two successional pathways were observed in the area and characterized by the early dominance of intolerant hardwood species or Picea mariana. With time elapsed since the last fire, composition converged towards either Picea mariana, Abies balsamea or a mixture of both species and the size structure of the coniferous dominated stands got more irregular. The environmental conditions varied between stands and explained part of the variability in composition. Their effect tended to decrease with increasing time elapsed since fire, as canopy composition was getting more similar. Gaps may be important to control forest dynamics in old successional communities.     

The fluctuating world of a tundra predator guild: bottom‐up constraints overrule top‐down species interactions in winter

Global warming is predicted to change ecosystem functioning and structure in Arctic ecosystems by strengthening top‐down species interactions, i.e. predation pressure on small herbivores and interference between predators. Yet, previous research is biased towards the summer season. Due to greater abiotic constraints, Arctic ecosystem characteristics might be more pronounced in winter. Here we test the hypothesis that top‐down species interactions prevail over bottom‐up effects in Scandinavian mountain tundra (Northern Sweden) where effects of climate warming have been observed and top‐down interactions are expected to strengthen. But we test this ‘a priori’ hypothesis in winter and throughout the 3–4 yr rodent cycle, which imposes additional pulsed resource constraints. We used snowtracking data recorded in 12 winters (2004–2015) to analyse the spatial patterns of a tundra predator guild (arctic fox Vulpes lagopus, red fox Vulpes vulpes, wolverine Gulo gulo) and small prey (ptarmigan, Lagopus spp). The a priori top‐down hypothesis was then tested through structural equation modelling, for each phase of the rodent cycle. There was weak support for this hypothesis, with top‐down effects only discerned on arctic fox (weakly, by wolverine) and ptarmigan (by arctic fox) at intermediate and high rodent availability respectively. Overall, bottom‐up constraints appeared more influential on the winter community structure. Cold specialist predators (arctic fox and wolverine) showed variable landscape associations, while the boreal predator (red fox) appeared strongly dependent on productive habitats and ptarmigan abundance. Thus, we suggest that the unpredictability of food resources determines the winter ecology of the cold specialist predators, while the boreal predator relies on resource‐rich habitats. The constraints imposed by winters and temporary resource lows should therefore counteract productivity‐driven ecosystem change and have a stabilising effect on community structure. Hence, the interplay between summer and winter conditions should determine the rate of Arctic ecosystem change in the context of global warming.     

Bottom‐up effects on a plant‐endophage‐parasitoid system: The role of flower‐head size and chemistry

The effects of water and nutrient addition on a trophic chain were studied in a plant‐endophage‐parasitoid system comprised of insects associated with flower heads of Chromolaena squalida (Asteraceae). Nine species of endophages associated with C. squalida flower heads were found, belonging to two families of Diptera – Tephritidae (Cecidochares sp1, Cecidochares sp2, Xanthaciura biocellata, X. chrysura, X. sp. and Neomyopites sp.) and Agromyzidae (Melanagromyza sp.), and two families of Coleoptera – Apionidae (Apion sp.) and Anthicidae (Anthicidae sp.). A factorial field experiment with water and nutrient addition showed that resource availability can affect the developmental process of flower heads. Fertilization increased flower‐head diameter and nitrogen and alkaloid concentrations. Although nutrient availability affected the size and chemistry of flower heads, endophage species did not respond consistently to the experimental treatments. This is contrary to other studies where endophages showed preference for larger flower heads. Thus, the plant vigour hypothesis was not corroborated for our study system. Our results also showed that coupled responses of plants to resource availability (i.e. tissue nutritional quality and investment in growth of the structure that serves as shelter for endophages) can represent distinct kinds of indirect interactions with opposing effects on the herbivore‐parasitoid interaction.     

The effect of manipulated shading on the colony abundance of two species of ants,Formica aserva and Leptothorax muscorum,in dead wood

Ants (Hymenoptera: Formicidae) are known to be a thermophilic group with fewer species at higher latitudes. They are ubiquitous in the sub‐boreal forests of west‐central British Columbia (Canada) in the early seral ages of forest succession, nesting almost exclusively in woody debris. Individual species begin to disappear with advancing seral ages as the canopy closes, suggesting the hypothesis that heat gain from insolation is critical to many species. We experimentally manipulated shading of two wood‐nesting ant species –Leptothorax muscorum (Nylander) and Formica aserva Forel – to emulate advancing seral age, in an environment documented to support their normal habitation. Shaded down woody debris (DWD) was found to be ca. 2 °C cooler than unshaded DWD, whereas stumps were ca. 1.5 °C cooler when shaded than when unshaded. We found significantly fewer colonies of both L. muscorum and F. aserva in shaded wood, as compared to unshaded, in a pattern consistent with how each species has been observed to respond to advancing seral age. Temperature monitoring during the experiment indicated that soil was cooler than either stumps or DWD, possibly explaining the absence of soil‐nesting ant species in these forests. The species‐specific differences in ant colony abundance in response to small temperature differences, found in this study, support other research on the significance of temperature in structuring ant assemblages.     

The role of resource dispersion in promoting the co‐occurrence of dominant and subordinate ant species

Dominant competitors govern resource use in many communities, leading to predictions of local exclusion and lower species diversity where dominant species are abundant. However, subordinate and dominant species frequently co‐occur. One mechanism that could facilitate resource sharing and co‐occurrence of dominant and subordinate competitors is fine‐scale resource dispersion. Here, we distributed 6 g of a food resource into 1, 2, 8, 32 or 64 units in small 0.40 m² areas centred on nests of the dominant ant Monomorium sydneyense. We tested three hypotheses. First, we hypothesized that the species richness and abundance of foraging ants would increase with increasing resource dispersion. Accordingly, species richness doubled and total ant abundance was two orders of magnitude higher in high resource dispersion treatments. Secondly, we hypothesized that increasing resource dispersion would reduce competitive interactions such as resource turnover events and lower the probability of food resources being occupied. Substantial support for this hypothesis was observed. Finally, we tested the hypothesis that the foraging time of each species would be proportional to the relative abundance of each species solely in high resource dispersion treatments. Expected and observed foraging times were statistically similar for only the dominant ant M. sydneyense. The subdominant Pheidole rugosula increased its foraging time much more than was expected, while two subordinate ants showed no relationship between observed and expected times. Thus, while increasing resource dispersion significantly increased overall species richness, this increase in co‐occurrence did not correlate with a significant increase in foraging time for the two subordinate species. Rather, changes in resource dispersion appeared to benefit only the subdominant species. Inter‐site variation appeared more important for other subordinate species indetermining co‐occurrence and foraging time. Multiple mechanisms facilitate co‐occurrence and resource sharing in this community, and probably in most other communities.     

Disturbance‐induced changes in a high‐alpine cushionfield community,south‐central New Zealand

Long‐term effects of two nearby disturbances, mechanical blading 34 years ago and an approximately century‐old sheep graveyard, in high‐alpine cushionfield share certain vegetation features despite their contrasting soil nutrient status. Secondary succession following blading has continued beyond that recorded over the first 24 years with continued trends towards the adjacent undisturbed cushionfield: increased cover of several cushion species and the lichen Cetraria islandica ssp. antarctica. Several mid‐seral species remain prominent while bare soil and stone pavement continue to decrease. Additional shelter, facilitated by tall snow tussocks planted locally at near‐natural density on part of the bladed site, allowed establishment of some larger, thin‐leaved species. Soil factors remain little affected by the mechanical disturbance compared with the general enrichment persisting at the graveyard. Here three small indigenous grasses, which dominated 50 years earlier (Trisetum spicatum, Agrostis muelleriana, Poa colensoi), remain prominent among the relatively diverse community (50 species, including several calciphilic lichens on the degrading bones). By contrast, frequency of the cushionfield dominant, Dracophyllum muscoides and the lichen Alectoria nigricans, are negatively correlated with the presence of bones. The post‐disturbance temporal community changes are assessed in the context of the initial productivity of the habitat, competitive exclusion and the intermediate disturbance hypothesis.     

A Field Test of the Intermediate Disturbance Hypothesis in the Soft‐Bottom Intertidal

The intermediate disturbance hypothesis has had success in explaining changes in local diversity in many habitats. Recent laboratory work has shown that disturbance may act to increase diversity in soft‐bottom marine communities as predicted by the hypothesis. In this paper, we present the results of a field experiment which tested the impacts of physical disturbance on soft‐bottom, intertidal macrobenthic communities. Five disturbance treatments were used, differing in the frequency of applied disturbance events; the highest frequency treatments were dug every week, lowest every eight weeks. The experiment was run for a total of 25 weeks over the winter of 1997/98. Our experiment controlled for differences in recovery time after disturbances. Abundances of Pygospio elegans, Macoma balthica, Hydrobia ulvae and Streblospio benedicti were all significantly reduced in high disturbance treatments, as was the total number of species. No species showed significant increases in abundance in disturbed treatments, and there was no evidence of an increase in diversity in any treatments. These results are discussed in the context of the intermediate disturbance hypothesis. Our results suggest that the interspecific competitive effects postulated by the hypothesis are not important in structuring this low diversity, intertidal community. However, unequivocally rejecting the hypothesis is difficult because it contains many ambiguities, and acts more as a conceptual model than as a falsifiable hypothesis.     

Comparison of invertebrate herbivores on native and non‐native Senecio species: Implications for the enemy release hypothesis

The enemy release hypothesis posits that non‐native plant species may gain a competitive advantage over their native counterparts because they are liberated from co‐evolved natural enemies from their native area. The phylogenetic relationship between a non‐native plant and the native community may be important for understanding the success of some non‐native plants, because host switching by insect herbivores is more likely to occur between closely related species. We tested the enemy release hypothesis by comparing leaf damage and herbivorous insect assemblages on the invasive species Senecio madagascariensis Poir. to that on nine congeneric species, of which five are native to the study area, and four are non‐native but considered non‐invasive. Non‐native species had less leaf damage than natives overall, but we found no significant differences in the abundance, richness and Shannon diversity of herbivores between native and non‐native Senecio L. species. The herbivore assemblage and percentage abundance of herbivore guilds differed among all Senecio species, but patterns were not related to whether the species was native or not. Species‐level differences indicate that S. madagascariensis may have a greater proportion of generalist insect damage (represented by phytophagous leaf chewers) than the other Senecio species. Within a plant genus, escape from natural enemies may not be a sufficient explanation for why some non‐native species become more invasive than others.     

Abundance and diversity of birch‐feeding leafminers along latitudinal gradients in northern Europe

Latitudinal patterns in biotic interactions, including herbivory, have been widely debated during the past years. In particular, recent meta‐analysis questioned the hypothesis that herbivory increases from the poles towards the equator. Our study was designed to verify this hypothesis by exploring latitudinal patterns in abundance and diversity of birch‐feeding insect herbivores belonging to the leafminer guild in northern Europe, from 59° to 69°N. We collected branches from five mature trees of two birch species (Betula pendula and B. pubescens) at each study site (ten sites for each of five latitudinal gradients) twice per season (in early and late summer of 2008–2011) and attributed all mines found on leaves of these branches to a certain taxon of insects. Latitudinal patterns were quantified by calculating Spearman rank correlation coefficients between both abundance and diversity of leafmining taxa and latitudes of sampling sites. In general, both abundance and diversity of leafminers significantly decreased with latitude. However, we discovered pronounced variation in patterns of latitudinal changes among study years and leafminer taxa. Variation among study years was best explained by mean temperatures in July at the northern ends of our gradients. During cold years, abundance of leafminers significantly decreased with latitude, while during warm years the abundance was either independent of latitude or even increased towards the pole. In the northern boreal forests (66° to 69°N), herbivores demonstrated larger changes in densities in response to temperature variations than in the boreo‐nemoral forests (59° to 62°N). Our data suggest that climate warming will result in a stronger increase in herbivory at higher latitudes than at lower latitudes.     

Long‐term tree fern dynamics linked to disturbance and shade tolerance

Question: Do New Zealand tree ferns have recognizable shade tolerance niches? Location: Lowland temperate rain forest of New Zealand (41°20′S, 174°58′E). Methods: Growth, death and recruitment of five tree fern species were estimated from a 38‐year record of stem heights, collected within a 2.25‐ha block of forest, and electron transport rates (ETR) of photosystem II of fronds were measured. Results: Two species of Cyathea were comparatively common (603 and 351 stems in total) and two were comparatively rare (155 and 17 stems in total) on the site. The common species had lower rates of growth, recruitment and mortality than the rare species, had skewed age distributions typical of shade‐tolerant species and were probably recruited soon after a catastrophic earthquake in 1855. The two rare species were failing to recruit under closed forests; their age distributions indicated that all had regenerated long after the earthquake. ETR were higher for faster‐growing than for the shade‐tolerant species. A tree fern that regenerates vegetatively from aerial buds, Dicksonia squarrosa, was common on the site (361 stems in total). Its age distribution suggested it was relatively shade tolerant, but its mortality and recruitment rates were much higher than those of the two shade‐tolerating Cyathea species, suggesting that this multi‐stemmed species functions differently from the monopodial Cyathea species. Conclusions: New Zealand Cyathea tree ferns occupy distinct niches along a shade tolerance spectrum and their relative abundances are strongly influenced by disturbance history. The study provides evidence that tree fern species differ strongly in their responses to canopy disturbance and are not ecologically equivalent.     

A long‐term record of Nothofagus dominance in the southern Andes,Chile

Abstract The general model of regeneration dynamics in Nothofagus forests of southern South America could have value in community ecology if predictive relationships between disturbance history, functional traits and site attributes could be identified. Examined here is the proposal that on favourable sites shade‐intolerant Nothofagus are likely not to survive in competition with shade‐tolerant, broad‐leaved evergreen taxa of temperate rain forests, and persistence, thus, is dependent on periodic coarse‐scale disturbance. Comparison of stand dynamics of three old‐growth Nothofagus forests at different elevations in the southern Andes, Chile where deciduous Nothofagus alpina dominates the upper canopy, and examination of the life history trade‐offs of this variation were made. Stem density of all stems ≥5.0 cm d.b.h. was 233–303 stems per hectare, and basal area was 123.9–171.0 m²ha^?1. Maximum lifespan of N. alpina was found to be greater than ca 640 years, exceeding all previously reported ages for this species in the region. Forests had a stable canopy composition for this long‐term, but some appeared to lack effective regeneration of N. alpina in recent years. Regeneration of N. alpina was generally greater in disturbed stands and higher elevation than in undisturbed stands and at lower elevation. Recruitment emerged to be strongly affected by competitive over‐ and understorey associates. There was a gradient of increasing dependence of N. alpina on disturbance towards the more productive end of the environment gradients, and hence less dependence of N. alpina on disturbance for its regeneration towards higher elevation. The study confirms that changes in forest composition may be explained by processes occurring in accordance with the predictions of the existing model of Nothofagus regeneration dynamics, providing stronger evidence specifically directed at mid‐tolerant N. alpina, and by factoring out regeneration dynamics on favourable sites. Thus, for N. alpina, trait differences probably contribute to the competitive advantage over its associates in productive habitats, and may be linked to small‐to‐intermediate‐sized disturbances which inevitably occur as older trees die, enabling N. alpina to persist in forests and therefore maintain species coexistence for the long‐term.     

Succession in sub‐boreal forests of West‐Central British Columbia

Abstract. Structural and compositional changes were analysed over the course of 400+ yr of post‐fire succession in the sub‐boreal forests of west‐central British Columbia. Using a chronosequence of 57 stands ranging from 11 to 438 yr in age, we examined changes in forest structure and composition with complementary PCA and DCA ordination techniques. To determine stand ages and timing of tree recruitment, approximately 1800 trees were aged. Most early successional forests were dominated by Pinus contorta, which established rapidly following fire. Abies lasiocarpa and Picea glauca × engel‐mannii were also able to establish quickly, but continued to establish throughout the sere. Few Pinus contorta survived beyond 200 yr, resulting in major changes in forest structure. In some stands P. contorta never established, which led to considerable variation among stands less than 200 yr old. The oldest forests converged on dominance by Abies lasiocarpa. Vascular plant diversity decreased during succession whereas canopy structure became more complex as gap dynamics developed. Although these sub‐boreal forests contain few tree species, successional changes were pronounced, with structure changing more than composition across the chronosequence.     

Effects of landslides on the mountain vegetation of Flores Island,Azores

Question: What are the consequences of frequently occurring landslides on vegetation dynamics, floristic and structural diversity? Location: 39°27′N; 31°13′W – Morro Alto, Flores Island, Azores, Portugal. Methods: Six comparable landslides were selected. Plots were placed at the top, slope and toe of landslides. Data on floristic composition and biovolume, demography and size structure of the dominant tree species (Juniperus brevifolia) were collected. Hierarchical agglomerative clustering and Principal Component Analysis were used in order to identify succession stages and compare succession pathways and vegetation recovery in different parts of the landslides. Results: Four stages of primary succession on substrates formed by landslides were identified: pioneer (Festuca‐Sphagnum grassland), assembly (Juniperus‐Festuca‐Sphagnum open scrub), building (Juniperus‐Sphagnum scrub) and mature (Juniperus‐Sphagnum woodland). Concerning J. brevifolia populations, the succession pathways are independent of location on the landslide. However, at the floristic level, there are some differences, mainly in the pioneer stage at the toes of landslides. Better abiotic conditions, resulting in a higher succession rate, are probably responsible for a faster vegetation recovery on landslide toes. Conclusion: Landslides trigger succession processes that enable a massive regeneration of the dominant tree species and existence of species not present in mature forests. They are also responsible for the simultaneous occurrence of vegetation of different structures. Overall, landslides increase the floristic and structural diversity of the vegetation, consequently increasing landscape heterogeneity.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Post‐fire succession and 20th century reduction in fire frequency on xeric southern Appalachian sites

Abstract. We document post‐fire succession on xeric sites in the southern Appalachian Mountains, USA and assess effects of 20th century reduction in fire frequency on vegetation structure and composition. Successional studies over 18 yr on permanent plots that had burned in 1976–1977 indicate that tree mortality and vegetation response varied with fuel load and fire season. In the first three years after fire, hardwood sprouts dominated tree regeneration. On sites where summer and autumn fires reduced litter depth to less than 1 cm, densities of shade‐intolerant Pinus seedlings increased steadily over this period. 4 to 8 yr after fire, large numbers of newly established seedlings and sprouts had grown to 1 – 10 cm DBH. By year 18 growth of these saplings led to canopy closure on most sites. Herbaceous cover and richness peaked in the first decade after fire, then declined. On similar sites that had not burned in more than 50 yr, regeneration of shade‐intolerant Pinus spp. and mean cover and richness of herbs were considerably lower than those observed on recently burned plots. Reconstructions of landscape conditions based on observed post‐fire succession and 20th century changes in fire regime suggest that reductions in fire frequency circa 1940 led to substantial changes in forest structure and decreases in cover and richness of herbaceous species.