Understory Plant Community Composition Research Articles

Variation in the Drought Tolerance of Tropical Understory Plant Communities across an Extreme Elevation and Precipitation Gradient.

Little is known about how differences in water availability within the "super humid" tropics can influence the physiology of understory plant species and the composition of understory plant communities. We investigated the variation in the physiological drought tolerances of hundreds of understory plants in dozens of plant communities across an extreme elevation and precipitation gradient. Specifically, we established 58 understory plots along a gradient of 400-3600 m asl elevation and 1000-6000 mm yr-1 rainfall in and around Manu National Park in southeastern Peru. Within the plots, we sampled all understory woody plants and measured three metrics of physiological leaf drought tolerance-turgor loss point (TLP), cuticular conductance (Gmin), and solute leakage (SL)-and assessed how the community-level means of these three traits related to the mean annual precipitation (MAP) and elevation (along the study gradient, the temperature decreases linearly, and the vapor pressure deficit increases monotonically with elevation). We did not find any correlations between the three metrics of leaf drought tolerance, suggesting that they represent independent strategies for coping with a low water availability. Despite being widely used metrics of leaf drought tolerance, neither the TLP nor Gmin showed any significant relationships with elevation or the MAP. In contrast, SL, which has only recently been developed for use in ecological field studies, increased significantly at higher precipitations and at lower elevations (i.e., plants in colder and drier habitats have a lower average SL, indicating greater drought tolerances). Our results illustrate that differences in water availability may affect the physiology of tropical montane plants and thus play a strong role in structuring plant communities even in the super humid tropics. Our results also highlight the potential for SL assays to be efficient and effective tools for measuring drought tolerances in the field.

Applying climate change refugia to forest management and old-growth restoration.

Recent studies highlight the potential of climate change refugia (CCR) to support the persistence of biodiversity in regions that may otherwise become unsuitable with climate change. However, a key challenge in using CCR for climate resilient management lies in how CCR may intersect with existing forest management strategies, and subsequently influence how landscapes buffer species from negative impacts of warming climate. We address this challenge in temperate coastal forests of the Pacific Northwestern United States, where declines in the extent of late-successional forests have prompted efforts to restore old-growth forest structure. One common approach for doing so involves selectively thinning forest stands to enhance structural complexity. However, dense canopy is a key forest feature moderating understory microclimate and potentially buffering organisms from climate change impacts, raising the possibility that approaches for managing forests for old-growth structure may reduce the extent and number of CCR. We used remotely sensed vegetation indices to identify CCR in an experimental forest with control and thinned (restoration) treatments, and explored the influence of biophysical variables on buffering capacity. We found that remotely sensed vegetation indices commonly used to identify CCR were associated with understory temperature and plant community composition, and thus captured aspects of landscape buffering that might instill climate resilience and be of interest to management. We then examined the interaction between current restoration strategies and CCR, and found that selective thinning for promoting old-growth structure had only very minor, if any, effects on climatic buffering. In all, our study demonstrates that forest management approaches aimed at restoring old-growth structure through targeted thinning do not greatly decrease buffering capacity, despite a known link between dense canopy and CCR. More broadly, this study illustrates the value of using remote sensing approaches to identify CCR, facilitating the integration of climate change adaptation with other forest management approaches.

Contrasting influences of two dominant plants, Dasiphora fruticosa and Ligularia virguarea, on aboveground and belowground communities in an alpine meadow.

Soil organisms are abundant, phylogenetically and functionally diverse, and interact to catalyse and regulate critical soil processes. Understanding what structures belowground communities is therefore fundamental to gaining insight into ecosystem functioning. Dominant plants have been shown to influence belowground communities both directly and indirectly through changes in abiotic and biotic factors. In a field study, we used piecewise structural equation modelling to disentangle and compare the effects of a dominant allelopathic plant, Ligularia virgaurea, and a dominant facilitative plant, Dasiphora fruticosa, on understory plant, soil microbial and nematode community composition in an alpine meadow on the Tibetan plateau. Dasiphora fruticosa was associated with changes in edaphic variables (total nitrogen, soil organic carbon, pH and ammonium), understory plant and soil bacterial communities, whereas Ligularia virguarea was associated with increased soil ammonium content and soil fungal richness relative to dominant plant-free control plots. Moreover, nematode richness was significantly greater under D. fruticosa, with no change in nematode community composition. By contrast, nematode richness under Ligularia virgaurea was similar to that of dominant plant-free control plots, but nematode community composition differed from the control. The effects of both plants were predominantly direct rather than mediated by indirect pathways despite the observed effects on understory plant communities, soil properties and microbial assemblages. Our results highlight the importance of plants in determining soil communities and provide new insight to disentangle the complex above- and belowground linkages.

Oak savanna vegetation response to layered restoration approaches: Thinning, burning, and grazing

Temperate savannas are unique, biodiverse ecosystems that have undergone extensive habitat conversion globally. In the midwestern United States, 99% of historic oak savanna area has been lost. Most remaining patches of savanna are degraded due to woody encroachment following the removal of both fire and large herbivore disturbances from the landscape. Restoring degraded savanna remnants is challenging because we lack an understanding of how to best apply contemporary restoration tools to mimic historic disturbance dynamics. To that end, we evaluated the outcomes of ongoing oak savanna restorations that have received a gradient of restoration actions: 1) no management, 2) tree thinning, 3) thinning + burning, and 4) thinning + burning + cattle grazing. We assessed several metrics of restoration success including canopy, shrub, herbaceous, and non-native cover, herbaceous diversity, and plant community composition. We found that layering restoration approaches achieved certain, but not all, structural vegetation goals. Compared to no management, thinning and fire successfully increased canopy openness, herbaceous cover, and herbaceous diversity, but had the unwanted effect of increased shrub cover. The addition of low-intensity cattle grazing did not improve structural outcomes. We also found that each restoration treatment left a unique signature on understory plant community composition. Unmanaged and thin-only treatments were characterized by tree saplings and woodland herbs, while burned and grazed treatments were defined by shrubs and savanna-associate species. We conclude that reintroducing multiple disturbances does not guarantee the successful restoration of disturbance-dependent ecosystems such as oak savannas. Restoration outcomes are not dictated by how many management approaches are applied, but rather, the nuances of how they are applied such as burn season and livestock density.

Direct and indirect interactions between shrubs and the flowering annual community in an arid ecosystem

Shrubs commonly shape the plant community in arid ecosystems such as deserts. Plant interactions between shrubs and the associated species can be both negative and positive, and direct and indirect. The importance of shrub effects on reproduction is also a key component of facilitation studies in deserts. Here, we tested the hypothesis that shrubs facilitate understory plant community composition and reproduction through both direct and indirect interactions mediated by the associated plant neighbourhood. Using the shrub species Larrea tridentata, we tested the effects of facilitation on understory plant density, floral density, and species richness in the Mojave Desert, California. We found that shrubs decreased the net community-wide plant assembly measures relative to open microsites including total density and species richness at the site tested. There was also evidence that shrubs directly decreased plant fitness by reducing floral density, but this effect depended on the species. The net density and diversity of the annual plant community also influenced plant and floral density under shrubs suggesting significant indirect effects between shrubs and all abundant annual plant species. While both direct and indirect interactions between shrubs and the annual plant community were species-specific, the effects of these interactions had greater impact on plant density than floral density measures. Annual plant density and to a lesser extent floral density are thus driven by both direct and indirect interactions with shrubs.

Increasing the intensity of regeneration treatments decreased beta diversity of temperate hardwood forest understory 20 years after disturbance

ContextIn temperate hardwood forests, increased intensity of soil and canopy disturbances tends to increase species richness due to the establishment of numerous early-successional plant species. However, while competitive pioneer species from early stages of succession can become recalcitrant and alter patterns of natural regeneration, very few studies have examined longer-term effects of these treatments on plant biodiversity.AimsIn this study, we investigated mid-term (ca. 20 years) effects of different regeneration treatments with varying soil and canopy disturbance intensities. We compared understory plant communities in temperate hardwood forests from all the South of Quebec (Canada).MethodsUsing circular experimental plots of 1962.5 m2 (radius = 25 m), we measured taxonomic and functional diversity indices and soil properties using four levels of disturbance intensity in six temperate hardwood forests of Quebec distributed along a longitudinal gradient. Reference forests, i.e. control forests with no silvicultural treatment known for ≥ 80 years, were compared to 20-year-old single-tree selection cuts, group-selection cuts and group-selection cuts with soil scarification.ResultsSpecies richness in both group-selection treatments was higher than that in reference forests. Plant equitability and beta diversity among sites in both group-selection treatments were lower than in single-tree selection cuts and control forests. More intense treatments contributed to the mid-term persistence of recalcitrant competitor species (e.g. Rubus idaeus L., Prunus pensylvanica L.f.) whereas soil scarification appears to have negative sustained effects on species known to be sensitive to regeneration treatments (e.g. Monotropa uniflora L., Dryopteris spinulosa Kuhn).ConclusionsIn temperate hardwood forests of Southern Quebec, silvicultural treatments of higher intensities resulted in detrimental effects on soil properties, especially in the surface horizon, 20 years after disturbance. This legacy, in turn, affected the composition and diversity of understory plant communities. The more intense silvicultural treatments contributed to the persistence of pioneer species better adapted to a wider range of environmental conditions and resulted in a decrease in understory plant community heterogeneity among sites. Conversely, single-tree selection cutting appeared to be the most appropriate silvicultural treatment for maintaining soil functions and heterogeneity of understory plant communities after 20 years; composition and structure being similar to long-undisturbed forests.

Scale‐dependent responses of understory vegetation to the physical structure of undisturbed tundra shrub patches

Much of the Arctic is experiencing rapid change in the productivity and recruitment of tall, deciduous shrubs. It is well established that shrub expansion can alter tundra ecosystem composition and function; however, less is known about the degree to which variability in the physical structure of shrub patches might mediate these changes. There is also limited information as to how different physical attributes of shrub patches may covary and how they differ with topography. Here, we address these knowledge gaps by measuring the physical structure, abiotic conditions, and understory plant community composition at sampling plots within undisturbed green alder patches at a taiga–tundra ecotone site in the Northwest Territories, Canada. We found surprisingly few associations between most structural variables and abiotic conditions at the plot scale, with the notable exceptions of canopy complexity and snow depth. Importantly, neither patch structure nor abiotic conditions were associated with the vegetation community at the plot scale when among-patch variation was accounted for. However, among-patch variation in plant community composition was significant and represented a gradient in the richness of tundra specialists and Sphagnum moss abundance. This gradient was strongly associated with mean patch snow depth, which was likely controlled at least in part by mean patch canopy complexity. Overall, natural variability in green alder patch structure had less of an association with abiotic conditions than expected, suggesting future changes in physical structure at undisturbed sites may have limited environmental impact at the plot scale. However, at the patch scale, increases in snow depth, likely related to canopy complexity, were negatively associated with tundra specialist richness, potentially due to phenological limitations associated with shortened growing seasons. In summary, our data suggest emergent properties exist at the patch scale that are not apparent at the plot scale such that plot-scale measurements do not represent variation in understory community composition across the landscape. The results presented here will inform future work addressing spatial variability in shrub impacts on ecosystem function and increase our understanding of understory community variation within alder patch habitats at the taiga–tundra ecotone.

Chronic browsing by an introduced mammalian herbivore in a tropical island alters species composition and functional traits of forest understory plant communities

AbstractMammalian herbivores have large‐scale impacts on vegetation, altering structure, and species composition, especially in tropical grasslands and savannas. However, there is limited understanding of the potential impacts of mammalian herbivores in tropical wet forests, where they are typically less abundant. We investigated the effects of an introduced mammalian herbivore chital (Axis axis) on vegetation structure, composition and leaf functional traits of tropical evergreen forests of the Andaman Islands, India. Across seven islands, representing a gradient of herbivore densities, increasing chital presence was associated with decreased understory richness, understory density and adult tree richness, but was not related to adult tree density or size class distributions. We also found a significant decrease in community level leaf palatability traits (specific leaf area decreased and leaf thickness increased) with increasing chital habitat use. This community level shift in leaf trait values was better explained by intraspecific variation in leaf traits across islands rather than changes in species composition. In summary, we show persistent long‐term impacts of an introduced mammalian herbivore on understory tropical tree communities although there is little impact on adult tree communities. Our results also show that functional traits of species can be altered in response to novel herbivory, even at herbivore densities where there are no detectable impacts on adult forest structure or composition. Such altered functional traits may potentially alter ecosystem functioning in these forests even without changes in vegetation structure.Abstract in Hindi is available with online material.

Understory plant diversity and composition across a postfire tree density gradient in a Siberian Arctic boreal forest

Cajander larch (Larix cajanderi Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect postfire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover. Canopy cover was also the most important variable affecting individual species’ abundances. In general, tall shrubs (e.g., Betula nana subsp. exilis) were more abundant in low-density stands with high light availability, and mosses (e.g., Sanionia spp.) were more abundant in high-density stands with low light availability. These results provide evidence that postfire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities.

Exotic Prosopis juliflora suppresses understory diversity and promotes agricultural weeds more than a native congener

Exotic invasive plant species alter ecosystems and locally extirpate native plant species, and by doing so alter community structure. Changes in community structure may be particularly important if invaders promote species with certain traits. For example, the positive effects of most invaders on soil fertility may promote species with weedy traits, whether native or not. We examined the effects of two co-occurring Prosopis congeners, the native P. cineraria and the exotic invader P. juliflora, on species identified as “agricultural weeds” and species that were not agricultural weeds in the United Arab Emirates. When compared to plots in the open, P. cineraria canopies were associated with lower richness and density of non-weeds while having no impact on agricultural weed species. In contrast, there was lower richness and densities of non-weeds under canopies of P. juliflora, but higher densities of agricultural weeds than in the open surrounding the canopies. These patterns associated with Prosopis congeners and understory plant community composition might be due to the much higher litter deposition, if litter is inhibitory, and shallow root biomass under P. juliflora, or the different soil properties that corresponded with the two Prosopis canopies. In general, soils contained more nitrogen under P. juliflora than P. cineraria, and both understories were more fertile than soil in the open. Our results suggest that evolutionary history may play a role in how exotic invasive species may select for some traits over others in plant communities, with an exotic invader potentially creating reservoirs of agricultural weeds.

Plant traits as indicators of recovery of reclaimed wellsites in forested areas: Slow but directional succession trajectory

Trait-based approaches can provide a generalizable mechanistic understanding of complex post-disturbance succession dynamics of plant communities. Much of our knowledge regarding successional trajectories of functional trait composition come from observations of natural disturbances that leave physical and biological legacy on site for self regeneration. We lack, however, understanding of the long-term recovery in severely degraded lands following reclamation through active revegetation. To address this gap, we examined changes in trait composition of forest understory plant communities in reclaimed oil and gas wellsites using chronosequence data (7–48 years since reclamation) to assess recovery towards that of post-harvest and natural post-fire reference forest sites. We used multiple traits associated with resource acquisition (e.g., fast growth) and performance ability metrics (e.g., shade-tolerance) with putative environmental factors to evaluate the trait-environment relationships underlying plant community recovery in reclaimed sites. We found an overall directional change in functional composition with time since reclamation towards that observed in reference sites, but even older reclaimed sites remained significantly different from reference sites. This could be related to differential trajectory patterns among traits where some trait values progressed towards those of reference (long-distance dispersal by wind decreased with time since reclamation) whereas some fast-resource acquisition traits and exotic species showed no change and remained dominant in older reclaimed sites. The strong link between traits and environment suggests a significant influence of time and subsequent developing site conditions (e.g., canopy cover) as well as enduring legacies of wellsite operation/reclamation (e.g., high soil bulk density and exotic species) on functional composition. Knowledge of functional composition trajectories in severely degraded ecosystems, such as wellsites, could improve our understanding of recovery processes and inform more effective reclamation practices by identifying putative underlying environmental factors and specific ecological attributes that may delay successful recovery.

Changes in Deadwood and Understory Vegetation 12 Years after Partial and Clearcut Harvesting in Mixedwood Stands of Western Quebec, Canada

AbstractEcosystem management promotes the integration of knowledge of natural ecosystem dynamics into forest-management planning and practices, and provides a useful framework for understanding and mitigating the impacts of timber harvesting on biodiversity and ecosystem functions. Understory vegetation and deadwood are involved in many ecosystem functions including nutrient and carbon, and tree-regeneration dynamics. The study objective was to compare the 12-year post-treatment effects on forest floor attributes of two intensities of partial harvesting (gap harvesting [62 percent basal area retention; BAR] and diameter-limit harvesting [35 percent BAR]) to clearcutting (10 percent BAR). We quantified the standing and downed deadwood abundance and the understory plant community composition in mixed, northern temperate forests, in western Québec, Canada. Our results suggest that moderate-intensity gap harvesting can best preserve a standing deadwood stem density and a diameter structure comparable to unmanaged stands. Downed deadwood abundance of all diameter classes was not altered, 12 years following the gap harvesting. Finally, understory plant communities in the gap treatment were more similar to those of unmanaged stands than those in diameter-limit and clearcutting treatments. These results illustrate the importance of tree retention levels for the maintenance of deadwood and understory plant species associated with closed-canopy or old-growth forests.

Patterns of Understory Community Assembly and Plant Trait-Environment Relationships in Temperate SE European Forests

We analyzed variation in the functional composition and diversity of understory plant communities across different forest vegetation types in Slovenia. The study area comprises 10 representative forest sites covering broad gradients of environmental conditions (altitude, geology, light availability, soil type and reaction, nutrient availability, soil moisture), stand structural features and community attributes. The mean and variation of the trait values were quantified by community-weighted means and functional dispersion for four key plant functional traits: plant height, seed mass, specific leaf area and leaf dry matter content. At each study site, forest vegetation was surveyed at two different spatial scales (4 and 100 m2) in order to infer scale-dependent assembly rules. Patterns of community assembly were tested with a null model approach. We found that both trait means and diversity values responded to conspicuous gradients in environmental conditions and species composition across the studied forests. Our results mainly support the idea of abiotic filtering: more stressful environmental conditions (e.g., high altitude, low soil pH and low nutrient content) were occupied by communities of low functional diversity (trait convergence), which suggests a selective effect for species with traits adapted to such harsh conditions. However, trait convergence was also detected in some more resource-rich forest sites (e.g., low altitude, high soil productivity), most likely due to the presence of competitive understory species with high abundance domination. This could, at least to some extent, indicate the filtering effect of competitive interactions. Overall, we observed weak and inconsistent patterns regarding the impact of spatial scale, suggesting that similar assembly mechanisms are operating at both investigated spatial scales. Our findings contribute to the baseline understanding of the role of both abiotic and biotic constraints in forest community assembly, as evidenced by the non-random patterns in the functional structure of distinct temperate forest understories.

Fire severity and changing composition of forest understory plant communities

AbstractQuestionsGradients of fire severity in dry conifer forests can be associated with variation in understory floristic composition. Recent work in dry conifer forests in California, USA, has suggested that more severely burned stands contain more thermophilic taxa (those associated with warmer and drier conditions), and that forest disturbance may therefore accelerate floristic shifts already underway due to climate change. However, it remains unknown how rapidly thermophilic taxa shifts occur following disturbance, how long such shifts are likely to persist, and how different thermophilic post‐disturbance communities are from pre‐disturbance communities.LocationColorado Front Range, USA.MethodsWe investigated these questions using a unique 15‐year vegetation plot dataset that captures pre‐ and post‐fire understory community composition across a gradient of fire severity in dry conifer forests, classifying taxa using the biogeographic affinity concept.ResultsThermophilization (defined here as a decrease in the ratio of cool‐mesic taxa to warm‐xeric taxa, based on biogeographic affinity of paleobotanical lineages) was observed as early as one year post‐fire for all fire severity classes, but was stronger at sites that burned at higher severity. The ratio of cool‐mesic to warm‐xeric taxa recovered to pre‐fire levels within 10 years in stands that burned at low severity, but not in stands that burned at moderate or high severity. The process of thermophilization after high‐severity fire appears to be driven primarily by the gain of warm‐xeric taxa that were absent before the fire, but losses of cool‐mesic taxa, which did not return during the duration of the study, also played a role.ConclusionsDecreases in canopy cover appear to be a main contributor to understory thermophilization. Fine‐scale heterogeneity in post‐fire forest structure is likely an important driver of floristic diversity, creating the microclimatic variation necessary to maintain floristic refugia for species mal‐adapted to increasingly warm and dry conditions.

Understory plant communities vary with tree productivity in two reclaimed boreal upland forest types in Canada

The dynamics of understory plant communities in forest ecosystems with different tree productivities that are reclaimed after surface mining is poorly studied. In the Athabasca oil sands region of Canada, the cover, composition, diversity and foliar nutrient concentrations of understory vascular plant communities were examined on reclaimed sites (at least 15 years old since site reconstruction) with low, medium and high productivity that were planted to lodgepole pine (Pinus contorta) and white spruce (Picea glauca). Understory plant communities showed different responses to tree productivity classes in both pine and spruce stands. On pine sites, higher shrub and grass covers contributed to higher total cover on medium than on low and high productivity sites; however, low and high productivity sites had higher species richness and Shannon-Wiener index than medium productivity sites. In addition, shrub, grass and total covers were all positively correlated with cover soil thickness, soil dissolved organic carbon and inorganic nitrogen concentrations, but there was no relationship between any understory plant community variable and tree growth parameters. On spruce sites, lower shrub and forb covers and higher grass cover on high than on low and medium productivity sites resulted in no difference in total cover among productivity classes; however, species richness and Shannon-Wiener index were higher on medium than on low and high productivity sites. Additionally, forb cover, species richness and Shannon-Wiener index were all negatively correlated with leaf area index, and grass cover and species evenness were positively correlated with tree growth parameters such as height, diameter at breast height and aboveground biomass increments. We conclude that the relationships between tree productivity and understory plant communities in reclaimed forest ecosystems were site specific. Reclamation strategies such as adopting a proper planted tree spacing or fertilization should be used on sites with different tree productivities to balance overstory tree growth with understory plant community development in reclaiming surface-disturbed mining areas.

Does burn severity affect plant community diversity and composition in mixed conifer forests of the United States Intermountain West one decade post fire?

BackgroundWildfire is an important ecological process in mixed conifer forests of the Intermountain West region of the USA. However, researchers and managers are concerned because climate warming has led to increased fire activity in recent decades. More area burned will result in larger land areas in early successional stages and will potentially limit tree establishment; therefore, evaluating long-term forest understory response to fire is important. We evaluate the impact of burn severity, overstory canopy cover, topography, and climate on understory plant community diversity and composition in seven wildfires, 9 to 12 years post fire, along a broad climate gradient from dry to moist mixed conifer forests in Idaho, Montana, and Washington, USA.ResultsClimate was the most important driver for species diversity and composition, but a burn severity gradient was detectable in the species data one decade post fire. A strong overlap in species composition between burn severity levels was documented, but dispersion was lower for high burn severity sites, indicating that those sites are still recovering. Local species richness and diversity had a nonlinear relationship with the burn severity index dNBR, with a maximum at low to moderate burn severity; the relationship was stronger in moist climates. Functional trait analysis revealed higher grass and forb cover in high-severity burns, higher cover of tree seedlings, residual and off-site colonizers in burned areas, and more shade-tolerant species in unburned areas. Of the 270 species recorded, 10% were introduced; however, only three were of noxious status and two were invasive annual grasses, generally occurring on dry sites.ConclusionsThe understory plant community was not fundamentally altered by these fires and fire contributed to increased species diversity both locally and regionally, suggesting that low to moderate burn severity fire is a treatment that contributes to long-term maintenance of a diverse and productive understory. Individual species traits were significant drivers of understory species assemblages and, as future change in climate and fire regimes leads to shifts in species composition, anticipation of consequences will be important. Although invasive species occurred at low cover levels, noxious weeds and invasive annual grasses will continue to be management challenges, particularly in dry regions of mixed conifer forests.

Fire legacies in eastern ponderosa pine forests.

Disturbance legacies structure communities and ecological memory, but due to increasing changes in disturbance regimes, it is becoming more difficult to characterize disturbance legacies or determine how long they persist. We sought to quantify the characteristics and persistence of material legacies (e.g., biotic residuals of disturbance) that arise from variation in fire severity in an eastern ponderosa pine forest in North America. We compared forest stand structure and understory woody plant and bird community composition and species richness across unburned, low‐, moderate‐, and high‐severity burn patches in a 27‐year‐old mixed‐severity wildfire that had received minimal post‐fire management. We identified distinct tree densities (high: 14.3 ± 7.4 trees per ha, moderate: 22.3 ± 12.6, low: 135.3 ± 57.1, unburned: 907.9 ± 246.2) and coarse woody debris cover (high: 8.5 ± 1.6% cover per 30 m transect, moderate: 4.3 ± 0.7, low: 2.3 ± 0.6, unburned: 1.0 ± 0.4) among burn severities. Understory woody plant communities differed between high‐severity patches, moderate‐ and low‐severity patches, and unburned patches (all p < 0.05). Bird communities differed between high‐ and moderate‐severity patches, low‐severity patches, and unburned patches (all p < 0.05). Bird species richness varied across burn severities: low‐severity patches had the highest (5.29 ± 1.44) and high‐severity patches had the lowest (2.87 ± 0.72). Understory woody plant richness was highest in unburned (5.93 ± 1.10) and high‐severity (5.07 ± 1.17) patches, and it was lower in moderate‐ (3.43 ± 1.17) and low‐severity (3.43 ± 1.06) patches. We show material fire legacies persisted decades after the mixed‐severity wildfire in eastern ponderosa forest, fostering distinct structures, communities, and species in burned versus unburned patches and across fire severities. At a patch scale, eastern and western ponderosa system responses to mixed‐severity fires were consistent.

Pervasive moose browsing in boreal forests alters successional trajectories by severely suppressing keystone species

AbstractLarge herbivores can shape young forest stands and determine the successional trajectory of forested ecosystems by selectively browsing palatable species at the sapling stage. Moose (Alces alces) is the dominant vertebrate herbivore in Fennoscandian boreal forests, and high population densities have raised concerns about potential negative effects on ecosystem functioning and properties including biological diversity and timber production. We used 31 herbivore exclosures in Norway to investigate how forests developed after clear‐cutting with or without moose present. We tested how tree demography, abundances of understory plant functional groups, community composition, and plant diversity (including bryophytes) across multiple scales varied with moose exclusion. After seven years, the exclosures were dominated by deciduous trees, including many large rowan (Sorbus aucuparia) individuals, a functionally important keystone species. In contrast, the open plots subject to moose impacts (browsing, trampling, defecation) were dominated by economically important coniferous trees and there was next to no rowan recruitment to taller height classes. The biomass of large herbs and ferns was much greater inside exclosures. This study emphasizes the large immediate effect of moose on early successional boreal forest stands. Landscape‐level alterations caused by reduced deciduous dominance, and a reduction in large flowering herbs is likely to lead to cascading effects on ecosystem functioning. The management of boreal production forests needs to account for the combined effects of silvicultural practices and ungulate herbivory to ensure ecosystem functioning, but this management goal may be jeopardized in our study regions due to drastically reduced abundance of keystone species.

Logging, exotic plant invasions, and native plant reassembly in a lowland tropical rain forest

AbstractHabitat modification and biological invasions are key drivers of global environmental change. However, the extent and impact of exotic plant invasions in modified tropical landscapes remain poorly understood. We examined whether logging drives exotic plant invasions and whether their combined influences alter understory plant community composition in lowland rain forests in Borneo. We tested the relationship between understory communities and local‐ and landscape‐scale logging intensity, using leaf area index (LAI) and aboveground biomass (AGB) data from 192 plots across a logging‐intensity gradient from primary to repeatedly logged forests. Overall, we found relatively low levels of exotic plant invasions, despite an intensive logging history. Exotic species were more speciose, had greater cover, and more biomass in sites with more local‐scale canopy loss. Surprisingly, though, exotic species invasion was not related to either landscape‐scale canopy loss or road configuration. Moreover, logging and invasion did not seem to be acting synergistically on native plant composition, except that seedlings of the canopy‐dominant Dipterocarpaceae family were less abundant in areas with higher exotic plant biomass. Current low levels of invasion, and limited association with native understory community change, suggest there is a window of opportunity to manage invasive impacts. We caution about potential lag effects and the possibly severe negative impacts of exotic plant invasions on the long‐term quality of tropical forest, particularly where agricultural plantations function as permanent seed sources for recurrent dispersal along logging roads. We therefore urge prioritization of strategic management plans to counter the growing threat of exotic plant invasions in modified tropical landscapes.

Fuel-reduction management alters plant composition, carbon and nitrogen pools, and soil thaw in Alaskan boreal forest.

Increasing wildfire activity in Alaska's boreal forests has led to greater fuel-reduction management. Management has been implemented to reduce wildfire spread, but theecological impacts of these practices are poorly known. We quantified the effects of hand-thinning and shearblading on above- and belowground stand characteristics, plant species composition, carbon (C) and nitrogen (N) pools, and soil thaw across 19 sites dominated by black spruce (Picea mariana) in interior Alaska treated 2-12years prior to sampling. The density of deciduous tree seedlings was significantly higher in shearbladed areas compared to unmanaged forest (6.4 vs. 0.1 stems/m2 ), and unmanaged stands exhibited the highest mean density of conifer seedlings and layers (1.4 stems/m2 ). Understory plant community composition was most similar between unmanaged and thinned stands. Shearblading resulted in a near complete loss of aboveground tree biomass C pools while thinning approximately halved the C pool size (1.2kg C/m2 compared to 3.1kg C/m2 in unmanaged forest). Significantly smaller soil organiclayer (SOL) C and N pools were observed in shearbladed stands (3.2kg C/m2 and 116.8g N/m2 ) relative to thinned (6.0kg C/m2 and 192.2g N/m2 ) and unmanaged (5.9kg C/m2 and 178.7g N/m2 ) stands. No difference in C and N pool sizes in the uppermost 10cm of mineral soil was observed among stand types. Total C stocks for measured pools was 2.6kg C/m2 smaller in thinned stands and 5.8kg C/m2 smaller in shearbladed stands when compared to unmanaged forest. Soil thaw depth averaged 13cm deeper in thinned areas and 46cm deeper in shearbladed areas relative to adjacent unmanaged stands, although variability was high across sites. Deeper soil thaw was linked to shallower SOL depth for unmanaged stands and both management types, however for any given SOL depth, thaw tended to be deeper inshearbladed areas compared to unmanaged forest. These findings indicate that fuel-reduction management alters plant community composition, C and N pools, and soil thaw depth, with consequences for ecosystem structure and function beyond those intended for fire management.