Understory Composition Research Articles

Prescribed burning of harvested boreal black spruce forests in eastern Canada: effect on understory vegetation

Ecosystem-based management advocates that forestry disturbances should aim to emulate natural disturbances to mitigate the landscape-level impact of forest management. This study compares the impact of clear-cuts followed by a prescribed burn (CCPB) with clear-cuts alone (CC) and current careful logging practices (CLAAG: “careful logging around advanced growth”) on understory composition within black spruce (Picea mariana Miller (BSP)) paludified forest stands at the plot, site, and treatment levels using a functional-type approach. Vascular and nonvascular taxa showed significant differences in composition at the plot level among treatments. We found that pioneer taxa occurred mainly in CCPB sites, while late-successional taxa characterized CC sites. CLAAG sites had higher taxa richness than CCPB and CC sites, and we found that CCPB treatments were most likely to promote vascular taxa compositions that are more similar to those observed after natural disturbances. Additionally, the relative abundance of Sphagnum spp., responsible for paludification, was significantly reduced in sites treated by prescribed burning. This study therefore presents results suggesting that prescribed burning might represent a sustainable alternative to current harvesting techniques in the Clay Belt of eastern Canada that could help in preserving biodiversity (in terms of understory species assemblage) while maintaining or even enhancing forest productivity.

Short-term effects of three commercial thinning treatments on diversity of understory vascular plants in white spruce plantations of northern New Brunswick

Plantation forestry is increasing in frequency and extent across the landscape, but can negatively impact biodiversity by promoting stand homogeneity and early-seral species dominance. Modifying silvicultural prescriptions in plantations, particularly with respect to creating or retaining more coarse woody debris (CWD), may improve their ability to support biodiversity, by mimicking some aspects of natural disturbance. We used a modified before-after-control-impact design to examine the response of understory vascular plants to four treatments in 25-year-old white spruce plantations of northwestern New Brunswick. Treatments included commercial thinning (CT) with enhanced, moderate, or no debris, as well as an unthinned control. Understory composition was analyzed using complementary methods that summarize biological communities at different resolutions: indicator species analysis, functional group responses, ordination, and biodiversity indices. Understory plants increased in overall richness and abundance after thinning, particularly in no-debris treatments. This was driven by (1) expansion of pre-established clonal forest herbs and (2) invasion of graminoids and long-distance dispersers. Several disturbance-sensitive species were significantly more abundant in unthinned controls. Few compositional differences were observed between the moderate and enhanced debris treatments, perhaps because the effects of CWD creation require longer to detect than those of thinning and ground-layer disturbance (from debris-removal). We recommend (1) continued monitoring of changes in moderate and enhanced treatments to determine the effects of debris-modification and (2) avoiding silvicultural prescriptions that remove branches, tree-tops, and other non-merchantable wood, which appear to facilitate early-seral species, and negatively impact disturbance-sensitive species.

Canopy cover and understory composition determine abundance of Vaccinium myrtillus L., a key plant for capercaillie (Tetrao urogallus), in subalpine forests in the Pyrenees

Background: The dwarf shrub Vaccinium myrtillus – with high cover, height, and fruit production – benefits capercaillie (Tetrao urogallus).Aims: Our aim was to quantify landscape (e.g. elevation, geographic location, and precipitation), site (e.g. overstorey cover and stoniness) and very fine scale factors (e.g. spatial associations in the understorey) that affect cover, height, and fruit production of V. myrtillus in subalpine forests in thePyrenees, with understorey usually dominated by Rhododendron ferrugineum.Methods: We sampled 155 plots (0.5 m × 5 m) in six sites. For each plot, in the understorey layer, we assessed species cover, height for R. ferrugineum and V. myrtillus, number of total fruits in V. myrtillus, and spatial associations among V. myrtillus and the remaining cover types.Results: Overstorey cover negatively influenced V. myrtillus cover, its height, and particularly, the number of fruits, which was also negatively influenced by R. ferrugineum cover. Associations between R. ferrugineum and V. myrtillus were site dependent, while V. myrtillus showed mostly positive associations with grasses and mosses.Conclusions: Reducing overstorey and R. ferrugineum cover has the strongest positive effect on increasing V. myrtillus fruit production, but with additional positive effects on V. myrtillus cover and height. Increases in grass and moss coverage could favour V. myrtillus.

Impact of introduced herbivores on understory vegetation along a regional moisture gradient in Patagonian beech forests

Introduced ungulates can alter understory structure and composition posing a serious threat to forest biodiversity. Yet how large-herbivore impacts in forested regions vary along major environmental gradients remains little explored. If ungulate effects shift with habitat conditions, then management could be tailored to protect most vulnerable forests. We tested the hypothesis that the extent of livestock impact on understory vegetation increases with habitat moisture across Nothofagus dombeyi forests in Nahuel Huapi National Park, NW Patagonia, Argentina. Understory composition and species diversity were compared for paired sites (N=5), which were historically used by cattle or remained free of livestock for more than 50yr, and were located along a regional precipitation gradient (1500–2800mm/yr). Long-term cattle presence reduced the cover of sub-canopy trees, shrubs and bamboo by 57–83%, and increased the relative cover of ground-layer herbs, regardless of habitat moisture. Livestock effects on species composition increased towards the wettest forests, which contained more species exclusive to either browsed or unbrowsed sites. Livestock presence increased species richness (α diversity) and within-site heterogeneity (β diversity) in some locations, but mostly reduced species evenness (30%) throughout the moisture gradient. Species turnover at the gradient scale was lower across browsed sites than for livestock-free sites. Our results indicate that the historical presence of domestic cattle induced region-wide changes in understory communities, highlighting the vulnerable nature of the local flora to ungulate disturbance. The greater impact of livestock browsing on the species composition of wetter forests was consistent with the role of plant growth–defence trade-offs along resource gradients. We suggest that the erosion of understory vegetation attributed to domestic herbivores in Patagonian beech forests can be mitigated by adjusting current animal stocks, while moister forests should be given the highest conservation priority.

Hotspots of Community Change: Temporal Dynamics Are Spatially Variable in Understory Plant Composition of a California Oak Woodland.

Community response to external drivers such climate and disturbance can lead to fluctuations in community composition, or to directional change. Temporal dynamics can be influenced by a combination of drivers operating at multiple spatial scales, including external landscape scale drivers, local abiotic conditions, and local species pools. We hypothesized that spatial variation in these factors can create heterogeneity in temporal dynamics within landscapes. We used understory plant species composition from an 11 year dataset from a California oak woodland to compare plots where disturbance was experimentally manipulated with the removal of livestock grazing and a prescribed burn. We quantified three properties of temporal variation: compositional change (reflecting the appearance and disappearance of species), temporal fluctuation, and directional change. Directional change was related most strongly to disturbance type, and was highest at plots where grazing was removed during the study. Temporal fluctuations, compositional change, and directional change were all related to intrinsic abiotic factors, suggesting that some locations are more responsive to external drivers than others. Temporal fluctuations and compositional change were linked to local functional composition, indicating that environmental filters can create subsets of the local species pool that do not respond in the same way to external drivers. Temporal dynamics are often assumed to be relatively static at the landscape scale, provided disturbance and climate are continuous. This study shows that local and landscape scale factors jointly influence temporal dynamics creating hotspots that are particularly responsive to climate and disturbance. Thus, adequate predictions of response to disturbance or to changing climate will only be achieved by considering how factors at multiple spatial scales influence community resilience and recovery.

Understory vegetation as an indicator for floodplain forest restoration in the Mississippi River Alluvial Valley, U.S.A.

In the Mississippi River Alluvial Valley (MAV), complete alteration of river‐floodplain hydrology allowed for widespread conversion of forested bottomlands to intensive agriculture, resulting in nearly 80% forest loss. Governmental programs have attempted to restore forest habitat and functions within this altered landscape by the methods of tree planting (afforestation) and local hydrologic enhancement on reclaimed croplands. Early assessments identified factors that influenced whether planting plus tree colonization could establish an overstory community similar to natural bottomland forests. The extent to which afforested sites develop typical understory vegetation has not been evaluated, yet understory composition may be indicative of restored site conditions. As part of a broad study quantifying the ecosystem services gained from restoration efforts, understory vegetation was compared between 37 afforested sites and 26 mature forest sites. Differences in vegetation attributes for species growth forms, wetland indicator classes, and native status were tested with univariate analyses; floristic composition data were analyzed by multivariate techniques. Understory vegetation of restoration sites was generally hydrophytic, but species composition differed from that of mature bottomland forest because of young successional age and differing responses of plant growth forms. Attribute and floristic variation among restoration sites was related to variation in canopy development and local wetness conditions, which in turn reflected both intrinsic site features and outcomes of restoration practices. Thus, understory vegetation is a useful indicator of functional progress in floodplain forest restoration.

Microhabitat associations of lichens, feathermosses, and vascular plants in a caribou winter range, and their implications for understory development

Vegetation–environment relationships are well understood for boreal lichen woodlands, but the mechanistic basis for small-scale understory patchiness (patches dominated by lichen, mosses, and vascular plants), and its implications for the prevalence of niche vs. neutral processes driving understory development, have not been explored. We asked whether predictable vegetation–environment associations exist at the microsite scale, with the goal of informing caribou range management. We sampled canopy and edaphic variables in patches of lichen, feathermoss, and vascular plants in subalpine lodgepole pine (Pinus contorta var. latifolia Engelm. ex S.Wats.) forests, in north-central British Columbia, Canada. Lichens positively associate with microsites that have high light and heat, and low moisture and nutrient availability; vascular plants positively associate with base-cation availability, sulphur and phosphorus availability, fine-textured soils, thick organic layers, and high light; feathermosses positively associate with low light and heat, and nitrogen availability. Understory composition is most strongly associated with canopy characteristics, but is also related to edaphic properties in predictable ways; soil and canopy attributes may further interact to define distinct intra-stand niches. The viability of caribou winter ranges can likely be extended by partially thinning or burning the canopy and organic layers, but the longevity of a lichen-dominant stage may ultimately depend on the soil texture, due to its influence on vascular plant growth.

Drivers of observed biotic homogenization in pine barrens of central Wisconsin.

Fire suppression throughout the 20th century greatly altered plant communities in fire-dominated systems across North America. Our ability to assess these effects over the long-term, however, is handicapped by the paucity of baseline data. Here, we used detailed baseline data from the 1950s to track changes in the over- and understory composition of pine-barrens vegetation growing on sandy, glacial lake-bed sediments in central Wisconsin. We expected fire suppression to favor succession to closed-canopy conditions, leading to decreases in shade-intolerant and fire-adapted species and consequent reductions in alpha and gamma diversity. We also expected beta diversity to decline due to increases in shade-tolerant, fire-sensitive, and exotic species. In fact, fire suppression has greatly altered the structure and composition of these pine-barrens communities over the past 54 years. Woody, wind-pollinated, and shade-tolerant species all increased in richness and abundance, as expected, with succession following fire suppression. Contrary to expectations, local and regional species richness increased by 12% and 26%, respectively, while Shannon beta diversity declined 24.1%. Increases in canopy coverage and number of native species appear to have driven this biotic homogenization. In contrast, increases in exotic species in our study did not promote biotic homogenization, reflecting their relative rarity across sites. Our findings highlight the key role fire plays in shaping the assembly of these pine-barrens communities.

Structural effects on understory attributes in second-growth forests of northern Wisconsin, USA

I sampled the understory and associated structural features in forty-four second growth forests of north-central Wisconsin, USA, to assess the influence of forest structure on four understory attributes: cover, richness, composition and evenness (variant of Simpson’s index). Eight structural features (overstory basal area and crown cover, percent conifer overstory, midstory basal area, shrub frequency, snag basal area, coarse woody debris volume, and maple seedling density) were measured. An additional derived variable, coefficient of variation (CV) for cover, was also tested. The objectives of the study were to determine the significance and impact (increase or decrease) of each structural feature on understory attributes. These were addressed with regression, canonical correspondence analysis, and two-sample tests. Ordination was used to assess differences in composition among forest types. The analyses indicated that structure had no effect on richness or cover, a modest influence on composition, and strong effect on evenness. An important revelation was the over-riding influence of understory structure, but a small role for the overstory. The severity and type(s) of historical disturbance interacted with species pools to strongly constrain richness in the early 20th century. Small changes related to succession have occurred since then. The amount of understory cover is still changing but appears to be succession-related. In contrast, more than 40% of the variation in evenness was explained by maple seedling density, and two interaction variables: maple density∗shrub frequency and overstory∗midstory basal area. Higher maple seedling density decreased evenness, probably by competitive effects. As the interaction variables increased, evenness was reduced. The regression analysis and CCA point to the shrub layer as having a direct, mechanistic effect on which species are present and their abundance; whereas the presence of shrubs is relatively benign. It is hypothesized that the abundance of upper strata resulted in varying levels of resource limitations, and influenced other environmental features to the extent that niche heterogeneity was affected. The proportion of conifer in the overstory had an important influence on understory composition, but hemlock assemblages did not differ from broadleaved forests in richness or cover. These results suggest that excess emphasis has been placed on the influence of the upper strata, but too little on the shrub and understory layers. Thus, the management of understory assemblages should carefully evaluate the sub-ordinate strata because they may be more important than upper layers.

Empirically validating a dense woody regrowth ‘problem’ and thinning ‘solution’ for understory vegetation

In landscapes with a short history of intensive land use, woody plant regrowth on cleared land is often favorably received as a shift back to a more natural state. However, it is common for these regrowth stands to be much denser than undisturbed forest. High stem density can adversely affect stand structure, understory composition, and habitat for dependent fauna. Thinning to reduce stem density is one common silvicultural method used to manage dense stands for ecological or restoration objectives. The effect of thinning on the stand structure is well understood but those on the understory vegetation are not. We address this knowledge gap in anticipation of an increasing call for public investment in ‘ecological’ thinning across public and private land. Our case study is from the eucalypt woodlands and forests of central Victoria, Australia, an ecosystem in which dense woody regrowth is common. From a broad survey of 98 sites, spanning a range of stem densities, we explored the effect of density on understory vegetation. High densities of small trees (<20cm DBH) caused the greatest suppression of native and exotic cover and species richness. We compared our observations with benchmarks and found that sites with stem densities exceeding their benchmark had median values approximately one-seventh of the benchmark native understory cover, which was also less than a quarter of the cover of those sites with benchmark or lower stem density. We conducted an additional targeted survey of 11 thinned sites paired with non-thinned sites to evaluate the effects of thinning. We built models combining broad and targeted survey data relating understory response to stem density, thinning, land tenure and environmental covariates. These models predicted that thinning is likely to elicit positive responses from the understory plant community in the short term. This is the desired response from native species, but we caution that thinning can equally favor exotic plant species.

Spectral reflectance patterns and seasonal dynamics of common understory types in three mature hemi-boreal forests

Due to the growing demand on more accurate prediction of biophysical properties (e.g., leaf area index) or carbon balance models based on remotely sensed data, the understory effect needs to be separated from the overstory. Reflectance models can provide possibility to model and retrieve understory reflectance over large scales, but ground truth data is needed to validate such models and algorithms. In this study, we documented the seasonal variation (April–September) and spectral changes occurring in understory layers of a typical European hemi-boreal forest. The understory composition was recorded and its spectra measured with an ASD FieldSpec Hand-Held UV/VNIR Spectroradiometer eight times at four site types during the growing period (from May to September) in 2013. The collected dataset presented within this study would be of much use to improve and validate algorithms or models for extracting spectral properties of understory from remote sensing data. It can be also further used as a valuable input in radiative transfer simulations that are used to quantify the roles of forest tree layer and understory components in forming a seasonal reflectance course of a hemi-boreal forest, and the upcoming phases of the RAdiation Model Intercomparison (RAMI) experiment.

Grazing affects shoot growth, nutrient and water status of Quercus ilex L. in Mediterranean open woodlands

& Context Mediterranean open woodlands (dehesa) have faced a dual process of intensification and abandonment of grazing which has resulted in alteration of the understory vegetation. & Aims We analysed the effects of land use changes on the physiological status of holm oak in different open woodlands (dehesa) in southern Iberian Peninsula. & Methods In an area of extensive grazing, we selected six paired plots (one grazed, one abandoned) and grouped them by habitat types according to understory composition (nearly all monospecific Cistus ladanifer L. shrub or mixed shrub).

Forest response to cumulative disturbance and stress: Two decades of change in whitebark pine ecosystems of west-central British Columbia

Forests dominated by the endangered tree species whitebark pine (Pinus albicaulis) are threatened by multiple stresses (fire suppression, climate change) and disturbances (white pine blister rust [Cronartium ribicola], mountain pine beetle [Dendroctonus ponderosae]). To gain insight into how these ecosystems respond, we quantified vegetation change over 2 decades (21–29 y) in xeric and submesic P. albicaulis ecosystems near the northern edge of the species' range on the leeward side of the Coast Mountains of British Columbia, Canada. We compared changes in overstory and understory vegetation composition of these stands to changes in mesic, non-whitebark pine ecosystems in the same region. Multi-response permutation procedure (MRPP) analysis showed that the overstory of xeric whitebark pine ecosystems became compositionally similar to mesic ecosystems, i.e., there was increased dominance by Abies lasiocarpa or Tsuga mertensiana. Yet understory composition in xeric whitebark pine stands changed little and there was continued regeneration of P. albicaulis. Submesic whitebark pine stands developed a dense canopy dominated by T. mertensiana, and although their understories did not become compositionally similar to mesic ecosystems, there was minimal P. albicaulis regeneration. Understory stability in xeric and submesic whitebark pine ecosystems over 21–29 y suggests compositional resilience in these ecosystems to multiple stresses and disturbances. However, ongoing disturbance affecting both overstory and understory P. albicaulis might still result in the loss of this keystone species.

Effects of post-windthrow management interventions on understory plant communities in aspen-dominated boreal forests

Understory plants are critical to diversity and ecosystem function in boreal forests. Significant advances have been made in understanding understory vegetation dynamics following fire and clearcutting but the effect of stand-replacing windthrow on understory plant abundance, diversity, and composition, especially when followed by management interventions such as salvage logging and forest renewal, is poorly understood. We examined herbaceous and woody understory plant communities in aspen stands in the first 5years following stand-replacing windthrow and subsequent salvage logging and forest renewal operations. Five windthrow–renewal options were included: windthrow (W), windthrow–salvage (WS), windthrow–salvage–windrow (planting) (WSW), and windthrow–salvage–windrow (planting)–herbicide application (WSWH). The understory vegetation in these treatment plots was compared with that in a typical clearcut (C) (without post-harvest management interventions). The windthrow created understory communities that generally had higher Shannon diversity index and species evenness values, but lower percent cover than those in the C; species richness, however, did not differ between W and C treatments. Low intensity salvage logging (WS) in winter did not affect understory plant species cover, richness, Shannon diversity index, or evenness values compared to those in non-salvage logged areas (W). Community composition, however, differed due to increases in some herbaceous species, shrubs, and deciduous trees and damage to advanced growth conifers during the salvage logging operation. Similar treatment effects were also found for WSW. The herbicide application after planting (WSWH) significantly reduced woody cover and increased herbaceous plant species richness. Our results demonstrated that post-windthrow management interventions influenced understory community composition, with the effects of herbicide application being strongest. The changes in understory composition have significant implications to long-term stand structure and composition. We expect increasing abundance of conifers from advanced growth in the W treatment areas to planted trees in the WSW and WSWH treatment areas and of deciduous trees from WS and WSW to C treatment areas.

Aspen (Populus tremuloides) stand dynamics and understory plant community changes over 46 years near Crested Butte, Colorado, USA

Aspen stands in the Rocky Mountains form especially productive and diverse plant understory communities. However, little is known about canopy–understory relationships or understory dynamics, especially in light of widespread aspen decline. The purpose of our research was to assess recent aspen plant community dynamics by resampling 19 sites sampled in 1964 and 1994 near Crested Butte, Colorado. In 2010, we replicated previous sampling methods to measure canopy structure and understory composition at each site.In contrast with the conclusions of previous work in the study area, sampled aspen stands did not exhibit long-term stability, nor were they succeeding to conifers. Over the 46-year sample period, live aspen density and basal area both decreased significantly (3151–1605 stemsha−1; 39.2–33.7m2ha−1), with more pronounced decreases over the most recent sampling interval, 1994–2010. These changes appear linked to natural patterns of stand maturation, but may also have been enhanced by recent, drought-triggered mortality, as well as constraints on aspen regeneration imposed by elk and livestock. A striking decrease in the cover and frequency of fireweed (Chamerion angustifolium) was consistent with expectations of aging stands initiated by historic fire, and the expansion of Kentucky bluegrass (Poa pratensis) indicated strong effects of ungulate herbivory.Recent overstory losses had little effect on understory communities, which were dominated by a suite of large perennial herbs that showed little change over the sampling interval, including Fendler’s meadowrue (Thalictrum fendleri), oshá (Ligusticum porteri), elk and dryspike sedge (Carex geyeri and C. siccata), and blue wildrye (Elymus glaucus). Understory community composition was not closely related to current stand conditions or light levels, but was linked with regional elevational and geographic gradients and historic (1994) stand structure.

Brood cover and food resources for wild turkeys following silvicultural treatments in mature upland hardwoods

ABSTRACTClosed‐canopy, upland hardwood forests with limited understory development provide suboptimal habitat for wild turkey (Meleagris gallopavo) broods and may lead to low recruitment. Various forest management practices have been used to stimulate understory development within upland hardwoods, but evaluation of such practices on cover and food resources for wild turkey broods is incomplete. Therefore, we compared effects of 7 silvicultural treatments (repeated fire, shelterwood harvest, shelterwood harvest with one fire, retention cut, retention cut with repeated fire, retention cut with herbicide, and retention cut with herbicide and repeated fire) on cover and food resources for wild turkey broods in mature upland hardwoods of the Ridge and Valley Physiographic Province, Tennessee, USA, during 2000–2009. Canopy reduction treatments enhanced understory conditions for wild turkey broods. Eight years following initial treatment, light infiltration in retention cuts with repeated fire was 6 times greater than that within control, 5 times greater than that within shelterwood harvests, and twice that within shelterwood harvests with one prescribed fire. Woody species dominated understory composition following all treatments and controls. Understory disturbance (prescribed fire and broadcast herbicide treatments) reduced density of stems &gt;1.4 m tall and &lt;11.4 cm diameter at breast height (dbh) and created less visual obstruction above 1 m compared with canopy reduction treatments without understory disturbance. Following canopy reduction without repeated prescribed fire, woody vegetation exceeded ideal height for wild turkey broods after 3 growing seasons and light infiltration returned to control levels within 7 years. Soft mast production was greatest following treatments that included canopy reduction, but varied by year and site. Invertebrate biomass did not increase following any treatment, but all treatment areas contained enough invertebrates to meet the protein requirement for a wild turkey brood (10.1 poults) for 28 days on &lt;30 ha. Where wild turkey is a focal species and understory structure in mature upland hardwoods is limiting for broods, we recommend reducing canopy coverage to 60–70% and using low‐intensity fire every 3–5 years to enhance and maintain brood cover and increase food availability. © 2014 The Wildlife Society.

Increased air humidity and understory composition shape short root traits and the colonizing ectomycorrhizal fungal community in silver birch stands

Climate change is predicted to bring about a rise in precipitation and air humidity at northern latitudes, which could have considerable impact on forest management. This paper investigates the effect of increased air humidity and understory composition on short root morphology and on the relative abundance of colonizing ectomycorrhizal (EcM) fungal associates in silver birch (Betula pendula Roth.) stands.Short root morphological traits of silver birch were analyzed at increased humidity and ambient conditions for two different understories (early-successional grasses and diverse “forest” understory) in three consecutive years (2009–2011). The fungal community was determined in 2010 (after three seasons of misting) using molecular methods. The study was conducted on the Free Air Humidity Manipulation (FAHM) experimental facility established in Estonia.Silver birches responded to the rise in air humidity by forming longer and thinner short roots, which can be interpreted as a morphological adaptation leading to an increase in the absorptive area. The response was stronger when humidification concurred with the species-poor understory of pioneer grasses. The inter- and intra-treatment variation in short root morphological parameters decreased by the third year. Using molecular methods, overall 64 EcM taxonomic units were distinguished. Hydrophilic fungal morphotypes dominated significantly in humidified plots, hydrophobic morphotypes in control plots.Our results suggest that rising air humidity causes a morphological stress response in EcM short roots. Young trees show the ability to adapt to climate change with great plasticity by modifying short root length, diameter and specific root length (SRL). Humidification leads to a shift in the fungal colonizers towards the dominance of hydrophilic taxa, which may alter ecosystem functioning.

Bioengineer effects on understory species richness, diversity, and composition change along an environmental stress gradient

Bioengineer effects on understory species richness, diversity, and composition change along an environmental stress gradient

Historic disturbance regimes promote tree diversity only under low browsing regimes in eastern deciduous forest

Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large‐scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade‐tolerant, fire‐intolerant species that replace and suppress many fire‐tolerant species); (2) the gap hypothesis (small gaps typical of today's forests promote dominance of a few shade‐tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few unpalatable species). We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size ∼255 m2), and browsing by deer, and we followed the fates of &gt;28000 seedlings and saplings for five years. Understory tree communities in control plots were dominated (up to 90%) by Fagus grandifolia, averaging little more than two species, whereas overstories were diverse, with 10–15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity, because browsers reduced the benefits of gaps and created depauperate understories following fire. Consequently, two major disturbances that once promoted tree diversity no longer do so because of browsing. Our findings appear to reconcile equivocal views on the role of fire and gaps. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances promote diversity (three‐ to fivefold). Our results apply to large portions of eastern North America where deer are overabundant, and we provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre‐European settlement forests could maintain high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.

Variability of historical forest structure and fire across ponderosa pine landscapes of the Coconino Plateau and south rim of Grand Canyon National Park, Arizona, USA

We undertook reconstructions of historical ponderosa pine forest structure and fire regimes across an entire landscape to expand understanding of spatial variability in forest structure and dynamics. The study area includes the ponderosa pine forests of the Coconino Plateau, Arizona, USA. Using General Land Office survey data and newly developed methods, we examined surveyor descriptions of overstory and understory composition and structure across 60,998 ha. For 41,214 ha of forests, we reconstructed density, basal area, diameter-class distributions, and fire severity and used GIS to analyse their relationships with topography. Ponderosa pine forests were continuous in only 34 % of the 60,998 ha landscape. In 40 %, they were mixed with pinon–juniper, 24 % was pure pinon–juniper and 2 % were grass or shrubs. In ponderosa pine forests, the focus of this study, mean tree density was 144.2 trees ha−1; 18.8 % of the landscape had low tree density ( 200 trees ha−1). Small trees ( 50 % of all trees on 47 % of the landscape and large trees (>40 cm) composed >50 % of all trees on 21 % of the landscape. Low-severity fire likely structured the forest on 59 % of the landscape while 39 % was structured by mixed-severity fire, likely including small, patchy crown fires. Forest parameters displayed wide variability across environmental gradients. Broader-scale reconstructions revealed a much more spatially complex forest that was structured by wide range of fire severities. Variability in ponderosa pine forests was often under-reported in past studies, but is important in perpetuating biological diversity.