Understory Species Abundance Research Articles

Abundance, diversity and composition of understory plants along the altitudinal gradient and dominant overstory composition types in the temperate Himalayan region

IntroductionThe Indian Himalayan forests are remarkable landforms experiencing tremendous climatic variation, constituting complex and diversified ecosystems with prominent vegetation zones. Despite their global significance and substantial research efforts focused on plant diversity in the temperate Himalayan region, only a few studies have explicitly assessed the distribution patterns of understory vegetation in relation to forest compositional types along altitudinal gradients.MethodsTo cover a wide range of altitudes and diverse overstory compositions, stands were sampled across four altitudinal ranges from 1500 to 3500 meters above mean sea level with increments of 500 meters in elevation steps. The overstory compositions were classified on the basis of dominant tree species in each stand on the similar sites. Vegetation in the shrub and ground layers was surveyed by visually estimating the percentage coverage within circular plots.ResultsA total of 99 understory species including 37 species each in the shrub layer, 62 species in the herb layer vegetation were recorded. The abundance, species diversity and composition of understory vegetation differed significantly along the altitudinal gradient and dominant overstory composition types. Moreover, distinct understory vegetation communities were observed at lower elevations compared to higher elevations, with middle elevations exhibiting intermediate vegetation characteristics. The study also highlighted the importance of dominant overstory composition types in shaping the pattern of understory vegetation abundance, species diversity and composition in the temperate Himalayan region. The higher resource conditions associated with broadleaved stands supported higher understory species abundance at lower elevations, while the heterogeneous conditions induced by the mixedwood stands promoted higher understory species diversity.ConclusionThe hump shaped pattern along the altitudinal gradient appeared to be the most dominant pattern of plant abundance and species diversity and call for more conservation concern towards the middle elevation zones in the temperate Himalayan region. Furthermore, the management interventions should aim at maintaining diverse range of overstory composition types for conserving biodiversity and their ecological functions in the temperate Himalayan region.

Changes to the understory vegetation community of the Acadian Forest a decade after herbicide use

Abstract Herbicides are commonly used in forestry to enhance conifer tree growth by reducing the abundance of competitive, undesired, early successional species. Reducing the abundance of understory species could also lead to changes in community composition that need to be documented to aid the understanding of any downstream ecosystem changes. We examined the effects of glyphosate-based herbicides on the abundance, diversity, and community composition of the understory vegetation community of forests located in the temperate-boreal transition zone. We sampled 37 blocks in two ecoregions of the Acadian Forest in eastern Canada that were harvested over the last 15 years. Species richness, Shannon’s diversity, or evenness did not differ among blocks with different herbicide history. However, community composition differed between the non-herbicide and herbicide blocks in both ecoregions. Overall, 26.5% of the plant community variation was explained by the factors herbicide use (10.6%), Biomass Growth Index/site quality (8.6%), time since harvest (3.6%), and ecoregion (1.7%). We found 16 indicator species that differentiated the non-herbicide (9 species) and herbicide blocks (7 species). Indicator species for non-herbicide blocks included two blueberry species, three shrubs (two flowering), and two ferns, whereas indicator species for herbicide blocks were largely perennial forbs. Together, our results indicate that herbicide use does not alter species richness but does reduce shrub abundance, a change that persists throughout the 10 years post-herbicide application captured in our study. The reduced shrub layer likely leads to other changes in the plant community. Herbicide use is associated with subtle changes to the understory plant community, and these changes are missed when only alpha diversity is used to examine the effects of herbicides use on community composition.

Impact of white-tailed deer on northern white cedar recruitment beyond the browsing vulnerability zone: A broad-scale study

By selectively feeding on the new vegetation from forest regeneration, cervids influence the abundance of understory species. This can lead to recruitment failure for the consumed species. The impact of cervids on regeneration is generally studied at fine scales, making it difficult to generalize about the species distribution scale for co-occurring cervid and tree species. This is especially problematic for species such as northern white cedar (Thuja occidentalis L.), which is susceptible to browsing and has been declining throughout much of its range. In our study, we aimed to determine the effect of the spatio-temporal variations in white-tailed deer (Odocoileus virginianus Zimmerman) abundance on northern white cedar recruitment over a broad spatial scale. We combined distinct forest and wildlife monitoring databases for two Canadian provinces, Québec and New Brunswick. We also surveyed cedar regeneration and deer browsing pressure in a subset of plots located in Québec. In both provinces, we detected a negative effect of deer abundance and deeryard proximity on the recruitment of cedar saplings. In Québec, local deer browsing pressure also had a negative effect on cedar seedling abundance. As deer browsing pressure can vary considerably over a short distance, we recommend the inclusion of browsing indicators in national forest surveys to precisely assess the impact of browsing on regeneration and recruitment. This would allow managers to identify areas that are highly vulnerable to browsing and adapt forest management accordingly to reduce browsing-induced regeneration failures.

Soil and shrub differentially determine understorey herbaceous plant richness and abundance in a semi-arid riparian meadow

It had been widely recognized that abiotic and biotic filters determined local community assembly and species diversity hierarchically. However, few studies distinguished the effects of abiotic and biotic filters on species richness and abundance, respectively. We evaluated the difference of abiotic (soil) and biotic (overstorey shrub) filters affecting richness and abundance of understory communities and further disentangle the direct and indirect effects of these filters, focusing on three habitats types defined along an elevational gradient (river bank, floodplain and upper terrace), in a semi-arid riparian vegetation dominated by Caragana aurantiaca Koehne, a dwarf legume shrub, in Yili valley, Sinkiang, China. Understorey herbaceous species richness presented a single effect of soil abiotic filter, which was significantly correlated with soil pH, but not with shrub height and cover, indicating that soil constitutes the main filter determining the species pool available for assembly of understorey vegetation. In contrast to species richness, understorey species abundance presented a joint effect of soil and shrub filters, but the variation explained by the shrub canopies was higher than that of soil variables, indicating overstorey shrub acted as the main filter determining species abundance. In addition, the direct effects of overstorey shrubs through competition and facilitation on understory species abundance are greater than the indirect effects through influencing soil status at local scale. Greater attention to shrub for understory vegetation assembly is needed to assess the role of biotic filter in the semi-arid riparian zone.

Herbaceous Understory Indicators of Post-Harvest Recovery in Coast Redwood (<i>Sequoia sempervirens</i>) Forests

Assessment of forest recovery following disturbance is enhanced by the use of biological indicators. One such indicator, the abundance of understory species, was examined in coast redwood (Sequoia sempervirens) forests using non-metric multiple dimensional scaling (NMDS) and indicator species analysis (ISA). Randomly distributed 10 m diameter circular plots were employed to record the abundance of all understory species across three treatments: actively managed (0 - 45 years since harvest); mature second-growth (~80 - 120 years since harvest); and unharvested old-growth stands. NMDS with perMANOVA analysis signified separation between treatments with the shade tolerant herbaceous species Trillium ovatum, Viola sempervirens, and Oxalis oregana positively correlated with mature second-growth and old-growth treatments. ISA supported the inclusion of T. ovatum, with the addition of Prosartes hookerii, as indicators of mature second-growth and old-growth. Both NMDS and ISA specified associations for Ceanothus thyrsiflorus and Stachys bullata with actively-managed stands. Occurrence of non-natives was low across treatments, though significantly higher on actively managed stands, with the shade intolerant invasive plants, Cortaderia sp. and Myosotis latifolia, occurring exclusively in actively-managed sites.

Plant strategies in extremely stressful environments: are the effects of nurse plants positive on all understory species?

ABSTRACT Facilitation by nurse plants likely increases from outside the canopy to the center of the canopy as stresses decrease towards the center of the canopy. These stress gradients may be important in controlling plant distributions, with stress-tolerant species specializing outside the canopy or at the canopy edge, and stress intolerant species specializing at the center of the canopy. We tested if interactions with nurse plants control the distribution of understory species, and if plants species specializing in the understory environment experience higher physiological stress when grown outside the canopy than species specializing in the open environments. We tested these predictions in field sites in the arid environment of Saudi Arabia. We measured the environmental conditions, understory species abundance, and functional and physiological traits of species found under nurse plant Acacia gerrardii. We found that Acacia trees have an overall facilitative impact on the understory species. Species found more commonly under nurse tree canopies experience significant physiological stress when growing outside canopies. In contrast, species found more commonly outside canopies do not experience significant physiological stress when growing either under canopies or outside canopies. Our results demonstrate that differences in species ability to tolerate environmental stresses are important in structuring herbaceous plant communities under nurse plants in these extremely stressful environments.

Invasive shrub removal benefits native plants in an eastern deciduous forest of North America

AbstractIn eastern deciduous forests of North America, invasive shrubs are increasing in richness and abundance at the expense of native species across taxa. Invasive shrubs create an understory that is more dense than both recent and historical preinvasion conditions. Interest in invasive shrub removal to restore native habitat is growing, but our understanding of natural regeneration following treatment of a diverse invasive shrub community is lagging. Using an invasive shrub removal experiment, we provide insight into the effect of repeated removal of a suite of 18 invasive shrub species dominated by border privet (Ligustrum obtusifolium Siebold & Zucc.). In 2009, invasive shrubs were removed from five 20-m-diameter treatment plots, each with a paired control plot. Seven years later, we find an increase in plant diversity, native understory species abundance, and overstory tree species regeneration for individuals under a meter in height. For plants 1 to 4 m in height, the removal treatment has a positive effect on understory woody species, but there has been no change in regenerating overstory trees. A lack of overstory tree regeneration to greater heights is not surprising, given the time frame and the closed-canopy conditions. However, other factors, such as white-tailed deer (Odocoileus virginianus Zimmermann) browse, could be serving as an impediment to taller tree regeneration in the forest understory. An ambient sampling approach in unmanaged, invaded, and uninvaded forest has been used in other studies to estimate the potential impacts of invasive shrub species to native plant communities. However, in this study the ambient sampling approach underestimated the impacts of invasive shrubs compared to their experimental removal. Overall, invasive shrub removal increased plant diversity and allowed passive natural regeneration of native plants that exceeded native cover in the unmanaged, ambient forest under minimal invasive shrub abundance.

Effects of Variable-Retention Treatments on Numbers of Singing Small Passerine Birds in Pacific Northwest Forests

Forest birds are sensitive to habitat change and may be suitable for measuring responses to retention forestry. I present the short-term effects of 6 treatments in a 6 block randomized design experiment on 15 breeding small bird species in mature Douglas-fir (Pseudotsuga menziesii) forests in western Washington and Oregon, 1994–2001. The 13-ha treatment stands contained aggregated (A) green-tree retentions of 100, 75, 40 and 15%, and a dispersed tree distribution pattern (D) of 40 and 15% retention. I compared numbers of entire singing bird territories (abundances) mapped inside sampling plots in post-(2-y x) and pre-treatments (1 y). Species richness significantly declined in both 15% treatments. In the 15%D treatment, significantly lower richness and lower species similarity were less than A treatments with ≥40% retention, and significantly lower species diversity was less than the other treatments. The size of decline of abundances of canopy-associated species (summed members) increased with successively lower tree retention; the cavity-nesting species declined with lower snag retention in treatments ≤40% retention; and there was no response for species associated with understory vegetation, but medians of the percentage change in abundance of understory species were negative in cut treatments. There was no detectable difference in treatment effects between tree distribution cut patterns. Greater amounts of tree retention helped maintain composition and abundance better than less retention, and overall the variety of treatments maintained all species. For maintaining richness, similarity, and diversity, the 15%A and 40%D treatments were transitional between the A treatments ≥40% retention, where these community parameters were maintained, and 15%D, which did not maintain natural diversity or species persistence.

Interactions of management and white pine blister rust on Pinus strobiformis regeneration abundance in southwestern USA

Modelling natural regeneration is complex, and both natural and anthropogenic disturbances can alter forest trajectories. Pinus strobiformis (southwestern white pine, SWWP) is an important component of mixed conifer forests in the Southwest and management recommendations related to natural and planted regenerations are needed to guide conservation of SWWP in the face of an invasive disease (white pine blister rust, WPBR). Regeneration was surveyed across six mountain ranges, three silviculture treatments and two levels of disease severity in the Southwest US. Key findings were: (1) SWWP regeneration in stands with no recent management (<20 years) and high disease severity had unsustainable WPBR infection, (2) SWWP regeneration was less abundant but less likely to be infected in stands with recent management, (3) stands with high disease severity had fewer SWWP seedlings than stands with no or low disease severity and (4) SWWP regeneration densities were best predicted by other understory species abundance. We recommend silviculture treatments that reduce basal area to 9–10 m² ha⁻¹ and leave large canopy openings to enhance natural SWWP regeneration. Without creating conditions for disease-free regeneration to reach reproductive maturity, some stands may lose SWWP as an overstory component. Results may help refine SWWP management guidelines and expand conservation efforts in forests threatened by WPBR.

Browsing rates and ratios provide reliable indices of ungulate impacts on forest plant communities

Ungulate browsing affects many plant species, shifting patterns of relative abundance, plant community diversity, and ecosystem processes. Despite the strength and diversity of ungulate impacts, we lack comprehensive methods and a unified network for tracking and comparing ungulate impacts across the Great Lakes region. The Great Lakes Network Office (GLN) of the National Park Service has identified white-tailed deer as a keystone herbivore that could threaten the health of Great Lakes forests. To supplement their permanent vegetation monitoring program, GLN monitors deer and moose browsing pressure on woody and herbaceous plants at nine national parks in the region. We used data from these surveys to investigate relationships among patterns of browsing across species and sites with the goal of inferring efficient and reliable indicators of ungulate impacts. We also investigated the relationships between patterns of browsing on woody vegetation and among-site variation in the presence and abundance of understory species. Our results showed that ungulates clearly prefer certain woody species, but these vary across the region in response to local community composition. Ungulate impacts on understory species are correlated with woody browsing impacts but also vary with local ecological conditions. To build a more efficient and accurate composite indicator of overall ungulate impacts, we combined data on woody plant browsing with data on the abundance of specific understory species to infer overall levels of threat from ungulate browsing. Such combined indicators improve our ability to monitor the impacts of ungulate browsing, particularly if such methods can be expanded and standardized across the region.

Partitioning vascular understory diversity in mixedwood boreal forests: The importance of mixed canopies for diversity conservation

Progress in understanding the patterns of plant diversity requires a conceptualization and quantification of the hierarchy of patch configuration that exists in plant communities across levels of observation. In order to identify the scales at which vascular plant diversity is maximized, we investigated the hierarchical organization of understory vascular plant diversity in relation to canopy patch types in mature boreal mixedwood forests in western Canada. In each of two study areas within a 30km2 landscape (55° N, 112° E) we sampled four canopy patch types: conifer, mixed conifer-broadleaf, broadleaf and canopy gaps. Understory diversity (richness and Shannon’s (H′) index) was additively partitioned in relation to these four canopy patch types across a hierarchy of four scales; α-individual patch+β1-among patches within canopy patch type+β2-among canopy patch types within area+β3-between areas. We also examined understory species abundance patterns by means of rank-abundance plots fitted to relative abundance models. The largest partition of species richness was among patches within canopy patch type (β1). For Shannon diversity index, the largest partition was at the within-individual patch level (α1) indicating that evenness was high at the patch level but that dominance increased at higher levels in the hierarchy. The assessment of relative abundance by means of rank abundance plots suggested that the canopy patch types differed in terms of the ecological mechanisms influencing diversity patterns. Considering plant diversity within a hierarchical framework is critical for the understanding and management of biodiversity as maximum levels of plant richness and evenness do not necessarily occur at the same observational scales. Management practices which retain the natural hierarchies of vegetation patches will help conserve plant community richness and diversity.

Tree canopy effects on understory species abundance in high-latitudePopulus tremuloidesstands, Yukon, Canada

To determine the effect of tree canopy composition on understory species abundance, three-hundred 2 m × 2-m quadrats from 30 high-latitude boreal forest stands were sampled. In addition, all trees within a 3 m radius of each quadrat center and ≥1 m tall were also measured for height, basal diameter, and canopy width (n = 3130). Stands were 33–178 years old, with canopies of Populus tremuloides (trembling aspen) and Picea spp. (spruce) in varying proportions. Arctostaphylos uva-ursi, Calamagrostis purpurascens, Chamerion angustifolium, Shepherdia canadensis. and Hylocomium splendens were the most frequent understory species among quadrats. Scatterplots of P. tremuloides and individual vascular understory species cover values lacked bivariate trends, but the understory species had distinct maxima that ranged from 20 to 90%. A moderately strong correlation (r = 0.52, P <0.001) occurred between P. tremuloides canopy and total vascular understory plant covers, but weak individual species correlations (r = 0.22–0.35, P <0.001), suggested understory species variation was primarily determined by factors other than the amount of immediately overhead canopy cover. Canonical correlation analysis (R = 0.82, P <0.001) indicated that greater vascular understory plant cover occurred when forest stands consisted of P. tremuloides with large canopies and large basal diameters, and lacked Picea. Maximum cover for vascular understory species declined when Picea cover exceeded 7–10%. In combination, P. tremuloides stem densities or a metric based on summed canopy areas converted to a diameter value (canopy-area diameter), and the vertical silhouette area of Picea canopies (canopy profile area), as independent linear regression variables, explained ~79% of the variance in total vascular understory plant cover. Several Picea basal area-derived metrics were strongly and positively associated with increasing H. splendens cover, but canopy profile area was more informative. Populus tremuloides canopy area and Picea canopy profile area, as indicators of shading, may be important determinants of vascular understory vegetation abundance in stands where solar radiation enters at angles of up to 52 during the summer.

Long‐term vegetation responses to reintroduction and repeated use of fire in mixed‐conifer forests of the Sierra Nevada

Nearly a century of fire suppression has changed fundamental aspects of the structure and functioning of fire‐adapted forests throughout the western U.S. Prescribed fire is increasingly used to restore forest structure and reduce surface fuels with limited consideration of its consequences for biological diversity. In this study, we used more than two decades of data from permanent plots in mixed‐conifer forests of Sequoia and Kings Canyon National Parks, California, to explore changes in plant diversity and abundance following reintroduction and repeated use of fire. Data on stand structure, fuel loading, fire severity and heterogeneity, and the richness and abundance of major growth forms were collected on 51 plots representing one of three treatments: control, first‐entry burn, and second‐entry burn. Understories showed distinct compositional changes over time in first‐ and second‐entry burns. Burned plots supported more than twice as many species as controls 10 yr after treatment; first‐entry plots showed a nearly threefold increase in richness by year 20. Burned plots supported four to five times as many shrub species as controls 5–10 yr after burning. Total plant cover (dominated by perennial forbs and shrubs) increased in first‐entry plots, but did not differ from controls until 20 yr after treatment. Following second‐entry, cover did not change through final sampling (year 10). Nonnative species were rare, occurring in only three plots at low abundance. Higher severity fires led to greater numbers of species and to greater plant cover. Species richness was not correlated with burn heterogeneity. Long‐term observations suggest that reintroduction of fire in previously unmanaged forests can gradually enhance the diversity and abundance of understory species. Repeated burning—necessary to achieve structural and fuel‐reduction objectives—does not appear to have a detrimental effect on plant diversity and may enhance the distributions of species that are adversely affected by fire exclusion. If fire is to play an important role in restoration, however, it will need to be maintained as a frequent and spatially dynamic process on the landscape.

Effect of Hydrologic Restoration and Lonicera maackii Removal on Herbaceous Understory Vegetation in a Bottomland Hardwood Forest

Abstract Amur honeysuckle (Lonicera maackii (Rupr.) Herder), a large deciduous shrub from China, has invaded many forests in eastern/central United States. The species was removed by cutting and herbicide application from a recently hydrologically restored section of a bottomland hardwood forest in central Ohio, and the response of understory plants, especially herbaceous species, was measured. Plots were established in uncleared and cleared sections, and percent cover of each herbaceous understory species was estimated monthly. One season after several years of Lonicera removal efforts, no significant association was discovered between percentage of Lonicera cover and total understory species abundance. There was, however, a direct correlation between elevation and honeysuckle abundance; L. maackii abundance was negatively associated with low elevations, likely due to hydrologic factors. Plant species diversity (H) and richness (s) increased with elevation but were not significantly different on plots with honeysuckle removal (H = 0.86 ± 0.08 vs. 0.78 ± 0.09 and s = 4.4 ± 0.19 vs. 4.2 ± 0.2 species/m2, respectively) despite the fact that understory light levels measured by densiometer were significantly higher (α= 0.003) in cleared versus uncleared sections. Native and invasive species were found in similar proportions in the two sections, and significant sprouting and regrowth of L. maackii were observed throughout the cleared section. Although the removal of L. maackii altered the characteristics of the plant species assemblage, the value of this management remains questionable in the years immediately following treatment.

Changes in stand structure and composition after restoration treatments in low elevation dry forests of northeastern Oregon

In many fire-dependent forests in the United States, changes occurring in the last century have resulted in overstory structures, conifer densities, down woody structure and understory plant communities that deviate from those described historically. With these changes, many forests are presumed to be unsustainable. Broad-scale treatments are proposed to promote stand development on trajectories toward more sustainable structures. Yet little research to date has identified the effects of these restoration treatments, especially in low elevation dry ponderosa pine ( Pinus ponderosa) and Douglas-fir ( Pseudotsuga menziesii) forests of northeastern Oregon. We report changes in tree structure, coarse woody debris (logs), and understory composition from an operational-scale, replicated ( N = 4), completely randomized experiment. Treatments included a single entry thin from below conducted in 1998, a late season burn conducted in 2000, a thin followed by burning (thin + burn), and a no action treatment which served as a control. Changes in live and dead tree structure and understory vascular plant community composition were compared between pre-treatment and 2004. Tree seedling density and composition and coarse woody debris structure were evaluated in 2004. Thin, burn, and thin + burn treatments reduced the density but not the basal area of live overstory trees. Thinning reduced the number of medium-diameter trees, and tended to decrease the abundance of shade tolerant, moist-site understory species yet increased the dominance of several rhizomatous species such as Calamagrostis rubescens, Symphoricarpos albus, and Spiraea betulifolia. Burning alone had little effect on large trees but reduced the number of small Douglas-fir and logs. Shade tolerant perennial species associated with fine textured soils such as S. albus, Spiraea betulifolia, C. rubescens, Carex geyeri, and Arnica cordifolia increased in frequency and average cover with burning. Conversely, cover of the bunch grass Festuca idahoensis was reduced while non-native invasive species establishment was little affected. Ordination scores suggested that burning increased the abundance of species representing greater shade tolerance and finer-textured soils. The thin + burn treatment left both ponderosa pine and Douglas-fir with modal or normal diameter distributions, and increased the abundance of understory species representing shallow, coarse texture soils. These results are discussed in the context of management options for restoration of ecosystem health in similar low elevation dry ponderosa pine and Douglas-fir forests.

Changes in Understory Species Diversity during the Nothofagus pumilio Forest Management Cycle

Human commercial activities must be compatible with sustainable management to achieve species conservation in the natural environment. After harvesting, diversity and abundance of understory species differ substantially from the pre-harvest situation. Have knowledge of the changes that logging produces in all components of the system allows us to develop management alternatives with reduced impacts. The objective of this work was to evaluate changes in density, biomass and diversity of understory species along a forest shelterwood cut system management cycle in a Nothofagus pumilio forest. Six forest management situations (postharvesting stands and different phases of forest development) were sampled in a pure N. pumilio forest of Tierra del Fuego (Argentina) of middle-high site class (from class I to III). Species number, density and biomass were modified substantially after a shelterwood cut. These were being greatest after cutting and falling substantially when the forest canopy closes during the initial growth phase, returning to values similar to this virgin forest when the mature phase was reached. The impact of the forest management on understory species diversity is analyzed and discussed. The silvicultural practices applied to N. pumilio forest in Tierra del Fuego permit the introduction of alien flora to the system, and produce great changes during the forest management cycle.

Modeling the distributions of shrub species in Pacific northwest forests

In Pacific Northwest forests, understory species are important elements of forest structure and composition, providing habitat and forage for wildlife and contributing to much of the floral diversity. Knowledge of the factors that control the distribution and abundance of understory species is central to understanding the ecology and dynamics of natural forests and how they will respond to management. Generalized linear models of the binomial family were used, at two spatial scales, to estimate the probability of occurrence of shrub species as a function of environmental variables and the cover of mature and regenerating tree species. Graphical analysis and resampling techniques were used to: (1) examine whether species show unimodal responses to environmental gradients; (2) explore the scale-dependence of model forms; and (3) predict areas of high probability of species occurrence within a multi-dimensional environmental space. Results suggest that the concept of unimodal response to environmental gradients can be used to improve predictions about species' distributions, and to strengthen the biological interpretation of statistical models. They also demonstrate the scale-dependence of model equations and the likelihood that key processes influencing plant distributions may change, or appear different, when viewed at different scales.

Effects of prescribed fire on the composition of woody plant communities in southeastern Texas

Abstract. The response to prescribed burning of plant communities ranging from dry to wet habitats was monitored using permanent plots sampled from 1989 to 1993. Temporal controls for fire effects were provided by matched sets of plots protected from fire by newly constructed fire breaks. Changes in species composition were studied by ordination of strata of trees (&gt; 5 cm DBH), small trees (2–5 cm DBH), large saplings (1–2 cm DBH), and small saplings and seedlings (50–140 cm tall). Results show that changes occurred largely in the small tree stratum, in which xeric species increased in importance. Although there were changes in sapling and seedling strata, no clear direction of change was recognized. Fire had little effect on the tree stratum.Of the seven community types under study, three types, sandhill, upland pine, and upperslope pine‐oak, were most strongly affected, as indicated by post‐fire change in positions of samples representing these communities in ordination space and reduction in understory species abundance. Samples representing the other four mesic and wet communities showed little or no change in their positions.These short‐term results indicate that changes in vegetation resulting from fire were small and were mostly restricted to the dry types in which possible compositional change is expected to occur. This differential effect of fire suggests that the influence of fire is secondary to that of topographical and soil gradients in determining vegetation pattern under current fire regimes. Fire seems to reinforce an overall vegetation gradient controlled by soil in southeastern Texas.

Understory vegetation as a useful predictor of natural regeneration and canopy dynamics in Pinus sylvestris forests in Italy

The relations between understory vegetation, canopy characteristics and natural regeneration have been studied in natural Scots pine forests growing in sub-Mediterranean conditions in Italy. Multivariate ordination techniques (detrended correspondence analysis, DCA, and detrended canonical correspondence analysis, DCCA) have been applied to extract vegetation gradients. The first four DCA axes accounted for 41% of the total variation in vegetation data and DCA ordination patterns have been interpreted by the variability of forest stands, ranging from pioneer pine communities to closed pine stands mixed with hardwood species. Characteristic indicator values (CIVs), computed by understory species abundance using the Ellenberg's species scores, have been tentatively used as estimators of environmental variability. Relating vegetation gradients extracted by DCA to CIVs allowed further interpretation of the multivariate ordination patterns. Geographic and edaphic factors had only a minor effect on plant communities in the present study. The competition exerted in mixed stands by hardwood species seems to be the main limiting factor for Scots pine recruitment in the study area. Multivariate synthetic variable and CIVs were found to predict a large proportion of variation in Scots pine recruitment. The application of CIVs for predicting ecological meaningful conditions and their use as a tool for management decisions is discussed.

Changes in the understory of Canadian southern boreal forest after fire

Abstract. We investigated changes in the composition and abundance of understory species after fire in the southern boreal forest around Lake Duparquet, Québec. Ten plots of 100 m2 were sampled in each of eight sites varying in post‐fire age from 26 to 230 yr, with 20 1‐m2 quadrats in each of these 80 plots. Variation in the understory was described by DCA ordination and interpreted as a regeneration succession series. Thickness of the organic layers, stand age and canopy composition were all correlated with vegetational change. This change was not constant throughout succession; some old sites showed an increase in the diversity and abundance of certain pioneer species. This was partly related to openings in the canopy resulting from a major outbreak of spruce budworm, which affected sites dominated by Abies balsamea. The ordinations were performed on both the 100‐m2 plots and the 1‐m2 quadrats. Heterogeneity within sites was larger at the 1‐m2 scale and there was a great deal of overlap in the position of the quadrats in ordination space. At the smaller scale of analysis, stand age and thickness of the organic layers were not correlated with the changes observed in the understory.