Overstorey Trees Research Articles

Predicting mortality of ponderosa pine regeneration after prescribed fire in the Black Hills, South Dakota, USA

Reduction of crown fire hazard in Pinus ponderosa forests in the Black Hills, SD, often focuses on the removal of overstorey trees to reduce crown bulk density. Dense ponderosa pine regeneration establishes several years after treatment and eventually increases crown fire risk if allowed to grow. Using prescribed fire to control this regeneration is hampered by the limited knowledge of fire-related mortality threshold values for seedlings (<1.4 m tall) and saplings (0.25 to 10 cm diameter at breast height). The present study was initiated to assess fire-related mortality of ponderosa pine seedlings and saplings on prescribed burns across the Black Hills. We established plots in several burn units after the first post-fire growing season to measure crown volume scorch, crown volume consumption, basal scorch, and ground char for ponderosa pine seedlings and saplings. Logistic regression was used to model the probability of mortality based on tree size, flame length, and direct fire effects. Tree size, flame length, crown damage, ground char, and basal char severity were all important factors in the prediction of mortality. Observed mortality was >70% for seedlings but was only 18 to 46% for sapling-sized trees. The differences in mortality thresholds for ponderosa pine seedlings and saplings highlight their susceptibility to different damage pathways and give managers several options when designing burn prescriptions.

Overstorey tree density and understorey regrowth effects on plant composition, stand structure and floristic richness in grazed temperate woodlands in eastern Australia

As natural woodlands decline in both extent and quality worldwide, there is an increasing recognition of the biodiversity conservation value of production landscapes. In low-input, low-productivity grazing systems in Australia, the modification of natural woodlands through overstorey tree and woody regrowth removal are vegetation management options used by landholders to increase native grass production for livestock grazing; however, there is little empirical evidence to indicate at what tree densities biodiversity attributes are compromised. We examined the effects of overstorey tree density and understorey regowth on the floristic composition, stand structure and species richness of eucalypt woodlands in a grazing landscape in the Traprock region of southern Queensland, Australia. We sampled 47 sites stratified according to vegetation type ( Eucalyptus crebra/Eucalyptus dealbata woodland; Eucalyptus melliodora/Eucalyptus microcarpa grassy woodland), density of mature trees (<6 trees/ha; 6–20 trees/ha; >20 trees/ha), and presence/absence of regrowth. Distinct patterns in composition were detected using indicator species analysis and non-metric multidimensional scaling, with low density areas compositionally indistinguishable, although distinct from other land management units. Within vegetation type, medium tree density woodlands were compositionally similar to high density and reference woodlands. Species richness ranged from 18 to 67 species per 500 m 2 across all sites. No differences in total or native species richness were detected across management units; however, some differences in exotic species richness were detected. Differences in grass cover existed between low and high density management units, yet no difference in grass cover was evident between low and medium density management units. Our results suggest that medium tree densities may provide biodiversity benefits concordant with more natural areas, yet not adversely impact on pasture production. Retaining trees in grazing landscapes provides significant landscape heterogeneity and important refuges for species that may be largely excluded from open grassland habitats. Maintaining a medium density of overstorey trees in grazed paddocks can provide both production and biodiversity benefits.

Long term accumulation of nitrogen in soils of dry mixed eucalypt forest in the absence of fire

In the Eden area in NSW, Australia, low fertility granitic surface soils were sampled from 156 sites and analysed for pH, organic C, total N, total P, available P, exchangeable bases and exchangeable Al. Fifty eight of these sites were also sampled to a depth of 40 cm. Time since fire ranged from 1 to 39 years and was used in the analysis as a surrogate for fire frequency. No information was available on fire intensity. No significant relationships were found between time since fire and P or base cations. However, the quantities of organic matter and total N (kg ha −1), and the C/N ratio were significantly related to both time since fire alone and to the combination of time since fire and soil total P. Based on these relationships, it was estimated that there were average net increases of between 11 and 21 kg N ha −1 year −1 in surface soil, the actual quantity depending on the level of soil total P. There was little change in N in the initial 10 years after fire and there was a peak in N accumulation about 24 years after fire. The C/N ratio and surface soil pH decreased with time since fire. Accumulation of N and reductions in pH and C/N ratio were studied further in a small scale paired plot analysis. The repeatedly burnt plots had lower levels of both litter and understorey and the overstorey trees generally had healthier crowns than in the unburnt plots. The differences between the repeatedly burnt and the unburnt plots matched the models developed from the general survey. There were no significant changes in the C/N ratio, but the unburnt sites had higher levels of extractable mineral N and the relationships between the mineral N and the C/N ratio for burnt and unburnt sites were statistically significant. The quantities of extractable mineral N in the unburnt soils (2.3 kg N ha −1) were about twice the levels in the burnt soils (1.2 kg N ha −1). The pH of the surface soil (4.4 in 1:1 water) in the regularly burnt area was higher than in the unburnt area (pH 4.1) and the exchangeable aluminium also differed (0.62 c mol −1 in the burnt area and 1.3 c mol −1 in the unburnt). The combined data indicate that changes occur in forest soils when there is a long period of exclusion of fire. It is suggested that these changes generally lead to secondary changes, such as in pH and availability of other elements such as aluminium. The study highlights a number of issues including the rates of inputs of N to the system and the question of N saturation and its long term interaction with plant species. It is hypothesised that reduced burning leads to increased N availability and other soil changes which negatively impact on tree health.

Wetland and terrestrial vegetation change since European settlement on the Fleurieu Peninsula, South Australia

Microfossil, sediment and documentary records provide a history of European land use and its impact on the vegetation of the Fleurieu Peninsula, South Australia. Two sedimentary cores were analysed for their fossil pollen and charcoal composition. Chronologies were established using a combination of 210Pb, 14C and microfossil markers. Primary and secondary evidence for the spatial expansion of land uses in the region were compiled providing local-, bioregional- and regional-scaled European settlement histories. The settlement and land-use histories of the major vegetation types in the region were different and were closely determined by the nature of the vegetation itself. The sedimentary and microfossil records indicate that wetland and terrestrial vegetation have undergone sequential changes of composition. There is evidence of a decline in fire-sensitive understorey species and the decline is likely due to intensive firing and grazing of scleropyllous woodlands and forests early in European settlement. Early-settlement native forestry practices were intensive, however they did not alter overstorey tree composition. Mid-twentieth-century wholesale vegetation clearance is clearly marked in the pollen record by a decline in Eucalyptus and increase in herbaceous species. Wetland vegetation was highly impacted by European land practices through changes to sediment inputs and hydrological conditions that began prior to catchment clearance, during the phase of intensive firing and grazing. Through the integration of multiscaled, ecosystem-specific historical settlement histories and palaeoecological analysis, correlations between past land uses and biotic responses can be confidently demonstrated.

Overstorey tree species regulate colonization by native and exotic plants: a source of positive relationships between understorey diversity and invasibility

ABSTRACTThe North American woody species, Prunus serotina Ehrh., is an aggressive invader of forest understories in Europe. To better understand the plant invasion process, we assessed understorey plants and Prunus serotina seedlings that have colonized a 35‐year‐old replicated common‐garden experiment of 14 tree species in south‐western Poland. The density and size of established (&gt; 1 year old) P. serotina seedlings varied among overstorey species and were related to variation in light availability and attributes of the understorey layer. In a multiple regression analysis, the density of established P. serotina seedlings was positively correlated with light availability and understorey species richness and negatively correlated with understorey species cover. These results suggest that woody invader success is adversely affected by overstorey shading and understorey competition for resources. Simultaneously, however, invader success may generally be positively associated with understorey species richness because both native and invasive plant colonization respond similarly to environmental conditions, including those influenced by overstorey tree species. Identification of characteristics of forests that increase their susceptibility to invasion may allow managers to target efforts to detect invasives and to restore forests to states that may be less invasible.

Association of reduced riparian vegetation cover in agricultural landscapes with coarse detritus dynamics in lowland streams

Studies were conducted on streams flowing through agricultural floodplains in south-eastern Australia to quantify whether reductions in riparian canopy cover were associated with alterations to the input and benthic standing stocks of coarse allochthonous detritus. Comparisons were made among three farmland reaches and three reaches within reserves with intact cover of remnant overstorey trees. Detritus inputs to these reaches were measured monthly over 2 years using litter traps. Direct inputs to streams within the reserves were relatively high (550–617 g ash free dry weight (AFDW) m–2 year–1), but were lower at farmland reaches with the lowest canopy covers (83–117 gAFDW m–2 year–1). Only a minor fraction of the total allochthonous input (&lt;10%) entered any of the study reaches laterally. The mean amounts of benthic detritus were lowest in the most open farmland reaches. Standing stocks of benthic detritus were found to be highly patchy across a large number of agricultural streams, but were consistently very low where the streamside canopy cover was below ~35%. Canopy cover should be restored along cleared agricultural streams because allochthonous detritus is a major source of food and habitat for aquatic ecosystems. Given the absence of pristine lowland streams in south-eastern Australia, those reaches with the most intact remnant overstorey canopies should be used to guide restoration.

Eucalypt health and agricultural land management within bushland remnants in the Midlands of Tasmania, Australia

In temperate Australia, sustainability of biodiversity in poorly-reserved woodland remnants within agricultural landscapes is a priority. Cornerstone species, such as those comprising the tree overstorey, are essential as a seed source for regeneration and for microclimate amelioration, soil erosion prevention and habitat provision in woodland ecosystems. However, trees within many woodland remnants are rapidly deteriorating in health. This has been attributed to several causes, including three decades of below average rainfall in temperate Australia. However, instances where trees are healthy and declining on either side of a management boundary indicate a key role of management in the condition of woodland remnants. We investigated the effects of past management on soil properties and vegetation understorey, and linked these with overstorey tree health across 49 sites within the agricultural Midlands of Tasmania, Australia. Sixty percent of the variation in overstorey tree health was associated with the cover of native shrubs, litter, moss and lichen in healthy sites and with cover of exotic pasture species in declining sites. Soil attributes explained 72% of the variation in tree health, with healthy sites having lower soil total nitrogen and pH, and higher soil organic carbon. A combination of soil and understorey vegetation attributes entirely separated healthy, declining and poor sites in a canonical analysis. Regression tree analysis indicated that grazing history (fencing, grazing frequency and intensity) was the primary management history factor in separating healthy and poor sites, whilst patch size, fire frequency and wood gathering were secondary, but significant, factors. Sites that were only lightly, if at all, grazed, had a fire frequency >10 years, and did not have coarse woody debris removed were healthy, indicating that the drying and warming climate of the past three decades is within the bioclimatic envelope of the species examined.

Comparison of community composition and species diversity of understorey and overstorey tree species in a dry tropical forest of northern India

The study focuses on understorey–overstorey plant community dynamics in a dry tropical forest to facilitate appropriate management decisions. We compare community composition and species diversity of the understorey vegetation among five dry tropical forest sites in northern India. A total of 1500 quadrats distributed over 15 one-ha permanent plots in five sites differing in the degree of disturbance, were used to enumerate the understorey tree species and the results were compared with overstorey tree layer. The non-metric multidimensional scaling (NMS) ordination revealed that human disturbance intensity, as well as the overall disturbance regimes, and soil water holding capacity controlled the organisation of dry tropical forest understorey composition through effects on soil organic matter. The α-diversity and its components decreased with increasing human disturbance intensity, reflecting utilisation pressure and decreased soil fertility, as also revealed by the analysis of overstorey tree layer. There was a significant positive relationship between overstorey and understorey diversity. Results suggest that in the future, the existing understorey tree communities may replace the current dry tropical forest communities under prevailing environmental conditions. The study also asserts that the rate of species accumulation will be greater in more disturbed sites as well as at small spatial scale within each disturbance level.

A dendro‐ecological study of forest overstorey productivity following the invasion of the non‐indigenous shrub Lonicera maackii

AbstractQuestion: Will a non‐indigenous, invasive, understorey shrub, such as Lonicera maackii (Amur honeysuckle) have an impact on the productivity of overstorey trees in hardwood forests?Location: Trees from 12 invaded and four non‐invaded sites were sampled in hardwood forests of southwestern Ohio, US.Methods: Changes in radial and basal area tree growth in the ten years prior to L. maackii invasion vs. ten years after invasion were examined using dendrochronological techniques. Intervention analysis was used to detect growth changes 25 years prior to and 25 years following invasion, and estimates of load impacts for L. maackii population and biomass were also calculated.Results: We found that the rate of radial and basal area growth of overstorey trees was reduced significantly in eleven out of twelve invaded sites. Non‐invaded sites did not exhibit this consistent pattern of reduced growth. For invaded vs. non‐invaded sites, the mean basal area growth was reduced by 15.8%, and the overall rate of basal area growth was reduced by 53.1%. Intervention analysis revealed that the first significant growth reductions were 6.25 ± 1.24 (mean ± SE) years after invasion with the greatest frequency of negative growth changes occurring 20 years after invasion. In invaded stands, 41% of trees experienced negative growth changes. In terms of invasive load estimates per 1000 L. maackii individuals, radial tree growth was reduced by 0.56 mm.a−1, and basal area growth was reduced by 0.74 cm2.a−1, Given these findings, significant economic losses could occur in hardwood forests of Ohio.Conclusions: To our knowledge, this is the first study using dendrochronological techniques to investigate the impact of a non‐indigenous, understorey plant on overstorey tree growth. Active management will likely be needed to maintain forest productivity in L. maackii impacted landscapes.

Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands

AbstractQuestions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands?Location: Minnesota, USA.Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long‐term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover.Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4‐grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3‐grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N‐fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover.Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire‐resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light‐demanding grasses and forbs.

Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands

Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands? Location: Minnesota, USA. Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long-term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover. Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4-grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3-grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N-fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover. Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire-resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light-demanding grasses and forbs.

A dendro-ecological study of forest overstorey productivity following the invasion of the non-indigenous shrub Lonicera maackii

Question: Will a non-indigenous, invasive, understorey shrub, such as Lonicera maackii (Amur honeysuckle) have an impact on the productivity of overstorey trees in hardwood forests? Location: Trees from 12 invaded and four non-invaded sites were sampled in hardwood forests of southwestern Ohio, US. Methods: Changes in radial and basal area tree growth in the ten years prior to L. maackii invasion vs. ten years after invasion were examined using dendrochronological techniques. Intervention analysis was used to detect growth changes 25 years prior to and 25 years following invasion, and estimates of load impacts for L. maackii population and biomass were also calculated. Results: We found that the rate of radial and basal area growth of overstorey trees was reduced significantly in eleven out of twelve invaded sites. Non-invaded sites did not exhibit this consistent pattern of reduced growth. For invaded vs. non-invaded sites, the mean basal area growth was reduced by 15.8%, and the overall rate of basal area growth was reduced by 53.1%. Intervention analysis revealed that the first significant growth reductions were 6.25 ± 1.24 (mean ± SE) years after invasion with the greatest frequency of negative growth changes occurring 20 years after invasion. In invaded stands, 41% of trees experienced negative growth changes. In terms of invasive load estimates per 1000 L. maackii individuals, radial tree growth was reduced by 0.56 mm.a−1, and basal area growth was reduced by 0.74 cm2.a−1, Given these findings, significant economic losses could occur in hardwood forests of Ohio. Conclusions: To our knowledge, this is the first study using dendrochronological techniques to investigate the impact of a non-indigenous, understorey plant on overstorey tree growth. Active management will likely be needed to maintain forest productivity in L. maackii impacted landscapes.

Diversity and spatio-temporal dynamics of dead wood in a temperate near-natural beech forest (Fagus sylvatica)

The diversity, spatial patterns and temporal dynamics of dead wood were examined within the near-natural beech forests (Fagus sylvatica) of Serrahn (North-eastern Germany). Data were collected in an 8 ha sample plot and in two permanent plots (0.36 and 0.25 ha) that had been established at the end of the 1960s. The mean volume of dead wood was 94 m3 ha−1, amounting to 14% of the total volume of all trees. The dead wood displayed a large variation in dead wood type, tree size and decay class. Standing dead wood accounted for about one-third of the total dead wood volume. The densities of standing dead trees were about 10% of the densities of the living trees over a wide range of diameters. The overall spatial distribution of dead trees exhibited a random pattern. Among the different dead wood types, standing entire dead trees and uprooted trees deviated from this pattern by displaying a significantly aggregated pattern. In the permanent plots a high mortality of overstorey trees was observed (1.3% year−1) and the average amount of dead wood increased greatly from 2.9 to 111.6 m3 ha−1 over the 35-year observation period. The near-natural beech forests of Serrahn have experienced a long period of low human interference. Nevertheless, our results suggest that the structure and dynamics of dead wood are strongly affected by the last major disturbance events that took place at the end of the Middle Ages. Information about the forest history is therefore a basic requirement when interpreting the results obtained in near-natural forests.

Impacts of restoration treatments on alien plant invasion in Pinus ponderosa forests, Montana, USA

Summary Invasion by alien plant species represents a challenge to land managers throughout the world as they attempt to restore frequent fire‐adapted ecosystems following decades of fire exclusion. In ponderosa pine Pinus ponderosa forests of western North America, the response of alien species to restoration treatments has not been well documented, particularly for alien species capable of altering environmental conditions (transformers). Understanding alien species dynamics is critical for developing treatments that accomplish restoration goals while minimizing alien invasion. We used a replicated, randomized block experiment to compare the effects of an untreated control and thin‐only, burn‐only and thin‐burn treatments on alien and transformer understorey species at multiple spatial scales (1 m2, 100 m2 and 1000 m2). Data were collected pre‐treatment and for multiple post‐treatment years. We compared richness and cover of alien species and transformer species among treatments, and identified environmental variables correlated with transformer species cover. Indicator species analysis was used to identify transformer species associated with specific treatments. Alien and transformer species richness and cover were significantly higher in the thin‐burn than in all other treatments at all spatial scales. Thin‐only and burn‐only treatments showed greater alien and transformer species responses than the control at the larger 100‐m2 and 1000‐m2 scales. Increased transformer cover was strongly correlated with increased tree crown scorch height and removal of overstorey trees. The thin‐burn treatment had four transformer species as indicators, the thin‐only had one, while the burn‐only and control had none. Synthesis and applications. The results show that alien species, including transformers, respond to restoration treatments, especially the combined thin‐burn treatment. Therefore monitoring for alien species invasion is an essential component of a restoration programme. Abundance of transformer species increased with increasing disturbance intensity, suggesting that less intense single‐disturbance treatments (burn‐only, thin‐only) or incremental treatments may be preferred in some applications. Where more intense treatments are required to meet management objectives, specific strategies, such as seeding of native species, limiting grazing before and after treatment and harvesting over a protective winter snowpack, may be necessary to limit alien invasion.

Pine savanna overstorey influences on ground‐cover biodiversity

AbstractQuestion:Does the overstorey of pine savannas influence plant species biodiversity in the ground cover?Location:Camp Whispering Pines (30°41’N; 90°29’W), eastern Louisiana (USA).Methods:We used ecologically sensitive restoration logging to remove patches ofPinus palustris(longleaf pine) in a second‐growth loess plainPinus palustrissavanna managed using frequent lightning season fires. Five years later, we measured numbers of vascular plant species and transmitted light in replicated 100‐m2plots. Treatments involved three different overstorey conditions: no overstorey for 5 years, no overstorey for several decades, and overstorey pines present for decades.Results:Both recent and long‐term openings contained, on average, about 100 vascular plant species per 100 m2, 20% more than in similar‐sized areas beneath overstorey trees. Responses varied with life form; more herbaceous species occurred in recent and older overstorey openings than beneath overstorey trees. Total numbers of all species and of less abundant forb species were positively and linearly related to light transmitted to ground level. Those species responding to openings in the overstorey and positively associated with increased transmitted light levels were monocarpic and shortlived perennial forb and grass species with a seed bank in the soil. In addition, community structure, as reflected in species composition and abundances, appeared to vary with canopy condition.Conclusions:Restoration involving ecologically sensitive removal of patches of overstorey pines in frequently burned pine savannas should benefit the ground cover and increase plant species biodiversity as a result of increased abundance of seed bank species.

Early regeneration of Fraxinus rhynchophylla in the understorey of Larix kaempferi stands in response to thinning

The effects of thinning were investigated on naturally regenerated Fraxinus rhynchophylla Hance (Korean ash) grown in the understorey of a Larix kaempferi (Lamb.) Carr. plantation, established on a site that previously supported old growth hardwood forests. The regeneration characteristics were evaluated after thinning of the plantation (about 60 per cent of relative density was removed). In the naturally regenerated and mixed stands, Quercus mongolica Fisch. was the most abundant tree species in the overstorey, but the understorey of the thinned plantations was dominated by F. rhynchophylla with six other minor tree species. Fraxinus rhynchophylla was regenerated by four propagation types including natural seedling, stump sprout, root sprout and layering. Before thinning, the difference of average height growth for each regeneration type was insignificant. One year after thinning, the stump sprouts showed the fastest growth among the regeneration types. Height growth of advance regeneration was slow in the year following thinning but then increased rapidly surpassing the other regeneration types 5 years after thinning. Thinning had an effect on the composition of regenerated trees also by seedling, stump sprout, root sprout and layering in the understorey. Advance regenerations occupied the upper understorey at 5 years after thinning. A large number of stump sprouts emerged in the plots 1 year after thinning, while at 5 years after thinning the occupation of stump sprouts decreased. Root sprouts and layers faded away. Considering the early growth and the composition of understorey F. rhynchophylla, its advance regeneration has a high potential to develop into overstorey trees at this site.

Lacunarity analysis to determine optimum extents for sample‐based spatial information extraction from high‐resolution forest imagery

Lacunarity analysis was evaluated as a means to determine multiple pattern scales that are inherent in high‐resolution imagery of forests and to specify an optimal spatial extent for spatial image information extraction. A series of 0.5 m pixel images of temperate hardwood and mixed boreal forests were analysed using lacunarity distributions calculated for spatial extents ranging from 7 m to 40 m. The optimal extent was taken as that which displayed the greatest number of distinct pattern scales. For the temperate hardwood forest dataset, 12–14 m extents were found to be optimal, detecting three pattern scales. For the boreal forest dataset, optimal extents were 14–18 m for five of six plots, detecting two or three pattern scales in each plot. The detected pattern scales ranged from 8 m to 14 m and showed some correspondence to tree crown size, but also responded to clusters of understorey and overstorey trees or to partially exposed tree crowns. The method can aid in determination of the sample extent that best captures the pattern scales present in the imagery. More generally, it can be useful in exploratory analysis of any spatial data for which the fundamental patterns are not known.

Pine savanna overstorey influences on ground-cover biodiversity

Question: Does the overstorey of pine savannas influence plant species biodiversity in the ground cover? Location: Camp Whispering Pines (30°41’N; 90°29’W), eastern Louisiana (USA). Methods: We used ecologically sensitive restoration logging to remove patches of Pinus palustris (longleaf pine) in a second-growth loess plain Pinus palustris savanna managed using frequent lightning season fires. Five years later, we measured numbers of vascular plant species and transmitted light in replicated 100-m2 plots. Treatments involved three different overstorey conditions: no overstorey for 5 years, no overstorey for several decades, and overstorey pines present for decades. Results: Both recent and long-term openings contained, on average, about 100 vascular plant species per 100 m2, 20% more than in similar-sized areas beneath overstorey trees. Responses varied with life form; more herbaceous species occurred in recent and older overstorey openings than beneath overstorey trees. Total numbers of all species and of less abundant forb species were positively and linearly related to light transmitted to ground level. Those species responding to openings in the overstorey and positively associated with increased transmitted light levels were monocarpic and shortlived perennial forb and grass species with a seed bank in the soil. In addition, community structure, as reflected in species composition and abundances, appeared to vary with canopy condition. Conclusions: Restoration involving ecologically sensitive removal of patches of overstorey pines in frequently burned pine savannas should benefit the ground cover and increase plant species biodiversity as a result of increased abundance of seed bank species.

Carbon and nitrogen elemental and isotopic patterns in macrofungal sporocarps and trees in semiarid forests of the south‐western USA

Summary Previous studies in humid forests have shown that the 13C and 15N isotopic composition differs between ectomycorrhizal (ECM) and saprotrophic (SAP) fungi, and that this ECM–SAP ‘divide’ may provide a useful tool for evaluating fungal trophic status. We evaluated whether this method could delineate the trophic status of fungi in two semiarid, temperate forests of the south‐western USA. This technique could be particularly valuable in arid regions where the functional roles of fungi can be difficult to assess because of infrequent sporocarp production. Our data were consistent with the existence of an ECM–SAP divide, although δ13C values were more useful than δ15N values in separating trophic status. Saprotrophic fungi consistently had higher δ13C values than their presumed substrates; however, the degree of 15N enrichment in SAP sporocarps was highly variable. Comparison of 11 sporocarp species common to both sites showed that δ15N values were higher in one of the forests, even though the δ15N values of foliage from common understorey and overstorey trees were similar between forests. We conclude that assessment of the isotopic compositions of fungal sporocarps and their substrates is helpful for elucidating ecological relationships in semiarid forests.

Overstorey Control of Understorey Species Composition in a Near-natural Temperate Broadleaved Forest in Denmark

Little is known about the importance of the forest overstorey relative to other factors in controlling the spatial variability in understorey species composition in near-natural temperate broadleaved forests. We addressed this question for the 19 ha ancient forest Suserup Skov (55°22′ N, 11°34′ E) in Denmark, one of the few old-growth temperate broadleaved forest remnants in north-western Europe, by inventorying understorey species composition and environmental conditions in 163 100 m2 plots. We use unconstrained and constrained ordinations, variation partitioning, and Indicator Species Analysis to provide a quantitative assessment of the importance of the forest overstorey in controlling understorey species composition. Comparison of the gradients extracted by unconstrained and constrained ordinations showed that the main gradients in understorey species composition in our old-growth temperate broadleaved forest remnant are not caused by variability in the forest overstorey, but are related to topography and soil, edge effects, and unknown broad-scale factors. Nevertheless, overstorey-related variables uniquely accounted for 15% of the total explained variation in understorey species composition, with the pure overstorey-related (Rpo), topography and soil (Rpt), edge and anthropogenic disturbance effects (Rpa), and spatial (Rps) variation fractions being of equal magnitude. The forward variable selection showed that among the overstorey-related variables understorey light availability and to a lesser extent vertical forest structure were most important for understorey species composition. No unique influence of overstorey tree species identity could be documented. There were many indicator species for high understorey light levels and canopy gap centres, but none for medium or low light or closed canopy. Hence, no understorey species behaved as obligate shade plants. Our study shows that, the forest overstorey has a weak control of understorey species composition in near-natural broadleaved forest, in contrast to results from natural and managed forests comprising both conifer and broadleaved species. Nevertheless, >20% of the understorey species found were indicators of high light conditions or canopy openings. Hence, variability in canopy structure and understorey light availability is important for maintaining understorey species diversity.