Greater Stem Density Research Articles

Natural forest regeneration over a fallow age chronosequence in central African moist forests

AbstractA better understanding of the natural regeneration in tropical forests could help develop more effective restoration strategies. This study examined relationships in structural and diversity attributes of natural regeneration in five forest fallow ages after slash‐and‐burn agriculture (6 months, 2 years, 5 years, 10 years, and 15 years) in the Republic of Congo. For each fallow age, all stems with a diameter ≥1 cm and below 5 cm (height ≥ 130 cm), corresponding to natural forest regeneration, were identified to the species level, and measured (diameter and height) in 12 plots (10 m × 10 m). Three structural attributes (stem density, maximum diameter, and maximum height) and five diversity indices (species richness, Shannon diversity, Simpson diversity, Fisher's alpha and Pielou's evenness) were estimated at the plot level. Our results revealed an increase in structural and diversity attributes with fallow ages, except stem density. The young fallow (6 months, 2 years and 5 years) had greater stem density (61 ± 48 stems), while the old fallow (10 years and 15 years) had higher maximum height (5.39 ± 2.36 m) and Shannon's index (0.94 ± 0.18). These results provided new insights into natural regeneration between young and old secondary forests.

Climate and soil effects on tree species diversity and aboveground carbon patterns in semi-arid tree savannas

Climatic and edaphic effects are increasingly being discussed in the context of biodiversity-ecosystem functioning. Here we use data from West African semi-arid tree savannas and contrasting climatic conditions (lower vs. higher mean annual precipitation-MAP and mean annual temperature-MAT) to (1) determine how climate modulates the effects of species richness on aboveground carbon (AGC); (2) explore how species richness and AGC relate with soil variables in these contrasting climatic conditions; and (3) assess how climate and soil influence directly, and/or indirectly AGC through species richness and stand structural attributes such as tree density and size variation. We find that greater species richness is generally associated with higher AGC, but more strongly in areas with higher MAP, which also have greater stem density. There is a climate-related influence of soils on AGC, which decreases from lower to higher MAP conditions. Variance partitioning analyses and structural equation modelling show that, across all sites, MAP, relative to soils, has smaller effect on AGC, mediated by stand structural attributes whereas soil texture and fertility explain 14% of variations in AGC and influence AGC directly and indirectly via species richness and stand structural attributes. Our results highlight coordinated effects of climate and soils on AGC, which operated primarily via the mediation role of species diversity and stand structures.

Using microphone arrays to investigate microhabitat selection by declining breeding birds

Understanding the microhabitat preferences of animals can help managers to develop better conservation and recovery strategies but this is challenging. Traditional methods are limited by cost, accuracy and human resources. In this study, we investigated avian microhabitat preferences using microphone arrays that are capable of accurately locating vocalizing birds. Our objective was to identify the microhabitat associations of two common species in steep population decline, the Boreal Chickadee Poecile hudsonicus and the Cape May Warbler Setophaga tigrina. We deployed 68 eight‐channel arrays at random locations in Labrador, Canada, during the 2016 avian breeding season. We returned in 2017 to the 18 array locations where the target species had been detected the previous year and characterized the microhabitat at the exact locations where they had been detected. We also characterized the microhabitat at randomly determined control locations. Results show that Boreal Chickadees select trees with greater diameter‐at‐breast‐height that are surrounded by greater stem density. We did not find evidence that Cape May Warblers exhibit microhabitat selection during song production. The study shows that microphone arrays are an effective tool for identifying preferred microhabitat that could be incorporated into future conservation or recovery strategies.

Summertime resource selection and reproductive effects on survival of Ruffed Grouse

ABSTRACT Theory suggests that animals should select resources on the basis of perceived fitness benefits, and avoid other available resources that may result in fitness costs. However, resource selection is both a scale- and a life-stage-specific process, in which the importance of different resources may vary among spatial scales of selection and phases of species' life cycles. We investigated multilevel resource selection and summer survival of Ruffed Grouse (Bonasa umbellus) in Maine, USA, to understand life-stage-specific resource selection and the cost of brood rearing. At the landscape level, male Ruffed Grouse selected areas with greater densities of woody stems taller than breast height, while females avoided these areas and selected areas with greater ground coverage by Rubus. Males further selected areas with greater stem densities at the local level, while females again avoided areas with greater stem densities and selected locally available areas with greater ground coverage by Rubus. Weekly s...

Fire Severity and Regeneration Strategy Influence Shrub Patch Size and Structure Following Disturbance

Climate change is increasing the frequency and extent of high-severity disturbance, with potential to alter vegetation community composition and structure in environments sensitive to tipping points between alternative states. Shrub species display a range of characteristics that promote resistance and resilience to disturbance, and which yield differential post-disturbance outcomes. We investigated differences in shrub patch size and stem density in response to variations in fire severity, vegetation community, and post-disturbance reproductive strategies in Sky Island forested ecosystems in the southwestern United States. Patterns in shrub structure reflect the effects of fire severity as well as differences among species with alternate post-fire reproductive strategies. Increased fire severity correlates with larger patch sizes and greater stem densities; these patterns are observed across multiple fire events, indicating that disturbance legacies can persist for decades. High severity fire produces the largest shrub patches, and variance in shrub patch size increases with severity. High severity fire is likely to promote expansion of shrub species on the landscape, with implications for future community structure. Resprouting species have the greatest variability in patch structure, while seeding species show a strong response to disturbance: resprouting species dominate at low disturbance severities, and obligate seeders dominate high severity areas. Differential post-fire reproductive strategies are likely to generate distinct patterns of vegetation distribution following disturbance, with implications for community composition at various scales. Shrub species demonstrate flexible responses to wildfire disturbance severity that are reflected in shrub patch dynamics at small and intermediate scales.

Repeated burning alters the structure and composition of hardwood regeneration in oak-dominated forests of eastern Kentucky, USA

The exclusion of anthropogenic fire is a primary factor responsible for the ‘mesophication’ of eastern oak (Quercus) forests and resultant oak regeneration problems. Consequently, the reintroduction of fire is increasingly used to promote the establishment and growth of oak and hickory (Carya) and control competition from shade-tolerant species (e.g., red maple (Acer rubrum)) in the forest understory. In this study, we examined the effects of fire frequency on the abundance of prominent species in the woody regeneration layer in oak-dominated forests of eastern Kentucky. Treatments included: (1) fire-excluded (FE); (2) frequent fire (FF) – five burns over nine years, and (3) less-frequent fire (LFF) – two burns over seven years. Prior to burning (2002) and again five and seven growing seasons following the cessation of burning in the FF and LFF treatments (2015), respectively, we inventoried tree species in the woody regeneration layer into three size classes: (1) small seedlings (stems<0.6m), (2) large seedlings (≥0.6m and <1.2m) and (3) small saplings (≥1.2m and <3.8cm diameter at breast height). Pre- and postburn, the regeneration layer was dominated by non-Oak-Hickory species, and although Oak-Hickory regeneration was abundant the majority of stems were <0.6m. For Oak-Hickory, significant treatment effects were limited to the large seedling and small sapling size classes. For large Oak-Hickory seedlings, density was significantly greater in the LFF than FE treatment. For small Oak-Hickory saplings, density in the LFF treatment was ∼17 and ∼4 times greater than in the FE and FF treatments, respectively. This study provides support for the notion that fire-free periods may be more of a factor controlling the abundance and composition of the woody regeneration layer than simply the number of burns. However, despite greater stem density, the fate of the Oak-Hickory regeneration layer that developed in response to the LFF treatment is uncertain, as the density of non-oak competitors remains high. Additional treatments (e.g., targeted herbicide application, additional burning) may be necessary to reduce the abundance of non-oak species and increase the likelihood of continued recruitment Oak-Hickory should natural or silvicultural release events occur.

Land use legacy effects on structure and composition of subtropical dry forests in St. Croix, U.S. Virgin Islands

Tropical dry forests are subject to intense human pressure and land change, including conversion to agricultural crops, pasture or agroforestry, and urban encroachment. Decades, and even centuries, of conversion, expansion, regrowth, and changing land-use practices can result in a mosaic of secondary growth patches with different land-use histories. Whereas post-agricultural landscapes may appear as contiguous areas of forest regeneration, the successional trajectory of forest patches from historic land cover to their current state may differ substantially, with consequences for species composition and ecosystem structure and function. We examined the effect of different land-use histories on current forest structure, biomass, and composition in subtropical dry forests in St. Croix, U.S. Virgin Islands. We sampled three types of secondary forests that followed different regeneration pathways after centuries of sugarcane agriculture: 40-year old secondary forests that naturally regenerated after sugarcane abandonment, 40-year old secondary forests that were reforested with timber plantations before management ceased and they were overtaken by natural succession, and 10-year old secondary forests that were intensive pasture prior to recent forest regeneration. Secondary forests that naturally regenerated after sugarcane had similar structural characteristics, in terms of basal area, stem density, aboveground biomass, and species diversity compared to secondary forests of the same age that were former plantations. Species composition, however, remained distinct. Compositional differences between the two types of 40-year old secondary forests could be partially attributed to plantation species, specifically Swietenia mahagoni, whereas naturally regenerated forests were dominated by common secondary forest species, such as Melicoccus bijugatus and Cordia alba. The effects of hurricane damage helped to explain structural similarity and compositional dissimilarity between naturally regenerated secondary forests and former plantations. Forest age had a significant effect on forest structure and composition. Differences between 10-year old and both types of 40-year old secondary forests were driven by a dominance of the nitrogen-fixing species Leucaena leucocephala, which rapidly established in 10-year old secondary forests and resulted in greater stem density and lower basal area, biomass, and species richness. Our results show that land-use history plays an important role in shaping species composition, especially in post-agricultural tropical dry forests. Although forests with differing land-use histories may structurally resemble one another within decades of abandonment, species composition may remain distinct for much longer. Understanding the legacy of human land use is important for dry forests that have a long history of disturbance and for predicting their response to future environmental change.

The influence of vegetation on turbulence and bed load transport

AbstractVegetation is ubiquitous in river channels and floodplains and alters mean flow conditions and turbulence. However, the effects of vegetation patches on near‐bed turbulence, bed load transport rates, and sedimentation are not well understood. To elucidate the influence of emergent vegetation on local and patch‐averaged bed load transport, we conducted a set of experiments in which we varied the mean flow velocity (U), total boundary shear stress (τ), or vegetation density between runs. We measured 2D velocity fields using Particle Imaging Velocimetry and bed load fluxes using high‐speed video. Simulated rigid vegetation caused bed load fluxes to vary spatially by an order of magnitude, causing distinct scour zones adjacent to, and depositional bed forms between stems. These local patterns of sedimentation could impact recruitment and survival of other plants. Large bed load fluxes were collocated with high near‐bed turbulence intensities that were three to four times larger than spatially averaged values. Higher vegetation densities increased the importance of inward and outward interactions, particularly downstream of vegetation. At the patch scale, greater stem densities caused either an increase or decrease in run‐averaged bed load fluxes, depending on whether U or τ was held constant between runs. This implies that sedimentation in vegetation patches is not only a function of bed grain size, sediment supply, and vegetation density and species, but whether vegetation significantly impacts mean and local flow properties, which could depend on vegetation location. Commonly used bed load transport equations did not accurately predict average sediment fluxes in our experiments unless they accounted for the spatial variability in the near‐bed Reynolds stress.

Landscape and Vegetation Effects on Avian Reproduction on Bottomland Forest Restorations

ABSTRACT Forest restoration has been undertaken on &gt;200,000 ha of agricultural land in the Mississippi Alluvial Valley, USA, during the past few decades. Decisions on where and how to restore bottomland forests are complex and dependent upon landowner objectives, but for conservation of silvicolous (forest‐dwelling) birds, ecologists have espoused restoration through planting a diverse mix of densely spaced seedlings that includes fast‐growing species. Application of this planting strategy on agricultural tracts that are adjacent to extant forest or within landscapes that are predominately forested has been advocated to increase forest area and enhance forested landscapes, thereby benefiting area‐sensitive, silvicolous birds. We measured support for these hypothesized benefits through assessments of densities of breeding birds and reproductive success of 9 species on 36 bottomland forest restoration sites. Densities of thamnic (shrub‐scrub dwelling) and silvicolous birds, such as yellow‐breasted chat (Icteria virens), indigo bunting (Passerina cyanea), and white‐eyed vireo (Vireo griseus) were positively associated with 1) taller trees, 2) greater stem densities, and 3) a greater proportion of forest within the landscape, whereas densities of birds associated with grasslands, such as dickcissel (Spiza americana) and red‐winged blackbird (Agelaius phoeniceus), were negatively associated with these variables. Vegetation structure, habitat edge, and temporal effects had greater influence on nest success than did landscape effects. Taller trees, increased density of woody stems, greater vegetation density, and more forest within the landscape were often associated with greater nest success. Nest success of grassland birds was positively related to distance from forest edge but, for thamnic birds, success was greater near edges. Moreover, nest success and estimated fecundity of thamnic species suggested their populations are self‐sustaining on forest restoration sites, whereas these sites are likely population sinks for grassland and open‐woodland species. We recommend restoration strategies that promote rapid development of dense forest stands within largely forested landscapes to recruit breeding populations of thamnic and silvicolous birds that have reproductive success sufficient to sustain their populations.

Density and habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas

Although the habitat requirements of breeding populations of Henslow's Sparrow (Ammodramus henslowii) have been examined, less is known about their habitat requirements and ecology during the nonbreeding season. We estimated population densities and quantified habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas. Densities of Henslow's Sparrows in the saline soil barrens were similar to those in the Longleaf Pine (Pinus palustris) Ecosystem of the southeastern United States, considered by many to be their primary wintering habitat. Henslow's Sparrows were closely associated with open areas with greater cover of Aristida spp. and globe beaksedge (Rhynchospora globularis), greater stem density at 11-20 cm above ground, more lichens, more herbaceous cover, more bare ground, greater occurrence of little bluestem (Schizacyrium scoparium )a s the tallest vegetation, less moss, and less shrub cover than randomly selected sites. In contrast to the results of studies conducted in the Longleaf Pine Ecosystem, the presence of Henslow's Sparrows in our study was not correlated with the height of the tallest vegetation. Our results indicate that saline soil barrens of southern Arkansas support a high density of wintering Henslow's Sparrows and do so for longer postdisturbance periods than longleaf pine savanna. We also found that stem density near the ground was similar to that reported from longleaf pine savanna, but only about half that observed on their breeding grounds. Areas used by Henslow's Sparrows had more lichen and less moss cover, suggesting that those areas were drier than random sites within the barrens. Further research is needed to determine if large populations of Henslow's Sparrows winter in other saline soil barrens and if fire influences habitat associations and densities in the barrens.

Integrating Grouse Habitat and Forestry: An Example Using The Ruffed Grouse Bonasa umbellus in Minnesota

We quantified forest stand attributes at ruffed grouse Bonasa umbellus drumming display sites to develop tree stocking guides as a tool for guiding ruffed grouse management. We estimated tree density and basal area surrounding grouse drumming sites and compared these with unused sites. We used model selection to assess predictions about whether tree density and basal area surrounding drumming sites varied by site classification (primary drumming site, alternate site, unused site) or forest type. We plotted the predicted values from the best model on tree stocking guides, which are tools commonly used by forest managers. Tree density and basal area varied by site classification and by forest type. Our results show that stem density was higher and basal area lower at both primary and alternate drumming sites compared to unused sites in all forest types. We also found that grouse site si n aspen stands had a greater stem density and lower basal area than grouse sites in pine and spruce/fir stands. Incorporating these results into a tree stocking guide suggested that management for grouse in aspen stands should attempt tomaintainstandswithaveragestem densityandbasalareaforthisspecies.In contrast, foresters who are managing for conifers and also wish to maintain some grouse habitat should favour wider spacing of trees in stands. Wider spacing will encourage the development of dense understory vegetation favoured by grouse as well as enhance the growth of quality saw-logs. Our study describes a method for incorporating habitat data on ruffed grouse and other wildlife into tree stocking charts, which are commonly used to facilitate management of forest stands.

Tree biodiversity in farmer cooperatives of a shade coffee landscape in western El Salvador

Conservation of tropical biodiversity in agricultural landscapes has become more important as the area covered by natural ecosystems decreases. We analyzed the effects of local livelihoods, cooperative types, and selected biophysical variables (elevation, slope, percent shade, distance to the forest, coffee density, and coffee age) on tree biodiversity in shade coffee cooperatives of El Salvador. Tree inventories from 51 quadrats in coffee cooperatives included 2743 individuals from 46 families and 123 identified tree species. Species richness and tree diameters differed among some cooperatives, with greater richness associated with greater stem density; other biophysical variables had little impact on diversity. The amount of shade in the coffee plantations differed among cooperatives, particularly in the wet season. Of the tree species reported in a recent study of a neighboring forest and in the cooperatives (N = 227 species combined), 16% were present at both sites. The three coffee plantations shared 35% of total species reported from all cooperatives. Our research shows that the number of tree species found in a coffee plantation increases with the density of shade trees included in the system. In turn, agroecological management, as influenced by farmer livelihood strategies and cooperative types, directly affects shade canopy composition. Important factors to take into account are the types of farmer organizations present, the cost of maintaining species of conservation concern, and the potential benefits that conservation could bring to the livelihood strategies of farm households.

Long-term impacts of goat browsing on bush-clump dynamics in a semi-arid subtropical savanna

The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush control were compared with plots unbrowsed for the same period of time. Differences in bush-clump density, structure and species composition were recorded. Bush-clump density did not differ between browsed and unbrowsed plots. Within individual bush-clumps, browsing was shown to impact more on structure than species composition, with smaller, shorter bush-clumps, containing fewer species but much greater stem-densities. Although species presence/absence was little affected by browsing, many species showed differences in abundance, growth and location within browsed and unbrowsed bush-clumps. Species reduced in abundance in browsed plots included Cussonia spicata, Ehretia rigida, Grewia occidentalis, Jasminum angulare and Senecio linifolius. Several species showed reduced growth in browsed plots, particularly those located at bush-clump edges. The relatively unpreferred Aloe ferox was a notable exception. Although browsing had little effect on the composition of the main clump founding species, emergents or late arrivals, there were twice as many single plants in browsed plots and emergence of several species was restricted to the middle of bush-clumps. Comparison of our findings with aerial photographic evidence and other literature suggest that browsing alone is unlikely to significantly reduce scrub cover, although it can clearly control further expansion. Combinations of fire and browsing, rather than one factor alone, are considered likely to act fastest and most effectively to significantly reduce or remove scrub cover altogether.

Spatial Distribution of Spartina pectinata Transplants to Restore Wet Prairie

Abstract Restoration of wet prairie in the central midwestern United States should include Prairie cordgrass (Spartina pectinata), but historic densities of this once dominant species are not easily achieved. Spartina pectinata was transplanted into a former agricultural field in three planting strategies: 20 small plugs, 4 medium‐sized plugs, and a single large plug, with each treatment area totaling 0.6 m2. Plugs were sampled annually for 3 years to determine which planting strategy produced highest survival, area, stem density (per unit vegetated area), and height of S. pectinata It was also determined whether these factors varied with microtopography, and the resulting differences in flooding, within the restoration site. Stem density and height of restored S. pectinata were also compared to reference S. pectinata communities, including both pristine populations and the source of transplants. Because soil particle size and soil nitrogen content can influence S. pectinata stem density and height, these soil characteristics were also tested in restored and reference sites. Three years after planting, S. pectinata survived in all planted plots regardless of planting strategy or microtopography, with an overall survival rate of 89.8% at the end of the study. The small plug treatment produced the greatest area after 3 years (3.99 m2), compared with the final area of medium (1.18 m2) and large plug treatments (0.79 m2). Stem density of small and medium plugs decreased dramatically during the sampling period, whereas density of large plugs increased. Height did not vary among planting strategies, and microtopography did not affect area, stem density, or height. Reference populations of S. pectinata had greater stem density and height than the transplanted plugs in the restoration, and these differences cannot be attributed to soil particle size or soil nitrogen. This study indicates that more than 3 years is needed to create area, density, or height of S. pectinata similar to established wet prairie populations. Planting many small plugs yields the greatest area of S. pectinata, and including some large plugs may add dense physical structure to wet prairie restoration.

Characteristics of Bolus Nests of Red Squirrels in the Pinaleno and White Mountains of Arizona

Red squirrels (Tamiasciurus hudsonicus) use cavity, bolus, and underground nests for sleeping, protection, and rearing of young. We compared occurrence and structural characteristics of bolus nests at midden sites (area surrounding the central cache) for red squirrels in the Pinaleno Mountains with those in the White Mountains of southeastern Arizona. Bolus nests were more prevalent in the White mountains, possibly as a result of differing habitat structure; forests of the Pinaleno Mountains had significantly (P < 0.05) larger basal area, greater stem densities, and there was less grass on the forest floor than in the White Mountains. Midden sites with more potential cavity nest sites were more likely to have only cavity nests present. Bolus nests were constructed mostly of lichens in the Pinaleno Mountains and of grasses in the White Mountains, whereas most other nest and nest tree characteristics were similar between locations, although nest trees were further from the main cache location at middens in the Pinaleno Mountains.

Late Fall Harvest, Winter Grazing, and Weed Control for Reduction of Alfalfa Weevil (Coleoptera: Curculionidae) Populations

This study was conducted during 1983-1987 to determine influence of late fall cutting and winter grazing in combination with control of cool-season weeds on egg deposition and seasonal occurrence of peak larval populations of the alfalfa weevil, Hypera postica (Gyllenhal), in Oklahoma. Alfalfaweevil egg numbers were reduced by an average of 55% by late fall cutting and 67% by grazing in winter by cattle compared with the ungrazed treatment. However, peak larval numbers were not lower due to fall cutting and were reduced by an average of just 25% with grazing. Numbers decreased least in years when the majority of eggs were laid in late winter rather than fall or early winter. There seems to be potential to delay occurrence of peak larval numbers up to 10 d by grazing if most eggs are laid in fall or early winter. Larval numbers per stem changed little with changing stem densities and the extent of weed infestation. As a consequence, larval numbers per 0.1 m2 tended to be higher with greater stem densities in treatment combinations that promoted stand longevity.