Remnant Trees Affect Species Composition but Not Structure of Tropical Second-Growth Forest

Remnant trees, spared from cutting when tropical forests are cleared for agriculture or grazing, act as nuclei of forest regeneration following field abandonment. Previous studies on remnant trees were primarily conducted in active pasture or old fields abandoned in the previous 2–3 years, and focused on structure and species richness of regenerating forest, but not species composition. Our study is among the first to investigate the effects of remnant trees on neighborhood forest structure, biodiversity, and species composition 20 years post-abandonment. We compared the woody vegetation around individual remnant trees to nearby plots without remnant trees in the same second-growth forests (“control plots”). Forest structure beneath remnant trees did not differ significantly from control plots. Species richness and species diversity were significantly higher around remnant trees. The species composition around remnant trees differed significantly from control plots and more closely resembled the species composition of nearby old-growth forest. The proportion of old-growth specialists and generalists around remnant trees was significantly greater than in control plots. Although previous studies show that remnant trees may initially accelerate secondary forest growth, we found no evidence that they locally affect stem density, basal area, and seedling density at later stages of regrowth. Remnant trees do, however, have a clear effect on the species diversity, composition, and ecological groups of the surrounding woody vegetation, even after 20 years of forest regeneration. To accelerate the return of diversity and old-growth forest species into regrowing forest on abandoned land, landowners should be encouraged to retain remnant trees in agricultural or pastoral fields.     

Tropical Rain Forest Structure,Tree Growth and Dynamics along a 2700-m Elevational Transect in Costa Rica

Rapid biological changes are expected to occur on tropical elevational gradients as species migrate upslope or go extinct in the face of global warming. We established a series of 9 1-ha plots in old-growth tropical rainforest in Costa Rica along a 2700 m relief elevational gradient to carry out long-term monitoring of tropical rain forest structure, dynamics and tree growth. Within each plot we mapped, identified, and annually measured diameter for all woody individuals with stem diameters >10 cm for periods of 3-10 years. Wood species diversity peaked at 400-600 m and decreased substantially at higher elevations. Basal area and stem number varied by less than two-fold, with the exception of the 2800 m cloud forest summit, where basal area and stem number were approximately double that of lower sites. Canopy gaps extending to the forest floor accounted for <3% of microsites at all elevations. Height of highest crowns and the coefficient of variation of crown height both decreased with increasing elevation. Rates of turnover of individuals and of stand basal area decreased with elevation, but rates of diameter growth and stand basal area showed no simple relation to elevation. We discuss issues encountered in the design and implementation of this network of plots, including biased sampling, missing key meteorological and biomass data, and strategies for improving species-level research. Taking full advantage of the major research potential of tropical forest elevational transects will require sustaining and extending ground based studies, incorporation of new remotely-sensed data and data-acquisition platforms, and new funding models to support decadal research on these rapidly-changing systems.     

Tree population changes in a tropical dry evergreen forest of south India over a decade (1992–2002)

Changes in species composition and density of trees >10 cm gbh in a tropical dry evergreen forest in Puthupet, south India are interpreted for the period between 1992 and 2002. A 1-ha plot was inventoried in 1992 and was recensused in 2002. During the 10-year interval tree taxa diversity as well as stand density increased, but the basal area value decreased. Tree species richness increased by 21% (from 24 to 29 species) by an addition of eight species and local extinction of three species. The tree density increased just by eight individuals (from 1330 stems ha⁻¹ in 1992 to 1338 ha⁻¹ in 2002), but the basal area decreased by 8% (from 37.5 to 34.5 m² ha⁻¹). Many species (11 numbers) have increased in abundance rather than decreased. Many surviving species seem to have considerable stability in abundance at the local scale. The density of smaller stems (10 29 cm gbh) increased by 15.3%, while that of the larger trees decreased drastically (81.6%). Ninety percent of the missing stems were from the middlestorey of the forest. Tree density changes among the three ecological guilds revealed a decrease in stem density and an increase in basal area in the lowerstorey; while the middlestorey exhibited a reverse trend. Family-wise, tree density changes revealed that the majority of families (67%) showed an increase in stem density. Long-term studies on tree population changes are essential to estimate tree mortality and recruitment rates, which will provide a greater insight in tropical forest dynamics.     

Species Richness,Forest Structure,and Functional Diversity During Succession in the New Guinea Lowlands

Much of the world's tropical forests have been affected by anthropogenic disturbance. These forests are important biodiversity reservoirs whose diversity, structure and function must be characterized across the successional sequence. We examined changes in structure and diversity along a successional gradient in the lowlands of New Guinea. To do this, we measured and identified all stems ≥5 cm diameter in 19 0.25 ha plots ranging in age from 3 to >50 yr since disturbance. We also measured plant functional traits related to establishment, performance, and competitive ability. In addition, we examined change in forest structure, composition, species diversity, and functional diversity through succession. By using rarefaction to estimate functional diversity, we compared changes in functional diversity while controlling for associated differences in stem and species density. Basal area and species density increased with stand age while stem density was highest in intermediate secondary forests. Species composition differed strongly between mature and secondary forests. As forests increased in basal area, community‐weighted mean wood density and foliar carbon increased, whereas specific leaf area and proportion of stems with exudate decreased. Foliar nitrogen peaked in medium‐aged forests. Functional diversity was highest in mature forests, even after accounting for differences in stem and species diversity. Our study represents one of the first attempts to document successional changes in New Guinea's lowland forest. We found robust evidence that as succession proceeds, communities occupy a greater range of functional trait space even after controlling for stem and species density. High functional diversity is important for ecological resiliency in the face of global change.     

Liana Abundance,Diversity, and Distribution on Barro Colorado Island,Panama

Lianas are a key component of tropical forests; however, most surveys are too small to accurately quantify liana community composition, diversity, abundance, and spatial distribution – critical components for measuring the contribution of lianas to forest processes. In 2007, we tagged, mapped, measured the diameter, and identified all lianas ≥1 cm rooted in a 50-ha plot on Barro Colorado Island, Panama (BCI). We calculated liana density, basal area, and species richness for both independently rooted lianas and all rooted liana stems (genets plus clones). We compared spatial aggregation patterns of liana and tree species, and among liana species that varied in the amount of clonal reproduction. We also tested whether liana and tree densities have increased on BCI compared to surveys conducted 30-years earlier. This study represents the most comprehensive spatially contiguous sampling of lianas ever conducted and, over the 50 ha area, we found 67,447 rooted liana stems comprising 162 species. Rooted lianas composed nearly 25% of the woody stems (trees and lianas), 35% of woody species richness, and 3% of woody basal area. Lianas were spatially aggregated within the 50-ha plot and the liana species with the highest proportion of clonal stems more spatially aggregated than the least clonal species, possibly indicating clonal stem recruitment following canopy disturbance. Over the past 30 years, liana density increased by 75% for stems ≥1 cm diameter and nearly 140% for stems ≥5 cm diameter, while tree density on BCI decreased 11.5%; a finding consistent with other neotropical forests. Our data confirm that lianas contribute substantially to tropical forest stem density and diversity, they have highly clumped distributions that appear to be driven by clonal stem recruitment into treefall gaps, and they are increasing relative to trees, thus indicating that lianas will play a greater role in the future dynamics of BCI and other neotropical forests.     

Vegetation Structure,Composition, and Species Richness Across a 56‐year Chronosequence of Dry Tropical Forest on Providencia Island,Colombia1

We compared vegetation structure and species richness across a 56‐yr chronosequence of six replicated age classes of dry tropical forest on the island of Providencia, Colombia, in the Southwest Caribbean. Stand age classes were determined using sequential, orthorectified panchromatic aerial photos acquired between 1944 and 1996 and Landsat 7 ETM + satellite imagery from 2000. Along the chronosequence we established 59 plots of 2 × 50 m (0.01 ha) to document changes in species richness, basal area, tree height, stem density, and sprouting ability. All woody trees and shrubs >2.5 cm diameter at breast height (DBH) were censused and measured. Although woody species density reached a peak in stands from 32 to 56 yr old, rarefaction analysis showed that species richness increased linearly with stand age and was highest in stands 56 yr old or greater. Nonparametric, abundance‐based estimators of species richness also showed positive and linear associations with age. Basal area and mean tree height were positively associated with age since abandonment, while sprouting ability showed a negative relationship. Our results indicate rapid recovery of woody species richness and structural characteristics along this tropical dry forest chronosequence.     

Successional Change and Resilience of a Very Dry Tropical Deciduous Forest Following Shifting Agriculture

We analyzed successional patterns in a very dry tropical deciduous forest by using 15 plots differing in age after abandonment and contrasted them to secondary successions elsewhere in the tropics. We used multivariate ordination and nonlinear models to examine changes in composition and structure and to estimate forest recovery rates and resilience. A shrub phase characterized early succession (0–3 yr); afterwards, the tree Mimosa acantholoba became dominant. Below its canopy, sprouts and seed-regenerated individuals of mature forest species slowly accumulated. Canopy height, plant density, and crown cover stabilized in less than 15 yr, whereas species richness, diversity, and basal area continued to increase. The pioneer species group has very low diversity and the long-lived pioneer phase typical of humid forests is absent; species composition may therefore recover soon as suggested by convergence toward mature forest species composition. The time trend of plant density also differed from humid forests for it lacked its characteristic density decline, presumably because of differences in regeneration mechanisms between very dry and other less water-stressed forest types. As opposed to the prevailing hypothesis, resilience was not higher than in moister forests, and thus factors other than structure relative simplicity must be accounted for when assessing resilience.     

Patterns and Correlates of Tropical Dry Forest Structure and Composition in a Highly Replicated Chronosequence in Yucatan,Mexico

Research on tropical dry forest (TDF) succession i0s needed for effective conservation and management of this threatened and understudied ecosystem. We used a highly replicated chronosequence within a 37,242‐ha TDF landscape to investigate successional patterns by plant size class and to evaluate the influence of stand age, topographic position, soil properties and spatial autocorrelation on forest structure and composition. We used a SPOT5 satellite image to obtain a land‐cover thematic map, and sampled woody vegetation (adults: >5 cm diam; saplings: 1–5 cm) and soil properties in 168 plots distributed among four vegetation classes: VC1 (3–8‐yr‐old forest), VC2 (9–15‐yr‐old forest), VC3 (>15‐yr‐old forest on flat areas), VC4 (>15‐yr‐old forest on hills). Stem density decreased with stand age and was lowest in VC3, while height, basal area and species density increased with age and were higher in older than in younger forests. Topographic position also influenced forest structure and composition. Basal area and height were largely determined by stand age, whereas stem and species density, and composition were influenced mostly by soil variables associated with fertility, and by spatial autocorrelation. Adults and saplings showed contrasting patterns and correlates of community structure, but similar patterns and correlates of composition, possibly due to the prevalence of coppicing. Our results show that our sampling approach can overcome several limitations of chronosequence studies, and provide insights in the patterns and drivers of succession, as well as guidelines for forest management and conservation. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Changes in Structure,Composition, and Nutrients During 15 Yr of Hurricane-Induced Succession in a Subtropical Wet Forest in Puerto Rico

The trajectory of hurricane-induced succession was evaluated in a network of forest plots measured immediately before and 3 mo, 5, 10, and 15 yr after the direct impact of a Category 4 hurricane. Comparisons of forest structure, composition, and aboveground nutrients pools were made through time, and between species, life-history groups and geomorphic settings. The hurricane reduced aboveground biomass by 50 percent, causing an immediate decrease in stem density and diversity indices among all geomorphic settings. After 15 yr, basal area and aboveground biomass returned to pre-hurricane levels, while species richness, diversity indices, and stem densities exceeded pre-hurricane levels. Differences in species composition among geomorphic settings had not returned after 15 yr but differences in stem densities and structure were beginning to emerge. Significant differences were observed in the nutrient concentration of the three species that comprised the most aboveground biomass, and between species categorized as secondary high-light species and primary, low-light species. Species whose abundance was negatively correlated with the mature forest dominant also had distinct nutrient concentrations. When total aboveground nutrient pools were compared over time, differences in leaf nutrients among species were hidden by similarities in wood nutrient concentrations and the biomass dominance of a few species. The observed successional trajectory indicates that changes in species composition contributed to fast recovery of aboveground biomass and nutrient pools, while the influence of geomorphic setting on species composition occurs at time scales >15 yr of succession.     

Forest Age and Plant Species Composition Determine the Soil Fungal Community Composition in a Chinese Subtropical Forest

Fungal diversity and community composition are mainly related to soil and vegetation factors. However, the relative contribution of the different drivers remains largely unexplored, especially in subtropical forest ecosystems. We studied the fungal diversity and community composition of soils sampled from 12 comparative study plots representing three forest age classes (Young: 10–40 yrs; Medium: 40–80 yrs; Old: ≥80 yrs) in Gutianshan National Nature Reserve in South-eastern China. Soil fungal communities were assessed employing ITS rDNA pyrotag sequencing. Members of Basidiomycota and Ascomycota dominated the fungal community, with 22 putative ectomycorrhizal fungal families, where Russulaceae and Thelephoraceae were the most abundant taxa. Analysis of similarity showed that the fungal community composition significantly differed among the three forest age classes. Forest age class, elevation of the study plots, and soil organic carbon (SOC) were the most important factors shaping the fungal community composition. We found a significant correlation between plant and fungal communities at different taxonomic and functional group levels, including a strong relationship between ectomycorrhizal fungal and non-ectomycorrhizal plant communities. Our results suggest that in subtropical forests, plant species community composition is the main driver of the soil fungal diversity and community composition.     

Effect of Hillslope Gradient on Vegetation Recovery on Abandoned Land of Shifting Cultivation in Hainan Island, South China

In the present study, we investigated the effect of hillslope gradient on vegetation recovery on abandoned land of shifting cultivation In Hainan Island, south China, by measuring community composition and structure of 25-year-old secondary forest fallows along a hillslope gradient （up-, middle-, and down-slope position）. A total of 49 733 free-standing woody plant stems higher than 10 cm and belonging to 170 species, 112 genera, and 57 families was found in the three l-hm2 investigation plots. Stem density was highest in the down-slope stand and lowest in the up-slope stand. Species richness and the Shannon-Wiener Index were both highest in the middle-slope stand, and lower In the down-slope and up-slope stands. The recovery forest fallows on different hiUslope positions were all dominated by a few species. The five most abundant species accounted for 70.1%, 58.8%, and 72.9% of total stem densities in stands in the up-, middle-, and down-slope positions, respectively. The five species with the greatest basal areas accounted for 74.5%, 84.3%, and 74.7% of total stem basal area for the up-, middle-, and down-slope positions, respectively. The number of low-density species （stem abundance less than five） Increased from the up-slope position downward. Of the nine local common species among three different functional groups, the short-lived pioneer species dominated the up-slope position, but long-lived pioneer species dominated the middle- and down-slope positions. The climax species of primary tropical lowland rain forest was found in the downslope position. Both the mean diameter at breast height （DBH） and mean height of the trees Increased with decreasing hillslope gradient. The stem density and basal area in different size classes were significantly different in stands In different slope positions. Our results indicated that the rate of secondary succession varies, even over small spatial scales caused by the hlllslope gradient, in early vegetation recovery.     

Recovery of floristic diversity and basal area in natural forest regeneration and planted plots in a Costa Rican wet forest

When compared to planted reforestation, natural unassisted regeneration is often reported to result in slow recovery of biomass and biodiversity, especially early in succession. In some cases, naturally regenerating forests are not comparable to the community structure of primary forests after many decades. However, direct comparison of the outcomes of tropical forest restoration and natural regeneration is hindered by differences in metrics of forest recovery, inconsistency in land use histories, and dissimilarities in experimental design. We present the results of a replicated reforestation experiment comparing natural regeneration and polyculture tree planting at multiple diversity levels (3, 6, 9, or 12 native tree species), with uniform land use history and initial edaphic conditions. We compare the recovery of basal area and floristic diversity in these treatments after 5 yr of succession. Total basal area was higher in planted plots than in naturally regenerating plots, but it but did not vary among the different planted diversity levels. The basal area of woody recruits did not differ among treatments. The diversity of woody recruits increased substantially over time but did not vary among planting treatments. Species composition trajectories showed directional turnover over time, with no consistent differences among treatments. The convergence of restoration trajectories and similarity of floristic community diversity and composition across all treatments, after only 5 yr, provides evidence of the viability of natural regeneration for rapid restoration of forest biodiversity.     

Conservation Thinning in Secondary Forest: Negative but Mild Effect on Land Molluscs in Closed-Canopy Mixed Oak Forest in Sweden

Secondary succession is changing the character of many temperate forests and often leads to closed-canopy stands. In such forests set aside for conservation, habitat management alternatives need to be tested experimentally, but this is rarely done. The Swedish Oak Project compares two often debated alternatives: minimal intervention and non-traditional active management (conservation thinning) on plots of each type replicated at 25 sites. We study responses of several taxa, and here report results for land molluscs. They are considered to be sensitive to more open, drier forest and we predicted a negative effect of the thinning (26% reduction of the basal area; mean value for 25 experimental forests). We sampled molluscs in the litter in ten 20 x 25 cm subplots, and by standardised visual search, in each plot. In total, we recorded 53 species of snails and slugs (24 369 individuals) and the mean species richness in plots was 17. Two seasons after thinning, mean (± SE) species richness had decreased by 1.4 (± 0.9) species in thinning plots, but increased by 0.7 (± 1.0) species in minimal intervention plots, a significant but small change with considerable variation among sites. In matched comparisons with minimal intervention, thinning reduced the overall abundance of molluscs. Most species responded negatively to thinning – but only five of the 53 species were significantly affected, and reproduction seemed to be negatively affected in only one species. An ordination analysis did not reveal any particular change in the species community due to thinning. Thus, the negative effect of conservation thinning on land molluscs was apparently mild – one reason was that many trees, shrubs and other forest structures remained after the treatment. Conservation thinning may be recommended, since other taxa are favoured, but minimal intervention is also a useful form of management for molluscs and saproxylic taxa.     

Forest succession in Kibale National Park, Uganda: implications for forest restoration and management

Forest succession was studied in four plots in former grasslands at the Ngogo study area in Kibale National Park, Uganda. The plots were located in areas that had been protected from fire for 0.58, 25, 9 and ≈30 years for plots 1, 2, 3 and 4, respectively. Species richness reflected the length of time that the plot had been protected from fire; it was highest in plot 4 and lowest in plot 1. Species density, stem density and basal area were all highest in plot 4 and lowest in plot 1. The species densities of plots 2 and 3 were not different. Similarly, plots 2 and 4 did not differ with regard to stem density or basal area. Animal seed dispersers played a vital role in the colonization of grasslands by forest tree species.     

Rapid recovery of tropical forest diversity and structure after shifting cultivation in the Philippines uplands

Shifting cultivation is a widespread land‐use in the tropics that is considered a major threat to rainforest diversity and structure. In the Philippines, a country with rich biodiversity and high rates of species endemism, shifting cultivation, locally termed as kaingin, is a major land‐use and has been for centuries. Despite the potential impact of shifting cultivation on forests and its importance to many people, it is not clear how biodiversity and forest structure recover after kaingin abandonment in the country, and how well these post‐kaingin secondary forests can complement the old‐growth forests. We investigated parameters of forest diversity and structure along a fallow age gradient in secondary forests regenerating after kaingin abandonment in Leyte Island, the Philippines (elevation range: 445–650 m asl). We first measured the tree diversity and forest structure indices in regenerating secondary forests and old‐growth forest. We then measured the recovery of tree diversity and forest structure parameters in relation to the old‐growth forest. Finally, using linear mixed effect models (LMM), we assessed the effect of different environmental variables on the recovery of forest diversity and structure. We found significantly higher species density in the oldest fallow sites, while Shannon’s index, species evenness, stem number, basal area, and leaf area index were higher in the old‐growth forest. A homogeneous species composition was found across the sites of older fallow age. Multivariate analysis revealed patch size as a strong predictor of tree diversity and forest structure recovery after shifting cultivation. Our study suggests that, secondary forests regenerating after shifting cultivation abandonment can recover rapidly. Although recovery of forest structure was not as rapid as the tree diversity, our older fallow sites contained a similar number of species as the old‐growth forest. Many of these species are also endemic to the Philippines. Novel and emerging ecosystems like tropical secondary forests are of high conservation importance and can act as a refuge for dwindling tropical forest biodiversity.     

Plant Community Structure in Tropical Rain Forest Fragments of the Western Ghats,India1

Changes in tree, liana, and understory plant diversity and community composition in five tropical rain forest fragments varying in area (18–2600 ha) and disturbance levels were studied on the Valparai plateau, Western Ghats. Systematic sampling using small quadrats (totaling 4 ha for trees and lianas, 0.16 ha for understory plants) enumerated 312 species in 103 families: 1968 trees (144 species), 2250 lianas (60 species), and 6123 understory plants (108 species). Tree species density, stem density, and basal area were higher in the three larger (> 100 ha) rain forest fragments but were negatively correlated with disturbance scores rather than area per se. Liana species density, stem density, and basal area were higher in moderately disturbed and lower in heavily disturbed fragments than in the three larger fragments. Understory species density was highest in the highly disturbed 18‐ha fragment, due to weedy invasive species occurring with rain forest plants. Nonmetric multidimensional scaling and Mantel tests revealed significant and similar patterns of floristic variation suggesting similar effects of disturbance on community compositional change for the three life‐forms. The five fragments encompassed substantial plant diversity in the regional landscape, harbored at least 70 endemic species (3.21% of the endemic flora of the Western Ghats–Sri Lanka biodiversity hotspot), and supported many endemic and threatened animals. The study indicates the significant conservation value of rain forest fragments in the Western Ghats, signals the need to protect them from further disturbances, and provides useful benchmarks for restoration and monitoring efforts.     

Tree community structure and stem mortality along a water availability gradient in a Mexican tropical dry forest

We document spatial changes in species diversity, composition, community structure, and mortality of trees across a gradient of water availability in a tropical dry forest in western Mexico. This gradient occurs along the main stream of a small watershed of less than 1 km in length. Four 30 × 80 m plots were established systematically to include the driest (ridge top of the watershed) to the wettest sites (watershed bottom) within this watershed. All stems larger than 5 cm were identified, and measured for diameter and height. Dead stems larger than 5 cm were measured and classified as: a) found on live or dead trees, and b) standing (“snags”) or lying (“downlogs”) on the ground. The number of recorded species per plot declined from 73 to 44 species as water availability decreased. A decline in estimated total richness, and in Shannon-Wiener and Simpson diversity indices was also observed in the drier plots. Species composition strongly changed along the gradient, with the two ends of the gradient sharing only 11% of the species. Stem density and percentage of dead stems and trees increased in abundance and basal area from the wetter to the drier sites. Tree and stem size (basal area, height and stem diameter) showed the opposite trend. Nonetheless, total basal area of live trees was largest at the two end gradient locations and oscillated between 12.22 m² ha⁻¹ and 7.93 m² ha⁻¹. Proportion of snags increased towards the driest site (from 46 to 72%), while that of down logs decreased. Overall, our results suggest that small-scale gradients of water availability play a paramount role in the spatial organization of tree communities in seasonal tropical environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Avian Habitat Preference in Tropical Forest Restoration in Southern Costa Rica

An important question for tropical forest restoration is whether degraded lands can be actively managed to attract birds. We censused birds and measured vegetation structure at 27 stations in young (6–9‐yr old) actively and passively restored pasture and old growth forest at Las Cruces Biological Station in southern Costa Rica. During 481 10‐min point counts, we detected a high diversity—186 species—of birds using the restoration area. Surprisingly, species richness and detection frequency did not differ among habitats, and proportional similarity of bird assemblages to old growth forest did not differ between restoration treatments. Bird detection frequency was instead explained by exotic grass cover and understory stem density—vegetation structures that were not strongly impacted by active restoration. The similarity of bird assemblages in actively and passively restored forest may be attributed to differential habitat preferences within and among feeding guilds, low structural contrast between treatments, or the effect of nucleation from actively restored plots into passively restored areas. Rapid recovery of vegetation in this recently restored site is likely due to its proximity to old growth forest and the lack of barriers to effective seed dispersal. Previous restoration studies in highly binary environments (i.e., open pasture vs. tree plantation) have found strong differences in bird abundance and richness. Our data contradict this trend, and suggest that tropical restoration ecologists should carefully consider: (1) when the benefits of active restoration outweigh the cost of implementation; and (2) which avian guilds should be used to measure restoration success given differential responses to habitat structure.     

Rain Forest Structure at Forest-Pasture Edges in Northeastern Costa Rica

Land-use change in the Sarapiquí region of Costa Rica has resulted in a fragmented forest landscape with abrupt edges between forest and pasture. Forest responses to edge effects vary widely and can significantly affect ecosystem integrity. Our objective was to examine forest structure at 20+ yr old forest-pasture edges in Sarapiquí. Three transects with 0.095-ha plots at seven distances from forest edges were established in each of six forest patches. Stem density, basal area, and aboveground biomass in trees and palms ≥ 10-cm diameter at breast height were measured in all plots. In addition, hemispherical photographs were taken to determine leaf area index, understory light availability, and percent canopy openness. Linear mixed-effects models showed significantly higher tree stem density at forest edges, relative to interiors, a pattern reflected by increased stem density, basal area, and aboveground biomass in small diameter trees (≤ 20 cm) growing near edges. No differences in total tree basal area, aboveground biomass, or hemispherical photograph-derived parameters were detected across the forest edge to interior gradient. The recruitment of small diameter trees following edge creation has contributed to the development of dense vegetation at the forest edge and has aided in the maintenance of similar tree basal area and aboveground biomass between edge and interior environments. These data reflect on the robustness of forest edges in Sarapiquí, a characteristic that will likely minimize future detrimental edge effects and promote a number of high-value environmental services in these forests.     

Forest community analysis and the point-centered quarter method

The point-centered-quarter (PCQ) method has been applied in community analysis since the publication of the method nearly 50 years ago. This and other distance methods offer increased sampling efficiency over fixed-area plots (FAP), but have long been known to produce biased density estimates when plant distribution deviates from random spatial patterns. Spatial indices have been developed to quantify the direction of this bias when plant distributions are aggregated or evenly distributed. Its continued use, especially in community analysis, requires additional scrutiny in measurements of community structure. We measured 14 forest stands of varying age, elevation and disturbance regime using FAP and PCQ methods. Density estimates were biased, with the point-centered quarter method lower than fixed-area plot estimates when stems were aggregated and higher when stems were evenly spaced. In general the PCQ method underestimated species richness. The efficiency of the PCQ method makes it popular for ordination studies, although comparison of community structure varied from 18% to 90% similarity between the measurements of species basal area in the same stands using the two different methods. The bias observed in calculations of stem density, species abundance and community similarity indicate that use of the PCQ method should be approached with caution when used in community level analysis.