Radius Circular Plots Research Articles

Effects of landscape complexity on the ecological effectiveness of agri-environment schemes

Agricultural intensification is a major cause for biodiversity loss. It occurs at field scales through increased inputs and outputs, and at landscape scales through landscape simplification. Agri-environment schemes (AES) of the European Common Agricultural Policy (CAP) aim at reducing biodiversity loss by promoting extensification of agricultural practises mostly at field scales. We present a conceptual model for the relationship between landscape complexity and ecological effectiveness of AES based on (a) non-linear relationships between landscape complexity and abundance and diversity at field scales and (b) four possible interactive scenarios between landscape- and field scale effects on abundance and diversity. We then evaluated whether and how effectiveness of AES interacted with landscape-scale effects of intensification along a landscape complexity gradient established in central Spain. Pairs of cereal fields with and without AES but with the same landscape context were selected in three regions differing in landscape complexity. Effectiveness of AES was measured as differences between paired fields in species richness and abundance of five target groups (birds, grasshoppers and crickets, spiders, bees and plants). Landscape metrics were measured in 500–m radius circular plots around field centres. Positive, negative and no effects of landscape complexity on effectiveness of AES were found, suggesting that effects of complexity on effectiveness of AES changes from positive to negative along gradients of landscape complexity. Effectiveness of AES for improving biodiversity was then constrained by landscape. Compulsory measures aimed at enhancing or maintaining landscape complexity would enhance the effectiveness of AES for preserving biodiversity in farmed landscapes.

Relationship between size hierarchy and density of trees in a tropical dry deciduous forest of western India

AbstractQuestions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density.Location: Western India.Methods: Density was measured as the number of trees in 10‐m radius circular plots (n= 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n= 1239 trees).Results: Size inequality between neighbours decreased with density but in a non‐linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ‘depletive’ resource such as below‐ground water (rather than a ‘pre‐emptive’ resource such as light), which is very plausible in a semi‐arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ∼−1/4), is higher than expected from existing models of allometric plant growth (γ=−1/3) which are based on above‐ground interactions alone. Seen in conjunction, these results suggest that above‐ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below‐ground resources, such as water, may be more important under semi‐arid conditions.Conclusions: The non‐linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above‐ground biomass becomes sluggish in semi‐arid, deciduous forests. Since current allometric models do not incorporate below‐ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.

A community analysis of sal ( Shorea robusta) forests in the western Terai of Nepal

Sal ( Shorea robusta) forest is found in an extensive array of conditions in lowland Nepal, and has been heavily used by both government and local people. Thus, we did a study to provide information on sal forests occupying a broad range of historical and environmental influences across an extensive range in the western Terai of Nepal and explore factors – both environmental and anthropogenic – that may be responsible for variation of forest structure. Trees, saplings, seedlings and shrubs were sampled along transects (2 km long) in two protected areas and two proposed community forests. The protected areas had three transects each, and a single transect covered two proposed community forests. Samplings were done every 200 m along the transects, a plotless technique sampled trees (>5 cm dbh). With tree sampling point as the center, shrubs and saplings (1–5 cm dbh and >1 m height) were sampled in 5 m radius circular plots and herbs and seedlings (<1 m height) were sampled in 1 m 2 circular plots nested within shrub plots. Altogether 131 species were recorded: 28 trees, 10 shrubs, 6 climbers and 87 herbs. The mean density across all plots was 220 trees/ha and the average basal area was 13.2 m 2/ha. Three different associations of sal forest were identified by cluster analysis. Community types distinguished in the classification analysis were clearly separated in the site ordination. None of the environmental variables measured (pH, percent organic matter, total nitrogen, available phosphorous, available potassium and soil texture) explained the distribution of plots in the site ordination. We concluded that rainfall and past disturbances (fire and anthropogenic use) are mainly responsible for different community types. Community types were different in structure and composition, thereby representing unique entities. The protection and maintenance of each of the different communities through forest management is important for biodiversity conservation.

Relationship between size hierarchy and density of trees in a tropical dry deciduous forest of western India

Questions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density. Location: Western India. Methods: Density was measured as the number of trees in 10-m radius circular plots (n = 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n = 1239 trees). Results: Size inequality between neighbours decreased with density but in a non-linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ʻdepletiveʼ resource such as below-ground water (rather than a ʻpre-emptiveʼ resource such as light), which is very plausible in a semi-arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ ≈ –1/4), is higher than expected from existing models of allometric plant growth (γ ≈ –1/3) which are based on above-ground interactions alone. Seen in conjunction, these results suggest that above-ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below-ground resources, such as water, may be more important under semi-arid conditions. Conclusions: The non-linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above-ground biomass becomes sluggish in semi-arid, deciduous forests. Since current allometric models do not incorporate below-ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.

Responses of forest herbs to available understory light measured with hemispherical photographs in silver fir–beech forest in Croatia

In this research we investigated response of forest herbs to available light conditions in the forest of silver fir ( Abies alba) and beech ( Fagus sylvatica) which are most frequent altimontane forest on Croatian karst, using the hemispherical photographs for measuring available light. In addition to this, we tested the possibility of estimation light condition parameters on the large areas using the remotely sensed data, as potential support to the spatial prediction of phytodiversity and particular plant species occurrences. Data about the floristic assemblage were gathered at two spatial resolution at each location consisted of 5 and 12.5 m radius circular plots. Hemispherical photographs of forest canopies were taken in the centre of each plot and analyzed with Gap Light Analyzer software calculating the two canopy characteristics: leaf area index (LAI) and canopy openness. For majority of sampled area satellite Landsat ETM+ image was available which enable comparison of LAI with spectral channels and Normalized Difference Vegetation Index (NDVI). Relations between mentioned variables (floristic, derived from hemispherical photographs and derived from satellite image) were investigated by redundancy analyses (RDA), classification trees (CT), logistic and linear multiple regression. Using the RDA, six herbaceous plant species that are most influenced by the light conditions were detected. According to CT analysis that included altitude, slope, northness (cosine of aspect) and LAI as predictor variables, LAI was important predictor variable for estimating the frequency of particular plant species. Besides CT, predictive logistic regression models were developed for selected species as well. Regression models developed with Landsat ETM+ bands as estimators explained moderate amount of variability in LAI data. Obtained results lead to the future modelling of silver fir–beech forest phytodiversity and spatial distribution of particular plant species as a function of environmental estimators.

Soil patchiness in juniper-sagebrush-grass communities of central Oregon

This study compared the sizes, spacings and properties (soil moisture, pH, nitrogen, soil arthropods and VAM) of soil resource islands and bare patches in sagebrush-grass communities invaded by western juniper versus those without juniper. We analyzed 1000 surface soil samples taken from nine 50-m radius circular plots sampled in December of 1991 and May of 1992 on ‘The Island’, one of the few undisturbed areas of sagebrush-grass shrubland in Oregon. Spatial structure was interpreted from correlograms (Moran’s I) and standardized semivariograms. The presence of juniper was associated with increased bare area and smaller, more widely spaced grass and sagebrush plants. Soil arthropod numbers and biomass in plots with juniper were only roughly one-fifth of those in sagebrushgrass plots in December. The dominant soil pattern in both sagebrush-grass and juniper-sagebrush-grass plots was regularly-distributed patches spanning a range of sizes and spacings. Plots with juniper had greater patchiness at shorter lags (<3 m), and patchiness was more developed for soil moisture, net nitrification, and net N mineralization, whereas sagebrush-grass plots had greater patchiness at longer lags (3 ‐ 9 m) and patchiness was more developed for NO3‐N, arthropod numbers and biomass. These differences in soil patterns with and without juniper indicate that juniper responds to, or causes, changes in the size of resource islands under sage and grass when it invades sage-grass communities.

Scale-dependent habitat use in three species of prairie wetland birds

We evaluated the influence of scale on habitat use for three wetland-obligate bird species with divergent life history characteristics and possible scale-dependent criteria for nesting and foraging in South Dakota, USA. A stratified, two-stage cluster sample was used to randomly select survey wetlands within strata defined by region, wetland density, and wetland surface area. We used 18-m (0.1 ha) fixed radius circular-plots to survey birds in 412 semipermanent wetlands during the summers of 1995 and 1996. Variation in habitat use by pied-billed grebes (Podilymbus podiceps) and yellow-headed blackbirds (Xanthocephalus xanthocephalus ), two sedentary species that rarely exploit resources outside the vicinity of nest wetlands, was explained solely by within-patch variation. Yellow-headed blackbirds were a cosmopolitan species that commonly nested in small wetlands, whereas pied-billed grebes were an area-sensitive species that used larger wetlands regardless of landscape pattern. Area requirements for black terns (Chlidonias niger), a vagile species that typically forages up to 4 km away from the nest wetland, fluctuated in response to landscape structure. Black tern area requirements were small (6.5 ha) in heterogeneous landscapes compared to those in homogeneous landscapes (15.4‐32.6 ha). Low wetland density landscapes composed of small wetlands, where few nesting wetlands occurred and potential food sources were spread over large distances, were not widely used by black terns. Landscape-level measurements related to black tern occurrence extended past relationships between wetlands into the surrounding matrix. Black terns were more likely to occur in landscapes where grasslands had not been tilled for agricultural production. Our findings represent empirical evidence that characteristics of entire landscapes, rather than individual patches, must be quantified to assess habitat suitability for wide-ranging species that use resources over large areas.

Efficacy of Pt® 370 Ascend™ (Avermectin Bl) Against the Red Imported Fire Ant, Solenopsis Invicta Buren, Fall, 1990

Abstract Efficacy of Avermectin Bl-based red imported fire ant (RIFA) bait was tested. Ten 15 m radius circular test plots (approx. 0.1 ha) containing 5-8 mounds were laid out in a single-queen RIFA-infested fallow lot. For each treated plot an adjacent plot was selected as control, being part of the same colony population. The mounds were staked and numbered for identification. A single broadcast application of 0.011% Avermectin B-l was made at the rate of 1.12 kg/ha (8.2 mg Al/plot) on 21 Sep, using a precalibrated, hand-held seed spreader. The plots were treated by walking along the circumference of circles subtended by 1.5, 4.6, 7.6, 10.6, and 13.7 m radii and applying insecticide pacing normally. The activity of ants in the five treated and untreated (control) plots (replicates) was monitored by thrusting five 20 ml Fluon®-rimmed scintillation vials into each mound and pooling the ants sampled. The samples were taken at the time of application (21 Sep) and at 10 d post-application intervals (2 Oct, 12 Oct, 22 Oct, 2 Nov, and 13 Nov). Ambient and soil temperatures during the sampling were 25.19 ° 5.97°C and 24.07 ° 5.87°C, respectively. The capture of workers was used as a parameter to deduce the decline in ant activity in the mounds as a result of insecticidal application.