Global Site Factor Research Articles

Estimation of forest canopy structure and understory light using spherical panorama images from smartphone photography

Abstract Accurate estimates of forest canopy structure are central for a wide range of ecological studies. Hemispherical photography (HP) is a popular tool to estimate canopy attributes. However, traditional HP methods require expensive equipment, are sensitive to exposure settings, and produce limited resolution which dramatically affects the accuracy of gap fraction estimates. As an alternative, hemispherical images can be extracted from spherical panoramas produced by many smartphone camera applications. I compared hemispherical photos captured with a digital single lens reflex camera and 180° lens to those extracted from smartphone spherical panoramas (SSP). The SSP HP method leverages built-in features of current generation smartphones to produce sharper images of higher resolution, resulting in more definition of fine canopy structure. Canopy openness and global site factor from SSP HP are highly correlated with traditional methods (R2 > 0.9), while leaf area index estimates are lower, especially in more closed canopies where traditional methods fail to capture fine gaps.

Defining functional groups and their vulnerability to the edge effect in a peri-urban forest in Mexico City

SUMMARYThe expansion of human settlements and primary-sector activities (agriculture and forestry) has resulted in the fragmentation of forests, but the impacts of this are still poorly understood. We examined the effect of patch size on the presence of plant functional groups along an edge–interior gradient. Plant species were classified based on a two-way indicator species analysis in order to determine their establishment thresholds and vulnerability along the gradient, while detrended correspondence analyses and canonical correspondence analyses were performed to identify environmental gradients related to vegetation distribution. Two groups of plant species were recognized in all patch sizes: one commonly found towards the edge and the other in the interior zone. The incidence of these groups was correlated with environmental factors associated with the edge–interior gradient, mainly with humidity, soil moisture and light (canopy opening and global site factor) in the edge zone and with litter cover, depth of litter, slope and soil and air temperature in the interior zone. Identifying the species’ threshold responses to fragmentation is key, as they provide tools to prevent the potential local extinction of species.

Traits associated with nutrient impoverishment and shade-tolerance in tree juveniles of three Bornean rain forests with contrasting nutrient availability

Abstract:In this study, we tested the hypothesis that functional traits associated with nutrient impoverishment contribute to enhancing shade-tolerance (survival at low light) for the juveniles of canopy tree species in Bornean rain forests. To test the hypothesis, survival and functional traits (biomass allocation, leaf dynamics and foliar nutrient concentration) were investigated as a function of light conditions for saplings of 13 species in three forests with different levels of nutrient availability. As predicted by the hypothesis, the species in the severely nutrient-poor site (a tropical heath forest on nutrient-poor soils) showed greater shade-tolerance (>91% survival for 8 mo at 5% global site factor) than in the other two sites (mixed dipterocarp forests) (54–87% survival). Across the species, greater shade-tolerance was associated with a higher biomass allocation to roots, a slower leaf production and a higher foliar C concentration, which are considered as C-conservation traits under nutrient impoverishment. These results suggest that the juveniles of the canopy species occurring on nutrient-poor soils can enhance shade-tolerance by the same mechanisms as the adaptation to nutrient impoverishments. Tree species in nutrient-poor environments may be selected for surviving also in shaded conditions.

Structural equation modeling as a tool to develop conservation strategies using environmental indicators: The case of the forests of the Magdalena river basin in Mexico City

A primarily descriptive approach has been used to develop studies in Mexico that use indicators to incorporate information on the conservation state of ecosystems. Consequently, these studies are not well supported by ecological data, or they lack an analysis of socio-ecological integration, making it difficult to implement the strategies derived from these conservation plans. Structural equations models (SEM) help with the understanding of direct and indirect interactions between variables and, consequently, allow the detection of root causes of change. In this study, a method to integrate indicators in relation to a pressure-state-response model was developed and applied to a forest in Mexico City. We developed the model using 21 environmental units, 17 state indicators of biodiversity (evaluating three aspects of the ecosystem: structure, composition and function), 14 environmental indicators (abiotic variables) and 6 anthropogenic pressure indicators. Subsequently, based on multicollinearity tests, redundant indicators were eliminated. A canonical correspondence analysis was carried out to establish the relationship between different indicators and the possible ecological connections of the SEM. With the previous information, an a priori model of the influence of pressure and environmental indicators on the structure, composition and function of the ecosystem was developed.The SEM results enabled us to evaluate the direct and indirect causes of degradation. Some environmental factors, such as the global site factor, humidity, air temperature, organic matter in the soil, slope, and soil pH, had a greater effect on the structure, composition and function indicators registered in the environmental units of the basin than other factors. The pressure factors that most affected the three aspects of the ecosystem were visitors, rubbish, fire incidence, and human activities. Pressure factors affected composition and structure indicators, though the effect on the function indicators was weak. It is possible to conclude that although the ecosystem appears to be changed by some disturbance factors, these factors have not yet altered the functionality of the ecosystem. Finally, from the model, a set of proposals and strategies for management was developed. These strategies can be used to sustain the biodiversity of each environmental unit.

Modelling the influence of light, water and temperature on photosynthesis in young trees of mixed Mediterranean forests

The composition of Mediterranean forests is expected to vary with ongoing changes in climate and land use. To gain a clearer understanding of the response to global change of growth and survival during regeneration it is necessary to take a closer look at the ecophysiological traits underlying seedling performance. Gas exchange, leaf water potential, chlorophyll fluorescence, soil moisture, temperature and global site factor were measured over 1 year in naturally regenerated young trees of three coexisting species (Pinus pinea, Quercus ilex and Juniperus oxycedrus) in two stands of different density. We modelled the photosynthetic response of plants to micro-climatic conditions via the parameterization of the non-rectangular hyperbolic model of photosynthesis, which relates gross photosynthesis to incident light through three biochemical parameters, and the subsequent expansion of these parameters as a function of environmental variables (light environment, soil moisture and temperature). We investigated the relationship between different photosynthetic performance and the species-specific strategies to cope with stress (stress tolerant or avoiders). The optimal light environment, defined through the global site factor (GSF), and the regeneration niche to maximize carbon assimilation differed between the three species. P. pinea showed high sensitivity to water availability in agreement with a drought avoidance strategy, attaining the maximum photosynthetic capacity of the three species following the spring rainfall. Q. ilex was the most thermophilic and light-demanding of the species. Under high light conditions, J. oxycedrus was more drought tolerant and displayed higher net CO2 assimilation than P. pinea over the course of a growing period. Optimal locations for P. pinea regeneration are below-crown environments, while for J. oxycedrus regeneration the optimal locations are open gaps. Q. ilex regeneration occupy open gaps where the other two species are unable to establish themselves because of excessive light, temperature or very low water availability. Competition between species will occur under a canopy gap fraction of 0.5. Higher GSF values will exclusively favour the regeneration of Q. ilex.

Modeling the environmental response of leaf net photosynthesis in Pinus pinea L. natural regeneration

Environmental factors as incident light, temperature and soil water content mainly determine the dynamics of growth and survival of natural forest regeneration in Mediterranean forests. The complex interactions among these factors highlight the need for physiology-based models which describe seedling and sapling performance under current and changing climatic conditions. These models should be flexible enough to take into account the effect of growth light environment changes on physiological processes and, therefore, to assist in the decision-making process on different silvicultural management alternatives under climatic uncertainty. In the present work the net photosynthesic rate of Pinus pinea L. natural regeneration is modeled as a function of light irradiance using the non-rectangular hyperbola function. The model fit was carried out following a two-step procedure, where the parameters of the original function were expanded over the most influential factors: leaf temperature, soil moisture, global site factor and needle type. The developed model allows defining the optimal niche conditions for the natural regeneration of the species and permits identifying the most limiting conditions which prevent natural regeneration. Regeneration niche was assessed by using the model to simulate net CO2 assimilation of a seedling over twelve contrasting light environments during both a normal and an extremely dry vegetative period. The model predicts that the most favorable carbon balance would be found in mid-shaded expositions, while fully exposed plants at midday during summer would exhibit longer periods of negative assimilation rates, especially on severe dry years.

Changes in light levels and stream temperatures with loss of eastern hemlock (Tsuga canadensis) at a southern Appalachian stream: Implications for brook trout

The exotic invasive insect, hemlock woolly adelgid (Adelges tsugae Annand), is causing mortality in eastern hemlocks (Tsuga canadensis [L.] Carr.) throughout the eastern U.S. Because hemlocks produce dense shade, and are being replaced by hardwood species that produce less shade, their loss may increase understory light levels. In the southern Appalachians, increases in light could increase stream temperatures, threatening species such as brook trout (Salvelinus fontinalis). We studied changes in light and stream temperature with eastern hemlock decline at a headwater southern Appalachian brook trout stream. Our results indicate that stream light levels have increased significantly with adelgid infestation. Leaf-on light levels are currently significantly higher (P<0.02) in plots containing high basal areas of hemlock (mean global site factor (GSF)(SE)=0.267(0.01)) compared with plots containing no hemlock (mean GSF(SE)=0.261(0.01)), suggesting that increases in light have occurred with hemlock decline. The Normalized Difference Vegetation Index (NDVI), a remotely sensed metric of vegetation density, decreased with hemlock decline from 2001 to 2008. In 2001, NDVI showed no relationship (R2=0.003; F=0.14; P=0.71) with hemlock basal area, but by 2008, there was a significant negative relationship (R2=0.352; F=19.55; P<0.001) between NDVI and hemlock basal area. A gap experiment showed that light levels may increase by up to 64.7% more (mean increase in GSF=27.5%) as hemlocks fall, creating gaps in the canopy. However, stream temperatures did not increase with hemlock decline during the study period, and we found that ground water inputs have a stronger influence on water temperature than light levels at this site. Linear regression showed a significant negative relationship between water temperature and proximity to ground water sources (R2=0.451; F=13.14; P=0.002), but no relationship between water temperature and light levels (R2<0.02; P>0.05). In addition, by comparing light levels between plots containing hemlock and those containing only hardwoods, we found that if hemlocks are replaced by hardwoods, light levels under an all-hardwood canopy (mean GSF(SE)=0.240(0.005)) are unlikely to be higher than they are under the current forest (mean GSF(SE)=0.254(0.007)). These results suggest that loss of hemlock along southern Appalachian headwater streams could have short-term impacts on light levels, but that long-term changes in light levels, increases in water temperature, and adverse effects on brook trout may be unlikely.

Thermal acclimation of leaf dark respiration of beech seedlings experiencing summer drought in high and low light environments

Little is known about how environmental factors shape the short- and long-term responses of leaf respiration to temperature under field conditions despite the importance of respiration for plant and stand carbon balances. Impacts of water availability and canopy cover on leaf dark respiration (R) and temperature sensitivity were assessed in beech (Fagus sylvatica L.) seedlings in a sub-Mediterranean population. We studied seedlings established within canopy gaps (39% global site factor; GSF) that were subject to either no watering (unwatered plants; UW) or regular watering (2-10% higher volumetric topsoil water content as summer progressed; W plants) and seedlings established beneath the adjacent understorey (12% GSF). Leaf R rose exponentially with diurnal increases in temperature; the same temperature sensitivity (Q(10): 2.2) was found for understorey and gap plants, irrespective of watering treatment. Respiration estimated at 25 degrees C (R(25)) was lower in the understorey than the gaps and was significantly lower in the unwatered than in the watered gap plants by the end of summer (0.65 versus 0.80 micromol m(-2) s(-1)). R(25) declined with increasing summer temperature in all plants; however, respiration estimated at the prevailing ambient temperature did not change through the summer. There were parallel declines in R(25) and concentrations of starch and soluble sugars with increasing summer temperature for gap plants. We conclude that seasonal shifts in temperature-response curves of beech leaf R occur in both low- and high-light environments; since leaf R decreased with increasing plant water deficit, such shifts are likely to be greater whenever plants experience summer drought compared to scenarios where plants experience high rainfall in summer.

Functional performance of oak seedlings naturally regenerated across microhabitats of distinct overstorey canopy closure

The extent to which seedling recruitment is limited by summer drought in Mediterranean-type ecosystems depends on the light microsite, yet the relationship between light availability and water status, functional performance, and survival of seedlings in these systems is still unclear. Over a 3-year period, we studied the pattern of survival and functional performance of seedlings of Quercus petraea (Matt.) Liebl. and Quercus pyrenaica Willd. in a montane forest in central Spain, which is the southern edge of the natural range of Q. petraea. After a mast year of the two species, 72 plots were established in six microhabitats spanning a range of overstorey canopy closure: closed, partial and open canopies dominated by either Q. petraea or Q. pyrenaica adult trees. Seedlings of each species naturally emerged beneath the conspecific-dominated canopies. The second and third years of study were extremely dry. Three years after emergence, the greatest seedling survival occurred beneath the partial canopy of Q. pyrenaica trees (8%) and the lowest (0%) beneath the closed canopies of Q. pyrenaica and Q. petraea. Survival for Q. pyrenaica increased linearly with understorey light across the range of 10–35% Global Site Factor. Plant water deficit (estimated by leaf water potential) was high across microhabitats, and increased with light availability for Q. pyrenaica. Potential for photosynthesis (estimated by the electron transport rate of photosystem II) decreased with canopy closure; and potential for light harvesting (e.g. specific leaf area (SLA) and chlorophyll concentration) increased with closure. Extreme water deficit could be the main contributor to seedling death in the more open microhabitats, whereas light level was insufficient to maintain carbon balance under the water-stressful conditions existing beneath the closed tree canopies. Seedling establishment appears to be a limiting factor for the recruitment of both oaks within this forest in a wide range of microhabitats, especially for the more drought-sensitive Q. petraea. Moderate reductions of tree canopy cover can improve seedling establishment, but extreme summer droughts can prevent the success of any silvicultural practice made.

Variation in canopy structure, light and soil nutrition across elevation of a Sri Lankan tropical rain forest

Mixed dipterocarp forests are perhaps the single most important rain forest type in the wet tropics. Only a few studies have purposefully examined differences in resource availability across mixed dipterocarp forest landscapes by simply measuring the abiotic variables of light, soil nutrition and soil water availability in relation to forest structure. We sought to directly measure the environment of canopy gaps across elevation and geology—from lowland mixed dipterocarp forest (100 m amsl) to lower montane dipterocarp forest (1200 m amsl) in southwest Sri Lanka. Middle elevation gap sites (300–900 m amsl) were subdivided into valley, mid-slope and ridge topographic positions. Eighteen natural disturbances all of which were canopy openings caused by tree fall, were randomly selected within primary rain forest that ranged across 100–1200 m elevation. Plots were placed in gap centers and in adjacent understories and measurements taken of forest structure (basal area, canopy height, canopy cover index, CCI), shade (light sensors—photosynthetically active radiation [PAR], canopy hemispherical photographs—global site factor [GSF]) and soil nutrition (pH, exchangeable Al, K, Mg and Ca; Total N; and plant available P). Soil moisture was measured at bi-weekly intervals for five years across middle elevation sites only (300–900 m amsl). Stand basal area, mean canopy height, and canopy cover index all declined with increase in elevation. Understory PAR and GSF decreased with increases in canopy height, basal area and CCI. Size of canopy opening decreased with increase in elevation, but PAR and GSF increased. Valley sites had significantly greater levels of mean percent soil water content as compared to mid-slope and ridge sites of middle elevation sites. However, at the onset of the southwest monsoons in May all sites were similar. Differences were most pronounced during the dry season (December–April). No differences in soil moisture content could be found between gap and understory microsites. K and Ca in gap centers and adjacent forest understories increased with increase in elevation and change in associated geology. pH increased and Al decreased with elevation and associated geology but only for forest understory conditions. Results demonstrate strong differentiation in soil and light resources with elevation that appears related to size of tree-fall disturbance, stature of the forest, topographic position and underlying geology and soil-weathering environment. This suggests that forest management and conservation practices need to develop and tailor techniques and treatments (silviculture) to the forest that emulate and/or account for change in elevation, geology and topographic position. Further studies are needed to identify which are the primary underlying mechanisms (e.g. temperature, wind, soil nutrients, soil moisture availability) defining change in forest structure across elevation.

Irradiance and oak seedling survival and growth in a heterogeneous environment

The Mediterranean region emerges highly heterogeneous at every spatio-temporal scale. Key species shape a mosaic of patches (landscape units) composed of different microhabitats. Within these ecosystems irradiance appears as one of the most changing factors affecting communities’ dynamic. However, the impact of irradiance on the species performance is still poorly known. In this study, we intended to explore whether irradiance heterogeneity influences early recruitment. We chose a patchy Mediterranean site. During 2001, 2002 and 2003 we marked naturally Quercus ilex emerged seedlings. We distinguished three landscape units ( Q. ilex woodlands, shrublands and afforestations) and six microhabitats (oak, broom, tall shrub, subshrub, pine and open). For each seedling, we measured (1) irradiance as global site factor (GSF); (2) acorn presence; (3) acorn depth; (4) burial layer; (5) distance to the nearest co-specific adult. As estimates of seedling performance, we measured seedling survival and height. Estimates of seedling performance were higher in 2001 and 2002 than in 2003. Oak or pine microhabitats achieved higher survival rates than brooms or open areas whilst under oaks, brooms or in open areas seedling height was lower than under tall shrubs. Irradiance significantly affected seedling survival and growth. The lower the irradiance, the higher the survival. Maximal growth was found at intermediate irradiance, indicating that, in this system, moderate shade is crucial for Q. ilex seedling recruitment. Some microhabitats provide intermediate irradiance that combines positive effects on survival and growth. As a final point, dispersers’ activity creates a heterogeneous distribution of acorn deposition filtering out part of the total irradiance present in the study site.

Acclimation to light in seedlings of Quercus petraea (Mattuschka) Liebl. and Quercus pyrenaica Willd. planted along a forest-edge gradient

Photosynthetic acclimation of two co-occurring deciduous oaks (Quercus petraea and Quercus pyrenaica) to a natural light gradient was studied during one growing season. In the spring of 2003, 90 seedlings per species were planted along a transect resulting from a dense Pinus sylvestris stand, an adjacent thinned area and a 10-m-wide firebreak (16.5–60.9% Global Site Factor (GSF)). In two dates of the following summer, we measured leaf gas exchange, carboxylation efficiency (CE), chlorophyll and nitrogen content, light–response curves of chlorophyll a fluorescence parameters, and leaf mass per area (LMA). Summer was mild, as evidenced by leaf predawn water potential (Ψpd), which reduced the interactive effect of water stress on the response of seedlings to light. Q. pyrenaica had higher LMA, CE, stomatal conductance (g s max) and photosynthesis per unit area $$(A_{\max }^a )$$ than Q. petraea at all growth irradiances. $$A_{\max }^a$$ , LMA, g s max and electron transport rate (ETR) all increased with light availability (GSF) in a similar fashion in both species. Light had also a clear effect on the organization of Photosystem II (PS II), as deduced by chlorophyll a fluorescence curves. Chlorophyll concentration (Chlm) decreased with increasing light availability in Q. pyrenaica but it did not in Q. petraea. Seedlings of Q. petraea acclimated to higher irradiances showed a greater non-photochemical quenching (NPQ) than those of Q. pyrenaica. This suggests a higher susceptibility to high light in Q. petraea, which would be consistent with a better adaptation to shade, inferred from the lower LMA or the lower rate of photosynthesis.

Altitude and woody cover control recruitment ofHelleborus foetidusin a Mediterranean mountain area

This study explores how variation of macro‐ and micro‐climatic conditions associated with changes in altitude affect early recruitment dynamics of the perennial herbHelleborus foetidus(Ranunculaceae). We also analyse the relevance of facilitation by woody vegetation on seedling recruitment along altitudinal gradient. We conducted a sowing experiment testing the effect of altitude (using three populations located at 1100, 1400 and 1650 m a.s.l.) and woody cover (open areas vs cover of woody species) on seedling emergence during two years and survival three years after sowing. Simultaneously, we characterised elevations and cover types in terms of climatic factors (surface air temperature and relative humidity) throughout a whole year, and light conditions (global site factor and red/infrared ligh ratio) using hemispheric photographs. We detected a significant effect of elevation on seedling emergence, with a higher emergence at lowest altitude. Woody cover greatly affected seedling survival and recruitment, both rates being higher under woody species than in open areas. Emergence was negatively correlated with winter stress factors, which increased with elevation. Survival and recruitment were negatively correlated with summer stress factors, which were ameliorated by woody cover and with altitude. Amelioration of climatic factors by woody cover was not influenced by altitude. Implications for species persistence in Mediterranean mountains under climate change scenarios are discussed.

Effect of plant architecture and hail nets on temperature of codling moth habitats in apple orchards

AbstractPlant architecture of apple trees in commercial orchards was rapidly changed from traditional tall trees to dwarf trees to optimize yield and fruit quality. Additionally, hail nets are widely used to prevent yield loss by hail. These changes are expected to considerably influence the orchard microclimate and thus the developmental rates of pest insects in apple. However, these relationships have not yet been fully elucidated. The present study was conducted over the seasonal cycle to investigate the influence of plant architecture and hail nets on the habitat temperatures of the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), in apple, Malus domestica Borkh. (Rosaceae). Within the canopies, leaf area index (LAI) and global site factor (GSF) were quantified using hemispherical photography. Temperature was analysed for the main habitats of the different codling moth stages, i.e., air within the canopy, bark of tree stems, and apple fruit. In dwarf trees, LAI was lower, leading to a higher GSF than in tall trees. Hail nets did not influence LAI and GSF. Results for dwarf trees compare as follows with those for tall trees: Average air temperatures within the canopy were 0.7 °C higher during daytime, whereas 0.4 °C lower at night. Mean surface temperatures of bark were 0.9 °C higher on sunny and 0.4 °C on overcast days. Mean surface temperatures of apple fruits were 1.8–2.7 °C higher on sunny days, but 0.6 °C cooler on overcast days. The effect of hail nets was confined to a reduction of the air temperature within the canopy by approximately 0.2–0.8 °C. Bark and apple surface temperatures were not significantly affected. Based on the temperature differences in the habitats considered, the calculated development of the codling moth in dwarf trees was on average 3 days faster than in tall trees. The calculations imply a negligible effect of hail nets on codling moth development.

Influence of environmental conditions on germinant survival and diversity of Scots pine (Pinus sylvestris L.) in central Spain

The aim of the study was to investigate dynamics and pattern of natural regeneration and the influence of seedbed and light on germination and initial seedling survival of Scots pine (Pinus sylvestris L.) during three growing seasons. Four 5-m2 plots in a natural Scots pine forest in central Spain were randomly established at the beginning of the regeneration process. Germination and seedling survival were recorded in 100 regeneration sampling subplots (0.25 m2) per plot, while seedbed type and the relative light intensity reaching the forest floor was characterised in 1-m2 subplots. The spatial correlations between survival, light conditions and organic matter depth were analysed through cross-variograms. Germination and survival were highly variable both within and between plots (ranging from 0 to 89%) and affected by high summer temperatures (33–35°C) in 2003. The spatial pattern characterisation of survival by Ripley’s K function showed a dominant cluster distribution. Occurrence tended to be clumped when abundance was greater than 15 seedlings, whereas for lower densities, seedlings were randomly distributed. Seedlings and herbs occupied the same sites where environmental conditions were appropriate for them to live. Spatial association occurred frequently for light and organic matter depth. Results suggested that the Scots pine seedlings in our study in central Spain preferred moderate light conditions (Global Site Factor <0.40). Thus, if this forest is to be naturally regenerated with Scots pine, the shelterwood system (whereby some mother trees are left to provide shelter for at least 5 years), along with mechanical disturbance of the seedbeds, would promote seed germination, as long as a seed source is present. However, regeneration success is affected by year-to-year conditions.

Canopy closure and amphibian diversity in forested wetlands

The crowns of trees and shrubs often overtop temporary wetlands in forested regions. By shading pond basins, canopy can dramatically change the conditions experienced by residents such as amphibians. In this study, we estimated the presence of 8 amphibian species across 17 temporary wetlands at the Yale–Myers Forest in northeastern Connecticut, USA. In addition, we quantified the light environment using a grid of hemispherical canopy photographs to calculate Global Site Factor (GSF) within each wetland. Amphibian richness was low in most wetlands, and most wetlands were relatively shaded. Amphibian richness increased in lighter wetlands. This result was not confounded by relationships with wetland size. Most amphibian species tended to be absent from heavily shaded wetlands (‘open canopy specialists’). However, three species were often found in the shadiest wetlands (‘canopy generalists’). Field transplant experiments using one canopy generalist and one open canopy specialist showed that development of the generalist was less affected by wetland light levels compared with performance of the specialist. These findings suggest that canopy may be an important determinant of amphibian diversity patterns across wetlands. Further, conservation strategies dependent on universally applied, inviolate shoreline vegetation buffers may inadvertently contribute to species loss. Because species differ in their sensitivity to changes in canopy, these losses may be predictable.

Forest mediated light regime linked to amphibian distribution and performance.

The vegetation in and around the basins of ephemeral wetlands can greatly affect light environments for aquatic species such as amphibians. We used hemispherical photographs to quantify the light environment in terms of the global site factor (GSF), the proportion of available solar radiation that actually strikes the wetland. We compared GSF to the distribution and performance of two amphibian species (Pseudacris crucifer and Rana sylvatica) within 17 ephemeral wetlands in northeastern Connecticut, USA. We found that P. crucifer is restricted to lighter wetlands (GSF >0.34) and that its abundance is proportional to GSF. By contrast, R. sylvatica is found across the light gradient and its abundance is unrelated to GSF. For both species, GSF is a strong predictor of larval developmental rate. In addition, P. crucifer growth rate is higher in lighter wetlands. Through thermal effects, changes in resources, or other influences, light appears to be an important predictor of the distribution and performance of amphibians. Because the structure of canopies can change rapidly, and because amphibians can be strongly impacted by these changes, vegetation mediated effects on wetland light environments may be critical to understanding the dynamics of amphibian populations within forested biomes.

The influence of canopy gap size on natural regeneration of Brazil nut ( Bertholletia excelsa) in Bolivia

To test whether Brazil nut ( Bertholletia excelsa) requires a specific canopy gap size in order to regenerate, 50 gaps of varying size (25–712 m 2) and 20 understorey sites (each of 141 m 2) were sampled in tropical moist forest at Riberalta, Bolivia. The density of Brazil nut seedlings (<1.30 m in height) and saplings (>1.30 m) was assessed at each site, and the light environment was determined using hemispherical photography to estimate the global site factor (GSF), which represents the proportion of total above canopy irradiance that is received below the canopy. Saplings only occurred in gaps >95 m 2 or >10.4% GSF, whereas seedlings occurred throughout a range of gap sizes and in the forest understorey. Regression analysis showed a highly significant relationship between Brazil nut sapling density against both gap size and global site factor (GSF). These results may contribute to the development of plans for sustainable management of this species, by defining the appropriate canopy gap size required for the successful natural regeneration of Brazil nut.

Landscape-scale evaluation of understory light and canopy structures: methods and application in a neotropical lowland rain forest

Light is a key resource controlling tree regeneration in the understory of closed-canopy old-growth forests. To evaluate the distribution of understory light environments at a landscape scale, we used stratified random sampling in a 500-ha stand of Costa Rican tropical rain forest. Fifteen 100 m long transects were placed using random coordinates within two soil–geomorphology units (flat alluvial terraces and dissected ridge-slope-swale terrain). At 2.5-m intervals we measured canopy height and slope angle, classified topographic position, and took canopy photographs with a fish-eye lens at 1 and 3 m above the ground (and at 0.6 and 5 m height at five stations per transect). Photographs were analyzed for global site factor (GSF), which is analogous to the percentage of full sun radiation reaching a point. Canopy height and GSF at 1 and 3 m above the ground were significantly autocorrelated (Moran's I) at 2.5-m intervals. The autocorrelation rapidly declined at greater intervals, reaching nonsignificance at ca. 20 m. Both canopy height and GSF at 3 m height had a weak tendency for negative autocorrelations at intervals of 25–50 m. Median canopy height (615 stations) was 23 m (range 0–37). Gaps (canopy height ≤2 m, Brokaw 1982) were only 1.5% of sample points. Gaps were more frequent on steep slopes than on terraces, ridgetops, swales, and gentle slopes. Canopy height varied significantly across this topographic gradient. At all four heights (0.6, 1, 3, and 5 m) median GSF was ≤2.4%. GSF values &gt;8% accounted for only 3% of the total sample (N = 1380). GSF was only weakly negatively correlated with canopy height and the relation was not monotonie. Under canopies 13-19 m tall, nearly all GSF values were ≤5%. Higher GSFs were more frequent under both shorter and taller canopies. Given the observed variance in GSF and canopy height, 100–200 points separated by intervals of ≥20 m are necessary to measure the forest-wide means of these variables to ±10%. We discuss implications of these results for current approaches to modeling understory light based on canopy characteristics. We compared the random background of light environments from 1–3 m above the ground at La Selva with those occupied by saplings of pioneer and nonpioneer tree species. The two pioneers (Cecropia spp.) occurred in microsites significantly brighter than random sites, while sapling microsites of all five nonpioneer species were significantly darker than random. Comparing the landscape-scale distribution of key resources with species' actual distributions at similar scales offers a quantitative method for assessing plant life histories within and among forests.

Comparative Analysis of Microhabitat Utilization by Saplings of Nine Tree Species in Neotropical Rain Forest

We examined the hypothesis that tropical rain forest tree species differ significantly in the degree to which their regeneration is associated with treefall gaps. Using static and dynamic analyses, we evaluated crown light environments and forest structure around 0.5-5 m tall saplings (N = 424) of two pioneer and seven nonpioneer tree species in old growth tropical wet forest at the La Selva Biological Station, Costa Rica. Canopy photographs were taken with a fisheye lens directly above all individuals. Each photograph was analyzed to calculate a Global Site Factor (GSF), which is analogous to the percentage of photosynthetically active radiation reaching a site relative to a totally open site (...)