Showing papers on "Tree canopy published in 2001"

Forest structure, canopy architecture, and light transmittance in tropical wet forests

Abstract: Identifying factors that influence variation in light availability within forested ecosystems represents an important component in our understanding of the complex determinants of tree seedling regeneration. We assessed the influence of forest structure and canopy tree architecture on spatial heterogeneity of understory light availability in three old-growth and three second-growth forests in lowland Costa Rica. Forest structure and understory light availability were measured within forest types using contiguous 10 × 10 m quadrats along three 130–160 m transects in each stand. Two 20 × 60 m plots in each forest type were sampled more intensively, including vertical profiles of light availability from 1 to 9 m height. Mean diffuse light transmittance increased from 2% at 1 m height to over 10% at 9 m height and did not differ significantly between forest types at any height. However, the relationships among height classes differed between forest types. Second-growth plots showed a negative spatial autocorr...

Fire and vegetation effects on productivity and nitrogen cycling across a forest-grassland continuum

Abstract: Mixed tree-grass vegetation is important globally at ecotones between grass- lands and forests. To address uncertainties vis-a`-vis productivity and nitrogen (N) cycling in such systems we studied 20 mature oak savanna stands, ranging from 90% woody dominated to 80% herbaceous dominated, growing on comparable soils in a 32-yr-old fire frequency experiment in Minnesota, USA. Fire frequencies ranged from almost annual burning to complete fire protection. Across all stands, aboveground net primary productivity (ANPP) ranged from 2 to 12 Mg·ha 21 ·yr 21 , decreased with fire frequency (r 2 5 0.59), increased with woody canopy dominance (r 2 5 0.83), and increased with soil net N min- eralization rates (r 2 5 0.79), which varied from 25 to 150 kg·ha 21 ·yr 21 . ANPP was positively related to total biomass (r 2 5 0.95), total canopy leaf N content (r 2 5 0.88), leaf area index (LAI; r 2 5 0.87), annual litterfall N cycling ( r 2 5 0.70), foliage N concentration (r 2 5 0.62), and fine root N concentration (r 2 5 0.35), all of which also increased with increasing tree canopy cover. ANPP, soil N mineralization, and estimated root turnover rates increased with woody canopy cover even for stands with similar fire frequency. ANPP and N min- eralization both decreased with fire frequency for stands having a comparable percentage of woody canopy cover. Fine root standing biomass increased with increasing grass dom- inance. However, fine root turnover rate estimated with a nitrogen budget technique de- creased proportionally more with increasing grass dominance, and hence fine root produc- tivity decreased along the same gradient. Via several direct and indirect and mutually reinforcing (feedback) effects, the com- bination of low fire frequency and high tree dominance leads to high rates of N cycling, LAI, and productivity; while the opposite, high fire frequency and high grass dominance, leads to low rates of N cycling, LAI, and productivity. Carbon and N cycling were tightly coupled across the fire frequency and vegetation type gradients.

Influence of logging, fire, and forest type on biodiversity and productivity in southern boreal forests

Abstract: The effects of logging on ecosystem sustainability are controversial. Sur- prisingly, existing data are inadequate to allow a comprehensive evaluation of logging effects on biodiversity, composition, and productivity since appropriate comparisons of stands of similar ages and differing disturbance histories are rare. We addressed this issue using a study of 2000 plots in 80 southern boreal forest stands in northern Minnesota, USA, wherein we contrasted naturally regenerated aspen (Populus tremuloides), jack pine (Pinus banksiana), and black spruce (Picea mariana) stands established following logging or the dominant natural disturbance, wildfire, for stands of two age classes (25-40 and 70-100 yr old). For young stands, those established postlogging had higher vascular plant diversity than those postwildfire. Otherwise, we found no evidence of differing species diversity (including canopy tree, shrub, herbaceous, and bryophyte species), composition, produc- tivity, or nitrogen cycling, in forest stands of comparable age and forest type that originated after logging compared to after wildfire. These variables, however, differed significantly among forest types, with aboveground net primary productivity and plant species diversity generally higher in aspen than jack pine stands, even when growing on comparable soils, and lowest in black spruce. Although there is evidence that logging has increased the proportional landscape dominance by aspen, a forest type with higher diversity, nutrient cycling, and productivity than other types, our evidence refutes the idea that disturbance by logging has diminished stand-scale productivity or plant diversity in comparison to the common natural disturbance, wildfire.

Effects of nutrients and shade on tree-grass interactions in an East African savanna

Abstract: Savanna trees have a multitude of positive and negative effects on understorey grass production. but little is known about how these effects interact. We report on a fertilization and shading experiment carried out in a Tanzanian tropical city savanna around Acacia tortilis trees. In two years of study there was no difference in grass production under tree canopies or in open grassland, Fertilization, however, indicate that trees do affect the nutrient limitation of the grass layer with an N-limited system in open grassland to a P-limited system under the trees. The NT ratios of grass gave a reliable indication of the nature of nutrient limitation, but only when assessed at the end of the wet season, Mid-wet season nutrient concentrations of grasses were higher under than outside the tree canopy, suggesting that factors other than nutrients limit grass production. A shading experiment indicated that light may be such a limiting factor during the wet season when water and nutrients are sufficiently available. However, in the dry season when water is scarce, the effect of shade on plant production became positive. We conclude that whether trees increase or decrease production of the herbaceous layer depends on how positive effects (increased soil fertility) and negative effects (shade and soil water availability) interact and that these interactions may significantly change between wet and dry seasons.

A strategy for estimating tree canopy density using landsat 7 etm+ and high resolution images over large areas

Abstract: Forest cover is of great interest to a variety of scientific and land management applications, many of which require not only information on forest categories, but also tree canopy density. In previous studies, large area tree canopy density had been estimated at spatial resolutions of 1km or coarser using coarse resolution satellite images. In this study, a strategy is developed for estimating tree canopy density at a spatial resolution of 30 m. This strategy is based on empirical relationships between tree canopy density and Landsat data, established using linear regression and regression tree techniques. One-meter digital orthophoto quadrangles were used to derive reference tree canopy density data needed for calibrating the relationships between canopy density and Landsat spectral data. This strategy was tested over three areas of the United States. In general, models derived using both linear regression and regression tree techniques were statistically significant. The regression tree was found more robust than linear regression, primary due to its capability of approximating complex non-linear relationships using a set of linear equations. This strategy will be recommended for use in developing a nation wide tree canopy density data set at a 30 m resolution as part of the Multi-Resolution Land Characteristics 2000 project.

Spatial distribution of leaf water‐use efficiency and carbon isotope discrimination within an isolated tree crown

Abstract: The spatial variations in the stable carbon isotope composition ( δ 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination ( Δ ) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO 2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in Δ . The main conclusions of this study are: (i) leaf Δ Δ can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf Δ , and (iii) the spatial variations in Δ Δ and water-use efficiency resulted from the short-term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long-term acclimation of leaf characteristics to the local light regime.

Does Rhododendron maximum L. (Ericaceae) Reduce the Availability of Resources Above and Belowground for Canopy Tree Seedlings

Abstract: Subcanopy shrubs and perennial herbs inhibit recruitment of canopy trees in forests around the world. Although this phenomenon is widespread, and can have significant effects on community dynamics, the mechanisms of inhibition are not well understood. In the southern Appalachian region, Rhododendron maximum inhibits the recruitment of canopy trees in forests of northern red oak (Quercus rubra). We have shown, in previous research, that processes occurring before canopy tree seed germination are not responsible for this inhibition. Therefore, post-germination processes, such as competition for resources are most important. In this study we show that the presence of a thicket of R. maximum in the understory reduced the availability of light by 80%, the frequency and duration of sunflecks by 96%, the availability of water by 20% and the availability of several soil nutrients (particularly cations) by variable amounts. Moreover, the survival of Q. rubra seedlings in the understory over 3 y was signif...

Fire impacts and crowning in the boreal forest: study of a large wildfire in western Quebec

Abstract: Within the context of studying the ecological impacts of wildland fires in the boreal forest, a spatial analysis of a major wildfire was conducted. The fire covered nearly 500 km 2 in the north-western part of Quebec's boreal forest in the summer of 1995. The spatial distribution of different fire impacts on the forest canopy was obtained using timber damage assessment maps. Fire impacts varied throughout the burned area, ranging from areas where trees had completely burned crowns (43%) to remaining patches of trees with green foliage (3%). The effects of local stand and site factors on crown fire, as assessed by the fire impacts, were evaluated using geographic information systems. Despite the large extent and high intensity of the wildfire created by extreme fire weather conditions, stepwise logistic regression and analysis by log-linear models indicated that variations in surface material, stand composition, and estimated stand age played a role in the presence or absence of crowning at the stand level. However, it appears that height and density of stand, as well as topography, did not have a significant influence. Our study presents the variability of fire impacts and its implications, and it provides a better understanding of the relationships between landscape components and fire crowning.

Woody vegetation and canopy fragmentation along a forest-to-urban gradient

Abstract: To identify patterns that can be used to predict vegetation and landscape characteristics in urban environments, we surveyed the species composition and size of woody plants, as well as the landscape structure of forest canopies, along a forest-to-urban gradient near Oxford, Ohio, USA. The gradient included six sites of increasingly urban land-use: a preserve, a recreational area, a golf course, a residential subdivision, apartment complexes, and a business district. We recorded species identity and stem diameter for all woody plants greater than 3 cm diameter at breast height (DBH) to examine the distribution of individual species as well as overall community composition. We used digitized aerial photographs to compare the spatial characteristics of the forest canopy at each site. We found predictable patterns in species diversity (Shannon index), spatial heterogeneity in species composition (mean percent dissimilarity), and all measures of patch fragmentation (canopy cover and patch number and size). There were clear differences in tree density and total basal area between forested sites and developed sites, but there was little resolution among developed sites. Species richness and average DBH showed no clear pattern, suggesting that landscaping preference largely determined these values. We present a modified version of an intermediate heterogeneity model that can be used to predict diversity patterns in urban areas. We discuss probable mechanisms that led to these patterns and the potential implications for animal communities in urban environments.

New Zealand's potential forest pattern as predicted from current species‐environment relationships

Abstract: New Zealand's potentid forest compoSition was predicted from regressions relating the distributions Of major canopy tree species to environment. Environmental variables, chosen for their correspondence to major tree physiological processes∗ inc1uded annual and seasod temperature and solar radiation, soil and atmospheric water deficit, soil leaching, slope, and Soil parent material and &ainage. Envkomend values were estimated both for a large set of irrgularly distributed plots describing fmest composition, and Points on a l‐km grid across New Zealand. Regressions were fitted to the plot data sPsies by Species with those for the four Nithofagus species also Including terms to correct for the effects of their geographic disjunctions. Regressions for other species contained both environmental variables and terms to account for their competitive interactions with the patchily distributed but strongly competitive Nothofagus species. Predictions of species abundance were then made for the grid data set...

Forest light and its influence on habitat selection

Abstract: Light filtered through the forest canopy is the most variable physical factor in tropical forests, both in space and time. Vegetation geometry, sun angle, and weather generate five light environments, which greatly differ in intensity and spectrum. Forest light spectra can directly affect photosynthesis, plant morphogenesis, visual communication, and the effectiveness of plant-animal interactions. For animals, the apparent simplicity of five light environments is complicated by different types of contrast with the optical background which greatly modify the conspicuousness of visual signals. The purpose of this paper is to describe peculiarities of light in tropical forest, and to review the effects of light intensity and especially quality on plants and animals. Ecophysiological adaptations of plants to cope with contrasting light environments operate at daily, seasonal and life time-scales. Ambient light quality acts as a signal for both animals and plants, and consequences on plant growth, colour display, and signal design are examined. An analysis of the range of spectral parameters along a deforestation gradient is presented, testing if sites with more variation in light could support more species which are light-environment specialists. It is suggested that light quality measurement may be used to estimate the structural impact of forest exploitation, and that gives us the information necessary for a functional explanation of anthropogenic effects on tropical forest diversity.

Forest canopy and community dynamics in a temperate old‐growth evergreen broad‐leaved forest, south‐western Japan: a 7‐year study of a 4‐ha plot

Abstract: Summary 1 Forest canopy gap and community dynamics were studied in a 4-ha permanent plot of an old-growth evergreen broad-leaved forest dominated by Castanopsis cuspidata var. sieboldii and Distylium racemosum in the Tatera Forest Reserve, Tsushima Islands, south-western Japan. The forest was affected by a powerful typhoon in 1987 and was monitored from 1990 to 1997. 2 In 1990, all woody stems ♢ 5 cm diameter at breast height (d.b.h.) in the plot were identified, mapped and marked, and the state of 1600 5 m 〈 5 m contiguous quadrats used to locate canopy gaps. Gaps occupied 17.1% of the plot, which contained 4494 tree and shrub stems (total basal area 63.48 m 2 ha ‐1 ). Gaps were re-censused in 1997 and both marked and newly recruited ( ♢ 5 cm d.b.h.) stems were recorded in 1992 and 1997. 3 Over 7 years the rates of canopy gap formation and closure were 0.72% year ‐1 and 1.61% year ‐1 , respectively, mortality and recruitment rates were 0.97% year ‐1 and 0.99% year ‐1 , and the rates of loss and gain in basal area were 0.95% year ‐1 and 0.83% year ‐1 . 4 The mortality of stems was size-dependent, with those in middle size classes having the lowest rates. Mortality of stems was lower in canopy and higher in the understorey, while the proportion of stems killed by disturbances increased with height. 5 Stems that died during the 7 years were predominantly located in newly created gaps, whereas stems were recruited into both established and new gaps. Deciduous broadleaved species were largely restricted to gaps that remained open throughout the study. 6 Both composition and structure of the forest changed in response to disturbancerelated effects on canopy dynamics.

Evaporation and transpiration from forests in Central Europe - Relevance of patch-level studies for spatial scaling

Abstract: Spatial scaling from patch to the landscape level requires knowledge on the effects of vegetation structure on maximum surface conductances and evaporation rates. The following paper summarizes results on atmospheric, edaphic, and structural controls on forest evaporation and transpiration observed in stands of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and European beech (Fagus sylvatica). Forest canopy transpiration (E c ) was determined by tree sapflow measurements scaled to the stand level. Estimates of understory transpiration and forest floor evaporation were derived from lysimeter and chamber measurements. Strong reduction of E c due to soil drought was only observed at a Scots pine stand when soil water content dropped below 16% v/v. Although relative responses of E c on atmospheric conditions were similar, daily maximum rates of could differ more than 100% between forest patches of different structure (1.5–3.0 mm d−1 and 2.6–6.4 mm d−1 for spruce and beech, respectively). A significant decrease of E cmax per leaf area index with increasing stand age was found for monocultures of Norway spruce, whereas no pronounced changes in were observed for beech stands. It is concluded that structural effects on E cmax can be specified and must be considered for spatial scaling from forest stands to landscapes. Hereby, in conjunction with LAI, age-related structural parameters are important for Norway spruce stands. Although compensating effects of tree canopy layers and understory on total evaporation of forests were observed, more information is needed to quantify structure-function relationships in forests of heterogenous structure.

Decaying scalars emitted by a forest canopy : A numerical study

Abstract: A large-eddy simulation is modified to include multiple scalars emitted by a plant canopy. Each of these scalars is subjected to varying rates of chemical loss. Presented is a detailed comparison between conservedspecies and species undergoing first- and second-order chemical loss.Profiles of mean mixing ratio, mixing-ratio variance and vertical mixing-ratio flux reveal the influence of chemical reactivity. Distribution of thescalar source through the depth of the canopy is shown to locally reducethe reaction rate for second-order species. Transport efficiencies, diffusioncoefficients, and mean source heights also exhibit chemical dependencies.Budgets of mixing-ratio variance and flux elucidate the mechanisms throughwhich chemistry modifies each. Instantaneous fields show the existence ofintermittently occurring coherent structures that are thought to enhancespecies segregation.

The unusual life history of alseis blackiana: a shade-persistent pioneer tree?

Abstract: Classifications of tree species into regeneration guilds or functional groups are often based on a limited number of life history characteristics indicative of requirements for early establishment, with the assumption that these traits are correlated with growth requirements of larger individuals. We tested the validity of this assumption for Alseis blackiana, a common canopy tree in central Panama, by examining its seed germination, seedling growth, and sapling distribution, growth, and mortality. We found that the early life history of Alseis is characteristic of pioneer species: the seed germination rate was much higher in forest gaps than in the forest understory, small seedlings were absent from the understory, and seedlings showed strong plastic responses to variation in light. Seedling relative growth rate, leaf area production rate, and maximum carbon assimilation rates of Alseis are much more similar to those of the pioneer tree Miconia argentea than to those of the non-pioneer tree Tetragastris panamensis. However, in contrast to Miconia, estab- lished seedlings of Alseis are capable of remarkable persistence in the understory; in a 1- ha plot on Barro Colorado Island, the mortality rate for 722 saplings ,1 cm dbh was ,4% per year between 1986 and 1998. We suspect that shade tolerance in Alseis is achieved by both physiological and morphological adaptations, e.g., efficient light interception by thin, nonoverlapping leaves, tolerance to litterfall damage, and a high resprouting capacity. These results show that existing functional group classifications do not account for the full range of seedling trait combinations found in tropical trees and indicate that a gap requirement for seedling establishment need not limit the abundance of adult individuals in a stand.

Canopy processes: implications for transpiration, interception and splash induced erosion, ultimately for forest management and water resources

Abstract: Some recent experimental and theoretical developments in research related to tropical forest canopies are reviewed. Deuterium tracing studies in India, which rely on the collection of samples of transpired water from leaves in the canopy, have established the importance of ‘tree size’ as an index of whole tree transpiration and the use of leaf area, basal area and sapwood area for scaling transpiration from the tree to plot scale is discussed.

The relationship between ice thickness and northern hardwood tree damage during ice storms

Abstract: The response of four tree species, Acer saccharum Marsh., Acer rubrum L., Populus tremuloides Michx., and Betula populifolia Marsh., to ice storm damage was studied in the northern hardwood forest of southern Quebec. The focus of the study was the impact of ice accretion on trees as a function of damage type and species at the stand and regional scales along a gradient of ice accumulation ranging from 2 to 88 mm radial thickness and to assess the role of the combined effect of ice and wind stress. Further, we estimate the return time for death resulting from ice storms in these forests. The study showed that the magnitude of ice accumulation was the primary determinant of tree damage (measured as the mean percentage of individual tree canopy removed) and that tree size was the primary determinant of damage type (bending, snapping, or substantial branch loss). Stand position (edge versus interior) did not influence susceptibility to damage. The research demonstrated that edge and slope trees bent or snapped in the direction dictated by crown asymmetry. We have no evidence that the modest winds during this icing event played a major role. Lastly, we couple the return time for a given ice thickness with the probability of severe damage to argue that (i) canopy tree mortality from icing is primarily due to glaze accumulations of moderate rarity (around 12-35 mm of ice) rather than extraordinary events such as 1998 and (ii) ice storms are likely the greatest single source of canopy tree mortality in the hardwood forests of southern Quebec with an estimated return time for death of about 250 years.

Methods to assess tropical rain forest canopy structure: an overview

Abstract: Forest canopy structure (sensu latu) is the combination of forest texture (the qualitative and quantitative composition of the vegetation as to different morphological elements), and forest structure (sensu strictu, the spatial arrangement of these elements). Scale is an aspect of major importance. At a regional scale forest types can be distinguished, like broadleaf or coniferous forest. At local scale, distribution and size and shape of tree crowns, and the spatial distribution of leaves and branches within tree crowns determine to a large extent the canopy structure. Which components and sub-components are used, and also the scale at which their spatial arrangements are studied, is of great importance for the possible outcome of the analysis of canopy structure. This is specially the case when canopy structure is needed as a correlate to ecological questions, e.g., on habitat specificity of animals, or epiphytes. Methods available for describing and analysing canopy structure are discussed. At large scale levels remote sensing data are used to describe differences in structure. High-resolution radar images are used to describe canopy structure in detail and over large areas. Repeated measurements over time can be used for monitoring purposes. Ways to measure the three dimensional structure of (components within) individual trees in detail are being developed, and are coupled to physiological models. Currently, use of such methods is only feasible for small plants. Forest tomography (where the vegetation occupation and empty spaces are determined in horizontal and vertical slices of the forest) is proposed as a way to describe vertical and horizontal structure. Vegetation cover and occupation is analysed above grid points in a forest. As an example the vertical structure of a Cameroonian forest is described at several levels of detail. The research question asked should govern completely the choice of the parameters and the methods used for the description of forest canopy structure.

Tree mortality and effects of release from competition in an old‐growth Fagus‐Abies‐Picea stand

Abstract: . In a montane mixed Fagus-Abies-Picea forest in Babia Gora National Park (southern Poland), the dynamics of an old-growth stand were studied by combining an 8-yr annual census of trees in a 1-ha permanent sample plot with radial increments of Abies and Picea growing in the central part of the plot. The mortality among the canopy trees was relatively high (10% in 8 yr), but the basal area increment of surviving trees slightly exceeded the losses caused by tree death. DBH increment was positively correlated with initial diameter in Abies and Picea, but not in Fagus. For individual trees smaller than the median height, basal area increment was positively related to the basal area of old snags and the basal area of recently deceased trees in their neighbourhood, but negatively related to the basal area of live trees. Dendrochronological analysis of the past growth patterns revealed numerous periods of release and suppression, which were usually not synchronized among the trees within a 0.3 ha plot. The almost normal distribution of canopy tree DBH and the small number of young individuals in the plot indicated that stand dynamics were synchronized over a relatively large area and, hence, were consistent with the developmental phase concept. On the other hand, the lack of synchronization among periods of growth acceleration in individual mature Abies and Picea trees conforms more closely to the gap-dynamics paradigm.

Forest canopy research: sampling problems, and some solutions

Abstract: Conducting research in forest canopies is accompanied by both opportunities and difficulties. Opportunities occur because the canopy is important in overall forest structure and function, for example in containing a high proportion of species richness, being the site of forest-atmosphere fluxes, and in strongly influencing understory microclimate. However, despite this importance, the canopy has been largely neglected by researchers, until recently. Many difficulties in forest canopy research apparently arise from problems related to access. In a survey of 112 canopy researchers, examples of problems cited include: bias in the selection of study species or sampling locations; difficulties in obtaining adequate replication; problems in collecting data in a three-dimensional, complex environment; logistical constraints in moving between canopy sampling points; and, disturbance, sometimes including interference of the object being studied (i.e., ‘demonic intrusion’) caused by access. Many of these problems have at least partial solutions. These include: using multiple access techniques to give greater sampling flexibility; identifying appropriate units of replication within the canopy; using ground-operated canopy sampling methods; and, using protocols for unbiased and systematic data collection in three-dimensional sampling space. Designing experimental protocols for work in the canopy requires at the outset the allocation of time and resources for anticipating and overcoming the particular sampling problems associated with the work.

Stochasticity of species assemblage of canopy trees and understorey plants in a temperate secondary forest created by major disturbances

Abstract: Detrended canonical correspondence analysis (DCCA) was applied to explore the species assemblage of plants in a temperate secondary forest that was created by major disturbances. The DCCA showed vague relationships between species dominance and environmental factors for canopy tree species even when rare species were excluded from the analysis. For the highest dominant species of the understorey, the scores of the first axis, which correlated with the species richness of overstorey trees, decreased. This fact showed that the assemblage of canopy trees affects, through the differences in leaf phenology or leaf characteristics of canopy trees, the life history of dominant understorey plants. The study’s results suggest that competition colonization might be more important for canopy trees during the developing stage of the forest if the disturbance occurs on a large scale, and that colonization from the local species pool determines species assemblage on a landscape scale.

Near‐surface climate in the boreal forest

Abstract: This paper addresses processes that affect near-surface climate over the boreal forest, using data from the Boreal Ecosystem-Atmosphere Study (BOREAS) northern study area just west of Thompson, Manitoba. The boreal forest is marked by a very large seasonal cycle with below-freezing temperatures for half the year. The freezing and thawing of the soil plays an important role in the climate at high latitudes. It moderates winter temperatures (together with the insulating snow cover), because during the freeze process, the effective heat capacity of the soil is greatly increased, and it introduces a significant lag into the climate system. Perhaps the most important consequence is that water is unavailable for evaporation and photosynthesis until snow melts and the ground thaws, which occurs late in spring. As a result, in April and in early May, relative humidity (RH) is a minimum, the surface sensible heat flux is large, and the daytime boundary layer (BL) is very deep, because of this unavailability of water. The situation reverses in the fall, when the ground is warmer than the cooling atmosphere, and mean RH is high and BL depths low. This asymmetry between spring and fall can be seen in both seasonal and diurnal cycles. The forest is heterogeneous, and there is a marked difference in summer in daytime evaporative fraction between the conifers and the deciduous forest and fens. However, above the forest the daytime BL has a strong homogenizing effect, and it is the dominant coniferous forest that controls the mean BL depth. The impact of recent rainfall, stored on the canopy, in the surface moss layer, and in the top soil layer can be readily seen in summer. BL depths rise on succeeding days without rain. A comparison of the fen and young jack pine sites shows the important role of the stomatal control by conifers on transpiration. Since evaporation goes down at high net radiation and low RH for conifer sites, it is clear that the low RH and high BL depth over the forest are a direct consequence of stomatal control. At night, however, temperature, relative humidity, and CO2 are quite heterogeneous under the stable BL. We show that uncoupling of the stable BL at night inside the forest canopy occurs at low wind speeds and high outgoing net radiation and can lead to a 5K cooling within the canopy.

Edge effects and tropical forest canopy invertebrates

Abstract: The term edge effect can be used to encompass a wide range of both biotic and abiotic trends associated with boundaries between adjacent habitat types, whether these be natural or anthropogenic. Edge effects have been shown to represent significant forces affecting both faunal and floral assemblages in fragmented ecosystems. Specific studies of faunal assemblages associated with habitat edges have revealed trends at all levels of biological organisation from individuals to communities. Studies of edge effects on invertebrates in tropical forests have been relatively scarce. In this paper we review the nature and organisation of edge effects, focusing upon the processes which may lead to detrimental consequences for both forest canopy invertebrates and the forests themselves. We present as a case study data illustrating the very large amount of variance (over 50%) in community structure that is predicted simply by abiotic (microclimatic) variables in both a tropical and a temperate forest edge. We summarise major features of edge effects amongst forest invertebrates, stress the inter-relatedness of edge and canopy biology, and present an agenda for study of the canopy as an edge.

Tree shape, forest structure and diversity of drosophilid community: Comparison between boreal and temperate birch forests

Abstract: Some models, based on the latitudinal variation in sun angle distribution, predict that trees at high latitudes have narrowly conical crowns and constitute simple-layered forests, whereas trees at low latitudes have shallowly dome-shaped and form more structurally complex multilayered forests. There is a hypothesis that structurally complex habitats can harbor potentially more species than simple ones. In this study, we examined latitudinal correlations between tree shape, forest structure and diversity in drosophilid communities, comparing boreal and cool-temperate forests. We selected secondary birch forests with a common canopy tree species, white birch (Betula platyphylla Sukatchev), as study sites. The crown shape of white birch tended to be spherical in the cool-temperate forest, but narrowly conical in the boreal forest. The foliage structure differed between the two forests. The cool-temperate forest was characterized by a clearly two-layered structure, whereas foliage in the boreal forest was less clearly stratified, being distributed somewhat continuously from the ground to the canopy at lower densities. The structural complexity expressed by foliage height diversity was greater in the cool-temperate forest than in the boreal forest. Various measures of drosophilid diversity were higher in the cool-temperate forest than in the boreal forest, probably resulting from the impoverishment of the canopy subcommunity in the boreal forest. Thus, a physical environmental factor (i.e. the angle of solar inclination) could be a potentially important factor in structuring latitudinal patterns of sylvan animal communities through changes in plant structure at the individual and community levels.

A plot-based method for rapid estimation of forest canopy chemistry

Abstract: In this study we present a rapid method to scale the leaf-level chemistry of forest stands to the whole-canopy level. The method combines simple leaf-level measurements of mass and chemistry with a...