Showing papers on "Tree canopy published in 1997"

Determination of mean tree height of forest stands using airborne laser scanner data

Abstract: The mean tree height of forest stands is a crucial stand characteristic in forest planning. Currently, the mean tree height is determined by field measurements or by photogrammetric measurements utilizing aerial photographs. In this study, mean tree height of 36 test stands is derived from tree canopy heights measured by means of an airborne laser scanner. On the average the laser recorded 505–1070 canopy heights per stand. First, the laser mean height is computed as the arithmetic mean of the canopy heights within each stand. The laser mean height underestimates the ground truth mean height by 4.1–5.5 m. Second, a weighted mean of the laser canopy heights is computed. The individual height values are used as weights. The weighted mean height underestimates the true height by 2.1–3.6 m. Finally, the laser mean height is computed as the arithmetic mean of the largest laser values within square grid cells with cell sizes of 15–30 m. The bias of the laser estimates is in the range −0.4 m to 1.9 m. The standard deviation for differences between the laser mean heights and the ground truth mean height is 1.1–1.6 m.

High spectral resolution remote sensing of forest canopy lignin, nitrogen, and ecosystem processes

Abstract: Remote sensing of foliar chemistry has been recognized as an important element in producing large-scale, spatially explicit estimates of forest ecosystem function. This study was designed to determine whether data from NASA's Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) could be used to determine forest canopy chemistry at a spatial resolution of 20 m, and if so, to use that information to drive an ecosystem pro- ductivity model. Foliage and leaf litter were sampled on 40 plots at Blackhawk Island, Wisconsin, and Harvard Forest, Massachusetts, to determine canopy-level nitrogen and lignin concentrations. At the time of the field sampling, AVIRIS data were acquired for both study areas. Calibration equations were developed, relating nitrogen and lignin to selected first-difference spectral bands (R2 = 0.87 and 0.77, respectively). Calibration equa- tions were evaluated on the basis of inter- and intrasite statistics. These equations were applied to all image pixels to make spatially explicit estimates of canopy nitrogen and lignin for both study sites. These estimates of nitrogen and lignin concentrations were then used with existing models to predict net ecosystem productivity at Harvard Forest and nitrogen mineralization rates at Blackhawk Island.

Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

Abstract: The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629°N 106.200°W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m 2 m -2 reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m -2 s -1 (8.4 mm s -1 ) (70% of the total forest conductance) and 113 mmol m -2 s -1 (2.9 mm s -1 ) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m -2 s -1 (30 mm s -1 ) and 910 mmol m -2 s -1 (23 mm s -1 ), respectively, and maximum stomatal conductances were 490 mmol m -2 s -1 (12.5 mm s -1 ) and 280 mmol m -2 s -1 (7 mm s -1 ), aspen and hazelnut, respectively. Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] α coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean α of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying α during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary laver feedbacks.

Microsite controls on tree seedling establishment in conifer forest canopy gaps

Abstract: Tree seedling establishment and growth were studied in experimental canopy gaps to assess the effect of heterogeneity of regeneration microsites within and among gaps in mature conifer forests. Seedlings were studied for two years in closed-canopy areas and in recently created gaps ranging in size from 40 to 2000 m2 in four stands of mature (90–140 yr) and old-growth (>400 yr) Douglas-fir forest in the western Cascade Range of central Oregon and southern Washington, USA. We examined the relative importance of substrate type, shade from logs and shade cloth, density of understory vegetation, gap size, and forest age on the success of Pacific silver fir (Abies amabilis), Douglas-fir (Pseudotsuga menziesii), and western hemlock (Tsuga heterophylla). Seedlings originating from seed sown on experimental microsites were compared with seedlings regenerating naturally on a range of microsites. Seedling establishment was greater on decayed wood than on forest floor or mineral soil in closed-canopy areas, particula...

Invasibility and effects of Amur honeysuckle in Southwestern Ohio forests.

Abstract: The Asian exotic Amur honeysuckle (Lonicera maackii [Rupr.] Herder) has become the dominant shrub in many forests in southwestern Ohio and in some other locations in the eastern United States. Our research focused on the invasibility of forest communities and relationships of L. maackii to the abundance of tree seedlings and herbs. We surveyed 93 forest stands near Oxford, Ohio (USA) to determine L. maackii cover, time since invasion, tree canopy cover, tree basal area, and a shade tolerance index. Stepwise multiple regression indicated that greater than one-half of the variation (r2 = 0.550) in Lonicera cover was correlated to five variables (in descending order of importance): tree canopy cover, distance from Oxford, shade tolerance index, tree basal area, and time since invasion. The results suggest that high light levels and proximity to an abundant seed source increase forest invasibility. Tree seedling density, species richness of seedlings, and herb cover were all inversely related to L. maackii cover. When Lonicera becomes abundant, future structure and composition of forests could be affected and local populations of herbs threatened.

Seasonal variation of energy and water vapor exchange rates above and below a boreal jack pine forest canopy

Abstract: Fluxes of energy and water vapor over boreal forest stands are expected to vary during the growing season due to temporal variations in solar energy, soil and air temperature, soil moisture, photosynthetic capacity, and leaf area. To investigate this hypothesis, we measured fluxes of energy balance components (solar, latent and sensible heat, and soil and canopy heat storage) over and under a boreal jack pine forest in central Canada during the 1994 growing season. Temporal trends of daily-integrated energy fluxes were significant during a 117 day period between spring and autumn. Mean fluxes of net radiation and latent heat peaked near the summer solstice. By the autumnal equinox their magnitudes were half of their peak values. On a day-to-day basis the presence or absence of clouds modulated solar energy fluxes, while evaporation rates were dependent on whether the canopy was dry or wet. When the canopy was dry, daily evaporation was generally less than 2.5 mm d−1. This amount was less than one-half the rate of equilibrium evaporation and was low compared to evaporation from vegetation in temperate zones. When the canopy was wet, daily evaporation approached 3 mm d−1 and exceeded predicted rates of equilibrium evaporation. Evaporation from the dry forest was weakly coupled to available energy and was restricted by the canopy AEs low-surface conductance. Biotic factors limiting the forest AEs surface conductance include the forest AEs low-leaf area index and partial stomatal closure. Abiotic and physiological factors restricting stomatal opening included a scarce supply of soil moisture, limiting vapor pressure deficits and the low photosynthetic capacity of the needles. The fluxes of solar energy and latent and sensible heat at the floor of the forest were a significant portion of energy exchange between the forest and the atmosphere. Typically, 20 to 40% of the total energy exchange by the jack pine stand originated at the understory. Since a substantial amount of energy occurs under the forest, two-layer, not a big-leaf AE, evaporation models are recommended as a tool for estimating water vapor fluxes from this open forest stand.

Decline of photosynthetic capacity with leaf age in relation to leaf longevities for five tropical canopy tree species.

Abstract: The effect of leaf aging on photosynthetic capacities was examined for upper canopy leaves of five tropical tree species in a seasonally dry forest in Panama. These species varied in mean leaf longevity between 174 and 315 d, and in maximum leaf life span between 304 and 679 d. The light-saturated CO2 exchange rates of leaves produced during the primary annual leaf flush measured at 7‐8 mo of age were 33‐65% of the rates measured at 1‐2 mo of age for species with leaf life span of , 1 yr. The negative regression slopes of photosynthetic capacity against leaf age were steeper for species with shorter maximum leaf longevity. In all species, regression slopes were less steep than the slopes predicted by assuming a linear decline toward the maximum leaf age (20‐80% of the predicted decline rate). Maximum oxygen evolution rates and leaf nitrogen content declined faster with age for species with shorter leaf life spans. Statistical significance of regression slopes of oxygen evolution rates against leaf age was strongest on a leaf mass basis (r 2 5 0.49‐0.87), followed by leaf nitrogen basis (r 2 5 0.48‐0.77), and weakest on a leaf area basis (r2 5 0.35‐0.70).

Simulating ozone effects on forest productivity: interactions among leaf-, canopy-, and stand-level processes

Abstract: Ozone pollution in the lower atmosphere is known to have adverse effects on forest vegetation, but the degree to which mature forests are impacted has been very difficult to assess directly. In this study, we combined leaf-level ozone response data from independent ozone fumigation studies with a forest ecosystem model in order simulate the effects of ambient ozone on mature hardwood forests. Reductions in leaf carbon gain were determined as a linear function of ozone flux to the leaf interior, calculated as the product of ozone concentration and leaf stomatal conductance. This relationship was applied to individual canopy layers within the model in order to allow interaction with stand- and canopy-level factors such as light attenuation, leaf morphology, soil water limitations, and vertical ozone gradients. The resulting model was applied to 64 locations across the northeastern United States using ambient ozone data from 1987 to 1992. Predicted declines in annual net primary production ranged from 3 to 16% with greatest reductions in southern portions of the region where ozone levels were highest, and on soils with high water-holding capacity where drought stress was absent. Reductions in predicted wood growth were slightly greater (3- 22%) because wood is a lower carbon allocation priority in the model than leaf and root growth. Interannual variation in predicted ozone effects was small due to concurrent fluc- tuations in ozone and climate. Periods of high ozone often coincided with hot, dry weather conditions, causing reduced stomatal conductance and ozone uptake. Within-canopy ozone concentration gradients had little effect on predicted growth reductions because concen- trations remained high through upper canopy layers where net carbon assimilation and ozone uptake were greatest. Sensitivity analyses indicate a trade-off between model sensitivity to available soil water and foliar nitrogen and demonstrate uncertainties regarding several assumptions used in the model. Uncertainties surrounding ozone effects on stomatal function and plant water use efficiency were found to have important implications on current predictions. Field measurements of ozone effects on mature forests will be needed before the accuracy of model predictions can be fully assessed.

Restoration of a Sri Lankan rainforest : using Caribbean pine Pinus caribaea as a nurse for establishing late-successional tree species

Abstract: In the moist tropics, studies have demonstrated poor seedling establishment of late-successional trees on lands cleared of forest. Our study examined the potential for establishing late-successional tree species that dominate the canopy of rainforest by planting within and adjacent to experimental openings that were created within a Pinus caribaea plantation. We tested five canopy tree species (Dipterocarpus zeylanicus, Mesua ferrea, Shorea disticha, S. megistophylla and S. trapezifolia) of tropical forest in south-western Sri Lanka. Seedlings were monitored for 2 years within treatments that removed either three rows or one row of Pinus canopy, a canopy edge treatment and a control that left the canopy intact. The greatest growth and dry mass for all species were in the canopy removal treatments. In particular, S. trapezifolia and S. disticha exhibited the greatest height growth in these treatments. In the three-row canopy removal treatment, M. ferrea had a significantly lower dry mass than the other species. Differences were shown in the number and area of leaves among species. Shorea trapezifolia and, to a lesser degree, S. disticha increased area by increasing leaf production. Dipterocarpus zeylanicus and, to a lesser degree, M. ferrea increased area by increasing the size of individual leaves. Guidelines based on results from this study recommend that species grow best when seedlings are planted within openings created by the removal of three rows of Pinus canopy. Where planting without canopy removal is required, S. disticha or S. megistophylla should be selected because of greater shade and drought tolerance. This experiment demonstrated that Pinus can be used as a nurse for facilitating the establishment of site-sensitive tropical forest tree species that are late-successional. In particular, results have application for similar mixed dipterocarp forest types in southeast Asia.

Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy

Abstract: The output of a large-eddy simulation was used to study the terms ofthe turbulent kinetic energy (TKE) budget for the air layers above andwithin a forest. The computation created a three-dimensional,time-dependent simulation of the airflow, in which the lowest third ofthe domain was occupied by drag elements and heat sources to representthe forest. Shear production was a principal source of TKE in theupper canopy, diminishing gradually above tree-top height and moresharply with depth in the canopy. The transfer of energy to subgridscales (dissipation) was the main sink in the upper part of the domainbut diminished rapidly with depth in the canopy. Removal ofresolved-scale TKE due to canopy drag was extremely important,occurring primarily in the upper half of the forest where the foliagedensity was large. Turbulent transport showed a loss at the canopytop and a gain within the canopy. These general features have beenfound elsewhere but uncertainty remains concerning the effects ofpressure transport. In the present work, pressure was calculateddirectly, allowing us to compute the pressure diffusion term. Wellabove the canopy, pressure transport was smaller than, and opposite insign to, the turbulent transport term. Near the canopy top andbelow, pressure transport acted in concert with turbulent transport toexport TKE from the region immediately above and within the uppercrown, and to provide turbulent energy for the lower parts of theforest. In combination, the transport terms accounted for over half ofthe TKE loss near the canopy top, and in the lowest two-thirds of thecanopy the transport terms were the dominant source terms in thebudget. Moreover, the pressure transport was the largest source ofturbulent kinetic energy in the lowest levels of the canopy, beingparticularly strong under convective conditions. These resultsindicate that pressure transport is important in the plant canopyturbulent kinetic energy budget, especially in the lowest portion ofthe stand, where it acts as the major driving force for turbulentmotions.

Snow ablation modeling at the stand scale in a boreal jack pine forest

Abstract: The purpose of this study is to predict spatial distributions of snow properties important to the hydrology and the remote sensing signatures of the boreal ecosystem. This study is part of the Boreal Ecosystems Atmosphere Study (BOREAS) of central Saskatchewan and northern Manitoba. Forested environments provide unique problems for snow cover process modeling due to the complex interactions among snow, energy transfer, and trees. These problems are approached by coupling a modified snow process model with a model of radiative interactions with forest canopies. Additionally, a tree well model describes the influence of individual trees on snow distribution on the ground. The snow process and energy budget model calculates energy exchange at the snow surface, in-pack snow processes, melting and liquid water flow, heat conduction, and vapor diffusion. The surface radiation model provides input on the radiation receipt at the snow surface for model runs in the jack pine forest. Field data consisted of measured meteorological parameters above and within the canopy, spatial variability of snow properties, and variations of incoming solar irradiance beneath the forest canopy. Results show that the area beneath tree canopies accumulated 60% of the snow accumulated in forest openings. Peak solar irradiance on the snow cover was less than one half that measured above the canopy. Model runs are compared between the open and the forested sites and show the open area ablating four days before areas beneath the canopy and eight days before forest openings and compare favorably with measured data. Physically based modeling of snow ablation was successful at the forested site and nearby open area.

Characterizing canopy nonrandomness with a multiband vegetation imager (MVI)

Abstract: A new method for measuring plant canopy nonrandomness and other architectural components has been developed using a 16 bit (65535 gray scale levels) charged-coupled device (CCD) camera that captures images of plant canopies in two wavelength bands. This complete system is referred to as a multiband vegetation imager (MVI). The use of two wavelength bands (visible (VIS) 400–620 nm and near infrared (NIR) 720–950 nm) permits identification of sunlit and shaded foliage, sunlit and shaded branch area, clouds, and blue sky based on the camera's resolution, and the varying spectral properties that scene components have in the two wavelength bands. This approach is different from other canopy imaging methods (such as fish-eye photography) because it emphasizes measuring the fraction of an image occupied by various scene components (branches, shaded leaves, sunlit leaves) under different sky conditions rather than simply the canopy gap fraction under uniform sky conditions. The MVI has been used during the Boreal Ecosystem-Atmosphere Study (BOREAS) in aspen (Populus tremuloides) and balsam poplar (Populus balsamifera) to estimate architectural characteristics of each canopy. The leaf area index (LAI), sunlit LAI, and degree of nonrandomness within a canopy are architectural properties that have been measured with the MVI. Using a crown-based Monte Carlo model for nonrandom canopies, nonrandomness factors are calculated from MVI data using two approaches (gap fraction and gap-size distribution theories) to correct total and sunlit LAI estimates from indirect methods that assume random foliage distributions. Canopy nonrandomness factors obtained from analyzing the gap-size distribution in a Monte Carlo model are shown to be a function of path length (angle) through the canopy (Ωe(θ)); thus we suggest that LAI-2000 indirect measurements of LAI be adjusted with the value of Ωe(θ) at θ=35° because this is the mean angle at which the canopy gap fraction is measured by the LAI-2000. In this study, values of Ωe(35)=0.69 in an aspen forest. Alternatively, corrections to indirect LAI measurements obtained with the MVI in this study are made using the value of Ωe(0) because the MVI is used to measure the canopy gap-size distribution and gap fraction within 15° of the zenith. Values of Ωe(0) obtained with the MVI in aspen are typically between 0.55 and 0.65; while in balsam poplar, average values of Ωe(0) are equal to 0.82. This study shows that the MVI provides an attractive indirect measurement technique to obtain accurate estimates of total LAI in aspen. Corrected canopy LAI and direct LAI measurements are greater than indirect estimates based on assuming the foliage to be randomly distributed: In aspen, total LAI is 45% larger (3.3 versus 2.0) and sunlit LAI (40° Sun zenith angle) 10% larger, while in balsam poplar, total LAI is 17% larger (2.3 versus 1.9) and sunlit LAI is only 1% larger. The importance of these clumping characteristics is best appreciated with estimates of canopy net CO2 assimilation derived from scaling leaf photosynthesis versus light relations. Aspen canopy assimilation accounting for clumping is 39% larger than estimates based on indirect measurements of total LAI and the assumption that foliage is randomly distributed.

Variation of snow cover ablation in the boreal forest: A sensitivity study on the effects of conifer canopy

Abstract: The duration and meteorological history of winter and thaw periods in the boreal forest affect carbon exchange during the growing season. Characteristics of conifer canopies exert important control on the energy exchange at the forest floor, which in turn controls snow cover processes such as melting. This analysis investigated the role of the conifer tree characteristics, including height and canopy density. Canopy and snow models estimated radiation incoming to the snow surface, the net energy budget of the snow, and melting rates of snow cover under conifer forests with different canopy density and tree height. This analysis assumed that canopy effects dominated snow surface energy exchange under conifers in the boreal forest. We used data layers of forest characteristics from the Boreal Ecosystem-Atmosphere Study (BOREAS) modeling subareas in Saskatchewan and Manitoba to guide the choice of modeled tree height and canopy density. Modeled stand characteristics assumed random location of trees and used a uniform tree height within a stand and regular crown geometry scaled to tree height. Measurements during winter and thaw in 1994 of incoming solar and longwave radiation, humidity, and wind speed above the forest canopy provided input to the models, along with air temperature measured in the canopy. Results showed the importance of canopy density and tree height as the first-order controls on cumulative incoming solar radiation at the forest floor for the range of these variables in the BOREAS test area. The combined canopy and snow models showed a large range of snow ablation within conifers, which showed the trade-offs between canopy density and tree height. Solar fluxes dominated the net transfer of energy to the snow in the north, while sensible heat exchange, net solar, and net longwave radiation played important roles in the south.

Forest regeneration in abandoned logging roads in lowland Costa Rica

Abstract: We characterized plant regeneration in four old logging roads (700-1000 m long), 12-17 yr after abandonment, in selectively logged forests in lowland Costa Rica. Sets of 4-m2 plots were laid out at 20-m intervals in three distinct microhabitats: road track (topsoil eliminated), road edge (where removed topsoil accumulates on the sides after road construction), and adjacent logged forest. Density of stems ?1 m tall and ?5 cm DBH (included canopy tree, midstory tree, liana, palm, shrub, and tree fern species) was highest in the road edge plots than either the track or logged forest plots. This "edge effect" is presumably due to buried seed germination of light-demanding trees and shrubs after moderate soil disturbance, less compaction, and higher substrate fertility than in road tracks. Species richness was the lowest, but relative dominance the highest, in the track plots of all roads: 6-9 species comprised alone 50 percent of the Importance Value Index (IVI), in contrast to 11-15 and 16-22 species required to reach 50 percent IVI in edge and forest plots, respectively. We found evidence of soil compaction in tracks of three out of four roads which, in addition to low substrate fertility, and initial lack of on-site plant propagules, could explain slower recovery of stem density and species richness compared to edge and logged forest plots. For stems >5 cm and ?20 cm DBH, density and basal area in the track plots averaged about one-fourth of edge and logged forest plot values. We estimated recovery of basal area in road tracks to take at least 80 yr to reach the status found in logged forest, and species richness over an even longer period. We suggest that abandoned logging roads serve as long corridors of relatively uniform and long-lasting floristic and structural characteristics that may confer particular ecological roles in selectively logged forests.

Seasonal Patterns in Soil Moisture, Vapour Pressure Deficit, Tree Canopy Cover and Pre-dawn Water Potential in a Northern Australian Savanna

Abstract: The wet–dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour pressure deficit, although temperatures are relatively constant throughout the year.This seasonal variation is associated with marked changes in tree canopy cover, although the exact determinants of these changes are complex. This paper reports variation in microclimate (temperature, vapour pressure deficit (VPD)), rainfall, soil moisture, understorey light environment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northern Territory, Australia. Patterns of canopy cover are strongly influenced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture that occur with the onset of wet season rains. Decreases in canopy cover coincide with decreases in soil moisture following the cessation of wet season rains and associated increases in VPD. Patterns of pre-dawn water potential vary significantly between species and between leaf phenological guilds. Pre-dawn water potential increases with decreasing VPD towards the end of the dry season prior to any increases in soil moisture. Decline in pre-dawn water potential coincides with both decreasing soil moisture and increasing VPD at the end of the dry season. This study emphasises the importance of the annual transition between the dry season and the wet season, a period of 1–2 months of relatively low VPD but little or no effective rainfall, preceded by a 4–6 month dry season of no rainfall and high VPD. This period is accompanied by markedly increased canopy cover, and significant increases in pre-dawn water potential, which are demonstrably independent of rainfall. This finding emphasises the importance of VPD as a determinant of physiological and phenological processes in Australian savannas.

Intra- and inter-specific variation in canopy photosynthesis in a mixed deciduous forest

Abstract: Within the same forest, photosynthesis can vary greatly among species and within an individual tree. Quantifying the magnitude of variation in leaf-level photosynthesis in a forest canopy will improve our understanding of and ability to model forest carbon cycling. This information requires extensive sampling of photosynthesis in the canopy. We used a 22-m-tall, four-wheel-drive aerial lift to reach five to ten leaves from the tops of numerous individuals of several species of temperate deciduous trees in central Massachusetts. The goals of this study were to measure light-saturated photosynthesis in co-occurring canopy tree species under field conditions, and to identify sampling schemes appropriate for canopy tree studies with challenging logistics. Photosynthesis differed significantly among species. Even though all leaves measured were canopy-top, sun-acclimated foliage, the more shade-tolerant species tended to have lower light-saturated photosynthetic rates (Pmax) than the shade-intolerant species. Likewise, leaf mass per area (LMA) and nitrogen content (N) varied significantly between species. With only one exception, the shade-tolerant species tended to have lower nitrogen content on an area basis than the intolerant species, although the LMA did not differ systematically between these ecological types. Light-saturated Pmax rates and nitrogen content, both calculated on either an area or a mass basis, and the leaf mass to area ratio, significantly differed not only among species, but also among individuals within species (P<0.0001 for both). Differences among species accounted for a greater proportion of variance in the Pmax rates and the nitrogen content than the differences among individuals within a species (58.5–78.8% of the total variance for the measured parameters was attributed to species-level differences versus 5.5–17.4% of the variance was attributed to differences between individual trees of a given species). Furthermore, more variation is accounted for by differences among leaves in a single individual tree, than by differences among individual trees of a given species (10.7–30.4% versus 5.5–17.4%). This result allows us to compare species-level photosynthesis, even if the sample size of the number of trees is low. This is important because studies of canopy-level photosynthesis are often limited by the difficulty of canopy access. As an alternative to direct canopy access measurements of photosynthesis, it would be useful to find an ”easy-to-measure” proxy for light-saturated photosynthetic rates to facilitate modeling forest carbon cycling. Across all species in this study, the strongest correlation was between nitrogen content expressed on an area basis (mmol m–2, Narea) and light-saturated Pmax rate (μmol m–2 s–1, Pmaxarea) (r2=0.511). However, within a given species, leaf nitrogen was not tightly correlated with photosynthesis. Our sampling design minimized intra-specific leaf-level variation (i.e., leaves were taken only from the top of the canopy and at only one point in the season). This implies that easy-to-measure trends in nitrogen content of leaves may be used to predict the species-specific light-saturated Pmax rates.

Remote sensing the biochemical composition of a slash pine canopy

Abstract: Airborne imaging spectrometers can record spatially-explicit information on the absorption features associated with foliar biochemicals in a forest canopy. The spectra of a single species pine canopy were recorded by the National Aeronautics and Space Administration's (NASA) Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). Up to three wavebands were correlated to the concentration of chlorophyll, nitrogen, lignin, and cellulose (R/sup 2/=0.96,0.94,0.93, and 0.61, respectively) and the content of these four biochemicals (R/sup 2/=0.98,0.91,0.88, and 0.92, respectively). The AVIRIS data were used, for the first time, to map the content of these biochemicals within the forest canopy and the accuracy was between 3-7% of the mean.

The use of remote sensing in the modeling of forest productivity.

Abstract: Section One: Stand-Level Analyses. 1. Assessing Leaf Area and Canopy Biochemistry of Florida Pine Plantations Using Remote Sensing H.L. Gholz, et al. 2. Modeling Radiative Transfer Through Forest Canopies: Implications for Canopy Photosynthesis and Remote Sensing T. Nilson, J. Ross. 3. Estimating Forest Canopy Characteristics as Inputs for Models of Forest Carbon Exchange by High Spectral Resolution Remote Sensing M.E. Martin, J.D. Aber. Section Two: Landscape/Regional-Level Analyses. 4. Detecting Structural and Growth Changes in Woodlands and Forests: The Challenge for Remote Sensing and the Role of Geometric-Optical Modeling D.L.B. Jupp, J. Walker. 5. Integrating Remotely Sensed Spatial Heterogeneity with a Three-Dimensional Forest Succession Model J.F. Weishampel, et al. 6. Combining Remote Sensing and Forest Ecosystem Modeling: An Example Using the Regional HydroEcological Simulation System (RHESSys) J.C. Coughlan, J.L. Dungan. 7. Forest Vegetation Classification and Biomass Estimation Based on Landsat TM Data in a Mountainous Region of West Japan N.J. Lee, K. Nakane. 8. Forest Structure and Productivity Along the Oregon Transect D.L. Peterson. 9. Use of Remote Sensing to Model Land Use Effects on Carbon Flux in Forests of the Pacific Northwest, USA D.O. Wallin, et al. Section Three: Global-Level Analyses. 10. Global Biospheric Monitoring with Remote Sensing S.N. Goward, D.G. Dye. 11. Energy Conversion and Use in Forests: An Analysis of Forest Production in Terms of Radiation Utilisation Efficiency (E) J.J. Landsberg, et al.

Seasonal leaf phenotypes in the canopy of a tropical dry forest: photosynthetic characteristics and associated traits

Abstract: &p.1: We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November‐December) of the rainy season in the canopy of a seasonally dry forest in Panama. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies than leaves produced during the early rainy season. Such seasonal phenotypic differentiation may be adaptive, since leaves produced immediately preceding the dry season are likely to experience greater light availability during their lifetime due to reduced cloud cover during the dry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for six common tree species with various ecological habits and leaf phenologies. Photosynthetic capacity was quantified as light- and CO 2 -saturated oxygen evolution rates with a leaf-disk oxygen electrode in the laboratory (O2max) and as light-saturated CO 2 assimilation rates of intact leaves under ambient CO 2 (A max ). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O 2max and A max per unit area and maximum stomatal conductances were significantly greater in pre-dry season leaves than in early wet season leaves. In two species, A max for a given stomatal conductance was greater in pre-dry season leaves than in early wet season leaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly different between seasons of leaf production in any species. In both early wet season and predry season leaves, mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiation observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns.

Forest ecosystem processes at the watershed scale: dynamic coupling of distributed hydrology and canopy growth

Abstract: The hydrological recovery of watersheds from disturbances such as fire and harvest can change the magnitude and distribution of flow paths as the canopy regenerates. The spatial distribution of net water input to the soil‐topography system is mediated by vegetation patterns through the processes of interception, evapotranspiration and snowmelt. We have previously described RHESSys, a distributed model of water and carbon flux with a prescribed canopy cover. Although the canopy structure varied spatially it did not change through time. We present an expanded model in which carbon and nitrogen are dynamically coupled with distributed hydrology. The model fixes and allocates canopy carbon annually to reflect changes in climate forcing. We test the interactions of the forest ecosystem to distributed hydrology through controlled experiments. In the first experiment, we prescribe canopy cover and examine the sensitivity of the hydrological outputs to the distribution of vegetation. The canopy distribution is found to have significant eAects on simulated hydrological outputs where evaporative demand exceeds available water. In a second experiment we simulate the canopy leaf area index (LAI) across the topography and through time. The model is executed over 100 years using repeated 10-year meteorological records to investigate spatial and temporal patterns of LAI. Annual precipitation and temperature diAerences result in temporally fluctuating LAI about a reasonably stable long-term mean. The topographical position has a strong eAect on local forest canopy characteristics. As expected, simulated ecosystem processes are found to be sensitive to rooting depth in more water limited environments. #1997 by John Wiley & Sons, Ltd.

Empirical relationships between structural and spectral factors of Yellowstone lodgepole pine forests

Abstract: Multiple regression analysis was used to examine the relationships between spectral and biotic factors within the lodgepole pine (Pinus contorta var. latifolia) forests of Yellowstone National Park. Field-sampled data on forest overstory and understory conditions were regressed against Landsat Thematic Mapper (TM) radiance values and transformed TM data for 70 stands. Factors relating to the physical structure of the forest canopy (height, basal area, biomass, and leaf area index (LAI)) are best predicted using a combination of visible and middle-infrared Thematic Mapper bands. Other overstory factors (density, size diversity, mean diameter, and number of overstory species) were not well explained by the TM data or by combinations of TM data with transformed spectral data. Understory factors [number of seedlings; number of understory species; total cover by forbs, grasses, and shrubs; and total living and nonliving cover) were poorly explained by regression models incorporating spectral and transformed spectral data.

Diversity of the understory vascular vegetation in 40 year‐old and old‐growth forest stands on Vancouver Island, British Columbia

Abstract: We studied plant diversity of the understory vascu- lar vegetation in 40 yr-old plantations (immature stands) and old-growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site-specific comparisons using several indices of species diversity were made between: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old- growth stands. There were no significant differences ( P < 0.05) among immature stands in species richness (S) and the Shan- non-Wiener index (H'), in relation to the canopy cover or in S, H' and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant diversity varied with edaphic conditions (represented by five site associations) and time (represented by two developmental stages). At both stand- and site levels, plant diversity increased with increasing soil moisture, from slightly dry to moist sites, and with in- creasing plant-available soil nitrogen in both immature and old-growth stands; and the plant diversity of immature stands across the sites studied was considerably lower than in old- growth stands, regardless of site association. The indices of plant diversity, floristic similarity indices, and species turno- ver rates indicated that the immature stands had their plant diversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in plant diversity to the absence of old-growth struc- tural features in immature stands. Several measures to foster the stand-level diversity were proposed.

Winter and Spring Thaw as Observed with Imaging Radar at BOREAS

Abstract: Measurements of the length of the growing season in the boreal regions, during which significant carbon exchange due to metabolic activity occurs, may improve current estimates of annual CO2 fluxes at high northern latitudes. For coniferous, evergreen forest species, the summer frost free period bounds the growing season length and period of net carbon uptake. Spring soil thaw bounds the period of soil respiration and decomposition and thus carbon release. The balance of these two exchanges determines whether the boreal region is a net carbon source or sink. Imaging radar data can potentially be used to monitor these periods of soil and canopy thaw due to the sensitivity of radar to surface freeze/thaw state. In considering the use of imaging radar, two issues must be addressed. First, the temporal relationship between the time of freezing and thawing of the forest canopy and soil and the periods of photosynthetic and respiration activity must be ascertained. Second, the sensitivity of imaging radar to freeze/thaw processes in each of the forest components must be assessed. Of particular interest is the extent to which radar is selectively sensitive to tree and soil thawing. In 1994, in situ soil, stem and root temperatures, and stem xylem flux were measured over a complete annual cycle at the Boreal Ecosystem-Atmosphere Study (BOREAS) test sites in Canada. Imaging radar data from the European Space Agency Remote Sensing (ERS-1) satellite were also acquired throughout 1994. The in situ temperature data show clear transitions in soil and stem thawing related to the start of soil respiration and canopy photosynthesis, respectively. The imaging radar data show clear shifts in backscatter related directly to soil thaw, and possibly to canopy thaw, as two independent transitions. These results are compared to seasonal ecosystem model results for carbon exchange.

Forest ecosystems of the Marietta Unit, Wayne National Forest, southeastern Ohio: multifactor classification and analysis

Abstract: Local forest ecosystem types were identified using an iterative approach consisting of reconnaissance, plot sampling, and multivariate data analysis. Canonical correspondence analysis (CCA) indicated that aspect and other environmental variables were highly correlated with the ground flora and canopy tree species. A multifactor classification comprising 10 ecological land type phases (ELTPs) was based on cluster analysis of the six most important physiographic and edaphic factors. The characteristic vegetation of each ELTP was specified by distinctive groups of mature canopy trees and of ground flora species identified by two-way indicator species analysis. Within this unglaciated area of southeastern Ohio, the majority of the landscape comprises moderately to steeply sloping terrain. Quercus dominated dry and dry-mesic ridgetops and slopes, while Liriodendron tulipiferaL. and Acer saccharumMarsh. dominated mesic slopes and wet-mesic ravine bottoms. In general, many of the 17 ecological species groups of ground flora plants were more closely related to the major environmental gradients than were six groups of canopy tree species. Multiple-response permutation procedures were used to corroborate the differences in these two vegetation layers among ELTPs. This classification system was developed to provide an ecological framework as the basis for ecosystem management on the Wayne National Forest. Resume : Des types d'ecosysteme forestier local ont ete identifies a l'aide d'une approche iterative comportant une reconnaissance, un echantillonnage par parcelles et une analyse multivariee des donnees. L'analyse canonique des correspondances a montre que l'exposition et d'autres variables environnementales etaient fortement correlees avec la flore