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A steady-state thermal radiance model to compute thermal exitance and energy balance within forest canopies is described. The model treats fully leafed canopies as discrete ensembles of leaves partitioned into slope-angle and heigh classes. Short-wave energy flux absorbed within the canopy is estimated by solving simplified radiosity equations. Sensible heat exchange is estimated using a logarithmic wind profile above the canopy and a modified exponential profile within the canopy. The latent heat boundary layer resistance is estimated from site-specific measurements summarizing the effects of solar irradiance, air temperature, and vapor pressure deficit on stomatal conductance Example comparisons for a dense spruce-fir forest study site in central Maine are given. For clear days the resulting root mean square error in modeled versu measured canopy temperatures was 1.2 [degrees] C. Corresponding errors in laten and sensible heat flux energy budget terms were 30 and 32 W/[m.sup.-2], respectively. For partly cloudy days the root mean square error in predicted temperature was 1.0 [degrees] C and corresponding errors in latent and sensible heat were 40 and 110 W/[m.sup.-2].

1 Spatial patterns of root biomass and plant cover were quantified in 10 late-successional, shortgrass steppe communities in which a large proportion of the soil is bare and regeneration is frequently limited by soil water. Our main objectives were to evaluate how patterns of root density were associated with previously documented variation in recruitment in canopy openings of different sizes and to estimate the abundance of openings with low root density. 2 Root biomass in the top 30 cm of soil was much lower in openings of all sizes than under plants and declined steeply as opening size increased. Biomass of light-coloured roots (i.e. those presumed to be functional) in centres of 10-, 20- and 60-cm openings was 62, 33 and 4%, respectively, of that under plants. 3 Openings more than 5 cm across made up 34% of the surface. Most were small: 86% of openings were 50 cm across), which are known to enhance regeneration, had low root density. Such openings (2% of total) occupied 2% of the area and nearly all were caused by disturbance. However, many openings caused by disturbance were 30-50 cm across, a size range where interference from neighbours changes from strong to weak, and some were even smaller. Most of the area (>99.5%) was within the range of the root system of the dominant grass, Bouteloua gracilis. 5 We infer that most openings large enough to support enhanced recruitment are explored by roots of dominant bunchgrasses and that gap dynamics in shortgrass steppe involves constraints on water use in B. gracilis root systems. Because large openings are rare, variation in below-ground competition in the abundant, smaller openings may be important for regeneration.

Cotton (Gossypium hirsutum L.) genotypes exhibit variation in leaf types and leaf area index (LAI). The objective of this research was to characterize canop development and yield of two cotton leaf types grown in varying plant populatio densities. Field experiments were conducted in 1991, 1992, and 1993 with both the okra-leaf (low LAI) and normal-leaf (high LAI) isolines of 'DES 24-8ne' cotton with selected plant population densities (5, 10, and 15 plants [m.sup.-2]). Photosynthetic photon flux density (PPFD) interception (1991-1992) and LAI (1991-1993) were determined periodically and final lint yield was determined at maturity. Averaged across years, yields of okra-leaf in narrow rows were 6 to 7% greater at 10 and 15 plants [m.sup.-2] than at 5 plants [m.sup.-2]. In 1991 and 1992, early-season PPFD interception of okra-leaf at 10 and 15 plants [m.sup.-2] was greater than at 5 plants [m.sup.-2]. In 1992 and 1993, okra-leaf at 10 and 15 plants [m.sup.-2] attained an LAI of 4.0 to 4.5 whereas the 5 plants [m.sup.-2] density failed to reach 4.0. Therefore, both PPFD interception and LAI limited yield of okra-leaf at 5 plants [m.sup.-2]. In contrast to okra-leaf, lint yields of normal-leaf were 4 to 8% greater at 5 plants [m.sup.-2] than at 10 and 15 plants [m.sup.-2]. In 1991 and 1992, the lower yields of normal-leaf at 10 and 15 plants [m.sup.-2] were apparently associated with late-season LAI values of 5.0 or greater, whereas the 5 plants [m.sup.-2] density developed late-season LAI values ranging from 4.0 to 4.8. These findings indicate that (i) optimal plant density was higher for okra-leaf than for normal-leaf and (ii) a maximum LAI between 4.0 and 5.0 was required to maximize cotton lint yields in this environment.

Optimization theory of leaf nitrogen distribution in canopies with respect to photosynthetic carbon gain predicts that vertical leaf nitrogen gradients are steeper in dense canopies than in more open canopies. We tested this prediction at the intraspecific and the interspecific level by studying the effects of two levels of nitrogen supply on canopy structure and vertical leaf nitrogen distribution in experimental populations of three Carex species that dominate in low-productivity fens (Carex diandra and Carex rostrata) and in high-productivity fens (Carex acutiformis), respectively. The high-productivity species had a significantly higher amount of leaf area per unit ground area, a higher absolute amount of nitrogen in the canopy, proportionally more leaf area in upper canopy regions, and steeper gradients of leaf nitrogen in the canopy than the low-productivity species. Similarly, increased nitrogen supply resulted for each species in a higher amount of leaf area per unit ground area and a higher absolute amount of nitrogen in the canopy. However, only in C. acutiformis did an enhanced N supply lead to proportionally more leaf area in upper canopy regions and to steeper gradients of leaf nitrogen in the canopy. It is suggested that the observed patterns in Carex acutiformis (a very high canopy with a large amount of leaf area) are not only directed towards maximization of photosynthetic carbon gain, but are also directed towards suppression of other species in the competition for light. The interspecific differences in canopy structure and leaf nitrogen distribution are in agreement with the optimization theory. However, at the phenotypic level the low-productivity species did not show proportionally more leaf area in upper canopy regions and steeper gradients of leaf nitrogen in the canopy enhanced N supply, despite a strong increase in the total amount of leaf area. This suggests that low-productivity species have a lower phenotypic plasticity with regards to their vertical leaf distribution pattern and their vertical leaf nitrogen distribution than high-productivity species.

Pressure fluctuations generated by coherent motions in a forest canopy at Camp Borden, Canada, are composed of mean shear wind components and velocity-phase relations inside the coherent structure. Pressure gradient and pressure transport interaction terms in the momentum flux and turbulent kinetic energy systems are involved in the transportation of the flux and kinetic energy in the forest canopy.

Patch depression is an essential premise of optimal patch use models. We studied the relationship between patch residence time and intake by Bos taurus. The hypotheses were: (1) grazing changes sward structure and thus, causes patch depression; (2) patches are grazed systematically, by horizon; (3) at equal herbage mass per unit area, tall and sparse swards yield larger bites and thus, exhibit faster and greater depletion than short dense ones. In one experiment, we constructed 0.46-m squared swards of wheat (Triticum aestivum L.) with two contrasting structures but equal herbage mass. In a second experiment we mowed a perennial ryegrass (Lolium perenne L.) field to produce 0.36-m squared patches of three heights (7, 12.5, and 18 cm). The animals were allowed to remove different numbers of bites from each patch type, and we measured bite mass, intake, and residence time. Bite dimensions and location (fresh or grazed area) were also recorded in the first experiment. In the first experiment, bite mass was initially larger and decreased more steeply with residence time in the tall sparse swards than in the short dense ones. Because bite dimensions and mass declined but time per bite did not change, instantaneous intake rate declined as patches were grazed. Animals grazed systematically and removed most of the top half of the canopy before biting into the lower horizon. A rectangular hyperbola described intake curves (cumulative intake vs. time) well but ignored the abrupt change in instantaneous intake rate when the top horizon was depleted. A model based on the horizon grazing style accurately depicted this change in slope but disregarded the reduction in bite mass within horizons. Intake curves for the field patches were more variable than for hand-constructed patches, but were well described by a rectangular hyperbola. Taller patches had more herbage mass and allowed greater bite mass than shorter ones. Therefore, intake curves had initially steeper slopes and approached higher asymptotes than in shorter patches. Grazers experienced patch depression at the scale of feeding stations. Plant height and density are the causal factors of this process.

The relative importance of interspecific interference and facilitation in natural plant communities is controversial, in part due to the difficulty of manipulating these interactions in the field. I investigated the relative intensity of facilitative and interfering effects of Arthrocnemum subterminale, a subshrub common in the upper zones of California salt marshes, on three species of winter annuals by measuring spatial associations, manipulating Arthrocnemum canopy cover and root distributions, and comparing aspects of the physical microhabitat under Arthrocnemum to that in the open. The annuals Hutchinsia procumbens and Parapholis incurva were positively associated with Arthrocnemum, and survival and aboveground biomass of these species declined when shrub canopies were removed. Spergularia marina was negatively associated with Arthrocnemum, and survival increased when shrub canopies were removed. Combined effects of facilitation and interference were observed for Parapholis survival and biomass, which were reduced by root interference even though the overall effect of Arthrocnemum on Parapholis was positive. Arthrocnemum canopies elicited direct effects on the winter annuals, but also altered substrate characteristics. Soil was mounded under the shrubs, averaging 3.14 cm higher in elevation than the surrounding open soil. Soil salinity was 27% lower under Arthrocnemum canopies, soil moisture was 13% higher under shrubs, and shrub understories received less light than open marsh soil. Although Arthrocnemum had species-specific interfering and facilitative effects, and interference and facilitation operated simultaneously, my results suggest that facilitative effects of Arthrocnemum were more significant than interference in the transition zone of the upper marsh. Physical conditions in the open reduced overall annual biomass and survival, but were ameliorated by Arthrocnemum, which altered the species composition and productivity of the upper marsh plant community.

Nimbus/SMMR 6.6 GHz data were used together with an extensive data base of surface moisture to study the influence of polarization on radiative transfer characteristics of savannah vegetation in south-eastern Botswana. Ratios of vertical and horizontal polarization optical depths and ratios of albedos were found to be almost constant throughout a three-year period. This conclusion led to a new approach to eliminate the vegetation components in the remotely sensed brightness temperatures, which is important for soil moisture monitoring from space. Using the same data set, this dual-polarization approach was compared with an earlier described synergistic approach, in which NDVI was used to eliminate the vegetation effects. The root mean square error between ground based soil moisture and satellite estimated surface soil moisture was reduced to 1.2% by volume from 5% by volume using the dual polarization approach.

1 Growth, leaf anatomy and stomatal responses were examined in seedlings of 12 rainforest tree species in northern Queensland. Plants were kept in a shade house for a period of 15 months at 37%, 10% and 2.5% photosynthetically active radiation (PAR), roughly corresponding to light environments in large gaps, small gaps, and forest understorey, respectively. The species were chosen to represent a wide array of taxa, ecological, and morphological characteristics, and were assigned regeneration niche (RN) index values based on observations of life history and spatial distribution of their seedlings and saplings in various forest microsites. 2 Growth, biomass allocation patterns, leaf anatomy and stomatal responses varied appreciably among species, as well as in response to shading. All species showed reduced growth with decreasing light intensity. Plants grown at 37% and 10% PAR exhibited morphological and physiological responses typical of plants found at forest edges and in large gaps: i.e. low leaf area ratio (LAR), high specific leaf weight (SLW), high root-shoot ratio (RSR), high relative growth rate (RGR), high net assimilation rate (NAR), high leaf conductance, and high chlorophyll a/b ratio. Those grown at 2.5% PAR showed the opposite trends. Comparing species within each light level, NAR and LAR showed positive correlation with RGR, but pooled data for all light levels suggests that the contribution of LAR and NAR to RGR are in opposite directions, depending on light conditions. 3 Sensitivity of seedling growth to shading was significantly correlated with mean seed reserved weight of the species: small-seeded species were more affected than large-seeded ones. Relationship between seed reserve weight and species RN index value, though linear, was weak and nonsignificant. RGR was significantly correlated with seed reserve weight at 37% and 10% PAR but not at 2.5% PAR, while LAR showed the opposite trend. 4 Plasticity of most parameters examined appeared highest in species that regenerate mostly in large gaps (> 200 m squared) and at forest edges, and least in species that regenerate mostly in forest understorey and small gaps (< 50 m squared). However, ordination of the study species, using seed size and various shade tolerance criteria for the seedlings, yielded a continuum rather than discrete groups. The first axis, which explained most of the variation (65.6%), was influenced by NAR, RGR, stomatal conductance and total biomass, and was strongly correlated with species regeneration status.

Estimation of the lower limit of the canopy-air temperature differential, (T(sub c)-T(sub a))(sub LL), is required for calculation of an empirically-based crop water stress, index. This research determined the complexity of model needed for accurate estimation of (T(sub c) - T(sub a))(sub LL) for several field grown cultivars of Kentucky bluegrass (Poa pratensis L.) and for creeping bentgrass (Agrostis stolonifera L. var. palustris (Huds.) Farw.). Regression models using vapor pressure deficit of the air (VPD), net radiation load (R(sub n)), and wind speed (WS) were developed for predicting (T(sub c) - T(sub a))(sub LL). The best one to three-variable regression models for predicting (T(sub c) - T(sub a))(sub LL) used variable groups of VPD (r squared = 0.47); VPD and R(sub n) (R squared = 0.66); and VPD, R(sub n), and WS (R squared = 0.82). Models developed for predicting (T(sub c) - T(sub a))(sub LL) on individual Kentucky bluegrass cultivars, across Kentucky bluegrass, and across both species were tested on a validation data set. Models using only VPD accounted for

Interception of solar irradiation by leaf canopies is influenced by the canopy architecture of crops, which is a function of shape, distribution, and orientation of the leaves that constitute the canopy. The objective of this study was to test the hypothesis that leaf azimuthal distribution in a maize (Zea mays L.) canopy is not influenced by plant density and row width. Experiments were conducted at Elora, Ontario, with maize grown at 4, 7, and 10 plants m to the -2 at a 0.76-m row width in 1991 and 1992 and with maize grown at 7 and 10 plants m to the -2 at a 0.5-m row width in 1992. Leaf orientation was recorded in 16 azimuthal classes for leaves from the bottom to the topmost leaf position. Results showed that the orientation of leaves in the topmost layer (twelfth to sixteenth leaf) was more perpendicular to the row for high than for low plant densities. An azimuthal shift of leaves from the bottom to the top in maize plants was apparent in all canopies, but the shift was larger for plants grown at high plant densities than for those grown at low plant densities and a similar trend was apparent for the comparison of maize grown at 0.76-m vs. 0.5-m row widths. Results of this study suggest that the orientation of leaves in a maize canopy is altered by intraspecific interference, thereby more effectively intercepting incident solar irradiance.

Plant competition is a process that occurs through the negative effects that individual plants have on resource availability to neighboring individuals. Therefore competition experiments need to examine how different species change resource availability to their neighbors, and how different species respond to these changes - allocationally, architecturally, and physiologically - through time. In a greenhouse study we used a model system of annuals to examine how canopies of species having differing morphologies differed in their architectures and light-interception abilities, and how different species performed when grown in these canopies. Abutilon theophrasti, Datura stramonium, and Polygonum pensylvanicum were grown as 'targets,' surrounded by neighbors of a single species. Neighbors could be any one of the target species or Setaria faberii. Plants were grown in pots, with one target plant and four neighbor plants. Detailed measurements of neighbor canopy structure and target plant canopy architecture were made at five harvests. Species with different morphologies showed large differences in canopy structure, particularly when grass and forb species were compared. Setaria, a grass, had a more open canopy than the other species (all forbs), and was a consistently weak competitor. Overall, however, the relative effects of different neighbors on target biomass varied with target species. Target biomass was poorly correlated with neighbor biomass and leaf area, but was highly correlated with a measure of target light-interception ability that took into account both target leaf deployment and neighbor light interception. Despite clear differences among neighbor species in canopy structure and effect on light penetration, the results suggest no broad generalizations about the effects of different species as neighbors. Knowledge of morphological, physiological, and life history characteristics of both the target and neighbor species may be necessary to explain the results of their competition.

In simulations, the computational method chosen to implement a model can critically affect quantitative results. Here I show that two reasonable implementations of a neighborhood model of plant growth produce predictions that differ quantitatively in several important respects. I conclude that for modeling forest canopies, pixel-based implementations should prove superior to implementations that are based on indices of neighborhood crowding.

Standard regression methods applied to canopies within a single homogeneous soil type yield good results for estimating leaf area index (LAI) but perform unacceptably when applied across soil boundaries. In contrast, the neural network reported here generally yielded absolute percentage errors of < 30%. The network was applied, without retraining, to a Landsat TM.

Canopy gaps in southern Appalachian mixed-Quercus forests were characterized to assess the impact of the 1985-1988 record drought on patterns of tree mortality in relation to topographic variables and changes in overstory composition. Using permanent transects, we sampled 68 canopy gaps within the Coweeta Basin. Among 1-5 yr old gaps, the most common gap type was the 1-yr-old single-tree snag, accounting for 49% of all gaps sampled; 65% of all gaps occurred within 2 yr of the drought peak. Gap area ranged from 40 to 850 m squared, averaged 239 m squared, with a median of 152 m squared. Analysis of 1988 color infrared (IR) aerial imagery yielded a gap formation rate of 0.8 gaps center dot ha to the -1 center dot yr to the -1 in mixed-Quercus stands affecting 2.0% of the area in this forest type in 1988. The most frequent gap-forming species were Quercus coccinea, Q. rubra, and Q. velutina, respectively. Evidence suggests that moisture stress brought on by severe drought increases the susceptibility of Quercus species to the shoe-string fungus Armillaria mellea, which may result in significant shifts in canopy composition in these forests.

The radiative transfer equations for a coupled atmosphere and canopy are solved numerically by an improved Gauss-Seidel iteration algorithm. The radiation field is decomposed into three components: unscattered sunlight, single scattering, and multiple scattering radiance for which the corresponding equations and boundary conditions are set up and their analytical or iterational solutions are explicitly derived. The classic Gauss-Seidel algorithm has been widely applied in atmospheric research. This is its first application for calculating the multiple scattering radiance of a coupled atmosphere and canopy. This algorithm enables us to obtain the internal radiation field as well as radiances at boundaries. Any form of bidirectional reflectance distribution function (BRDF) as a boundary condition can be easily incorporated into the iteration procedure. The hotspot effect of the canopy is accommodated by means of the modification of the extinction coefficients of upward single scattering radiation and unscattered sunlight using the formulation of Nilson and Kuusk. To reduce the computation for the case of large optical thickness, an improved iteration formula is derived to speed convergence. The upwelling radiances have been evaluated for different atmospheric conditions, leaf area index (LAI), leaf angle distribution (LAD), leaf size and so on. The formulation presented in this paper is also well suited to analyze the relative magnitude of multiple scattering radiance and single scattering radiance in both the visible and near infrared regions.

Dehesa ecosystems of the southwestern Iberian Peninsula are characterized as a savanna-like rangeland dominated by scattered mediterranean evergreen oak trees. We investigated whether isolated trees modify the water balance of this ecosystem and if so, what implications this finding might have on models that assume homogeneity of soil water resources. The water balance of the two ecological components of the dehesas - (1) the tree-grass component, and (2) the open areas between the tree canopies with unshaded grass vegetation - was studied for three consecutive years in three locations in the Sierra Norte de Sevilla region of Andalusia in southern Spain. In this region, annual rainfall was generally between 600 and 800 mm, and the summer drought lasted approximately equal to 130 d. Soil water storage was measured with a neutron moisture gauge outside and under the tree canopy. Deep drainage between two consecutive census dates was calculated using field-measured drainage characteristics. Evapotranspiration (Ea) and surface runoff were computed from the water balance equation assuming that Ea is limited by Penman potential evapotranspiration. Monthly Ea by annual species in open areas was poorly correlated with rainfall levels in the autumn and was limited during the spring by availability of water in the top 40 cm of soil. During summer, monthly Ea by trees ranged from 30 to 50 mm. Mean annual Ea was 400 mm outside and 590 mm under the tree cover. In open areas, water yield (WY), defined as the sum of deep drainage and surface runoff, ranged from 65 to 100% of total Ea, whereas under the tree canopy WY was only 20 to 40% of the Ea. Under the tree canopy, when annual precipitation was < 570 mm, WY was negligible and all precipitation was lost by evapotranspiration. Outside the tree canopy, WY occurred as soon as annual precipitation exceeded 250 mm. Models of competition between trees and grass generally assume a spatial homogeneity of soil hydrodynamic properties. Our results, however, show that both soil water storage and evapotranspiration are greater for the tree-grass component. Consequently, these models must account for this spatial variability in water resources according to species.

We explored the effect of post-fire microsites on seed and seedling distribution and hence their potential role in community restoration. A summer wildfire and control burn in a sclerophyll shrubland in mediterranean Australia produced mosaics of physically and chemically contrasting microsites of litter and sand. Most seeds (> 75%) of all species released from the burnt canopies fell, or were redispersed by wind, into the litter patches after both fires. Data on microsite characteristics and wind exposure (fire intensity), height of fruits, time of release, and seed properties were required to interpret relative distribution between the litter and sand patches. Seeds remained equally viable (up to 100%) over summer-autumn in the litter and sand and had equally high rates and levels (up to 100%) of subsequent winter germination. However, seedlings were 2-3 times less likely to survive in the litter and survivors were 35% smaller than those in the sand by the end of the first summer. Banksia hookeriana was particularly vulnerable to microsite properties, whereas the needle-leaved Hakea poly-anthema showed only minor responses. Pre-summer thinning of seedlings in the litter increased survival of the remainder by 2 times and size of the survivors by 31%. The fire-sensitive, small-seeded B. hookeriana had 17 times more seeds in the backburn litter than the resprouting, larger-seeded B. attenuata, which more than compensated for its 3 times greater seedling mortality levels over the dry summer. Recruitment of species prone to density-dependent mortality in the litter was enhanced by the retention of some seeds in the sand where competition for water was minimal, as indicated by the 2.2 times greater stomal conductance of their seedlings in early summer.

1. The contribution of seed rain to the seed bank and to the seedling cohort was evaluated for three species, Abies balsamea, Acer saccharum and Betula alleghaniensis, in a deciduous forest near Quebec, Canada. Through experimental manipulations, seed-bank turnover time and rate of seed incorporation into the soil were estimated for these tree species. The forest floor was also modified in small quadrats to simulate various natural microsites; seedling emergence and early survival, and microsite characteristics were then monitored. 2. All Abies and Acer seedlings, and most Betula seedlings, originated from the previous year's seed rain. Abies and Acer were characterized by a winter-transient seed bank, but Betula maintained a persistent reserve of seeds in the soil. Turnover time of the Betula seed bank was c. 2 years in humus, but

1. The generation of shoot thrust was measured in eight contrasted herbaceous species (Arrhenatherum elatius, Bromus erectus, Campanula rotundifolia, Cerastium fontanum, Hypericum perforatum, Koeleria macrantha, Poa trivialis and Urtica dioica) using a framework of weighted windows that could be opened by an expanding shoot. 2. The species that occur as vegetation dominants in their usual habitats (A. elatius, B. erectus and U. dioica) were found to have the greatest ability to generate thrust. 3. To test the prediction that shoot thrust was positively related to competitive ability, additional data were obtained by growing all eight species together in a productive turf in a glasshouse. 4. The rate of development of thrust was positively correlated with an index of competitive ability obtained in the productive, undisturbed conditions of the mixture experiment. 5. When associated with certain other attributes such as a tall leaf canopy, shoot thrust may be an important characteristic influencing the ability to forage for resources during competition and the potential of a plant to attain dominance in vegetation.

Estimation by aerodynamic surrogate surfaces anf throughfall of the resuspension rates of particulate chemical species from forests are discussed. Vertical variation in resuspension was estimated by placing surrogate surfaces at different heights. The resuspension rate of the whole canopy was estimated by throughfall measurements. It was found that the resuspension rates within the crown were only slightly less than those above the canopy in contrast with the mean speed wind.

To assess the diversity of tropical tree life histories, a conceptual framework is needed to guide quantitative comparative study of many species. We propose one such framework, which focuses on long-term performance through ontogeny and over the natural range of microsites. For 6 yr we annually evaluated survival, growth, and microsite conditions of six non-pioneer tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. The species were: Lecythis ampla, Hymenolobium mesoamericanum, Dipteryx panamensis, Pithecellobium elegans, Hyeronima alchorneoides (all emergents), and Minquartia guianensis (a canopy species). The study was based on long-term measurement of individuals from all post-seedling size classes. Trees were sampled from 150 ha of primary forest spanning several watersheds and soil types. To evaluate individuals' microsites we recorded the number of overtopping crowns, forest phase (gap, building, mature), and crown illumination index (an estimate of the tree's light environment). For comparison, we also evaluated the microsites of three species that have been categorized as pioneers (Cecropia insignis, C. obtusifolia) or high-light demanders (Simarouba amara). For the six species of non-pioneers, mortality rates declined with increasing juvenile size class. As a group, these emergent and canopy trees showed a much lower exponential annual mortality rate (0.44%/yr at > 10 cm diameter) than has been found for the La Selva forest as a whole. Growth rates increased with juvenile size class for all six species. As adults (trees >30 cm in diameter), all five emergent species showed substantial annual diameter increments (medians of 5-14 mm/yr). Small saplings and adults of all species had significant year-to-year variation in diameter growth, with much greater growth occurring in the year of lowest rainfall. Passage time analysis suggests that all six species require > 150 yr for growth from small saplings to the canopy. Evaluation of all nine species revealed four patterns of microsite occupancy by juveniles. Among the non-pioneers, one species pair (Lecythis and Minquartia: Group A) was associated with low crown illumination and mature-phase forest in all juvenile stages. For two species (Dipteryx and Hymenolobium: Group B) the smallest saplings were in predominantly low-light, mature-forest sites, but crown illumination and association with gap-or building-phase sites increased with juvenile size (Simarouba also showed this pattern). Two species (Pithecellobium and Hyeronima: Group C) were strongly associated with gap or building phase as small juveniles [is less than or equal to] 4 cm diameter) and again as subcanopy trees (> 10-20 cm diameter), but were predominantly in mature-phase sites at intermediate sizes. Juveniles of the two pioneer species (Cecropia: Group D) showed the highest crown illumination and association with gap or building sites. Among the six non-pioneer species, only one aspect of juvenile performance clearly varied according to microsite group. The smallest saplings ([is less than or equal to] cm diameter) of Groups B and C showed significant mortality differences across a small gradient in crown illumination; neither of the Group A species showed this pattern. Otherwise, juvenile performance was strikingly similar among the six species. All showed a capacity for growth responses to small increases in light, substantial height and diameter increments at higher light levels, equal ability to survive 4-yr periods of no growth, and very low mortality rates at intermediate-to-large juvenile sizes. Species differed significantly in growth rates, but relative differences shifted with tree size and were unrelated to microsite group. These findings do not support prevailing paradigms concerning trade-offs and correlated suites of traits. For non-pioneer tropical trees, life history classification based on generalized concepts such as gap dependence and shade tolerance is inadequate to describe the complex size-dependent patterns of life history differences and similarities that exist among species.

1. The leaf growth dynamics of five species of rain-forest tree in New South Wales, Australia, were studied over the lifespan of several cohorts of leaves (ranging in longevity from 6 months to over 10 years), to determine the patterns of emergence, growth, mortality and herbivory in the canopy. Leaf population dynamics were examined for variability within and between several spatial scales: canopy heights, individual trees, species, and rain-forest formations. 2. Representative species were selected within each of the three major rain-forest formations of New South Wales: Nothofagus moorei and Doryphora sassafras in cool-temperate or montane or mossy microphyll forest, D. sassafras and Ceratopetalum apetalum in warm-temperate or simple notophyll vine forest, and Dendrocnide excelsa, D. sassafras and Toona australis in subtropical or complex notophyll vine forest. Technical climbing apparatus was used to gain access to the canopy, where leaves were permanently marked and monitored on a monthly basis for up to 12 years. 3. Three main leafing patterns were observed: seasonal growth - leaves flushed synchronously (e.g. N. moorei, T. australis); intermittent growth - leaves flushed over several months (e.g. D. sassafras, C. apetalum); and continuous growth - leaves flushed throughout the year (e.g. D. excelsa). 4. Leaf lifespans were variable, ranging from short-lived leaves (D. excelsa, approximately 6 months), to annual leaves (T. australis), and longer-lived leaves (e.g. D. sassafras, ranging from 2 to 12 or more years). Shaded leaves lived longer than sun-exposed leaves, and those on trees at montane sites generally lived longer than leaves on individuals of the same species at lower altitudes. 5. Significantly different proportions of leaf area were lost throughout the lifespan of a leaf in various species: T. australis (4.5%); D. sassafras (16.6%); C. apetalum (22%); N. moorei (31%); and D. excelsa (32.5%). Shade leaves and young leaves were more extensively grazed than sun leaves and old leaves, respectively. Leaves located nearer to ground level suffered greater herbivory, although this may be in part an interaction with light regime. 6. The measurements on herbivory, senescence, and longevity were used to calculated annual turnover of canopy material to either primary decomposers (through leaf fall), or primary consumers (through herbivory) in the three rainforest formations. In a hectare of the subtropical forest, approximately 1.76 t (14%) and 5.59 t (46%) were allocated each year to herbivores and decomposers, respectively, from a total canopy of 12.26 t. In the warm-temperate forest, 2.5 t (22%) and 4.05 t (35%) were allocated annually to herbivores and decomposers, respectively, from a canopy of 11.41 t. In the cool-temperate forest, an estimated 2.63 t (27%) and 3.53 t (37%) went to herbivores and decomposers, respectively, from a total canopy of 9.69 t.

1. Epiphytes on Quercus petraea and Fraxinus excelsior were studied in the native woodlands around Loch Sunart, western Scotland, to obtain information on community variation and habitat requirements of the Lobarion pulmonariae alliance at an unpolluted site. 2. Percentage cover data for 119 species were obtained from 207 randomly located 20-cm x 20-cm quadrats on Quercus and Fraxinus and from a further 18 quadrats subjectively sited in Lobarion communities on Quercus. Bark pH and cation levels and direct and diffuse radiation inputs (hemispherical photography) were measured for each quadrat. 3. The bark chemical factors and radiation estimates, respectively, showed many strong inter-correlations. Mean bark pH was 0.7 units higher and mean bark Mg and K were, respectively, 1.8 and 3.7 times higher in Fraxinus than in Quercus. Bark pH was positively correlated with bark Ca in Quercus, whereas in Fraxinus, bark pH increased with girth but was unrelated to cation levels. In Quercus, bark Mg content declined with increasing altitude probably due to increasing insect defoliation of the canopy at higher altitudes. Bark pH or cation concentrations were positively correlated with soil exchangeable cation levels. 4. Correlations between the epiphytic flora and environmental variables were explored using numerical classification (two-way indicator species analysis), indirect ordination (detrended correspondence analysis) and direct ordination (canonical correspondence analysis). 5. Height on the bole and bark angle were identified as the most important factors influencing epiphytes. They probably determine the degree to which water is available on, or evaporated from, the bark surface. Bark pH correlated strongly with the second ordination axes but radiation was a relatively unimportant predictor of epiphyte community composition. 6. Classification of the floristic data resulted in the recognition of eight quadrat groups. Species of the phytosociological Lobarion pulmonariae alliance were distributed between two groups, one predominantly on Quercus, the other, with richer representation of cyanobacterial lichens, on Fraxinus. Both groups were characteristic of bark with high pH and high cation levels, and were best developed in low-altitude sites, possibly indicating that acidic inputs may restrict their colonization at higher altitudes. Neither group was strongly associated with high light levels which were previously thought to be important for Lobarion lichens. Key-words: correspondence analysis, light, Lobarion pulmonariae alliance, TWINSPAN Journal of Ecology 1992, 80, 163-179

The goal of this study was to explore how leaf and canopy level differences interact in determining total canopy productivity. Maximum rates of carbon gain and transpiration were found to be higher in leaves of the deciduous oak Quercus lobata than in the co-occurring evergreen oak Quercus agrifolia. However, the efficiency of water and nitrogen use is greater in leaves of Q. agrifolia. A biochemically based model of annual leaf photosynthesis using environmental field data indicates that the evergreen habit in Q. agrifolia partially compensates for the lower photosynthetic capacity of leaves and increases the amount of photosynthate produced per unit water or nitrogen. Simulations indicate that with low N or water availability, the evergreen species can produce canopies with greater annual production than the deciduous species; the converse is true at higher levels of resource supply. Some of the patterns of distribution of these species in northern California may be explained by examining the relationships between resource availability and estimated annual production. Key words: biochemical model; canopy model; deciduous; evergreen; leaves; photosynthesis; Quercus agrifolia; Quercus lobata.

(1) Time of reproduction in Oenothera biennis was controlled experimentally by gibberellic acid applications in a glasshouse experiment. Performance of vegetative plants and plants reproducing at various ages was compared by means of growth analysis. (2) Regardless of age, reproduction increased stem allocation and decreased root and leaf allocation. Early reproduction reduced growth relative to vegetative plants, while late reproduction had no effect on growth in the long term and enhanced growth in the short term. Seed output increased with age at reproduction up to a point after which there was little change. (3) The reduced growth of plants reproducing at an early age was due to a decrease in leaf area ratio, and the enhanced growth of older plants to a temporary increase. Specific leaf area in older plants increased with reproduction, temporarily over-compensating for the decrease in leaf allocation. There was a temporary increase in unit leaf rate in plants reproducing at a young age but not in plants reproducing at a late age. (4) Changes in specific leaf area were the result of a switch in leaf morphology between vegetative and reproductive plants. Vegetative plants produced leaves with petioles which increased in length with age. This morphology minimized self-shading in the vegetative rosettes. Leaves produced after reproduction had short petioles and, consequently, a higher specific leaf area. Plants reproducing at an early age produced few new leaves and exhibited little change in specific leaf area. (5) It is suggested that the beneficial effects of reproduction on canopy structure and photosynthesis that are observed in many species can compensate for the cost of reproduction, providing the accumulated nutrient reserves are sufficient to supply the reproductive structures.

To understand how populations of salmonberry (Rubus spectabilis) persist in common forest stand types, we studied the above- and belowground structure of salmonberry clones and populations in upland alder and conifer stands, in riparian stands, and in 2-yr-old and 13- to 18-yr-old clearcuts in the central Oregon Coast Range. On undisturbed sites individual clones and populations replace aerial stems by rhizome extension, by production of new genets, and by sprouts from buds on old aerial stems. It appears that frequent initiation of aerial stems from these three sources enables salmonberry populations to persist and maintain a dense cover. The size distribution of aerial stems on these sites resembled that of an uneven-aged stand of trees, with stem numbers decreasing from small to large size classes. Within the first two growing seasons after disturbance to overstory trees and the understory, salmonberry populations maintained themselves by a rapid initiation of new rhizomes (1.0-2.5 m/[m.sup.2] annually) and aerial stems (25-50 stems/[m.sup.2]). These populations apparently continue to grow well, since the greatest rhizome and aerial stem biomass occurred in 13- to 18-yr-old clearcuts with no overstory trees. Rhizome density of salmonberry populations, as well as rhizome and aerial stem biomass, was significantly and negatively related ([r.sup.2] = 0.71-0.83) to the basal area of overstory trees. It appears that population structure, rhizome length and aerial stem, rhizome, and total biomass, can be predicted from measures of preharvest overstory trees, and also from salmonberry stem number and basal area. Clonal architecture varied with stand type. Salmonberry clones in alder stands were larger (18 m total rhizome) than those in conifer stands and on riparian sites (5-6 m rhizome). Clones in alder stands also produced more ramets and more aerial stems than those in conifer and riparian stands.

In order to develop a model of the carbon cycle for mature slash pine (Pinus elliottii) stands in north Florida, we studied seasonal variation in leaf area index (LAI, all-sided), aboveground biomass increment and litterfall, and light penetration through the forest canopy, over a 3-yr period. The primary approach to establishing monthly LAI included annual destructive analyses and monthly measurements of needle fall and elongation. Imagery from the Landsat Thematic Mapper (TM) and patterns of light penetration were also used in attempts to derive less arduous estimates; the TM imagery was most promising. LAIs ranged from 3.0 to 6.5 on control plots over the 3 yr, with repeated fertilization increasing maximum LAI by >40%. Seasonal variation was high (40%), as was variation from year to year. An average of 31% of the incident photosynthetically active radiation (PAR) penetrated the canopies annually, ranging from 18 to 42% seasonally. Seasonal light penetration could not be described using a simple application of the Beer-Lambert law, perhaps due to the highly aggregated nature of the canopies. Models incorporating more information on canopy structure are necessary to predict light penetration through slash pine stands accurately. A model of needle litterfall was derived that could account for much of the seasonal and annual variation using stand basal area and climate conditions from the spring of the previous year; this model may be useful for developing climate-driven predictions of LAI. Efficiencies of use of incoming and intercepted PAR were low compared to other forest types. Low light interception and high nutrient-use efficiencies (demonstrated in earlier studies) are important adaptive characteristics of slash pine stands to these relatively warm and nutrient-poor sites.

Treefall gaps are thought to contribute to the diversity of plants in tropical forests by providing opportunities for niche differentiation in modes of regeneration. To examine this hypothesis, we studied the survival, diameter growth, and recruitment of saplings in > 100 species of woody plants in a 50-ha permanent plot of moist tropical forest on Barro Colorado Island, Panama, from 1982 to 1985. The performance of saplings in low-canopy sites ( Most of the common (> 10 saplings/ha) species appeared to be generalists. Many rare (

Leaves that show paraheliotropic movement are inclined at a shallow angle with respect to the horizontal near dawn and dusk, and at a steep angle near solar noon. However, the mean inclination angle is only a first approximation to the distribution of leaf angles, and, as such, is not a good measure of the complexity of the underlying distribution. In order to ascertain the accuracies of statistical summaries of angular data used to measure the orientation of plant leaves for intercepting light, I used cellulation plots to analyze the distribution of leaf angles or cosines of the angle of incidence. Examination of these plots shows that inclinations of leaves in the top canopy layer of individual heliotropic plants are distributed over a wide range of angles during a period of several hours before and after solar noon, but that there is a sharp lower limit to this distribution of angles. Experimental observations show that near midday leaves are inclined at least at some minimum angle with respect to the horizontal. In order to interpret these data, I used a simple model of plant canopy photosynthesis and optimized the distribution of leaf angles to maximize total canopy photosynthesis. The results of this model suggest that the existence of a limit angle may be a function primarily of stress factors, and that the distribution of leaf angles more vertical than the limit angle may be important primarily for maximization of whole-plant photosynthesis.