Search

Landscape heterogeneity influences large herbivores by altering their feeding rates, but as herbivores attempt to maximize feeding rates they also create spatial heterogeneity by altering plant growth. Herbivore feeding rates thus provide a quantitative link between the causes and consequences of spatial heterogeneity in herbivore-dominated ecosystems. The fractal geometry of plant canopies determines both the density and mass of twigs available to foraging herbivores. These properties determine a threshold distance between plants ([d.sup.*]) that distinguishes the mechanisms regulating herbivore intake rates. When [d.sup.*] is greater than the actual distance between plants (d), intake is regulated by the rate of food processing in the mouth. But when [d.sup.*] < d, intake is regulated by the rate at which the herbivore encounters new plants. Alterations to plant geometry due to past browsing could change the rate at which herbivores encounter and process bites of plant tissue, modify [d.sup.*] relative to d, and thus change intake rates and the distribution of mechanisms regulating it across landscapes. We measured changes in the geometry of aspen (Populus tremuloides) and balsam fir. (Abies balsamea) saplings along gradients of moose browsing from 2001 to 2005 at Isle Royale National Park, Michigan, USA. For aspen saplings, fractal dimension of bite density, bite mass, and forage biomass responded quadratically to increasing moose browsing and were greatest at ~3-4 g x [m.sup.-2] x [yr.sup.-1] consumption. For balsam fir, in contrast, these same measures declined steadily with increasing moose browsing. The different responses of plant canopies to increased browsing altered [d.sup.*] around plants. In summer, [d.sup.*] [greater than or equal to] d for aspen saplings at all prior consumption levels. Food processing therefore regulated summer moose feeding rates across our landscapes. In winter, changes in bite mass due to past browsing were sufficient to cause [d.sup.*] < d for aspen and balsam fir. Therefore, travel velocity and food processing jointly regulated intake rate during winter. Browsing-induced changes in the small-scale geometry of plant canopies can determine intake rate at larger spatial scales by changing [d.sup.*] relative to d and, hence, which mechanisms determine intake rate, essentially altering how herbivores sense the distribution of their food resources. Key words: Abies balsamea; Alces alces; allometry; aspen; balsam fir; browsing; fractal dimension; herbivory; moose; plant geometry; Populus tremuloides; threshold distance.

We resurveyed the under-and overstory species composition of 94 upland forest stands in southern Wisconsin in 2002 2004 to assess shifts in canopy and understory richness, composition, and heterogeneity relative to the original surveys in 1949-1950. The canopy has shifted from mostly oaks (Quercus spp.) toward more mesic and shade-tolerant trees (primarily Acer spp.). Oak-dominated early-successional stands and those on coarse, nutrient-poor soils changed the most in canopy composition. Understories at most sites (80%) lost native species, with mean species density declining 25% at the 1-[m.sup.2] scale and 23.1% at the 20-[m.sup.2] scale. Woody species have increased 15% relative to herbaceous species in the understory despite declining in absolute abundance. Initial canopy composition, particularly the abundance of red oaks (Quercus rubra and Q. velutina), predicted understory changes better than the changes observed in the overstory. Overall rates of native species loss were greater in later-successional stands, a pattern driven by differential immigration rather than differential extirpation. However, understory species initially found in early-successional habitats declined the most, particularly remnant savanna taxa with narrow or thick leaves. These losses have yet to be offset by compensating increases in native shade-adapted species. Exotic species have proliferated in prevalence (from 13 to 76 stands) and relative abundance (from 1.2% to 8.4%), but these increases appear unrelated to the declines in native species richness and heterogeneity observed. Although canopy succession has clearly influenced shifts in understory composition and diversity, the magnitude of native species declines and failure to recruit more shade-adapted species suggest that other factors now act to limit the richness, heterogeneity, and composition of these communities. Key words: biotic homogenizution; oak forest; shade tolerance; species loss; succession; understory.

Species responses to disturbance (mortality of dominant individuals within a community) influence many aspects of that ecological community. To trace the responses of vegetation to one particular type of disturbance, I sampled vegetation in 250 canopy gaps in 1976/1977, 1983, and 1990/1991. These gaps were located in three sites in the southern Appalachians of eastern North America: Great Smoky Mountains National Park, Walker Cove Research Natural Area, and Joyce Kilmer Wilderness Area. Each gap was sampled thrice for sapling composition (stems ≥ 1 cm in diameter at breast height). Additional measurements included the extension growth of border trees into the gap, recent mortality rates of border trees, deterioration rates of gap makers that had been standing dead in earlier samples, and the composition of understory plots. The mean extension (branch) growth of border trees was 12 cm/yr, with slower growth by longer branches. Gap makers that had been tall stumps tended to deteriorate, although 35% stayed intact over the 14 yr of the study. Border-tree mortality averaged 0.60%/yr, with higher rates for larger stems and with much interspecific variation. Stem density of saplings in gaps increased during the first sampling interval and decreased during the second as self-thinning counteracted increased establishment. Basal area increased during both sampling intervals. The death of border trees increased basal area per unit gap area. The four main species (Acer saccharum, Tsuga canadensis, Fagus grandifolia, and Halesia carolina) showed different patterns of correlation to gap size and age. Species in general showed more correlations with gap age for the first sample than afterwards; gap size was more consistently related to species importance. Species patterns also were affected by the presence or absence of border-tree mortality. The stands studied seem nearly at equilibrium. Some small changes are likely to occur, but the species present dominated all size classes: gap saplings, border trees, other canopy trees not related to gaps, and understory saplings. Species differed in their relative growth rates in the understory and in gaps of different sizes. Species also differed in their survival rates in the understory and in the canopy. Key words: Acer saccharum; border trees; canopy gaps; Fagus grandifolia; Halesia carolina; long-term studies; old-growth forests; repeated disturbance, impacts on vegetation/species; southern Appalachians (USA); tree species, relative growth and survival; Tsuga canadensis., INTRODUCTION Many studies have documented the response of species to gaps formed in the forest canopy by the death of a large branch, a whole tree, or a small group [...]

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation of the experiment). Measurements in the control plots indicated that the interval of stand development from age 6 to 23 yr was marked by a peak in basal area and leaf area of pin cherry at about age 17 yr, followed by a steady decline in P. pensylvanica dominance thereafter. Fertilization increased and prolonged the dominance of P. pensylvanica, indicating that nutrient limitation accelerates the demise of this species during the second and third decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrations and often higher specific leaf area (area:mass ratio), and these responses were most pronounced for P. pensylvanica. Although the light-response curve for photosynthesis of P. pensylvanica was altered by fertilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The marked growth response of P. pensylvanica was accompanied by changes in its canopy architecture, as the trees had more leaf area per unit stem basal area, and proportionally more of this leaf area was in the upper canopy. In contrast, height and leaf area of B. papyrifera were similar in the control and fertilized plots. Seed deposition of P. pensylvanica also increased in the fertilized plots during one year of high seed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its physiological performance, canopy growth, and ability to compete for light. Leaf area index of the fertilized plots was only slightly higher than the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided competition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all species occurred. The increased dominance of the fast-growing P. pensylvanica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probably associated with reduced C allocation to roots in some of the fertilized plots, but this pattern was not consistent across all the stands. The results indicate that the outcome of interspecific competition in mesic forests, where co-limitation by light and soil resources prevails, depends upon the effect of site quality upon the relative intensity of one-sided competition (for light). Key words: birch; canopy architecture; co-limitation; competition; leaf area index; northern hardwood forests; photosynthesis; pin cherry; productivity; Prunus pensylvanica; resorption; specific leaf area., INTRODUCTION Natural vegetation exhibits a complex three-dimensional structure (i.e., spatial distribution of leaves, roots, and stems) that influences its function both in ecosystem energetics and material cycling, and in the [...]

This study identifies patterns in the gap disturbance regime along a successional gradient in the southern boreal forest and uses this information to investigate canopy composition changes. Gaps were characterized in hardwood, mixed-forest, and conifer stands surrounding Lake Duparquet in northwestern Quebec. From 39 to 80 gaps were evaluated along transects established in each of these stands. The abundance of gap makers and gap fillers and total regeneration was evaluated by species, as well as the size of each gap encountered along the transects. The percentage of the forest in canopy gap was calculated directly from the proportion of the transect in gap and by using gap area and line-intercept techniques. Changes in composition were evaluated from gap-maker and gap-filler distributions and by using transition matrices based on species mortality and regeneration in canopy gaps. The percentage of the forest in canopy gap ranges from 7.1% in a 50-yr-old forest dominated primarily by aspen to 40.4% in a 234-yr-old fir-dominated forest. Gap events are due to individual or small-group tree mortality in the early successional forest but become species-specific events controlled by spruce budworm outbreaks in the later stages of succession. Due to the high latitude, direct light only reaches the forest floor in the very largest gaps of the conifer-dominated stands. However, these gaps form slowly as budworm-caused mortality occurs over a number of years, whereas in aspen-dominated stands gaps are formed quickly by the snapping of tree stems. Balsam fir is the most abundant gap-filling species; however, its abundance is negatively correlated to gap size in all stand types. Markovian transition matrices suggest that in the young aspen-dominated forests small gaps lead to species replacement by more shade-tolerant conifers but that in the oldest forests the larger gaps will result in maintenance of the intolerant species and an increase in the abundance of cedar. Key words: balsam fir; boreal forest disturbance regimes; canopy composition; canopy gaps; canopy turnover times; cedar; gap characteristics along a successional gradient; gap size distributions; gap makers and gap fillers; regeneration of trees and forest; spruce budworm-caused gaps; trembling aspen., INTRODUCTION In forests where large-scale disturbances are rare, stand dynamics are controlled by the creation of gaps due to single or multiple overstory-tree mortality. It is within these openings that [...]

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.

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.

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.

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.

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.

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.

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.

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.

There is still limited understanding of the processes underlying forest dynamics in the world's tropical rain forests, ecosystems of disproportionate importance in terms of global biogeochemistry and biodiversity. Particularly poorly documented are the nature and time scale of upward height growth during regeneration by the tree species in these communities. In this study, we assessed long-term height growth through ontogeny for a diverse group of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual height measurements of large samples of individuals in all postseedling size classes, over a 16-yr period ([is greater than] 11 000 increments). The study species were seven nonpioneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Simarouba amara, Dipteryx panamensis, Balizia elegans, and Hyeronima alchorneoides) and two pioneers (Cecropia obtusifolia and Cecropia insignis). For each species, inherent height growth capacity was estimated as the mean of the five largest annual height increments (from different individuals) in each juvenile size class (from 50 cm tall to 20 cm in diameter). All species showed marked ontogenetic increases in this measure of height growth potential. At all sizes, there were highly significant differences among species in height growth potential. The two Cecropia species consistently showed the highest observed maximum height increments as smaller juveniles ([is less than or equal to] 4 cm in diameter). Among the nonpioneers, Simarouba had the highest growth potential across all juvenile sizes. For all species, mean annual height increments in all juvenile size classes were very much lower than the species' potential growth rates and reflected the impacts of frequent periods of zero growth and major height losses from physical damage. Because of the same factors, maximum net height increments declined over increasing measurement intervals (1-15-yr periods). With only one exception (Simarouba amara saplings [is less than or equal to] 1 cm in diameter that survived 10 yr), the annual height growth increments of these species showed no significant temporal autocorrelation. For the seven nonpioneer species, we estimated the minimum time required to grow from 50 cm tall saplings to 10 cm diameter trees, based on their greatest net height increments over increasing intervals. Estimated passage times increased from 7-33 yr, when based on maximum 5-yr height increments, to 37-83 yr, when based on maximum net height growth over 15 yr for the six longest studied (nonpioneer) species. Given the erratic height growth trajectories found for these juvenile trees in this 16-yr study, it is clear that much longer term observation is required. Still unknown are the actual growth trajectories that characterize successful regeneration in each of our focal species, how much time successful trees spend as suppressed juveniles, and the number of times they experience and recover from major physical damage. Key words: Costa Rica; inherent growth capacity; La Selva; ontogenetic switching; physical damage; pioneers; tree height growth; tree regeneration; tropical rain forest; suppression, growth of juvenile trees.

Forest succession was studied in a species-rich cove forest of the Great Smoky Mountains, Tennessee. Long-term compositional and structural changes in secondary stands, 15-63 yr after agricultural abandonment circa 1920, were compared to an adjacent old-growth stand. Later successional trends were projected by computer simulation. The colonizing stand (15 yr) was dominated by Liriodendron tulipifera and Robinia pseudoacacia. Common old-growth species absent at this early stage included Aesculus octandra, Fagus grandifolia, and Tsuga canadensis. By year 40, Liriodendron dominated the overstory and all old-growth dominants were represented in the understory. After 60 yr, Acer saccharum was clearly the most abundant understory species; gap model projections indicated that after 200 yr it will dominate the forest. The actual old-growth stand was dominated by Acer saccharum. Species richness peaked at mid-succession ([is nearly equal] 50 yr), while diversity (H') and evenness (J') of species biomass were highest at the old-growth stage. Richness peaked with the coexistence of shade-intolerant colonizers in the overstory and shade-tolerants in the understory. Canopy-gap processes of the secondary and old-growth stands differed sharply. The old-growth canopy was composed of large, spreading, and overlapping crowns, while the second-growth stand was composed of narrow, nonoverlapping crowns. Gaps in the secondary stand tended to be small (

When sunlight passes through openings in leaf or overstory plant canopies, strong penumbral effects (e.g., penumbral light-spreading) can result in major changes in the total sunlit area, the irradiance level, and exposure duration at the incident surface. These effects are based on well-known optical principles whereby light that originates from a non-point source (e.g., sunlight) and passes through a circular aperture will project an image composed of a bright center portion (numbra) circumscribed by a image composed of a bright center portion (numbra) circumscribed by a shadow-edge (penumbra). A hypothetical opening diameter (G) and distance to the incident surface (D) was expressed as the ratio G/D (= GDR) and used to quantify changes in the projected sun spot diameter ([G.sub.p]) and area ([G.sub.p.sup.area]), and the relative irradiance levels and durations of the numbra and penumbra. When GDR was 80% of the exposure period. Beyond a GDR of 0.05, penumbral effects were negligible. Definitions of a sunfleck, sunpatch, gap, and clearing are proposed based on the specific sunlight regimes generated by the corresponding GDR values. These penumbral and latitudinal effects on sunlight penetration through canopy openings are discussed for a variety of leaf canopy and forest overstory types using representative gap sizes and vegetation heights.

Seasonal changes in leaf nitrogen distribution were examined in the canopy of a Carex acutiformis stand in a wet meadow area. Although there was a tendency for leaf nitrogen concentration to decrease with increasing leaf age in any one layer of the canopy, nitrogen concentration increased significantly with plant height despite increasing age of leaf portions higher in the canopy. This suggests a predominant effect of the light climate on the nitrogen distribution within the canopy. During the growing period, standing crop dry mass increased significantly, while the increase in the standing crop of nitrogen was marginal. The amount of nitrogen decreased in the lower layers and increased in the upper layers, and a strongly decreasing gradient of nitrogen concentration developed from the top to the bottom of the canopy. It is suggested that this gradient resulted mainly from leaf tips with high nitrogen concentrations being lifted to higher positions because of growth at the base, with some retranslocation of nitrogen downwards from senescing tips to active parts. The distribution of nitrogen concentration became less uniform during the growing period, thus supporting the prediction that nitrogen concentration should become less uniformly distributed with development of the canopy.