Showing papers in "Ecological Monographs in 1994"

Mechanisms of Primary Succession Following Deglaciation at Glacier Bay, Alaska

Abstract: In primary succession following deglaciation at Glacier Bay, Alaska, we tested the hypothesis that the major effect of initial nitrogen-fixing colonizers is to facilitate establishment of late-successional dom- inants and that other possible causes of successional change (e.g., life history factors governing seed rain and competitive interactions among species) need not be invoked. Environment changed dramatically through the first 200 yr of succession. Soil organic matter increased 10-fold in the upper mineral soil with corresponding increases in soil moisture, total nitrogen (N), and capacity to support plant growth and declines in bulk density, pH, and total phosphorus (P). Plant growth in pioneer soils tended to be simultaneously limited by both N and P, as well as by unknown factors (perhaps lack of mycorrhizae), whereas only P limited growth in older soils. Light availability to seedlings declined through succession. Early-successional species (Epilobium latifolium, Dryas drummondii) had smaller seeds, younger age at first reproduction, shorter life-span, and shorter height at maturity than did mid-successional (alder, Alnus sinuata) and late-successional species (sitka spruce, Picea sitchensis). Seed rain of alder and spruce was negligible in the pioneer stage, increased prior to the stage in which a species was dominant, and was greatest in the stage in which a species dominated. Vegetation in each successional stage inhibited germination and initial establishment of sown alder and spruce seeds (except a tendency of the "black-crust" algal/microbial community in the pioneer stage to enhance survivorship). Removal of the surface litter layer generally enhanced germination and survi- vorship, particularly of alder. Comparisons of germination in the greenhouse and the field indicated that climatic or indirect vegetation effects (e.g., differential seed predation) and allelopathy also reduced germination and establishment in vegetated communities. Naturally occurring spruce seedlings grew most rapidly in the Dryas and alder stages and most slowly in the spruce stage. Similarly, growth of spruce seedlings transplanted into each successional stage was facilitated by the Dryas (nonsignificantly) and alder stages but inhibited by the spruce stage, relative to earlier successional stages. Facilitation of growth of natural and transplanted spruce seedlings by Dryas and alder stages was associated with higher N and P uptake and tissue nutrient concentrations, whereas nutrient uptake and concentration in spruce seedlings declined in the spruce stage. By contrast, transplanted alder seedlings grew rapidly and accu- mulated most nutrients in the pioneer stage and were strongly inhibited by subsequent stages. The facilitative effect of Dryas and alder comes primarily from inputs of organic matter and associated N. Addition of alder litter stimulated nutrient uptake and growth of transplanted spruce seedlings in the pioneer and Dryas stages, whereas shading had no effect on growth of spruce seedlings. Root trenching and planting of spruce near isolated alders indicated that, although the net effect of alder is facilitative, alder also inhibits growth of spruce seedlings through competition for soil resources. Strong root competition also occurs in the spruce stage. Alder competitively inhibits Dryas, primarily by shading but also through the physical and allelopathic effects of its litter. In general, both at Glacier Bay and elsewhere, life history traits determine the pattern of succession. Changes in competitive balance accompanying successional changes in environment provide the mechanism for changes in species dominance. Initial site conditions (and facilitation, where present) influence the rate of change and final state of community composition and productivity. We conclude that no single factor or mechanism fully accounts for primary succession at Glacier Bay.

The Keystone Species Concept: Variation in Interaction Strength in a Rocky Intertidal Habitat

Abstract: The usefulness and generality of the keystone species concept has recently been questioned. We investigated variation in interaction strength between the original keystone predator, the seastar Pisaster ochraceus, and its primary prey, mussels (Mytilus californianus and M. trossulus). The study was prompted by differences in community structure at two low zone sites along the central Oregon coast, Boiler Bay (BB) and Strawberry Hill (SH). Predators, especially seastars, were larger and more abundant at SH than at BB. Further, sessile animals were more abundant and macrophytes were less abundant at SH. Predators were more abundant at wave—exposed sites at both sites, and at SH, sessile invertebrates were more abundant at the wave—exposed location and sand cover was high at the wave—protected location. To test the hypothesis that variation in predation strength explained some of these differences, we examined the seastar—mussel interaction at locations with high and low wave exposure at both sites. Predation intensity was quantified by determining the survival of mussels in clumps (50 mussels per clump, shell length 4—7 cm) transplanted to large plots (18—163 m2) with or without seastars in the low intertidal zone. Predation effects were quantified by determining prey recolonization rates in marked quadrats in the same large plots. Spatial variation in interaction strength was quantified by examining predation at scales of metres (among transplants within plots), 10's of metres (between replicate plots within each exposure at each site), 100's of metres (between wave exposures within locations), and 10 000's of metres (between sites). Temporal variation was evaluated by performing the experiments in 1990 and 1991. The relation between prey (mussel) recruitment and growth to differences in community structure was evaluated by quantifying recruitment density in plastic mesh balls (collectors) and growth of individually marked transplanted mussels, respectively, at each site ° exposure ° tide level combination each month for 4 yr. Predation intensity varied greatly at all spatial scales. At the two largest spatial scales (10's of kilometres, 100's of metres), differences in both survival of transplanted mussels and prey recolonization depended on variation in seastar abundance with site, wave exposure, prey recruitment and growth, and at SH protected, the extent of sand burial. Variation at the two smallest scales (metres, 10's of metres) was high when seastars were scarce and low when seastars were abundant. Transplanted mussels suffered 100% mortality in 2 wk at wave—exposed SH, but took >52 wk at wave—protected BB. Seastar effects on prey recolonization were detected only at the SH wave—exposed site. Here, where prey recruitment and growth were unusually high, the mussel M. trossulus invaded and dominated space within 9 mo. After 14 mo, whelks, which increased in both size and abundance in the absence of Pisaster, arrested this increase in mussel abundance. Similar changes did not occur at other site ° exposure combinations, evidently because prey recruitment was low and possibly also due to whelk predation on juveniles. Longer term results indicate that, as in Washington state, seastars prevent large adult M. californianus from invading lower intertidal regions, but only at wave—exposed, not wave—protected sites. Thus, three distinct predation regimes were observed: (1) strong keystone predation by seastars at wave—exposed headlands; (2) less—strong diffuse predation by seastars, whelks, and possibly other predators at a wave—protected cove, and (3) weak predation at a wave—protected site buried regularly by sand. Comparable experimental results at four wave—exposed headlands (our two in Oregon and two others in Washington), and similarities between these and communities on other West Coast headlands suggest keystone predation occurs broadly in this system. Results in wave—protected habitats, however, suggest it is not universal. In Oregon, keystone predation was evidently contingent on conditions of high prey production (i.e., recruitment and growth), while diffuse predation occurred when prey production was low, and weak predation occurred when environmental stress was high. Combining our results with examples from other marine and non—marine habitats suggests a need to consider a broader range of models than just keystone predation. The predictive and explanatory value of an expanded set of models depends on identifying factors distinguishing them. Although evidence is limited, a survey of 17 examples suggests (1) keystone predation is evidently not distinguished from diffuse predation by any of 11 previously proposed differences, but (2) may be distinguished by rates of prey production. Further, (3) differential predation on competitively dominant prey does not distinguish keystone from nonkeystone systems, since this interaction occurs in both types of community. Instead, differential predation on dominant prey evidently distinguishes strong—from weak—predation communities. While the keystone predation concept has been and will continue to be useful, a broadened focus on testing and developing more general models of community regulation is needed.

Density dependence in time series observations of natural populations: estimation and testing'

Abstract: We report on a new statistical test for detecting density dependence in uni- variate time series observations of population abundances. The test is a likelihood ratio test based on a discrete time stochastic logistic model. The null hypothesis is that the population is undergoing stochastic exponential growth, stochastic exponential decline, or random walk. The distribution of the test statistic under both the null and alternate hy- potheses is obtained through parametric bootstrapping. We document the power of the test with extensive simulations and show how some previous tests in the literature for density dependence suffer from either excessive Type I or excessive Type II error. The new test appears robust against sampling or measurement error in the observations. In fact, under certain types of error the power of the new test is actually increased. Example analyses of elk (Cervus elaphus) and grizzly bear (Ursus arctos horribilis) data sets are provided. The model implies that density-dependent populations do not have a point equilibrium, but rather reach a stochastic equilibrium (stationary distribution of population abundance). The model and associated statistical methods have potentially important applications in conservation biology.

Woodland Expansions in the Platte River, Nebraska: Patterns and Causes

Abstract: This research was conducted to identify the factors that have permitted Populus—Salix woodland to expand into the formerly active channels of the Platte River and its two major tributaries, the South and North Platte rivers. The research included: pre—settlement vegetation reconstruction based on the General Land Office survey notes, a statistical comparison between historic rates of woodland expansion from aerial photographs and environmental variables, and a field study of seedling demography to isolate the factors controlling recruitment and survival in the modern river. Woodland expansion began in the South and North Platte rivers around 1900 and spread downstream into the Platte River. By the late 1930s, vegetation had occupied most of the former channel area of the South and North Platte rivers and was expanding into Platte River channels. Rates of channel loss in the Platte River have been as great as 10%/yr during droughts. By 1986, channel—to—woodland proportions were relatively uniform throughou...

Recruitment of a Mast‐Fruiting, Bird‐Dispersed Tree: Bridging Frugivore Activity and Seedling Establishment

Abstract: The recruitment of Phillyrea latifolia L. (Oleaceae), a bird-dispersed tree of Mediterranean forest, is described. Fruit removal by birds, seed rain, post-dispersal seed predation, seed germination, and seedling emergence, survival, and establishment were studied. The main objective was testing whether seed dispersal by birds produced a pre- dictable seedling shadow as a result of coupled patterns of seed rain, seedling emergence, and seedling establishment. P. latifolia is a mast-fruiting species and large fruit crops were produced in only 2 (1981 and 1989) out of 15 yr (1978-1992). We report here on the 1989 fruiting event at one scrubland and one forest site. Ripe fruits were available from mid-September to early June. Extensive removal by birds started after fruit crops of other species were depleted. Seed dispersers were more abundant, and fruit predators more scarce, in scrubland than in forest. P. latifolia fruits were a major component in the diet of principal seed dispersers (Sylvia atricapilla and Erithacus rubecula) that depended almost exclusively on them for food late in the season. Fruit removal levels were higher, crops were depleted earlier, and individual plants dis- persed more seeds in scrubland than in forest. Crop size was the best predictor of number of seeds dispersed by individual plants in scrubland, while fruit characteristics were more influential in forest. Seed dispersal was largely a within-population phenomenon, as no seed fall occurred in traps set beyond the distributional limits of P. latifolia in the study region. Frugivores produced a spatially predictable seed rain at the two sites. Seed rain was greatest beneath fleshy fruit-producing species (under female individuals in dioecious species) in scrubland and at forest-gap interfaces in forest. Post-dispersal seed predation was low at the two sites (39 and 54% after 1-yr exposure). In forest, seed survival was lower in gaps than in forest interior or forest edges. In scrubland, seed survival differed widely among microhabitats (defined by overlying plant species), ranging from 19% (open ground) to 61% (beneath Rosmarinus officinalis). In forest, density of emerging seedlings was unrelated to location in the habitat mosaic (gap, forest edge, interior). Seedling density did differ among microhabitats in scrubland, where emergence was greatest under fleshy fruit-producing species. Seedling survival was higher in forest than in scrubland, where seedlings incurred greater mortality due to desiccation. In both sites, seedling survival depended significantly on microhabitat and was depressed under adult conspecifics. The activity of frugivores directly impacted seedling distribution in scrubland, as spatial patterns of seed deposition were not overshadowed by later-acting factors, such as rodent seed predation or variation in germination. In forest, there was spatial discordance between seed rain and seedling distribution, as a consequence of uncoupled seed rain and seedling emergence. Spatial patterns of seed deposition by birds may thus have a lasting impact on the population dynamics of P. latifolia, but this will vary among populations depending on the extent of coupling of the different stages in the recruitment process (dispersal-seed rain-germination and seedling establishment).

Hurricane impacts to tropical and temperate forest landscapes

Abstract: Hurricanes represent an important natural disturbance process to tropical and temperate forests in many coastal areas of the world The complex patterns of damage created in forests by hurricane winds result from the interaction of meteorological, physiographic, and biotic factors on a range of spatial scales To improve our understanding of these factors and of the role of catastrophic hurricane wind as a disturbance process, we take an integrative approach A simple meteorological model (HURRECON) utilizes meteorological data to reconstruct wind conditions at specific sites and regional gradients in wind speed and direction during a hurricane A simple topographic exposure model (EXPOS) utilizes wind direction predicted by HURRECON and a digital elevation map to estimate landscape—level exposure to the strongest winds Actual damage to forest stands is assessed through analysis of remotely sensed, historical, and field data These techniques were used to evaluate the characteristics and impacts of two important hurricanes: Hurricane Hugo (1989) in Puerto Rico and the 1938 New England Hurricane, storms of comparable magnitude in regions that differ greatly in climate, vegetation, physiography, and disturbance regimes In both cases patterns of damage on a regional scale were found to agree with the predicted distribution of peak wind gust velocities On a landscape there was also good agreement between patterns of forest damage and predicted exposure in the Luquillo Experimental Forest in Puerto Rico and the town of Petersham, Massachusetts At the Harvard and Pisgah Forests in central New England the average orientation of wind—thrown trees was very close to the predicted peak wind direction, while at Luquillo there was also good agreement, with some apparent modification of wind direction by the mountainous terrain At Harvard Forest there was evidence that trees more susceptible to windthrow were felled earlier in the storm This approach may be used to study the effects of topography on wind direction and the relation of forest damage to wind speed and duration; to establish broad—scale gradients of hurricane frequency, intensity, and wind direction for particular regions; and to determine landscape—level exposure to long—term hurricane disturbance at particular sites

Landscape Evolution, Soil Formation, and Ecological Patterns and Processes in Sonoran Desert Bajadas

Abstract: Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene—aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene—aged surfaces containing soils with strongly developed argillic (clay—rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weathering—resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long—lived Larrea tridentata. Individuals of this shrub species can exhibit clone—like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long—lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata. Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay—rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought—deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal variability in water availability. Syntheses involving the study of various ecological processes (e.g., plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of arid land systems.

Mechanical Consequences of Size in Wave‐Swept Algae

Abstract: The intertidal zone of wave-swept rocky shores is characterized by high velocities and exceedingly rapid accelerations The resulting hydrodynamic forces (drag, lift, and the accelerational force) have been hypothesized both to set an upper limit to the size to which wave-swept organisms can grow and to establish an optimal size at which reproductive output is maximized This proposition has been applied previously to inter- tidal animals that grow isometrically, in which case the accelerational force is the primary scaling factor that constrains size In contrast, it has been thought that the size of wave- swept algae is limited by the interaction of drag alone with these plants' allometric pattern of growth Here we report on empirical measurements of drag and accelerational force in three common species of intertidal algae (Gigartina leptorhynchos, Pelvetiopsis limitata, and Iridaea flaccida) The drag coefficients for these species decrease with increased water velocity, as is typical for flexible organisms For two of these species, this decline in drag coefficient is dramatic, leading to small drag forces with concomitant low drag-induced mortality at plant sizes near those observed in the field However, all three species have surprisingly large inertia coefficients, suggesting that these plants experience large acceler- ational forces in surf-zone flows Preliminary calculations show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these algae This study further models the interplay between size-dependent survivorship and re- productive ability to predict the size at which reproductive output peaks This "optimal size" depends on the strength distribution and morphology of the algal species and on the flow regime characteristic of a particular site This study shows that the optimal size predicted for G leptorhynchos, calculated using velocities and accelerations typical of the moderately protected location where this species was collected, closely matches its observed mean size Similarly, the predicted optimal sizes of P limitata and I flaccida at the exposed site where these plants were sampled also match their mean observed sizes These data, although preliminary, suggest that mechanical factors (in particular the accelerational force) may be important in limiting the size of intertidal macroalgae and that attention solely to biological constraints may be inappropriate

Woody plant regeneration in four floodplain forests

Abstract: Between 1987 and 1990, we estimated seedfall and recorded age, growth, and survival of 10 933 tree and vine seedlings growing in the understories of four bottom- land hardwood forests in South Carolina. The forests differed in flood frequency, soils, and vegetation structure but had a number of woody plant species in common. Several demographic processes were consistent for all four forests as well as for flood- plain forests described in other published studies. Smaller seeded species had larger numbers of seeds dispersed, germinants, and established seedlings. Seed size, however, was not clearly related to seedling survival. Published rankings of shade and flood tolerances were also unrelated to survival, at least during the first growing season after germination. Seedling survival rates were least during the first growing season and greater in subsequent years. Within a growing season, early germinants had greater survival. For some species, survival was negatively related to basal area of neighboring conspecific adult trees. Some aspects of the regeneration process were more site specific. Within species, seedfall densities relative to adult tree abundance differed across forests by an order of magnitude. First-year seedling mortality rates were significantly affected by site and site x species interactions. Mortality in subsequent years was also significantly affected by site. Although the role of flooding in site-specific mortality was not clear, small elevation changes within flooded sites were correlated with changes in germination and survival for some species.

Local Adaptation to Regional Climates in Papilio Canadensis (Lepidoptera: Papilionidae)

Abstract: Papilio canadensis encounters shorter, cooler summers in interior Alaska than in northern Michigan: average thermal sums are 583 vs. 985 Celsius degree—days (10°C base); mean daily temperature is 14.4 vs. 18.8°. The temperature physiology of P. canadensis could be evolutionarily conserved, or the species may be a composite of regionally adapted populations. We evaluated these hypotheses by comparing the developmental physiology of P. canadensis from Alaska and Michigan across a range of temperatures in the laboratory and field. Higher temperatures generally resulted in more rapid larval development, but the effects varied with insect population and host. At low temperatures (12°), Alaskan larvae grew faster than Michigan larvae (fifth instars doubled their fresh mass in 5.8 vs. 9.1 d), primarily due to 40% higher consumption rates. At high temperatures (30°), Alaskan larvae grew slower, faster, or the same as Michigan larvae, depending upon host. Effects of host quality were greatest at high temperatures. Elevated respiratory expenses in Alaskan larvae (35% higher than Michigan larvae) made them especially sensitive to host quality at high temperatures. Dry matter digestibility and nitrogen use efficiency differed across hosts, but not between populations or across temperatures. Molting accounted for 35—51% of development time. Alaskan larvae completed their fifth molt faster than Michigan larvae at 12° (11.8 vs. 17.8 d), but not at 30° (3.1 vs. 2.9 d). In both populations, molt was more temperature sensitive than growth at low temperatures (Q10 of 5.65 vs. 3.04 from 12 to 18°), but less temperature sensitive at high temperatures (Q10 of 1.60 vs. 2.06 from 18 to 30°). Survival differed across temperatures, but not between populations. Under ideal basking conditions, larvae in the field were able to elevate body temperatures °10° above ambient, but such conditions were rare in Alaska and larvae were usually near ambient temperature. Alaskan larvae were no better than Michigan larvae at selecting high radiation microsites or converting solar radiation into heat. Growth rates of Alaskan larvae were the same in the field and laboratory when fed the same foliage and exposed to the same mean daily air temperature. We incorporated P. canadensis temperature responses into a life history development model, then used a 48—yr climatic record to evaluate the fitness contributions of apparent adaptations to Alaskan summers. On a good host, at Alaskan temperatures, Alaskan P. canadensis had an estimated fitness 3.0 times greater than Michigan P. canadensis. Furthermore, the Michigan population was predicted to go extinct in 31 of 48 yr at Alaskan temperatures. Changes in growth temperature responses made the greatest contribution to enhanced fitness in Alaska, followed by increased mass of neonates, enhanced molting abilities at low temperatures, and a reduced size threshold for pupation. Analysis of fitness trade—offs suggested that extreme summers have been more important than average summers in shaping adaptive responses. Regional adaptation to climate allows P. canadensis to maintain a broader geographic distribution than would otherwise be possible, but northern distribution limits are probably still constrained by summer temperatures.

A Spatial Patch Dynamic Modeling Approach to Pattern and Process in an Annual Grassland

Abstract: Landscapes are hierarchical mosaics of patches that differ in their age, size, shape, content, and other aspects. The Jasper Ridge serpentine grassland exemplifies hierarchical patchiness and pattern—process interactions that are common features of natural ecosystems. Gopher mounds formed each year destroy all the plant individuals underneath and result in conspicuous spatial pattern in the landscape. A snapshot of the system is, therefore, a reflection of the patch mosaic of gopher mounds that are different in age and species composition and abundance. Based on a patch dynamics perspective, we have developed a spatially explicit patch—based modeling approach to studying landscape pattern and process dynamics. The simulation model (PatchMod) has two major components: a spatially explicit, age— and size—structured patch demographic model and a multiple—species plant population dynamic model. We use this simulation model to examine the spatiotemporal dynamics of the disturbance patches and of populations of...

Biomass, Productivity, Leaf Longevity, and Forest Structure in the Central Himalaya

Abstract: Patterns of leaf characteristics, forest structure, tree species diversity, bio- mass, and productivity across a gradient of 3300 m and 15.70C in mean annual temperature in Kumaun, in the Indian central Himalaya, were summarized and compared to values from other similar forests. Throughout the elevational gradient, the annual rainfall was high (100-300 cm), but not correlated with elevation. Evergreen species with a 1-yr leaf life-span dominated most of the elevational transect; above 1800 m, species with deciduous and multiyear evergreen leaves were also well represented. Although variability among sites within forest types was high, a number of consistent patterns were apparent. Forests of Pinus roxburghii and those at high elevations were most consistently different from other forest types. Leaf life-span was not strongly correlated with leaf mass, specific leaf mass, or leaf production efficiency (net primary productivity per unit leaf mass), contrary to relationships presented in the literature. Tree species richness and basal area were lower than for most similar types in Nepal. Biomass and productivity of the forests in Kumaun were relatively high, compared to mean values for similar forest types elsewhere. Measured values for most variables describing these forests (but not all) fell within the ranges for the variables in similar forests worldwide. The maximal values for forest biomass remained high, 500-600 Mg/ha, up to 2600 m elevation, but declined sharply in birch forest (_ 170 Mg/ha) above 3100 m. Net primary productivity (NPP) varied little (15-20 Mg* ha- I.yr- 1) below 2700 m, despite a 10WC gradient in mean annual temperature and marked changes in basal area, tree density, growth form, and leaf char- acters. The level of productivity appeared not to be limited by rainfall, forest structure, leaf type, or temperature above an annual mean of 1 PC. Leaf mass (LM) varied consid- erably among forest types, being 3.7-8.6 Mg/ha for deciduous species, 5.7-8.9 Mg/ha for P. roxburghii, and 10.0-28.2 Mg/ha for evergreen broad-leaved species. Leaf mass duration (leaf mass x months of the year with leaves present) was related directly to NPP and inversely to leaf production efficiency (NPP/LM). These data add substantially to the data base for forest properties, especially for broad-leaved evergreen forests.

Spatial and Temporal Variation in the Structure of an Intermittent-Stream Food Web

Abstract: Food webs from the Lerderderg River, an intermittent stream in Victoria, Australia, were compiled with the aim of examining changes in food web structure in a highly variable habitat. Emphasis was placed on a high degree of taxonomic precision. Spatial and temporal variation in the food webs was assessed by partitioning the study area into three sites, located ≈1.5 km apart along the river. Sites differed in overall stream width and the length of the low streamflow period during summer. Three separate webs for each site were compiled for four different times of the year. Relatively little spatial variation in community structure was observed. In contrast, temporal variation was considerable, with species composition and the number of species in the community changing considerably over the year. The number of species increased dramatically as the period of constant streamflow lengthened. Detritivores dominated the community, both in terms of species and individual numbers. The proportion of predators in the community increased slightly by the end of the year, suggesting that recolonization of the community by predators lags behind that of detritivores. The increase in the number of predator species also resulted in an increase in the mean food chain length through the year. Patterns observed in the food webs tended to fall within the range of values reported from several previous studies, suggesting that underlying constraints may structure certain aspects of food webs. However, the constancy of certain food web statistics was attributable either to methodological decisions made during compilation, or to an inherent property of the statistic itself. The potential sensitivity of several food web statistics to the methodology used to compile a food web render between—web comparisons difficult due to the confounding effects of methodology. This suggests that comparisons between food webs should be restricted to webs derived from similar habitats using a comparable methodology.

Late quaternary history of low- and mid-elevation vegetation in the white mountains of new hampshire'

Abstract: Pollen and plant macrofossil analyses of sediments from an altitudinal series of lakes in the White Mountains of New Hampshire, USA were used to reconstruct the history of vegetation on the mountain slopes and to identify the factors responsible for vegetation change. Six sites, Mirror Lake (213 m), Lost Pond (625 m), Little East Pond (793 m), Lonesome Lake (831 m), Carter Notch Pond (1004 m), and Lake of the Clouds (1538 m), provide paleoecological records of changes in the altitudinal limits of species, the species abundances within communities, and the vegetation zones. These are supple- mented by previously published data from three high-elevation sites (Spear 1989). Although past plant communities were different from modem ones, differentiation of vegetation along the slopes has always existed. At low elevations the sequence of vegetation change was: 13 700-11 500 yr BP, tundra; 11 500-9000 yr BP, transitional mixed-conifer wood- lands of first spruce (Picea) and then fir (Abies balsamea), larch (Larix laricina), poplar (Populus), and paper birch (Betula papyrifera); 9000-7000 yr BP, forests dominated by pine (Pinus) and oak (Quercus); 7000 yr BP-present, mixed-hardwood forests. No late-glacial paleoecological records exist at mid-elevation sites (700-1200 m). The steep slopes at these elevations stabilized by 10 000 yr BP and an early woodland of spruce was replaced by fir, larch, and paper birch. The altitudinal limits of both white pine (Pinus strobus) and hemlock (Tsuga canadensis) expanded to mid-elevations during the mid- Holocene, suggesting greater warmth 6000-4000 yr BP. The modem spruce/fir forests of mid-elevations became established 2000 yr ago as spruce expanded at all elevations, sug- gesting cooler, moister climate similar to today. A tree line dominated by balsam fir and black spruce (Picea mariana) was established at its modem position 10 000 yr ago and has varied little since then, although it appears to have been slightly higher than now during the early Holocene. Changes in the vegetation at low and mid-elevations have not been synchronous with those at tree line and result from a more complex set of environmental factors and climatic variables acting on several different species. At low and mid-elevations disturbance by wind and frost action was important during the late-glacial. Disturbance by fire was im- portant during the period of spruce woodland and later in the early Holocene pine and oak forests. From 7000 yr BP to the present the primary factor disturbing New Hampshire forest was again probably wind, especially from 7000 to 4000 yr BP when higher temper- atures than present may have been associated with increased frequency and intensity of thunderstorms and tropical hurricanes. The vegetation history reveals that different lapse rates have occurred along the mountain slopes. This provides evidence that the source of air masses reaching the White Mountains has varied. The boundary between alpine tundra and subalpine fir forest (tree line) most likely has always been governed by temperature (summer insolation) and wind. The montane plant communities result from individual species response rather than community response to the numerous climatic forces that have affected the mountains over the past 14 000 yr.

Obtaining birth and mortality patterns from structured population trajectories

Abstract: A method is presented for unravelling the demographic equation for structured populations. A solution to the McKendrick-von Foerster equation is constructed using spline functions and this is fitted to stage-structured population data in such a way that the solution is smooth positive and does not imply negative death rates. The smoothness of the surface and hence the complexity of the population model is determined in a statistically optimum manner using cross validation. Time- and age-dependent death rates can be obtained as well as time-dependent birth rates. Confidence intervals are obtained for population size and death rates that give a 95% probability that the true population dynamics are within the intervals. Practical application of the method is demonstrated and comparison made with three alternative methods. (EXCERPT)

Effects of a Specialist Herbivore (Altica Subplicata) on Salix Cordata and Sand Dune Succession

Abstract: Although effects of insect herbivory on host plant growth and reproduction are well documented, few studies have examined how decreases in host plant fitness might influence community—level patterns, such as plant succession. The purpose of this study was to examine how a specialist flea beetle, Altica subplicata, influences (1) the growth and survivorship of its host plant, sand—dune willow (Salix cordata), and consequently, (2) patterns of plant succession on sand dunes. Growth and mortality of S. cordata were compared over a 3—yr period in experimental plots in which host plants were either protected from beetle feeding (with mesh cages) or exposed to beetle feeding (without cages). The changes in the abundance and biomass of other plant species in these experimental plots were also monitored for the 3—yr period. Growth rates of host plants were significantly influenced by the herbivore exclusion treatment. Plants protected from beetle feeding added 2.2 times as much height and 2.0 times as much diameter as did plants exposed to beetle feeding. Differences between growth rates of plants with and without beetles were greater on the west dune, which had significantly greater amounts of beetle damage, than on the east dune. Furthermore, within plots exposed to beetle feeding, areas with greater amounts of beetle damage had significantly lower height and diameter growth rates than did areas with lesser amounts of beetle damage. A separate experiment showed that mesh cages per se did not influence height growth, diameter growth, or number of shoots added over a 2—yr period. Plant mortality was also strongly affected by the herbivore exclusion treatment. Eighty—four percent of the plants that died over the 3—yr period of the study were in plots with beetles present. Mortality was 3x and 6x higher for plants with beetles than for plants without beetles on the east and west dune, respectively. On the west dune, which had higher beetle damage and plant mortality, the amount of beetle damage incurred by plants explained 88% of the variation in plant mortality. Herbivory on S. cordata also had significant, indirect effects on the changes in abundance of neighboring, non—host plants. For both monocot and dicot herbs, there were greater increases in numbers of plants in plots with beetle feeding than in plots with beetles absent, but only on the west dune with its greater beetle damage. Numbers of herbaceous monocots and herbaceous dicots increased 2.5 times and 1.5 times more, respectively, in plots with beetles than in plots without beetles. The changes in abundance of other Salix plants displayed the same trends, but differences were not statistically significant. In contrast, other woody plants showed significantly greater decreases in abundance in plots with beetles than in plots without beetles on the west dune. Individual plant species displayed both positive and negative changes in abundance in response to exclusion of herbivores from S. cordata. Biomass of two common herbaceous monocots (Juncus spp.) increased more in plots with beetles present, but for the most common herbaceous dicots (Aster and Solidago spp.), increases were greater in plots with beetles absent. Biomass of Salix myricoides, the most common other willow, on the east dune increased more in plots with beetles than in plots without beetles. In general, species that did not show differential increases in abundance also did not show differential growth to already—present plants. After 3 yr of herbivore exclusion, there were no significant differences in species richness, species diversity, or species evenness in plots with and without beetles. Thus, herbivory on S. cordata caused increases in the abundance of herbaceous plants and decreases in the abundance of other woody plants, but over 3 yr did not influence overall species diversity. It appears that selective herbivory by Altica subplicata directly decreases growth and increases mortality of S. cordata and that damage of this single plant species also exerts an indirect impact on the patterns of plant succession on sand dunes.

Crown Construction, Leaf Dynamics, and Carbon Gain in Two Perennials with Contrasting Architecture

Abstract: We used two co—occurring plant species of similar size and leaf morphology but with contrasting architecture to test if they would show specific patterns of leaf demography and leaf ecophysiology when they were grown in a range of experimental environments. Aster lanceolatus, a species with branched shoots, had exponentially increasing leaf populations. This led to high leaf turnover rates and to crowding of leaves at the top of the plants. Solidago canadensis, a species with unbranched shoots and rapid height growth, had linearly increasing populations of leaves, leading to a uniform vertical distribution of leaves and to lower leaf turnover rates. In comparison with the short—lived leaves of A. lanceolatus, the leaves of S. canadensis lived longer, contained more nitrogen and calcium per dry mass, and responded to changing light conditions within the plant crown by adjusting their angles towards the sun. Leaves of A. lanceolatus had an early peak in gas exchange activity followed by a rapid decline, whereas gas exchange activity in leaves of S. canadensis at the beginning of leaf development was lower than in A. lanceolatus but decreased only slightly with leaf age. Within the narrow vertical band in which leaves of A. lanceolatus were concentrated, young leaves at the top had higher mass per area, higher chlorophyll a/b ratios, and much higher rates of photosynthesis and conductance than the older leaves beneath. In S. canadensis, in which leaves were distributed over much of the vertical dimension along the plant, upper leaves, in contrast to lower (older) leaves, had lower mass per area, were held less perpendicular to the sun, and had higher rates of photosynthesis in high but not in low light. Because leaf populations grew exponentially in A. lanceolatus, assimilates could be re—invested; thus leaf number during the second half of the growing season was a good predictor of final biomass. In S. canadensis, with linearly growing leaf populations, the assimilates that were not re—invested in new leaves were allocated to the stem, and stem height or volume were good predictors of final biomass. Compared with the strong influence of architecture on the structure and dynamics of leaf populations, the effects of background species and fertilizer application were weak but nevertheless significant. Background species of similar aboveground mass but different stature (the tall Solidago altissima and the short Poa pratensis) affected leaf deployment (internode elongation, leaf orientation, and leaf survival), probably via competition for light, but not leaf quality (morphology, physiology) of the two target species A. lanceolatus and S. canadensis. Further, diameter—height allometries, which varied considerably among plants, tended to be flatter if the target species were grown in the tall than in the short background. Fertilizer application accelerated plant growth and phenological development. It increased light competition among leaves and leaf turnover; nevertheless, the photosynthetic water use efficiency of old leaves was higher in fertilized than in unfertilized plants. We suggest that the characteristic leaf dynamics of A. lanceolatus and S. canadensis are typical for the contrasting aboveground architectures these species represent. Based on the results of this and previous studies we suggest that an important driving force in the evolution of these complex adaptations is the degree of mixing within canopies of leaves of different plant genotypes. Leaves (and branches) are expected to be more autonomous if they frequently interact with leaves from other genotypes, as in the polyclonal patches of A. lanceolatus with its long and intermingling belowground rhizomes, than if they usually interact with leaves from the same genotype, as in the monoclonal patches of S. canadensis with its short rhizomes and compact belowground architecture.

A simulation model of lagoon algae based on nitrogen competition and internal storage

Abstract: Simulation modeling was used to explore the importance of resource com- petition to algal community structure of shallow coastal lagoons. We used modified Mi- chaelis-Menten equations to describe the kinetics of nitrogen uptake for three co-occurring algal functional forms: phytoplankton, foliose algae, and cyanobacterial mats. Uptake rates and subsequent growth were modified by differential internal storage and recycling of nitrogen from decaying tissues. Analysis of model behavior indicated that a two-step model uncoupling nutrient uptake and growth was capable of producing a wide variety of dynamic behavior not possible with simple uptake kinetics. A nutrient storage term allowed for time lags between nutrient input and growth, and resulted in periodic changes in dominance between foliose algae and mats. Analyses of the most important parameters of the model, nutrient uptake and growth rates, indicate that our model can be adapted for algal com- munities composed of different dominant algal forms. The model demonstrates a dynamic response in both magnitude and timing of algal growth when the parameter values for nutrient uptake are manipulated. From this we hypothesize that the dynamic relationships between the algal groups are similar in different communities, but that the parameter values in the Michaelis-Menten equations for each algal form must be tailored to each system. We validated the model with three field experiments from the published literature. First, we compared model predictions to results of a replicate microcosm experiment modeling lagoonal communities treated with 20 nitrogen loading rates spanning three orders of magnitude. The model adequately predicted the dynamics of the simplified microcosm community within the range of loading rates where nitrogen was limiting (0-0.270 mg L- I *d- 1). As predicted by the model and confirmed with the experiments, microalgal mats dominated when nitrogen addition was low, but were replaced by foliose algae at higher N loading levels (0.068-0.090 mg -L-I d-'). Phytoplankton never dominated the community. Second, we compared the observed and predicted communities for a com- petition experiment excluding two of the algal forms. The magnitude of the phytoplankton biomass in each treatment is accurately predicted but predictions of timing are less accurate. The final biomass of foliose algae and mats is predicted accurately. Finally, we applied the model to an experiment using algal communities of San Francisco Bay. Results matched predictions when the kinetic parameters for foliose algae were changed to those for UOva, the green macroalga from San Francisco Bay. These results support the hypothesis that nitrogen storage is important in understanding growth and nutrient utilization patterns of foliose algae and benthic cyanobacterial mats and the subsequent impacts on community structure.