bluuebunny/crossref_metadata_2025 huggingface dataset:
Competition and Nitrogen Loss from Plants in Grassland Ecosystems
Authors: Berendse F. , Elberse W. Th. , Geerts R. H. M. E.
Published: May 2006

Adaptation of plant populations to nutrient—poor environments may follow two essentially different pathways: maximizing the assimilation of nutrients or minimizing the loss of nutrients. Our hypothesis was that plant species that are adapted to nutrient—poor soils can replace species that are characteristic for relatively nutrient—rich habitats by features that enable them to restrict their nutrient losses. We chose Festuca rubra and Arrhenatherum elatius as species to test this hypothesis. In a long—term field experiment in a hayfield it was observed that Festuca was very common and Arrhenatherum was almost absent in the unfertilized plots, whereas after fertilization Festuca disappeared and Arrhenatherum increased strongly. It was hypothesized that differences in nutrient and carbon losses resulting from mowing were responsible for the different success of the two species in the unfertilized plots. A pot experiment was carried out in a greenhouse with clipped and unclipped and with unfertilized and fertilized treatments in a factorial design with four series. In the fertilized treatments Arrhenatherum replaced Festuica; its heavier seeds resulted in more rapid early growth and its more erect architecture enabled it to overtop Festuca after some time. In the two unfertilized treatments Festuca replaced Arrhenatherum, but much more rapidly in the clipped pots than in the unclipped ones. Losses of nitrogen due to clipping were much greater in Arrhenatherum than in Festuca because of the strongly increased allocation of nitrogen to aboveground parts in Arrhenatherum in response to clipping. It was concluded that differences in nitrogen loss resulting from clipping have an important impact upon the competitive balance between plant species under nutrient—poor conditions.
The Effect of Leaf Litter on Early Seedling Establishment in a Tropical Forest
Authors: Molofsky Jane , Augspurger Carol K.
Published: May 2006

The presence of leaf litter of different depths within a tropical forest creates many different microsites for plant establishment. The amount and distribution of leaf litter within a forest can influence patterns of plant establishment. In this study, we determined the spatial variability in leaf litter in the forest understory, and investigated how different litter depths (bare, 1, 6, and 12 cm) affected the establishment of several tropical tree species in both growth house (sun and shade) and field (gap and understory) experiments in the semideciduous tropical forest of Barro Colorado Island, Panama. The tree species used in this study (Aspidospermum cruenta, Ceiba pentandra, Cordia alliodora, Gustavia Superba, Luehea semmannii, Ochroma pyrimidale) were chosen to represent a range of seed masses and a gradient in the light requirement for establishment of the species. The spatial distribution of leaf litter was not correlated between adjacent sampling points within the forest understory, suggesting that the establishment environment for seedlings, with respect to litter, is highly variable at scales of 1—20 m. The presence of litter affected five of the six species, but the nature and the magnitude of the effect were species specific. The smaller seeded shade—intolerant species had fewer seedlings establishing under leaf litter than on bare ground. The species ranged from strongly nagatively affected (Luehae) to moderately negatively affected (Cordia, Ochroma) to affected only by extreme amounts of litter (Ceiba). The presence of litter influenced Gustavia, one of the larger seeded shade—tolerant species, but did not affect Aspidospermum, the other large seeded species. The effect of litter on Gustavia depended on the light environment. Gustavia had more seedlings establishing under litter in the sun, but the presence of litter had no effect in the shade. Differences among the smaller seeded shade—intolerant species in the amount they were negatively influenced by litter were not correlated with seed mass. Data from our field study were consistent with our growth house results for the shade—intolerant species. Additional data from the field study indicated that these species with similar habitat requirements differed in the developmental stage at which they were affected by the presence of litter. Luehea had fewer seeds germinating under litter while the other two species, Ochroma and Cordia, were affected only after germination. Interspecific comparisons done for each light level and litter depth indicated that the presence of litter caused reversals in the relative ranking of species success. For example, Gustavia preferentially established under relatively deep litter depths in the sun where Luehea could not establish. In conclusion, the presence of litter can potentially increase seedling diversity within the forest by creating heterogeneity in the establishment environment and by causing reversals in species' rankings.
The Role of Food Chain Composition and Nutrient Availability in Shaping Algal Biomass Development
Authors: Hansson Lars-Anders
Published: May 2006

With the aim of assessing the principal structuring forces of algal biomass development, I conducted field studies along a productivity gradient of Swedish and Antarctic larkes. In accordance with predictions from current ecological theory, the regression line of planktonic algal biomass vs. the concentration of total posphorus shows a steeper slope for Swedish (functionally three trophic levels) than for Antarctic lakes (functionally two trophic levels). This difference suggests that, besides the effect of nutrients, the food chain composition in aquatic systems has a crucial impact on the biomass development of planktonic algae. However, at very high productivity, phytoplankton biomass in the Antarctic lakes approached levels similar to those in Swedish lakes, suggesting that the algae "grew away" from being grazer regulated and instead became nutrient limited. The mechanistic connections between the components in the food chain, suggested by theory and the descriptive field study, were evaluated in an enclosure experiment. It may be concluded that classical food chain theory can explain a great deal of the variation in the relation between phosphorus concentration and phytoplankton chlorophyll, frequently used by limnologists.
Complex Dynamics in Ecological Time Series
Authors: Turchin Peter , Taylor Andrew D.
Published: May 2006

Although the possibility of complex dynamical behaviors–limit cycles, quasiperodic oscillations, and aperiodic chaos–has been recognized theoretically, most ecologists are skeptical of their importance in nature. In this paper we develop a methodology for reconstructing endogenous (or deterministic) dynamics from ecological time series. Our method consists of fitting a response surface to the yearly population change as a function of lagged population densities. Using the version of the model that includes two lags, we fitted time—series data for 14 insect and 22 vertebrate populations. The 14 insect populations were classified as: unregulated (1 case), exponentially stable (three cases), damped oscillations (six cases), limit cycles (one case), quasiperiodic oscillations (two cases), and chaos (one case). The vertebrate examples exhibited a similar spectrum of dynamics, although there were no cases of chaos. We tested the results of the response—surface methodology by calculating autocorrelation functions for each time series. Autocorrelation patterns were in agreement with our findings of periodic behaviors (damped oscillations, limit cycles, and quasiperiodicity). On the basis of these results, we conclude that the complete spectrum of dynamical behaviors, ranging from exponential stability to chaos, is likely to be found among natural populations.
Foraging Behavior and Physiological Adaptation for Diving in Thick‐Billed Murres
Authors: Croll Donald A. , Gaston Anthony J. , Burger Alan E. , Konnoff Daniel
Published: May 2006

Foraging behavior and physiological adaptations for diving were studied in Thick—billed Murrer, Uria lomiva, in the field and laboratory. Electronic, light—emitting diode, and capillary recording devices were used to measure foraging behavior. Individual dives were a flattened U shape in profile, and occurred in bouts lasting °15 min. Dive patterns were nocturnal; most dives occurred between 2000 and 0400. Murres probably concentrate their foraging effort at times when prey is most available as it migrates closer to the surface in the evening as part of the deep scattering layer. Although dives averaged 18 m in depth and 55 s in duration, most time—at—depth was spent between 21 and 40 m. Thus, murrers made a large number of shallow short—duration dives. Maximum dive dpeth was 210 m, while maximum dive duration was 224 s. Descent and ascent rates averaged 0.94 and 0.85 m/s, respectively. Hematocrit, hemoglobin, blood volume, and pectoralis myoglobin levels were measured in the laboratory as 52.8%, 18.0 g/100 mL, 12.3% body mass, and 1.9 g/100 g, respectively. Total usable oxygen store was calculated as 44.8 mL/kg, giving an estimated aerobic dive limit (ADL) of 47 s. Murres exceeded the calculated ADL in 48% of their dives. Long—duration diving is probably a more efficient foraging strategy for murres given their relatively small size and limited oxygen storage capabilities. The observed dive depths raised questions of potential problems with decompression sickness (bends) and lung collapse.
Results of Experimental and Natural Food Reductions for Breeding Black‐Throated Blue Warblers
Authors: Rodenhouse Nicholas L. , Holmes Richard T.
Published: May 2006

We examined effects of natural and experimentally created reductions in food abundance on the reproductive ecology of Black—throated Blue Warblers (Dendroica caerulescens). The study was carried out between 1982 and 1985 on four 30—ha plots of temperate deciduous forest within and near the Hubbard Brook Experimental Forest, New Hampshire, USA. Experimental food reduction using aerially sprayed Bacillus thuringiensis created significant differences in caterpillar biomass between one sprayed and two unsprayed plots throughout the breeding season in 1983. Where caterpillar abundance was reduced, Black—throated Blue Warblers made significantly fewer nesting attempts and diets of neslings included fewer caterpillars. Clutch size, hatching success, and numer of young fledging per nest did not differ among the food reduction site and controls. Also, the reduced number of nesting attempts per pair on the food reduction site in 1983 did not significantly lower production of young per pair. No detectable differences in caterpillar biomass between sprayed and unsprayed sites were created in 1984 or 1985 because natural caterpillar abundances were already low. Natural declines in food abundance for Black—throated Blue Warblers occurred from 1982 through 1985, primarily because of a decrease in caterpillar abundances. Significant reductions in number of young fledging per nest, nestling growth rates and survival, and number of nests attempted per pair corresponded with the natural decline in food abundance. Of those measures, a reduced number of nesting attempts per pair lowered annual production the most. When effects of food limitation were calculted separately from those of nest predation, food limited annual breeding productivity to below that needed to balance annual mortality in at least one of the four years of this study. These findings indicate that neotropoical migrant bird species are probably limited periodically by food when breeding in north—temperate habitats.
Foraging Ecology and Energetics of the Frugivorous Bat Artibeus Jamaicensis
Authors: Morrison Douglas W.
Published: May 2006

In Panama that bat Artibeus jamaicensis feeds primarily on figs (fruits of the genus Ficus). One hundred forty—two Ficus trees in 25 ha were mapped and their fruit production monitored for 2yr. Radio—tracking observations of foraging A. jamaicensis revealed that each bat carries away 9 ± 2 figs per night, enough to provide 40% more than the 43.9 kJ needed daily for basal metabolism and minimum maintenance flying. The estimated 6 ± 2 A. jamaicensis per ha carried away °7% of the 200, 000 figs (650kg) produced annually per hectare. Artibeus jamaicensis appears to be the most important dispersal agent for fig seeds in this area.
Leaf Quality and Enemy Avoidance by the Larvae of a Pyralid Moth
Authors: Damman Hans
Published: May 2006

Low nutritional quality of mature plant leaves generally leads to slow growth of herbivores. Slow growth, in turn, is thought to increase the vulnerability of herbivores to their annual enemies. I investigated the relative importance of nutritional quality and enemy avoidance to the larvae of a pyralid moth, Omphalocera munroei, that feed almost exclusively on the old leaves of their host plant (Asimina spp.). The effect of leaf age on development rate, the importance of predators and parasitoids as mortality agents, and the interactions between leaf quality and enemy avoidance were studied in field manipulations using larvae placed on naturally growing plants. Larvae of O. munroei developed 20% more slowly on old leaves than they did not young foliage. Predators, which accounted for most of the mortality of O. munroei caterpillars, reached maximum densities at the same time that O. munroei populations peaked. Two aspects of larval feeding behavior, leaf tying and gregarious feeding, reduced the impact of natural enemies. Leaf ties built of old leaves were more effective at reducing predating than were those built of young leaves, probably because old leaves maintained their shape, while young leaves wilted, following feeding by O. munroei. The greater rigidity of old leaves, however, required that O. munroei feed in groups of at least 20 caterpillars, which could then pull the leaves together. A group size of 20 coincided with the mean size of egg masses laid by females in the field. Given a choice, larvae preferred nutritionally poor old leaves arranged to form a shelter rather than nutritionally superior young leaves without shelter, indicating that protection from enemies was more important than nutrient to the larvae. The results also suggest that slow growth need not invariably lead to greater exposure of herbivores to natural enemies.
Optimizing Great Tit Clutch Size in a Fluctuating Environment
Authors: Boyce Mark S. , Perrins C. M.
Published: May 2006

Great Tits (Parus major) nesting Wytham Wood near Oxford, England, lay clutches averaging 8.53 eggs, although the most productive clutch size is 12. We evaluated various mechanisms that have been suggested to account for this apparent disparity, including (1) cost of reproduction, (2) Mountford's "cliff—edge effect," and (3) year—to—year environmental variability. Data on 4489 clutches, including 603 manipulated clutches, from the past 24 yr, offer weak or contradictory support for the first two mechanism. There are marked annual environmental fluctuations, and a geometric mean estimate of relative fitness best predicts the observed mean clutch size. Years that are poor for survival of young affect individuals laying layer larger clutches much more than they affect those laying smaller clutches. This "bad—years effect" reduces the mean and increases the variance in fitness for individuals laying large clutches more than it does for individuals laying smaller clutches. Hence it is more advantageous, in the long term, for birds to lay clutches smaller than the most productive clutch size. The cost of reproduction as it affects adult survival is neither necessary nor sufficient for clutch size optimization in Great Tits.
Garter Snake Diets in a Fluctuating Environment: A Seven‐Year Study
Authors: Kephart Donald G. , Arnold Stevan J.
Published: May 2006

Diets of the terrestrial garter snake (Thamnophis elegans) and the common garter snake (T. sirtalis) were studied during seven consecutive summers at a lake in northern California, USA. The snakes were opportunities feeders; their diets varied dramatically from year to year, parallel with the abundance of prey. Snakes foraged more successfully, judging from the incidence of stomachs containing prey, in years when metamorphic toads (Bufo boreas) were available. Breeding failure in toads with associated with lowering lake level.
A Neighborhood Model of Annual‐Plant Interference
Authors: Weiner Jacob
Published: May 2006

A model is constructed in which seed production by individual annual plants within a population is a function of the number and species of individuals within each of several concentric neighborhoods. The effect of increasing competition is to reduce seed production in a hyperbolic fashion, and the contribution of each individual to this effect is inverse proportion to the square of its distance from the test individual. A simple monospecific version of this model was tested on populations of two annual knotweeds. A least—squares fit of the model accounted for over 80% of the variation in seed production. This model provides an alternative to density in describing plant populations. A monospecific aggregated version can be seen as an extension of the inverse—yield law, which has been widely applied to monocultures.
Microhabitat Utilization and Foraging Behavior of Bipedal and Quadrupedal Hetermoyid Rodents
Authors: Thompson Steven D.
Published: May 2006

Direct observations of free—living individuals were employed to quantify foraging behavior in Dipodomys deserti, D. merriami, and Perognathus longimembris in two Mojave Desert communities. D. merriami and P. longimembris do not partition shrub and open microhabitats, and all three species concentrate their foraging efforts (>75%) near or beneath the canopies of desert shrubs. Results of livetrapping assessments of microhabitat usage (conducted concurrently with foraging observations) resemble those of earlier studies but differ greatly from observational data; trapping may not produce accurate representations of spatial utilization. P. longimembris avoids open areas and utilizes shrubs in a relatively coarse—grained fashion. D. merriami and D. deserti frequently traverse open areas at high speeds and hence utilize shrubs in a relatively fine—grained manner. Beneath shrubs, D. merriami and D. deserti are more active and discrete in their foraging than is P. longimembris; both Dipodomys apparently search for large clumps of seeds, while P. longimembris forages slowly and continuously, taking individual seeds from scattered dispersions. The distinctive morphology of Dipodomys is interpreted as an adaptation that allows fine—grained use of desert shrubs in the face of high predation risk during intershrub transits. Differences in heteromyid body sizes may also reflect two foraging strategies: small size would reduce energy demands, and thus lower the need to move between shrubs and reduce the need for predator avoidance; larger size would permit year—round activity (avoidance of torpor), reduce susceptibility to starvation (B. K. McNab, personal communication), increase the range of thermal tolerance, and generally increase abilities to cope with variations in environmental parameters (Boyce 1978). It is concluded that the morphological and behavioral differences between Dipodomys and Perognathus reflect divergent foraging strategies which may reflect equivalent but alternative strategies for exploitation of desert seeds.
"Ecological Release," Seasonal Variation in Food Supply, and the Hummingbird Amazilia Tobaci on Trinidad and Tobago
Authors: Feinsinger Peter , Swarm Lee Ann
Published: May 2006

Resident populations of the Coppery—rumped Hummingbird, Amazilia tobaci, occupy the West Indian Islands of Trinidad and Tobago. Trinidad has 15 other hummingbird species, Tobago only 4 others. We predicted that A. tobaci on Tobago would have a broader feeding niche and a denser population than would A. tobaci on Trinidad. We monitored populations of Amazilia tobaci for 13 mo at study sites in the Arima Valley of northern Trinidad and in central Tobago. The two sites had nearly identical climates, and similar assemblages of flowers visited by hummingbirds. At both sites, food available to nectar—feeding birds varied > 100 fold over the year. For 9 mo (December—August), neither A. tobaci population faced a severe food shortage. From September through November, nectar was scarce at both sites. During this period, Tobago A. tobaci faced only two other, uncommon species of nectar—feeding birds, which had very different feeding habits. The Trinidad population endured the food shortage along with six other hummingbird species. At this time Tobago A. tobaci utilized variously sized and shaped flowers of numerous plant species. On Trinidad, where many flowers were visited by the other hummingbird species, A. tobaci primarily used flowers that matched their bill size, which resulted in narrower feeding niche than on Tobago. These effects of "ecological release" on Tobago were apparent for only 3 mo, however, and when data for an entire year are compared there were few statistically significant contrasts between Tobago and Trinidad populations. At both sites, niche (diet) breadth responded in density—dependent fashion to changes in food density, broadening with high densities of A. tobaci relative to food, narrowing when food was abundant. Median values for niche breadth, population density, and population density per unit resource were not significantly greater on Tobago than on Trinidad. Ecological differences between the two populations during the food shortage may be linked to a phenotypic difference. We found no increase on Tobago in the intrapopulation variance of any morphological parameter important to feeding. However, Tobago, A. tobaci had longer wings than Trinidad conspecifics, resulting in lower wing disc loadings (ratio of body mass to area swept out by the wings). Among hummingbirds in general, low wing disc loading indicates a capacity to forage at a wide variety of flowers, which suggests that the Tobago population has undergone "within—phenotype nich expansion." Two experiments showed that individuals from the Tobago population of A. tobaci used a wide variety of artificial flowers, and a wide range of flower densities in field manipulations, than did conspecifics from Trinidad. The results suggest that occasional food shortages play a crucial role in the ecology and evolution of island populations, and that this might be undetected in short—term studies.
Subalpine Tree Growth, Climate, and Increasing CO_2: An Assessment of Recent Growth Trends
Authors: Graumlich Lisa J.
Published: May 2006

LaMarche et al. (1984) hypothesized that recent trends of increasing ring widths in subalpine conifers may be due to the fertilizing effects of increased atmospheric CO2. Five tree—ring series from foxtail pine (Pinus balfouriana), lodgepole pine (P. murayana), and western juniper (Juniperus occidentalis) collected in the Sierra Nevada, California, were analyzed to determine if the temporal and spatial patterns of recent growth were consistent with the hypothesized CO2—induced growth enhancement. Specifically, I address the following questions: (1) Can growth trends be explained solely in terms of climatic variation? (2) Are recent growth trends unusual with respect to long—term growth records? For three of the five sites, 20th—century growth variation can be adequately modeled as a function of climatic variation. For the remaining two sites, trends in the residuals from the growth/climate models indicate systematic underestimation of growth during the past decade that could be interpreted as either CO2 fertilization or as a response to extreme climatic events during the mid 1970s. At all five sites, current growth levels have been equalled or exceeded during some preindustrial periods. Taken together, these results do not indicate that CO2—induced growth enhancement is occurring among subalpine conifers in the Sierra Nevada. While the results presented here offer no support for the hypothesized CO2 fertilization effect, they do provide insights into the response of subalpine conifers to climatic variation. Response surfaces demonstrate that precipitation during previous winter and temperature during the current summer interact in controlling growth and that the response can be nonlinear. Although maximum growth rates occur under conditions of high winter precipitation and warm summers for all three species, substantial species—to—species variation occurs in the response to these two variables.
Sapling Survival, Growth, and Recruitment: Relationship to Canopy Height in a Neotropical Forest
Authors: Welden Charles W. , Hewett Steven W. , Hubbell Stephen P. , Foster Robin B.
Published: May 2006

Treefall gaps are through 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 (<10 m) was compared to that of saplings in high—canopy sites (°>10 m), and performance of common species was compared to that of rare species. Of the 108 species for which all three parameters of performance were measured, 104 fell into four response groups, each with characteristic patterns of survival, growth, recruitment, and response to canopy height. Pioneers (six species) survived poorly in both canopy—height categories, and survivors grew rapidly in low—canopy sites. Sapling recruitment was skewed toward low—canopy sites. Understory specialists (three species) survived well in high—canopy sites and poorly in low—canopy sites. They grew slowly and recruited poorly in both situations. Generalists (79 species) survived well and grew slowly in both canopy—height categories. Per—adult recruitment was usually low, and often skewed toward low—canopy sites. Poorly performing species (16 species) survived poorly, grew slowly, and recruited infrequently in both canopy—height categories. Most of the common (>10 saplings/ha) species appeared to be generalists. Many rare (<1 sapling/ha) or occasional (1—10 saplings/ha) species survived significantly (P ≤ .05) less well than the average survivorship of saplings, while many common species survived significantly better than average. Some rare or occasional species grew rapidly, either in low—canopy sites or in both canopy—height categories, while most common species grew slowly in both situations. Rare and occasional species had significantly more recruits per adult than did common species, but often this did not balance their higher mortality. Large differences in survival, growth, and recruitment between canopy—height categories were found only among rare and occasional species.
Resource‐Use Efficiency and Drought Tolerance In Adjacent Great Basin and Sierran Plants
Authors: DeLucia Evan H. , Schlesinger William H.
Published: May 2006

Using several adjacent Great Basin desert and Sierran montane trees and shrubs, we tested the prediction that water—use efficiency (WUE) is lower in more drought—tolerant species. We also measured maximum net photosynthetic rates and photosynthetic nitrogen—use efficiency (PNUE) to determine if these attributes decrease at high WUE. Differences in the physiological responses to resource availability were more clearly evident between growth forms, trees vs. shrubs, than between vegetation types, montane vs. desert. The shrubs (Anelanchier alnifolia, Arctostaphylos patula, and Artemisia tridentata) maintained higher rates of net photosynthesis at lower predawn water potential but had lower WUE than the trees (Juniperus osteosperma, Pinus monophylla, P. jeffreyi, and P. ponderosa). PNUE was highest in the shrubs that also had the highest foliage N content, and PNUE was inversely related to WUE. However, trees had higher nutrient retranslocation efficiency and growth—based NUE than shrubs. Low WUE and high drought tolerance of shrubs suggests that maintaining high WUE under competitive water—limited conditions may not be advantageous. In contrast, conservative use of water (high WUE) and high growth—based NUE by trees lengthens the season of net carbon assimilation and more closely matches growth with low rates of nutrient availability on nutrient—poor sites.
Nitrogen Sharing Among Ramets Increases Clonal Growth in Fragaria Chiloensis
Authors: Alpert Peter
Published: May 2006

Transport of resources among connected ramets can increase the growth of clonal plants when ramets are located in microsites with contrasting light, water, or total soil nutrient availability. To test whether this is also true for soil nitrogen availability alone, two greenhouse experiments were conducted using the stoloniferous herb Fragaria chiloensis (beach strawberry). First, pairs of ramets were prepared in which the two ramets were connected by a stolen but rooted in separate pots. Ramets in a pair were: (1) severed from each other and given contrasting levels of soil nitrogen (high vs. low); (2) left connected and given contrasting levels of soil nitrogen; or (3) left connected and given a uniform level of soil nitrogen (both high or both low). Second, single ramets were given the high, the low, or one of three intermediate levels of soil nitrogen. Maintaining the vascular connection between ramets given contrasting levels of the soil nitrogen resulted in a greater production of new stolons and new ramets by the ramet given low nitrogen level, and in slightly greater leaf growth of the ramet given the low nitrogen level if it was also younger than the connected ramet. However, a ramet given high soil nitrogen grew much more than a ramet given low soil nitrogen, even when connected. Maintaining the vascular connection between ramets given a uniform level of soil nitrogen had no significant effect on growth. Single ramets given a higher soil nitrogen had greater total and component masses, more stolons and new ramets, higher nitrogen concentrations, higher specific leaf area, and a higher proportional mass of leaves and of stolons and new ramets. Experiments suggested that: (1) transport of nitrogen among ramets can increase growth in F. chiloensis, but to a limited extent; (2) transport of nitrogen through a stolon may be either acropetal or basipetal but tends to promote the growth of younger ramets; (3) higher soil nitrogen availability alters plant architecture by increasing allocation to leaves and to new stolons and ramets; and (4) physiological integration among ramets modifies colonal architecture because nitrogen acquire by roots promotes growth of both a rooted ramet and its new ramets, whereas nitrogen acquired from another ramet promotes the growth of new ramets almost exclusively.
On the Adaptive Value of Physiological Integraton in Colonal Plants
Authors: Caraco Thomas , Kelly Colleen K.
Published: May 2006

In clonal plants, genetically identical ramets arise from a common stolon or rhizome. Anatomical connection often allows physiological integration, the translocation of resources from a larger mother ramet to a developing daughter ramet. Translocation of a limiting resource can reduce the mother's growth while increasing the daughter's growth. Our models predict patterns in resource translocation; the models assume that fitness increases with the expected biomass a genet attains over a season of vegetative growth in a stochastic environment. In each model a ramet's growth depends nonlinearly on its level of a limiting resource. If resource availability varies both spatially and temporally, and a ramet's growth does not depend on its size, an analytical approximation for total genet growth leads to several new predictions. If a ramet's growth increases as a concave function (i.e., a function with decreasing positive slope) of resources level, physiological integration should increase when spatial variance increases and spatial convariance is negative. If a ramet's growth increases as a convex function (increasing positive slope) of resource level (the less likely case), spatial variance—convariance in resource availability has the opposite effect on translocation. Independently of the concavity or convexity of the growth function, increasing temporal variance in the mother ramet's resource availability reduces translocation, and increasing temporal variance in the daughter ramet's resource availability increases translocation. When a ramet's growth increases with both its resource level and its size, translocation and growth in one time interval influence the value of future physiological integration. For this case a stochastic dynamic programming model demonstrates how translocation can depend on time and the sizes of the mother and daughter ramets, as well as on spatiotemporal resource variability. The predictions qualitatively match those deduced from the model of size—independent growth, although translocation often declines late in the season of vegetative growth. The dynamic model also indicates that a large mother ramet should always share resource with a daughter ramet. But a smaller mother ramet should often abandon a daughter and allocate all its available resource to its own growth.
Is Physical Space a Soil Resource?
Authors: McConnaughay K. D. M. , Bazzaz F. A.
Published: May 2006

We examined effects of differing levels of physical belowground space on the performance of several colonizing annual species in two experiments. Individual plants were grown in controlled greenhouse conditions under an experimental regime that independently varied nutrient addition rates and the physical space available for the deployment of roots while maintaining water supply at adequate rates. When equal amounts of nutrients were given, plants grown in greater volumes had greater vegetative growth and often higher reproductive output. Species differed in patterns or response to physical space in both vegetative and reproductive parameters. Abutilon theophrasti given larger soil volumes increased allocation to reproductive tissues relative to vegetative tissues. Setaria faberii, however, responded through a phenological shift: earlier flowering and greater reproductive output in smaller volumes. These results indicate that physical underground space, released in the formation of certain types of gaps, may influence the performance of colonizing annuals beyond providing access to other soil resources, such as nutrients and water.
Interactions Among the Effects of Herbivory, Competition, and Resource Limitation on Chaparral Herbs
Authors: Swank Sarah E. , Oechel Walter C.
Published: May 2006

Older chaparral stands are frequently characterized by gaps in the canopy and by a lack of herbaceous vegetation in these openings. Many causes for the lack of herb growth in gaps have been postulated, including lack of seeds in the soil, limited nutrients and water, competition with shrubs for resources, interference competition (i.e., allelopathy), and herbivory. An experiment was conducted to assess the individual and combined effects of four main factors (trenching to reduce root competition, caging to reduce herbivory, nutrient addition, and water addition) on herb germination, survival, and growth in chaparral dominated by chamise, Adenostoma fasciculatum. We found strong interactions among these treatments. Therefore, no single factor can be regarded as causing the absence of herbs in chaparral. Herb establishment was limited by herbivory and root competition with shrubs for limited nutrients and/or water. Survival was enhanced by trenching, caging, and water. Growth was enhanced by nutrient addition, but the magnitude of this nutrient effect became appreciable only within trenched, caged plots. The vegetative cover of annual herbs in gaps was limited by the combined effects of herbivory, competition with the roots of shrubs, inadequate nutrients, and inadequate water.
Climatically Induced Change in Fire Frequency in the Southern Canadian Rockies
Authors: Johnson E. A. , Larsen C. P. S.
Published: May 2006

The purpose of this study was to partition the components of a mixed fire frequency, and empirically relate these components to temporal and spatial differences in fire frequency. The method of reconstructing the fire frequency was to build a stand—origin map, from this estimate the mixed time—since—fire distribution, and then use a graphic technique to partition the mixed distribution into two homogeneous fire—frequency distributions. The fire frequency for the last 380 yr in the 495—km2 Kananaskis Watershed showed a temporal change in fire frequency at °1730. This change in fire frequency is related to a change from a warmer and drier climate before 1730 to a cooler and moisture climate since then. When the fire—frequency data were partitioned by a graphic technique, the two resulting fire frequencies fit negative exponential distributions. For the period 1730—1980 the fire cycle (the time required to burn an area equal to the area of study, or one divided by the scale parameter of the exponential distribution) was 90 yr. For the period before 1730 the fire cycle was 50 yr. The Watershed could not be subdivided into smaller, spatially homogeneous fire—frequency units. The negative exponential distribution of fire frequencies suggests a constant hazard function (mortality force). This constant hazard was corroborated by a spatial correlation (Moran's I) tests which found no spatial pattern other than ones expected by chance between forest ages on either side of all fire boundaries. Thus, there was no tendency for young stands to be associated either with only younger— or only older—aged stands. Other studies on fire behavior in the Watershed corroborate the short fire cycle, constant hazard, and lack of spatial fire—frequency differences, and suggest that the regional climate control of the temporal fire frequency is related to a characteristic synoptic weather pattern and the resulting high intensity and high rate—of—spread of fires.
The Response of Subalpine Forests to Spruce Beetle Outbreak in Colorado
Authors: Veblen Thomas T. , Hadley Keith S. , Reid Marion S. , Rebertus Alan J.
Published: May 2006

Spruce beetle (Dendroctonus rufipennis Kirby) outbreaks are important disturbances affecting subalpine forests of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern Rocky Mountains. However, little is known about the influences of these outbreaks on overall forest dynamics. We used age—structure analyses and dendrochronological techniques to investigate the effects of a major spruce beetle outbreak on stand composition, dominance, tree age and size structures, radial growth, and succession in subalpine forests in Colorado. This outbreak, which occurred in the 1940s, caused a shift in dominance from spruce to fir and a reduction in average and maximum tree diameters, heights, and ages. The outbreak did not favor new seedling establishment of the seral lodgepole pine. Thus, in seral stands spruce beetle outbreak accelerates succession towards the shade—tolerant tree species. The predominant response to the outbreak was the release of previously suppressed small—diameter spruce (not attacked by the beetle) and subalpine fir (a non—host species). Following the 1940s outbreak, growth rates of released trees remained high for >40 yr. The relative increases in growth rates were similar for both species. Both spruce and fir will continue to codominate the affected stands. The predominance of accelerated growth following a spruce beetle outbreak, instead of new seedling establishment, is a major contrast to the pattern of stand development following fire. In some Colorado subalpine forests the effects of disturbance by spruce beetle outbreaks appear to be as great as those due to fire.
Principal Component Analyses of Assemblage Structure Data: Utility of Tests Based on Eigenvalues
Authors: Grossman Gary D. , Nickerson David M. , Freeman Mary C.
Published: May 2006

We examined the ability of eigenvalue tests to distinguish field—collected from random, assemblage structure data sets. Eight published time series of species abundances were used in the analysis, including data sets for: fishes, birds, mammals, stream benthos, and crabs. To test the efficacy of eigenvalue tests, we constructed 1000 randomly generated data sets for each real data set, whose means and variances were identical to the means and variances of the original data matrices. The data sets were then subjected to a principal components analysis (PCA) and eigenvalue tests used to identify significant eigenvalues for both correlation and covariance matrix solutions. We also examined the effects of: (1) number of species (= number of variables), (2) number of samples (= replication), (3) variance structure, on the performance of the test. Using PCA's based on the correlation matrix and with sample sizes typically encountered in the field, the eigenvalue tests generally performed at the .05 level when a = .01. Slightly poorer results were obtained with the covariance matrix. Increasing the number of samples to at least three times the number of species generally gave a level coverage of an a level test (i.e., a = .05, .01). Increasing variance in the data set only affected test outcomes at levels of replication less than twice the number of species. We conclude that the eigenvalue tests can be used to detect patterns in PCA's of assemblage structure data, if the number of samples is at least three times the number of species and either a covariance or correlation matrix solution is used. It is assumed that these patterns represent ecologically meaningful patterns of variation.
The Ecology of Sex Expression in Red Buckeye
Authors: Bertin Robert I.
Published: May 2006

I examined the reproductive biology of the andromonoecious shrub, red buckeye (Aesculus pavia, Hippocastanaceae) in order to describe the distribution of the two flower types in space and time, and to explain these patterns in an ecological context. Bisexual flowers were rare relative to male flowers, comprising typically <2% of the total flowers at woodland sites and 5—40% at field sites. Light availability was apparently the most important environmental factor influencing among—plant differences in ratios of flower types and production of mature fruit, though variability of the former may also have had a large genetic component. Within inflorescence, bisexual flowers were most common basally, especially near the central inflorescence axis, and on the side of the inflorescence away from the branch bearing the inflorescence. Predator and pollinator behavior, and requirements for fruit support were potentially important in determining this pattern. Bisexual flowers tended to occur in those inflorescences receiving the most light, and having the most subtending leaves and greatest yearly twig elongation. Abundance of bisexual flowers through time showed (1) a strong negative correlation with intensity of predation on fruits produced from flowers borne at these times, (2) a negative correlation with intensity of predation on flowers, and (3) a positive correlation with probability of pollinator visitation. I suggest that spatial and temporal distributions of flower types are often selected for by patterns of resource availability and behavior of predators, pollinators, or dispersers, and that selection on monoecious species will generally favor individuals in which sex expression and fruit production respond to the same environmental factors.
Clutch Size, Egg Size, and the Consequences of Single Vs. Multiple Parasitism in Parasitic Finches
Authors: Payne Robert B.
Published: May 2006

Clutch size and number of eggs in a season were determined for 11 species of parasitic African finches by examination of the ovaries. Clutch size averaged 3.14 for 10 species of Vidua and 2.91 for the Cuckoo—finch Anomalospiza imberbis. No significant species differences or latitudinal variation were found. Some Village Indigobirds V. chalybeata ♀ ♀ mate repeatedly for as long as 5 mo; 90 days is an average season. A ♀ lays 22—26 eggs in a breeding season. The eggs of the parasitic viduines and Anomalospiza are relatively smaller than those of other nesting finches (Ploceinae and Passerinae), and mean clutch size is larger than in the Ploceinae, the presumed ancestral group. Clutch size in V. chalybeata corresponds with the largest number of parasite young that a pair of foster parents Lagonosticta senegala rear in nature, and survival data (Morel 1973) indicate no net density—dependent effect on success of the parasites from egg to fledging in multiply parasitized nests. The frequency distribution of parasite eggs in all host nests in two wild populations is contagious, with more instances of multiple parasitism than if eggs had been laid randomly. An aviary ♀ laid one to three eggs in sequence in individual host nests. Nesting V. chalybeata are tolerant of other nestlings as expected if parasite young were often siblings. It is suggested that clutch size has likely been selected to correspond to the number of parasite young that the foster parents can rear. Nevertheless, half of all parasitized nests have a single parasite egg, and the most common laying behavior is scatter—laying. The results of scatter—laying parallel a probabillistic strategy that minimizes total failures, rather than one that maximizes a maximal number of successes. The observed mixed or conditional combined strategy of (1) scatter parasitism adapted to escaping high losses to predators and (2) multiple parasitism adapted to exploiting parental care of the foster parents may be peculiar to the finches among the parasitic birds.
Interactions of Nutrients, Plant Growth and Herbivory in a Mangrove Ecosystem
Authors: Onuf Christopher P. , Teal John M. , Valiela Ivan
Published: May 2006

The effect of nutrient enrichment of red mangroves (Rhizophora mangle) was studied by comparing two mangrove—covered islands in the Indian River at Ft. Pierce, Florida, USA, one (high nutrient) with and one (low nutrient) without a breeding colony of pelicans and egrets. Repeated measurements taken on > 100 tagged branches in each area revealed higher growth rates at the high nutrient site. Trees at the high nutrient site showed greater additions of (1) leaves; (2) reproductive parts; (3) new lateral branches; and (4) larger increments to existing stems. Growth in the fertilized stand also began earlier in the year and had a second maximum not shared by the low nutrient area. Both leaves and fruits at the high nutrient site were richer in nitrogen. More striking than the effects on plant growth, however, was the proportionately much greater stimulation of herbivory by insects in response to nutrient enrichment. Larvae of the five lepidopteran species that we observed feeding on buds or leaves were either more abundant or only present in the high nutrient area, as was the scolytid beetle that infested seedlings before they dropped from the parent tree. This difference in herbivory between sites disappeared when the birds seasonally migrated away from their nesting areas at the high nutrient site. This observation and the demonstration that the mangrove skipper Phocides pigmalion attains a higher growth efficiency on high nutrient leaves are both consistent with the hypothesis that increased nutritive value of vegetation (correlated with the increased concentration of nitrogen) is responsible for the 4x greater losses to herbivores in the high nutrient. Implications for environmental management and in more complex communities are discussed.