摘要:

Reproductive effort including metabolic costs (REmet) was calculated for two wild strawberries, Fragaria virginiana and F. vesca. Five populations were studies for 2 or 3 yr each on sites comprising stages in succession near Ithaca, New York. Field and laboratory studies of biomass, environment, and CO2exchange characteristics wee integrated using a model of whole—plant carbon dynamics. Leaves supplied 96—99% of total yearly plant photosynthesis; reproductive structures also had some photosynthetic capacity. Results of the model suggest that photosynthetic carbon gain is the major factor limiting growth. Sexual REmetdecreased from 12% in an open, recently disturbed habitat to 4% in a shady forested site, while vegetative REmetdecreased from 29 to 9% over the same series of habitats. Initial capital, harvest time, and the ratios of respiration costs to costs of new structures in different habitats all contributed to variation in calculated reproductive effort based on different definitions. Variation in REmetamong sites and among years on the same site suggest substantial control of reproductive effort by environmental conditions within some genetically determined limit.

ISSN:0012-9658    

DOI:10.2307/1937488

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

In comparison with collections from the early 1900's when oxidant air pollution was essentially absent, 50% fewer lichen species were found on conifers during 3 yr (1976—1979) of collecting and sampling in the mountains of Southern California. Among the five mountains ranges studies, the San Bernardino Mountains, the region with the highest oxidant levels, had lower lichen frequency and cover values. Within the San Bernardino Study sites, lichen cover was inversely related to estimated oxidant doses. Furthermore, at sites with high oxidant levels, marked morphological deterioration of the common species Hypogymnia enteromorpha was documented. Transplants of this species from the relatively unpolluted Cuyamaca Rancho State Park into the San Bernardino Mountains exhibited similar deterioration after a year's exposure.

ISSN:0012-9658    

DOI:10.2307/1937489

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Eastern redcedar (Juniperus virginiana) is a sun—adapted, drought—resistant pioneer species common to pastures, abandoned fields, fence rows, and calcareous rock outcrops throughout the eastern United States. However, it is also a frequent component of the understory in mature oak—hickory forests in central Missouri, where light levels are typically<10% of full sunlight during much of the growing season. This is below eastern redcedar's reported optimum for photosynthesis. The competitive survival of understory eastern redcedar under such environmental conditions was apparently due to it being an evergreen conifer in a deciduous forest. Hence, its foliage was able to maintain a positive carbon dioxide balance throughout much of the year, with maximum net photosynthetic rates occurring during periods when the overstory was leafless. The greatest daily average net photosynthetic rates (Phn) occurred during overstory leaf emergence when temperatures were moderate and light levels to the understory trees were annually the highest. Furthermore, since leaf temperatures and tree water deficits were relatively low at this time, daily gas exchange rates were not greatly limited by midday stomatal closure. After the overstory foliage had fully developed, understory light levels averaged °50—80% below levels observed in early spring. Thus, photosynthesis was severely light limited during the day, resulting in Ph?nthat were °15—45% of the springtime maxima. The greatest daily average transpiration rates (TR) occurred during the summer due to the high evaporative demand. Increasing leaf temperatures and tree water deficits became more important by late summer, causing stomatal closure during some afternoons, which reduced Phnand TR to °30 and 40%, respectively, of the early summer levels. During the autumn, winter, and early spring, understory light levels were normally above the saturation point for photosynthesis. The light saturation point for an understory study tree (expressed as flux of photosynthetically active photons) was °800 @mmol°m—2°s—1, less than half of that reported for open—grown eastern redcedar. This relatively lower light saturation point suggested an adjustment to shade conditions. During the autumn overstory defoliation period, light levels to understory trees progressively increased, and Phneventually reached 80% of the springtime maximum. In contrasts, TR only reached °25% of the summer maximum, owing to relatively low evaporative demands. During the late autumn and winter, low leaf and oil temperatures combined to limit gas exchange severely. The major controlling factors seemed to be cold air temperatures directly inhibiting Phnand cold soil temperatures indirectly producing tree water deficits due to reduced water uptake at the soil—rate interface. Such conditions promoted persistent stomatal closure, resulting in Phnnear zero. However, a temporary warming trend during the winter caused an increase in Phnto a level °30% of the springtime maximum. Higher net photosynthetic rates probably were not possible due to the effects of low soil and air temperatures on the especially mechanism and on the photosynthetic apparatus.

ISSN:0012-9658    

DOI:10.2307/1937490

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

The role of herbs in affecting tree seedling patterns was investigated in an old—growth hemlock—hardwood forest in Cathedral State Park, West Virginia. Herb species cover, tree seedling density, and overhead foliage were sampled along with midsummer soil pH, soil moisture, and light intensity in 20 selected herb patches and 440 30 x 30 cm plots. Analyses of the patch data showed significant correlations of seedlings of major tree species: Prunus serotina, Acer rubrum, Tsuga canadensis, and Betula spp., with leading herb species (Dennstaedtia, Mitchella, Lycopodium, and Oxalis). Each tree species had few seedlings in some herb species patches, but was independent of or concentrated in other patches. Herb species were spatially related to other herb species, but showed little correspondence to soil pH, soil moisture, and light intensity patterns. Analyses of plot samples supported each of the above results. In addition, total tree seedling density was inversely correlated with total herb cover, but neither total seedling density nor number of seedlings of leading tree species was correlated with the abiotic factors. Seedling density and herb cover varied with the height and composition of the overhead canopy foliage. Based on the data from the two sampling approaches in this old mesic woods, we conclude that herb patches play a major role in determining the density and distribution of seedlings of leading tree species, and that the distribution of herb patches is significantly affected by both tree canopy foliage and other herb patches. Dynamic models of forest growth, species diversity, and composition must include herbs.

ISSN:0012-9658    

DOI:10.2307/1937491

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Diurnal and Seasonal water relations were monitored in a population of Prosopis glandulosa var. torreyana in the Sonoran Desert of southern California. Prosopis glandulosa at this research site acquired its water from a ground water source 4—6 m deep. Measurements of diurnal and seasonal cycles of aboveground environmental conditions, soil moisture, and soil water potential (to 6 m depth) were taken to ascertain environmental water availability and water stress. Leaf water potential, leaf conductance, leaf transpiration, relative saturation deficit of leaves, osmotic potential, and turgor potential were measured to evaluate plant adaptations to environmental water stress. Soil water potential was low (—4.0 to —5.0 MPa) in surface soil in relation to deep soil (—0.2) MPa). This difference was due to high surface soil salinity and low surface water content. the climatic conditions at the research site produced extreme water stress conditions in summer months when temperatures reached 50°C, vapor pressure deficit (VPD) reached 8 kPa, and surface soil water potential was below —4.5 MPa. Although considerable plant water stress developed in these trees (midday leaf water potential —4.8 MPa), osmotic adjustment occurred and turgor was maintained on a diurnal and seasonal cycle. Prosopis glandulosa has adapted to avoid water stress by utilizing deep ground water, but this phreatophyte has also evolved physiological adaptations, such as osmotic adjustment and seasonally changing stomatal sensitivity to VPD, which result in greater tolerance of water stress.

ISSN:0012-9658    

DOI:10.2307/1937492

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

The first 40 yr of forest succession on permanent plots at the Harvard Forest in central New England followed the initial floristic composition model forest succession. After the 1938 hurricane removed the previous white pine (Pinus strobus) canopy, species regenerated within 4—6 yr by sprouts, buried seed, and wind—blown seed, with no method of regeneration uniformly contributing more to species success than another. Hemlock (Tsuga canadensis) was the only species successfully regenerating after 1948. Pin cherry (Prunus pennsylvanica) was the early dominant in size and numbers (5000 stems/ha). At year 10, pin cherry, red maple (Acer rubrum), white ash (Fraximus americana), and red oak (Quercus rubra) were dominant. Species diversity had reached a maximum. By year 40, red oak and paper birch (Betula papyrifera) showed strong canopy dominance, making up only 7.5 and 4.9%, respectively, of total density but 37.5 and 12.5%, respectively, of the size—dominant stems. Red maple and white pine were also codominant in 1978. Some evidence for an intermediate stage dominated by red maple and gray birch (B. populifolia) was found. On one previously hardwood plot, the same species were present, and similar trends in species composition and dominance were followed, but there was more surviving hemlock advance regeneration and a lower density of some shade—intolerant early dominant species. The canopy structure was loosely multilayered, and at any given point in succession, species tended to be found in characteristic layers, although these relative positions could change with time.

ISSN:0012-9658    

DOI:10.2307/1937493

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Ovipositing Battus philenor butterflies discriminate among conspecific host plants in the field. Discrimination has no detectable effect on larval growth rates or on predispersal mortality but does appear to enhance larval survival by increasing larval size at dispersal. A comparison of these results with those of a previous study suggests that the effects of discrimination among conspecific hosts on larval survival are similar in magnitude to analogous effects of discrimination between host species. The results of this study suggest that models and discussions of the evolution of host selection behavior that treat plant species as the unit of discrimination may be seriously flawed because they overlook significant aspects of insect behavior.

ISSN:0012-9658    

DOI:10.2307/1937494

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

The hypothesis that ants attracted to the nectar secreted by bracken fern (Pteridium aquilinum Kuhn) foliar nectaries protect the frond from herbivore damage was tested experimentally in three New Jersey populations. There was no difference in the amount of herbivore damage experienced by control ferns and those from which ants were excluded. Sugar secretion rates were highest in fiddleheads just prior to the start of frond enlargement, and ant activity was highest on fronds in this developmental stage. Both sugar secretion and ant activity decreased during frond enlargement to low levels at maturity. Despite considerable variability among fronds in sugar secretion, population sugar production and ant activity remained constant through most of the summer. The developmental and seasonal patterns of nectar secretion therefore fit those that would be predicted if bracken and ants were engaged in a mutualistic interaction, but there is no evidence that the ferns in fact derived any benefit from their ant visitors.

ISSN:0012-9658    

DOI:10.2307/1937495

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Replicated pond communities were established in 27 tanks 1.52 m in diameter and 0.62 m deep. The communities included a sample from a pooled mixture of zooplankton from nine small ponds, 20 stems of the aquatic perennial Myriophyllum heterophyllum, and 300 hatchlings of the toad Bufo woodhousei fowleri. All nine combinations of three densities of a predator (0, 4, or 8 adult Notophthalmus viridescens in an equal sex ratio) and three densities of prey (larvae of the frogs: Rana sphenocephala, Bufo terrestris, and Scaphiopus holbrooki, in constant relative abundance) were randomly represented in each of three blocks arranged in a Graeco—Latin square design. In the absence of predators, the anurans had density—dependent survival, lengths of larval period, and sizes at metamorphosis. There was evidence of competition among, as well as within, species. Rana sphenocephala had wintering tadpoles only in the highest—density populations. In this system community structure is affected by a gape—limited predator, Notophthalmus, that acts early in the anuran larval period, probably before competition occurs. In the presence of Notophthalmus all anurans had low survival, large mean size at metamorphosis, and short mean larval period. The survivors of predation probably have enhanced success as terrestrial juveniles compared to the survivors of competition in communities where there is a low risk of predation.

ISSN:0012-9658    

DOI:10.2307/1937496

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

In artificial—pond communities, a doubling of the density of the salamander Notophthalmus viridescens dorsalis resulted in decreased adult growth and reduced production of metamorphs. Experimental manipulations of the abundance of a trophic resource, hatchling tadpoles, had no effect on density—dependent growth and reproductive success in Notophthalmus. However, experimental differences in salamander density depressed the abundance and altered the species composition of tadpoles and microcrustacean. Accidental introductions of small numbers of Notophthalmus eggs from a permanent pond resulted in the production of neotenic individuals in some control populations without adult newts. These results document the probable importance of intraspecific density in population regulation and life history variation in a keystone vertebrate predator with a complex life cycle.

ISSN:0012-9658    

DOI:10.2307/1937497

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY