摘要:

In the northern hardwood forest, growth of vernal photosynthetic herbs is temporally restricted to the period between spring snowmelt and summer canopy development. This characteristic suggests that several unique adaptations exist which allow the species to complete their life cycles, and that temporal separation of production in the herbaceous layer may add to structural and functional complexity of the ecosystem. Erythronium americanum Ker. (Liliaceae) was examined in central New Hampshire with respect to its natural history, growth characteristics and influence on energy flow and mineral cycling in the deciduous forest ecosystem. Growth leading to the early spring development of photosynthetic tissue begins with fall root growth and continues through a long winter phase during which the shoot elongates from the perennating organ, through the soil and into the snowpack. Following snowmelt, the shoots begin rapid unfurling and maturation of the photosynthetic tissue. The length of the mature leaf phase is controlled by the timing of snowmelt and canopy development, and may be quite variable between successive years. During the short period of production, total biomass increased by 190% in 1972 and 338% in 1973; however, plant weight at the end of the winter period in 1973 had decreased to 28% of the spring 1972 maximum. In the annual energy cycle, biomass losses during the nonphotosynthetic period may amount to more than production during the preceding spring. In comparison with summer green herbs, Erythronium shoot tissue contained significantly higher concentrations of N but lower levels of K, Mg and Ca, suggesting that the spring adaptation may be oriented toward higher N levels of the soil during the spring period as well as higher light levels at the forest floor. Significant correlations of biomass of vernal photosynthetic herbs with summer green species imply that temporally separated species may utilize the same physical site and resources. This adds to the structural complexity and production of the herbaceous layer; however, the vernal photosynthetics account for only 0.5% of total aboveground primary production of the ecosystem. The temporal character of Erythronium's growth and its capacity for rapid biomass accumulation combine to make it a significant factor in nutrient dynamics in the deciduous forest. Uptake of N and K during spring flushing of nutrients from the ecosystem and later release through senescence of shoot tissue appear to reduce gross ecosystem losses of these elements.

ISSN:0012-9615    

DOI:10.2307/2937357

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

About 800 Microtus ochrogaster were live—trapped at biweekly intervals from May 1971 through March 1973 in 3 grassland study areas in eastern Kansas, USA. Details of reproduction were determined by autopsy. Population density increased through the first winter, reached a peak in April 1972, then declined sharply during that summer and beyond. Body length, but not body mass, tended to be positively related to density. Both sexes matured at about the same weight; development was somewhat prolonged during the winter months, especially in the winter preceding the population peak. Pregnancy rates were high, approaching maximal iteroparity, throughout the study, with intervals of nonbreeding in both Julys, and in August and December of the population decline. Embryo counts increased significantly during the months of peak density but did not diminish in the period of the population decline. There was no association between either maternal weight or parity and number of embryos. Corpora counts were not higher in heavy or multiparious ♀ ♀, nor did the level of prenatal mortality increase during the period of greatest density. Except for a few individuals taken in July and August 1971, ♂ ♂ with a mass 30 g or more were judged to be fertile, as were a majority of the 20 to 29—g ♂ ♂. Body weight and testes weight were significantly correlated in fertile ♂ ♂ but not in nonfertile ♂ ♂. These reproductive findings were applied to a model of population regulation of microtine cycles. The pattern of reproduction of Kansas and Indiana prairie voles was compared. Three concordant lines of evidence seem to indicate that Kansas and possibly other geographical populations of prairie voles have adjusted the breeding schedule to accommodate both hot and dry summers and cold, snowy winters. Pregnancy rate, litter size, and adjusted testes weight all show a significant increase in March–April and September–October and a decrease in midsummer and midwinter. Thus the pattern of reproduction of Kansas prairie voles may represent a composite of the patterns shown by the dry—adapted species, such as Microtus californicus, and the cold—adapted microtines, such as Microtus pennsylvanicus. Based on the observations of this study, Kansas prairie voles seem to have adjusted to both harsh seasons by breeding more or less continuously, with the midsummer depression possibly reflecting the Great Plains origin of the species, and the winter depression being a facultative response to weather, density, or quality of habitat.

ISSN:0012-9615    

DOI:10.2307/2937358

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Aspects of community structure and nutrient circulation are described for the cypress (Taxodium distichum) forest in Okefenokee Swamp, Georgia, USA. This bog environment is characterized by low nutrient availability and large peat accumulations. The tree stratum of the forest is dominated by cypress, which is probably due to recurrent understory fires which eliminate other swamp species. In stands throughout the cypress forest, the density and total basal area of living stems>4 cm diameter vary greatly, but mean values (1,465 stems/ha; 52 m2/ha) are high compared to upland forests. Natural thinning appears to be unimportant. Abundant standing dead trees suggest that differences in density among stands are due to past differences in the frequency and intensity of forest fires during periodic droughts. At an intensive study site, biomass and net primary production were measured using techniques of dimension analysis. Cypress trees compose 98% of the total forest above—water biomass of 307 tonnes/ha. Most of the cypress biomass is in tree boles (96%), little is in foliage (0.8%). Presumably due to the acid, nutrient—poor conditions in the swamp, total above—water net primary productivity is low (692 g°m—2°yr—1); the high biomass is the result of the old age (150 yr) of the stand and the high density of trees. Cypress net production is largely channeled into bole wood (41%) and current twigs with needles (41%). It appears that in recent years, above—water biomass has been maintained at a constant level by fire—induced thinning. The nutrient pools in the above—water community are large (666 kg/ha Ca, 111 kg/ha Mg, 230 kg/ha K, 996 kg/ha N and 46 kg/ha P). These nutrients are largely contained in cypress boles and do not freely circulate. The annual nutrient uptake in the forest is rather small compared to that in upland forests. The nutrient uptake (54 kg°ha—1°yr—1Ca, 11 each for Mg and K and 2.3 for P) is largely channeled to the small amount (231 g/m2) of very efficient cypress foliage. Foliage abscission and foliar leaching by rainfall return 73 to 91% of the annual nutrient uptakes each year; therefore, there are only small permanent additions of nutrients to the above—water pools each year. Except for K which is apparently reabsorbed from foliage before abscission, the cypress trees do not appear to conserve foliar nutrients. However, nutrient conservation may be effected by minimizing the amount of foliage. Litterfall from the forest has resulted in a large accumulation of peat. Although some nutrient regeneration apparently occurs, these peat deposits contain large, permanent nutrient losses from the community. Nutrients cycling is dissolved form (e.g., foliar leachates) and nutrients received from atmospheric precipitation are likely to be of special importance to the nutrition of this closed—basin swamp forest community.

ISSN:0012-9615    

DOI:10.2307/2937359

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

This paper analyzes the factors controlling the development and persistence of patterns of distribution, abundance, and diversity of space users in the low rocky intertidal zone of New England. The spatial structure of this community changes along a wave exposure gradient. Mussels (Mytilus edulis) dominate at headlands exposed to wave shock, the alga Chondrus crispus (Irish moss) dominates at sites protected from wave shock, and both are abundant at areas intermediate in exposure to waves. Using a combination of experiments (exclosures, enclosures, removals) and observations, we evaluated the effects of several factors on this system, including (1) predation, (2) herbivory, (3) plant—plant competition, (4) plant—animal competition, and (5) physical disturbance from high—energy waves. The interaction having the greatest effect on the structure of this low zone association was predation. At protected sites, the starfish Asterias forbesi, Asterias vulgaris, and the snail Thais lapillus prey heavily on Mytilus, which is the functionally dominant competitor in the low (and mid) zone(s). When secondary succession is initiated by removal of all erect animals and plants, community development in the absence of these predators (predator exclusion) results in competitive elimination of both the barnacle Balanus balanoides and Chondrus by Mytilus. A similar result occurs if predators are excluded from unaltered stands of Chondrus. Controls in these experiments (i.e., with predators present) usually either developed to, or remained as stands of Chondrus. At intermediate sites, patches of Mytilus occassionally escaped from predation, suggesting predation intensity is patchy in space and time. Persistence of Chondrus is thus a by—product of the activities of predators at protected sites. At exposed sites, predators do not control the mussels. As a consequence, Mytilus outcompetes Chondrus and Balanus for space and achieves structural dominance. Periwinkle abundance decreases, and abundance and seasonality of ephemeral algae increase with increasing wave shock. Results of manipulations during both primary and secondary succession indicate that Littorina littorea, the only large, abundant herbivore in the low zone, has no direct effect on perennating (regrowing vegetatively) or established Chondrus, or on its extensive, encrusting holdfast. However, this periwinkle exerts an important indirect effect by consuming seasonally abundant ephemeral algae, which slow the rate of succession by suppressing growth of Chondrus. Once Chondrus is established, L. littorea damps variations in its abundance by cropping epiphytic ephemeral algae. The role of other herbivores seems negligible. Experiments show that limpets and sea urchins potentially could control Chondrus (and its holdfast), but they are normally too scarce to have a detectable effect on the algae. Chondrus thus monopolizes space at protected areas because (1) its competitors (mussels and epiphytic ephemeral algae) are removed by their consumers (predators and herbivores), (2) it has escaped control by herbivores, and (3) it can outcompete other perennial algae by virtue of its ability to perennate, and thus maintain its occupancy of space. The organization of this portion of the New England rocky intertidal region is thus similar in important ways to that of the mid zone (Menge 1975, 1976). Predation intensity, at least partly a function of wave shock, is great at relatively protected sites and determines the observed structural pattern (domination of space by algae). Since consumers are ineffective in controlling prey at exposed sites, mussels outcompete other space users and monopolize space on the shore. Thus, predators apparently determine the "trajectory" followed during succession. The persistence of algae is strongly dependent on the removal of mussels by predators. Herbivores, though ineffective in controlling the structurally dominant perennial algae, control the abundance of ephemeral algae and hence both determine the rate at which a Chondrus bed develops and stablize established beds of Irish moss by reducing variability in its abundance. Thus, herbivores evidently control the rate of community development (succession) and enhance the persistence of this alga. Disturbance from wave shock seems to operate in a fashion similar to both types of consumers by removing mussels and ephemeral algae. However, this sort of removal tends to be more catastrophic, frequently clearing large areas of space and initiating secondary succession, especially at exposed sites. The role of disturbance is thus largely that of inducing, rather than suppressing, variability in this system.

ISSN:0012-9615    

DOI:10.2307/2937360

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY