摘要:

Pollen data were used to reconstruct the past 340 yr of vegetational changes at Great South Beach, a system of barrier beaches east of Fire Island Inlet, Long Island, New York. The data came from cores taken along 16 transects, 6 of which are described in detail. Dates and stratigraphy from individual cores provided local environmental histories that were combined to reconstruct temporal and spatial patterns of vegetation on local and regional scales. Changes in inlet status and local disturbances accounted for much of the dynamism in vegetation. Salt marshes fringed back—barrier lagoons only when many inlets were open and saline conditions prevailed (1760 to 1835; 1931 to the present). Cyperaceae dominated marshes throughout the remainder of postsettlement time. Inlets affected vegetation by altering tidal range and salinity in back barrier lagoons and provided new substrates for marsh establishment when flood—tide deltas were abandoned by inlet channels. At least once every 100 yr along the six transects, sufficient sand was deposited to destroy all vegetation. The composition of invading assemblages on primary substrates was regulated mainly by physical factors. Annual herbs, shrubs, or high—marsh plants each were "pioneers" at different localities and times. Putative "pioneer" annuals were not limited to the early stages of succession. Fluctuating water tables played an important role in determining composition of freshwater wetlands. Biotically controlled trends in plant composition could not be positively identified. The reconstructions on Great South Beach indicate tight environmental control of plant distribution but emphasize that vegetation does not achieve an equilibrium with the physical environment. Present distributions are largely the product of historical events acting together with existing conditions.

ISSN:0012-9615    

DOI:10.2307/1942504

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.

ISSN:0012-9615    

DOI:10.2307/1942505

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The temporal and spatial patterns of larval settlement of the bluehead wrasse, Thalasoma bifasciatum, were documented in the San Blas Islands of Panama. Daily censuses indicated that larvae settle onto coral reefs in brief episodes that tended to occur around the new moon and peak in intensity between August and December of each year. The magnitude of settlement each day was unrelated to changes in the local population size but was found to be significantly correlated with the nightly catch of planktonic reef fish larvae in the waters over the reef. The spatial pattern of settlement was examined on three scales. On a large geographic scale, 24 reefs within 1000 km2, there was tremendous variability in settlement intensity that was very consistent from one year to the next. The best correlate of recruit density was exposure to the onshore current. It is suggested that some large—scale oceanographic process determines the intensity of settlement of bluehead wrasses on this scale. The densities of adults on these reefs directly reflected the densities of recruits. Counts of daily otolith increments indicated that those larvae that settled onto low—density reefs subsequently grew significantly faster. On an intermediate scale, sites within 1 km2, there was no consistency among sites or months in settlement intensity, suggesting that variance in settlement intensity on this scale may be the product of random processes. On the smallest scale, habitat selection within a patch reef, it was found that recruits had distinct preferences in their choices of settlement sites. The daily mortality schedule of bluehead wrasses indicated that mortality was extremely high for the first 3 d on the reef. Juvenile mortality was, furthermore, found to be independent of density. As a result, the patterns of larval settlement of bluehead wrasses persisted into the adult population relatively unchanged. It is therefore proposed that the distribution and abundance of bluehead wrasses in this region are more a product of the external processes controlling larval settlement than of any biological interactions within the reef assemblage.

ISSN:0012-9615    

DOI:10.2307/1942506

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Two experiments were designed to test which hypothesis of population regulation provided the most consistent explanation of population fluctuations in the California vole. The first experiment tested the effects of spacing behavior and of initial breeding—season density on population regulation. The second experiment tested the effects of resource level, of population history, and of population substructure. A factorial design was used for each experiment. The first experiment was analyzed as two separate factors. The second experiment was analyzed with a 2 × 2 × 2 factorial analysis of variance. The results indicate that naturally occurring populations appear to be regulated by two separate but equally important mechanisms. When habitat occupation is patchy, spacing behavior dominates and densities are regulated below 200 individuals/ha. Individuals from these populations can be characterized as coming from low or early—increase phases. Survival is low due to very high dispersal losses and a moderate mortality rate. Individuals have a greater tendency to be reproductive. Males have lower masses. When habitat occupancy is more uniform, increasing overall density reduces the dispersal rate and, as a result, densities increase further. In this situation, resource limitation dominates the regulation process. Peak densities of these populations range from 500 to 800 individuals/ha. Individuals from these populations can be characterized as coming from the peak phase. Survival is high due to low dispersal losses and a moderate mortality rate. Individuals tend less to be reproductive, and males are heavier.

ISSN:0012-9615    

DOI:10.2307/1942507

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY