ISSN:0012-9658    

DOI:10.2307/1941545

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941546

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941547

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941548

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

At Monteverde, Costa Rica, the plants Besleria triflora (Gesneriaceae) palicourea lasiorrachis, and Cephaelis elata (Both Rubiaceae) share hummingbirds (Lampornis calolaema) as pollinators. To investigate the effect that intervening visits to Cephaelis flowers have on pollen transfer between plants of self—compatible Besleria or of self—in—compatible Palicourea, we presented captive Lampornis with series of hand—held flowers: first, 2 or 15 flowers of either Besleria or Palicourea as pollen donors, followed by 0, 2, or 10 Cephaelis flowers, followed by 20 recipient flowers of the first species. In protandrous Besleria, we used male—phase flowers as donors and female—phase flowers as recipients. In distylous Palicourea, we used pin (long—styled) flowers as donors and thrum (short—styled) flowers as recipients. We assessed pollen transfer by counting pollen tubes in recipients' styles and, in Besleria, by counting pollen grains deposited on stigmas. Intervention of Cephaelis flowers strongly reduced pollen transfer among Palicourea flowers. Increasing the number of donor Palicourea flowers significantly increased the absolute amount of pollen transferred but did not overcome the negative effect of Cephaelis. Pollen transfer among Besleria flowers also declined with the intervention of Cephaelis flowers, the decline was less strongly significant than for Palicourea, perhaps because pollen loads on stigmas of some female Besleria flowers can contain self pollen left over from their male phase. Under controlled conditions, then, strong competition may exist where plants share pollinator individuals, a situation faced by the experimental species and many others at Monteverde. Other studies on these plants reveal that despite its demonstrability in the laboratory, however, competition among individual plants varies in its ability to explain density dependence in pollination among plant populations in the field. Furthermore, community—level patterns of coexistence among plant species at Monteverde feature few "ghosts of competition past." The collective results demonstrate that studying the role of particular interactions in community structure must involve examinations of mechanisms at the level of individuals, processes at the population level, and patterns of coexistence rather than just one level of this hierarchy. Studying mechanisms alone could exaggerate their importance to community structure, whereas studying coexistence patterns along could misjudge the strength of interactions among individuals.

ISSN:0012-9658    

DOI:10.2307/1941549

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In a tropical cloud forest at Monteverde, Costa Rica, three understory shrub species overlap greatly in flowering seasons and share hummingbird pollinators (Lampornis calolaema). We conducted two field experiments to determine if a plant's pollination and subsequent reproductive output reflect local densities of conspecific and heterospecific flowers. We controlled floral composition in the neighborhoods surrounding 12 focal plants of self—compatible Besleria triflora (Gesneriaceae) and of self—incompatible Palicourea lasiorrachis (Rubiacaea), producing prescribed mixtures of flowers of the focal plants' conspecifics and of Cephaelis elata (Rubiaceae). Each experiment subjected each focal plant to four different treatments; a floral neighborhood of 10 conspecific flowers and 10 of Cephaelis within a radius of 20 m (Treatment A); 90:10 (B); 10:90 (C); and 50:50 (D). Focal plants themselves always had 10 open flowers. We assigned the sequence of treatments to each focal plant in a Latin Square crossover design using three balanced blocks of four plants each. Dependent variables were: frequency of hummingbird probes per flower; pollen tubes in styles; and seed output per flower. From standard models of competition for pollination, we predicted that pollination levels and seed production would be ranked among treatments: B(90:10)>D(50:50)>A(10:10)>C(10:90). These overall predictions were supported for both Palicourea and Besleria, based on tests for directionality showing that the rank order among adjusted treatment means did not differ from the order predicted. Separate a priori treatment contrasts indicated that in both species increasing flower density above base levels (Treatment A, 10:10) by adding conspecific flowers alone (B, 90:10) or along with Cephaelis (D, 50:50) increased the rate at which hummingbirds visited focal plants' flowers. In Palicourea, both the number of pollen tubes in styles and seed output per flower increased in the same fashion. In Besleria, the only effect was increased pollination (but not seed output) from A (10:10) to B (90:10). In contrast, enriching the basic neighborhood (A. 10:10) with Cephaelis flowers (C, 10:90) did not alter frequencies of hummingbird probes to either species of focal plant. Heterospecific enrichment did, however, decrease both pollination levels and seed output in Palicourea, and seed output in Besleria. In general, altering the floral neighborhood affected Palicourea more strongly than Besleria. Apparently the intensity of density dependence, at least in terms of female reproductive success, varies among plant species sharing pollinators. Seed outputs in self—incompatible plants such as Palicourea may closely reflect flower densities of neighbors, but interspecific effects on fertilization in self—compatible plants such as Besleria may be density vague.

ISSN:0012-9658    

DOI:10.2307/1941550

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The interaction between two species is usually assigned as though they were in isolation from all other species. Here we use a (known) method that determines species interactions more realistically, within the framework of the community to which they belong. This "inverse" method evaluates all the effects that one species experiences from another, both direct and indirect. We use this method to study the classical (though highly controversial) "competition community," where each species is considered (in the "isolated pair" approach) to suffer from the presence of every other. The model we use takes account of the fluctuations in interaction coefficients that one must expect in the real world, both from one species pair to another, and as the effect of ambient environmental variations. Remarkably, the "inverse" method finds that generally a high proportion (20—40%) of the interactions must be beneficial, or "advantageous," when not lifted out of the community context in which they actually occur. The contrary case, called here "hypercompetitive," in which each species suffers from every other species, can occur only if the environment is nearly constant, and the species closely akin to each other, with both of these conditions holding and persisting to a degree that must be considered implausible. The results of the model remain valid, even after incorporating a number of major structural modifications, thus indicating robustness in the predictions. We survey the available field data and show that they are in good general agreement with the conclusions reached, on the high proportion of interactions which must be "Advantageous in a Community Context" (ACC).

ISSN:0012-9658    

DOI:10.2307/1941551

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

This study illustrates the physiological and biochemical responses to temperature of maternal phenotypes of plants of Echinochloa acrus—galli that result in different progeny phenotypes. Our results provide a physiological scenario that accounts for strong, nongenetic, maternal effects. At low temperature, the photosynthetic rates of flag leaves were reduced, while concentrations of sugars and starch were increased. These physiological modifications suggest reduced phloem translocation to the reproductive structures in plants grown under cold conditions. At high temperature, high respiration rates of inflorescences were observed during seed development. To offset the effect of ear (fruit) respiration, mother—plants grown under warm conditions would have to translocate some 30% more carbohydrates than at low temperature. Our study therefore suggests that environmentally induced modifications in simple physiological traits can account for an important reduction in resources available to reproduction. Changes in leaf starch, reducing sugars, and protein contents induced by temperature persist through the life of a second generation of plants. Biochemical modifications due to maternal effects may influence overall plant performance. For example, the higher protein concentration of seedlings grown from cold—matured seeds may enhance their cold tolerance.

ISSN:0012-9658    

DOI:10.2307/1941552

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In order to characterize the fire regime of the southern boreal forest and to understand the way in which landscape and fire regime interact, a detailed study of fire history was undertaken in two adjacent contrasting landscapes in northwestern Quebec. The fire history for the islands of Lake Duparquet was reconstructed and compared to that of the surrounding lakeshore. Fire occurrence was determined by archival search and by collection of information from the fire scars. Dendroecological techniques were used to determine years of stand initiation on the lakeshore and for a subsample of the islands. Stand initiation data were used to estimate fire frequency, fire cycle, and fire intensity (lethal and non—lethal fires). Tree composition and surficial deposits were sampled in order to assess the possible effect of topography and forest cover on fire frequency. Dated fire scars (n = 273) revealed 82 fire years, with the earliest dated to 1593. Islands experienced more fire years (56) than the lakeshore (37), and fire years were uncorrelated. Most 72%) of the lakeshore stands were initiated by a few large fires (eight), whereas the fire size on islands was variable, ranging from<10% to 100% of the total surface of the island. Despite these differences in fire regime, fire occurrence and fire frequency decreased concurrently on the islands and the lakeshore starting °120 yr ago. The lakeshore fire cycle was 63 yr before 1870 and 99 yr since 1870, whereas the fire cycle on the islands was slightly longer (74 yr and 112 yr) during the same intervals. The long—term decrease in fire frequency of both the lakeshore and the islands was possibly driven by climatic change. Fire frequency was generally similar for all surficial deposits and fuel types on the lakeshore, whereas fires were more abundant in pine woodland growing on bedrock on the islands. Lakeshore fires were generally of lethal intensity, whereas both a long cycle of lethal fires and a shorter cycle of non—lethal fires were observed on 50% of the islands. The morphology and location of islands with both types of fires suggest that a higher frequency of lightning strikes resulted in a regime characterized by more frequent fires and fires of variable intensity. The specific fire regime observed in the island landscape may be the cause of the abundance of pines, whose presence may contribute, in turn, to the continuation of this regime of less intense and more frequent fires. We suggest that fire regime in the boreal forest is controlled by long—term climate change at the regional scale, and by a strong interaction with landscape at the local scale, both components having a great impact on the distribution and the dynamics of boreal vegetation.

ISSN:0012-9658    

DOI:10.2307/1941553

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The study reported here investigates the effects of fire intensity and combustion products (ash and char) on shrub and herbaceous seedling emergence in the field in a mixed southern California chaparral, which was dominated by a facultative seeder, Adenostoma fasciculatum, and by an obligate seeder, Ceanothus greggii. Fire effects were studied in two winter burns, one of which as manipulated by adding brush to experimental plots to increase fire intensity. Adenostoma was very sensitive to increasing fire intensity, with decreasing seedling production as fire intensity increased. This was detected within the normal range of fire intensities of a winter burn, and to a greater extent when intensities were experimentally modified. Ceanothus germination was either stimulated by, or very resistant to, increasing fire intensity. Increasing fire intensity also promoted earlier germination of Ceanothus. Herb species were also differentially affected by increasing fire intensities. Among the abundant herbs, fire annuals like Phacelia brachyloba were more resistant to increasing fire intensity than opportunistic annuals like Cryptantha muricata. Seedling production of one opportunistic annual, Lotus strigosus, was however, stimulated by all levels of increased fire intensity. In most cases, ash and char did not affect field germination. Fire intensity can profoundly modify seedling production of shrubs and herbs after a fire. By substantially altering post—fire seedling populations, fire intensity may represent a major factor in controlling species composition and diversity in chaparral.

ISSN:0012-9658    

DOI:10.2307/1941554

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY