摘要:

To assess the diversity of tropical tree life histories, a conceptual framework is needed to guide quantitative comparative study of many species. We propose one such framework, which focuses on long—term performance through ontogeny and over the natural range of microsites. For 6 yr we annually evaluated survival, growth, and microsite conditions of six non—pioneer tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. The species were: Lecythis ampla, Hymenolobium mesoamericanum, Dipteryx panamensis, Pithecellobium elegans, Hyeronima alchorneoides (all emergents), and Minquartia guianensis (a canopy species). The study was based on long—term measurement of individuals from all post—seedling size classes. Trees were sampled from 150 ha of primary forest spanning several watersheds and soil types. To evaluate individuals' microsites we recorded the number of overtopping crowns, forest phase (gap, building, mature), and crown illumination index (an estimate of the tree's light environment). For comparison, we also evaluated the microsites of three species that have been categorized as pioneers (Cecropia insignis, C. obtusifolia) or high—light demanders (Simarouba amara). For the six species of non—pioneers, mortality rates declined with increasing juvenile size class. As a group, these emergent and canopy trees showed a much lower exponential annual mortality rate (0.44%/yr at>10 cm diameter) than has been found for the La Selva forest as a whole. Growth rates increased with juvenile size class for all six species. As adults (trees>30 cm in diameter), all five emergent species showed substantial annual diameter increments (medians of 5—14 mm/yr). Small saplings and adults of all species had significant year—to—year variation in diameter growth, with much greater growth occurring in the year of lowest rainfall. Passage time analysis suggests that all six species require>150 yr for growth from small saplings to the canopy. Evaluation of all nine species revealed four patterns of microsite occupancy by juveniles. Among the non—pioneers, one species pair (Lecythis and Minquartia: Group A) was associated with low crown illumination and mature—phase forest in all juvenile stages. For two species (Dipteryx and Hymenolobium: Group B) the smallest saplings were in predominantly low—light, mature—forest sites, but crown illumination and association with gap— or building—phase sites increased with juvenile size (Simarouba also showed this pattern). Two species (Pithecellobium and Hyeronima: Group C) were strongly associated with gap or building phase as small juveniles (≤4 cm diameter) and again as subcanopy trees (>10—20 cm diameter), but were predominantly in mature—phase sites at intermediate sizes. Juveniles of the two pioneer species (Cecropia: Group D) showed the highest crown illumination and association with gap or building sites. Among the six non—pioneer species, only one aspect of juvenile performance clearly varied according to microsite group. The smallest saplings (≤1 cm diameter) of Groups B and C showed significant mortality differences across a small gradient in crown illumination; neither of the Group A species showed this pattern. Otherwise, juvenile performance was strikingly similar among the six species. All showed a capacity for growth responses to small increases in light, substantial height and diameter increments at higher light levels, equal ability to survive 4—yr periods of no growth, and very low mortality rates at intermediate—to—large juvenile sizes. Species differed significantly in growth rates, but relative differences shifted with tree size and were unrelated to microsite group. These findings do not support prevailing paradigms concerning trade—offs and correlated suites of traits. For non—pioneer tropical trees, life history classification based on generalized concepts such as gap dependence and shade tolerance is inadequate to describe the complex size—dependent patterns of life history differences and similarities that exist among species.

ISSN:0012-9615    

DOI:10.2307/2937114

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Stage—classified matrix models are important analytical and theoretical tools for the study of population dynamics; in particular, these models may be appropriate for populations in which survivorship and fecundity are dependent on size or developmental stage, populations in which the age of individuals is difficult to determine, and populations in which there are multiple types of newborns. Nevertheless, methods for analyzing the implications of a population's stage—transition matrix have been limited in comparison to methods available for age—structured models (life tables or Leslie matrices). In this paper we show that all of the standard age—based measures of life history traits can be derived from a stage—transition model. By decomposing the transition matrix into separate birth, survival, and fission matrices we derive simple, direct formulas for age—based life history traits such as the discrete survivorship function, lx, maternity function, fx, mean age at maturity, and net reproductive rate, Ro, and also population parameters, including the stable age distribution, age—specific reproductive value, and generation time. These provide a common set of parameters for comparing age—structured and stage—structured populations or comparing populations with differently structured life cycles. In addition, we define four measures of age and life—span that summarize the relationship between stage and age in a stage—structured population: age distribution and mean age of residence for each stage class, expected remaining life—span for individuals in each stage class, and total life—span conditional on reaching a given stage class. We illustrate the use of our methods to address specific ecological questions by applying them to several previously published demographic data sets. These questions include: (1) what are the demographic effects of crowding on the tropical palm Astrocaryum mexicanum?; (2) how important is the initial rosette size in determining life history of teasel, Dipsacus sylvestris?; and (3) how old are reproducing adults in a stage—classified population of pink lady's—slipper, Cypripedium acaule? Our results may also be useful for evaluating the adequacy of a given stage—transition model.

ISSN:0012-9615    

DOI:10.2307/2937115

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Variation in leaf life—span has long been considered of ecological significance.Despite this, quantitative evaluation of the relationships between leaf life—span and other plant and ecosystem characteristics has been rare. In this paper we ask whether leaf life—span is related to other leaf, plant, and stand traits of species from diverse ecosystems and biomes. We also examine the interaction between leaf, plant, and stand traits and their relation to productivity and ecological patterns. Among all species, both mass— (Amass) and area—based (Aarea) maximum net photosynthesis decreased with increasing leaf life—span, but the relationship was stronger on a mass (P<.001, r2= 0.70) than an area (P<.05, r2= 0.24) basis. Similarly, mass—based leaf nitrogen (leaf Nmass) decreased (P.25, r2= 0.01). Specific leaf area (SLA, leaf area/leaf dry mass) and leaf diffusive conductance also decreased with increasing leaf life—span. Decreasing Amasswith increasing leaf life—span results from the impact of decreasing Nmassand SLA on Amass. Variation in leaf traits as a function of leaf life—span was similar for broad—leaved and needle—leaved subsets of the data. These leaf—scale data from several biomes were compared to a data set from a single biome, Amazonia. For several leaf traits (e.g., SLA, Nmass, and Amass) the quantitative relationship with leaf life—span was similar in the two independent data sets, suggesting that these are fundamental relations applicable to all species. Amasswas a linear function of Nmass(P .001, r2= 0.74) with a regression similar to previous analyses, while Aareawas not significantly related to Narea. These results suggest that the photosynthesis—leaf N relationship among species should be considered universal when expressed on a mass, but not on a leaf area, basis. Relative growth rates (RGR) and leaf area ratio (LAR, the whole—plant ratio of leaf area to total dry mass) of seedlings decreased with increasing leaf life—span (P<.001, r2= 0.61 and 0.89, respectively). LAR was positively related to both RGR and Amass(r2= 0.68 and 0.84, respectively), and Amassand RGR were also positively related (r2= 0.55). Absolute height growth rates of young trees decreased with increasing leaf life—span (P<.001, r2= 0.72) and increased with Amass(P<.001, r2= 0.78). It appears that a suite of traits including short leaf life—span and high leaf Nmass, SLA, LAR, and Amassinteractively contribute to high growth rates in open—grown individuals. These traits interact similarly at the stand level, but stands differ from individuals in one key trait. In closed—canopy forests, species with longer lived foliage (and low LAR as seedlings) have greater foliage mass per unit ground area (P<.001, r2= 0.74) and a greater proportion of total mass in foliage. The aboveground production efficiency (ANPP/foliar biomass) of forest stands decreased markedly with increasing leaf life—span or total foliage mass (P<.001, r2= 0.78 and 0.72, respectively), probably as a result of decreasing Amass, Nmass, and SLA, all of which were positively related with production efficiency and negatively related to total foliage mass. However, high foliage mass of species with extended leaf life—spans appears to compensate for low production per unit foliage, since aboveground net primary production (ANPP, in megagrams per hectare per year) of forest stands was not related to leaf life—span. Extended leaf life—span also appears to compensate for lower potential production per unit leaf N per unit time, with the result that stand—level N use efficiency is weakly positively related to leaf life—span. We hypothesize that co—variation among species in leaf life—span, SLA, leaf Nmass, Amass, and growth rate reflects a set of mutually supporting traits that interact to determine plant behavior and production, and provide a useful conceptual link between processes at short—term leaf scales and longer term whole plant and stand—level scales. Although this paper has focused on leaf life—span, this trait is so closely interrelated with several others that this cohort of leaf traits should be viewed as casually interrelated. Generality in the relationships between leaf life—span and other plant traits across diverse communities and ecosystems suggests that they are universal in nature and thus can provide a quantitative link and/or common currency for ecological comparisons among diverse systems.

ISSN:0012-9615    

DOI:10.2307/2937116

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The extent to which insect communities are organized is poorly understood because few studies have examined both population—level processes and community—level patterns. Furthermore, our view of phytophagous insect populations is probably biased by the more frequent attention given to economic pests whose dynamics might be expected to be somewhat exceptional. Herein, we report on the population dynamics and community—level features in a diverse, native fauna of phytophagous insects associated with goldenrod (Solidago altissima), a native perennial plant that forms a dominant, long—persisting element in old—field succession. The data consist of censuses taken in six consecutive years at 16 old fields in the Finger Lakes Region of New York; five additional stands were followed for four consecutive years. Our analyses address two questions: (a) to what extent is the functional structure of the community (as reflected in such attributes to the herbivore load, guild spectrum, and dominance hierarchy) maintained by compensatory changes in the densities of the member species and (b) are certain traits associated with a species' tendency to fluctuate in density or to dominate the community? In general, populations of goldenrod insects fluctuated less than those of insects reported in the literature. Few were abundant; only 7 of the 138 species in the goldenrod fauna ever reached densities at which their biomass exceeded 0.1% of the leaf biomass. The functional structure of the community, as reflected in the total herbivore load and the proportions of that load contributed by various guilds, varied widely in space and time. A small subset of the fauna remained dominant throughout the study; these species, whose populations occasionally irrupted, retained their high rank in relative abundance even during those periods when their populations were waning. There was no evidence for compensatory changes in the densities of species within guilds, as would be expected if a relatively distinct set of limiting resources were available to insects with different feeding styles. Species were no more likely to exhibit negative correlations with their guild mates than they were with members of other guilds. There was also little evidence of sporadic or diffuse competition within the fauna as a whole; even during population outbreaks, the dominant species rarely engendered decreases in the biomass or diversity of the remaining species, and increases in the total herbivore load were seldom associated with losses of species from the community. Several species were positively associated with one another on the basis of similar habitat requirements or the use by one species of conditions created by the presence of another. The community was predictable only in the sense that dominant species remained dominant and rare species rare. The abundance and population variability of species were not correlated with such traits as body size, generation time, or host range. Taxon—related trends, on the other hand, stood out; coleopterans tended to be abundant whereas lepidopterans were rare, and hemipterans tended to fluctuate widely whereas dipterans remained relatively constant. Furthermore, species that were clumped in space (onto stems within fields) tended to be abundant and variable. The "boom and bust" dynamics of these aggregating species may be related to the ability of colonies to survive the inroads of generalist predators. The low degree of regional synchrony in the performance of populations at different sites suggests that large—scale, weather—driven fluctuations are uncommon. Our analysis of this well—developed, native insect fauna reveals a community that has a somewhat predictable structure that reflects the idiosyncracies of the component species. These species do not fluctuate in an integrated community—structuring manner, but as an outcome of each species performing at its "individualistic" level, the community displays a characteristic dominance hierarchy. The members of this diverse fauna rarely achieve densities at which they compete with one another. Our results are consistent with Hairston, Smith, and Slobodkin's hypothesis (1960) which predicts that terrestrial herbivores rarely deplete plant resources to levels at which there is widespread interspecific competition.

ISSN:0012-9615    

DOI:10.2307/2937117

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

This paper addresses questions of community and patch stability as defined by the population biology of dominant plants in the context of different areas within a large kelp forest. We ask (1) "Do large—scale episodic events override biological mechanisms as major community structuring processes?", (2) "Are different local areas characterized by different processes?", and (3) "How persistent are the patches or biological structure over decal and local spatial scales?" We evaluate these questions with regard to the effects of various types of disturbance for as much as three decades on the populations of several species of kelp in the large kelp forest off Point Loma, San Diego, California. The most sensitive population factors we studied include recruitment, density, and survivorship. Patch stability was evaluated with regard to the persistence of patches already well established in 1971—1972. The study sites offer a cross—shore transect through the central part of a large kelp forest at depths of 8, 12, 15, 18, and 21 m; two additional sites at the north and south ends of the forest offer a longshore transect along the 18—m contour. There were marked differences among the decades with regard to the intensity of the disturbances. Compared with the 1980s, the two preceding decades were relatively benign. The 1980s had two extreme disturbance events: the 1982—1984 El Gino—Southern Oscillation (ENSO) was the most severe El Nino event in the last century, which included very warm, nutrient—depleted water, and a short but intense storm in January 1988 appeared to have been the most severe in perhaps 200 yr. The storm changed age—specific kelp mortality patterns and caused the first large—scale understory mortality in several decades. By sweeping away drift algae it caused intense local urchin grazing. The storm was followed by a strong La Nina event marked by cool, nutrient—rich water in 1988—1989. Differences in kelp recruitment and survivorship in different areas of the kelp forest are influenced by gradients in longshore currents, temperature, light, wave energy, floc, planktonic propagules, and physical disturbance. The areas are characterized by different plant population patterns and the effects of several species of herbivores. The massive disturbances of the 1980s obliterated much of the structure in the kelp forest. Certainly the disturbances caused many lag effects including outbreaks of understory algae such as Desmarestia ligulata, intraspecific competition, changes in grazing patterns, etc., which in turn resulted in between—area variation in recovery rates. However, in all cases this variation was overshadowed by the overwhelming competitive dominance of Macrocystis pyrifera. Most of the understory patches on the transect lines, some of which had persisted for 7 yr, died out by the end of 1990. The population biology of Macrocystis was remarkably similar in most areas, as the cohort longevity and survivorship curves were very similar, and the plant and stipe densities tended to level off in only a few years. Thus large—scale episodic events such as El Ninos, La Ninas, and rare storms exert dramatic impacts, but small—scale responses such as density—vague recruitment (neither density dependent nor density independent) and survival allow prompt recovery, often to preexisting patterns. The one exception was the southern site, which was marked by sea urchin grazing and poor kelp recruitment through the latter half of the 1980s, but a recent sea urchin disease event has led to kelp recruitment in fall 1991. A seeming paradox to the observed Macrocystis dominance is that in almost all areas, some understory patches of old plants have persisted through the 1980s.

ISSN:0012-9615    

DOI:10.2307/2937118

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY