摘要:

The goal of this study was to explore how leaf and canopy level difference interact in determining total canopy productivity. Maximum rates of carbon gain and transpiration were found to be higher in leaves of the deciduous oak quercus lobata than in the co—occurring evergreen oak Quercus agrifolia. However, the efficiency of water and nitrogen use is greater in leaves of Q. agrifolia. A biochemically based model of annual leaf photosynthesis using environmental field data indicates that the evergreen habit in Q. agrifolia partially compensates for the lower photosynthetic capacity of leaves and increases the amount of photosynthate produced per unit water or nitrogen. Simulations indicate that with low N or water availability, the evergreen species can produce canopies with greater annual production than the deciduous species; the converse is true at higher levels of resource supply. Some of the patterns of distribution of these species in northern California may be explained by examining the relationships between source availability and estimated annual production.

ISSN:0012-9658    

DOI:10.2307/1938716

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Two of the central hypotheses of the triangular model of primary plant strategies were tested by a movel technique involving seven grasses of contrasted ecology grown in pure stands and an additive mixture on an experimental matrix of crossed gradients of mineral nutrient stress and vegetation disturbance. The experimental design allowed reductions in vegetative and reproductive vigor resulting from interspecific competition to be distinguished from those arising from direct effects of nutrient stress and vegetation disturbance. It was also possible to determine the extent to which competitive suppression of each species was affected by stress and disturbance. In isolation, all species showed maximum vegetative and reproductive vigor at high soil fertility and low disturbance. In the mixture, absolute reductions in biomass and flowering due to competition were greatest at high soil fertility and low disturbance, and the species of most extreme strategy became restricted to areas of the matrix broadly consistent with those predicted by strategy classification. When standardized for differences in biomass in pure stands, the effect of competition remained relatively constant across the stress—disturbance matrix for all species except Poa annua, which was less restricted by competition at high intensities of stress. There were marked and consistent differences between species in their susceptibility to competition. At both high and low soil fertility, two species of natural occurrence on infertile soils (Festuca ovina, Bromus erectus) were poor competitors relative to Arrhenatherum elatius, a widespread dominant of productive grasslands. The effect of competition was least severe on flowering of annuals in low—stress portions of the matrix. If competition is assessed simply as the percentage of reduction in biomass between pure and mixed stands it appears that competition intensity is constant across different intensities of stress and disturbances. However, observations that maximum reductions in biomass coincided with low stress and low disturbance, that competition decreased in importance as a factor reducing yield and flowering (relative to stress and disturbance) as stress and disturbance intensities increased, and that there was a consistently inferior competitive ability of plants from infertile soils at all positions on the matrix all support the hypothesis that competition declines in importance as a vegetation determinant in the vegetation of infertile soils.

ISSN:0012-9658    

DOI:10.2307/1938717

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Mechanisms of interactions between Douglas—fir (Pseudotsuga menziesii) and red alder (Alnus rubra) seedlings were assessed in experimentally manipulated stands. The density of each species was varied systematically, creating a matrix of competitive regimes that consisted of five monoculture densities and 25 mixtures of all possible pairwise combinations of the monoculture densities. Response surfaces for growth rates, leaf area, photon flux density, soil moisture content and depletion, and plant water potential were generated within the matrix. Regression coefficients quantified the effects of species densities on response variables, and correlation analysis yielded insight into interrelationships between variables. Tree performance, leaf area per square metre of ground surface area, resources, and physiological variables were all quantitatively altered by alder density, Douglas—fir density, and the interaction between species densities. Alder was the dominant competitor and overtopped the Douglas—fir. Competition for light was mediated by density effects on the leaf area of each species per square metre of ground surface area. Increasing alder leaf area reduced the light reaching the understory Douglas—fir. In contrast, increasing Douglas—fir leaf areas increased the light penetrating through to the understory conifers, due to Douglas—fir's suppression of alder leaf area per square metre. Soil moisture limitations were also created by increasing the density of both species and resulted in increasingly negative leaf water potentials for both species. Growth rates concurrently declined as plant water stress increased. Response variables were assembled into a conceptual modeling proposing how species density regulated growth through the interactions between resource limitations and impairment of physiological function.

ISSN:0012-9658    

DOI:10.2307/1938718

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Adaptation of plant populations to nutrient—poor environments may follow two essentially different pathways: maximizing the assimilation of nutrients or minimizing the loss of nutrients. Our hypothesis was that plant species that are adapted to nutrient—poor soils can replace species that are characteristic for relatively nutrient—rich habitats by features that enable them to restrict their nutrient losses. We chose Festuca rubra and Arrhenatherum elatius as species to test this hypothesis. In a long—term field experiment in a hayfield it was observed that Festuca was very common and Arrhenatherum was almost absent in the unfertilized plots, whereas after fertilization Festuca disappeared and Arrhenatherum increased strongly. It was hypothesized that differences in nutrient and carbon losses resulting from mowing were responsible for the different success of the two species in the unfertilized plots. A pot experiment was carried out in a greenhouse with clipped and unclipped and with unfertilized and fertilized treatments in a factorial design with four series. In the fertilized treatments Arrhenatherum replaced Festuica; its heavier seeds resulted in more rapid early growth and its more erect architecture enabled it to overtop Festuca after some time. In the two unfertilized treatments Festuca replaced Arrhenatherum, but much more rapidly in the clipped pots than in the unclipped ones. Losses of nitrogen due to clipping were much greater in Arrhenatherum than in Festuca because of the strongly increased allocation of nitrogen to aboveground parts in Arrhenatherum in response to clipping. It was concluded that differences in nitrogen loss resulting from clipping have an important impact upon the competitive balance between plant species under nutrient—poor conditions.

ISSN:0012-9658    

DOI:10.2307/1938719

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Many yeasts in the genus Pichia are killers (yeasts that secrete proteins toxic to other yeasts). The genus is subdivided into complexes comprised of species with similar morphologies, physiological abilities, and percentage of (guanine + cytosine) comprising their nuclear DNA. Here, we test the hypothesis that ecological factors influence the types of toxins produced by members of two Pichia complexes (the Pichia kluyveri and Pichia opuntiae complexes). Members of each complex are separated by geography or host range, but all live in decaying cactus tissue. Similarities among killer toxins were investigated by testing strains from each complex in the laboratory for their ability to kill a standard set of 70 yeasts. Principal components analysis demonstrated that the killer phenotype was constant within a species, but not within the complexes. The probability that a toxin would kill a particular yeast depended on some ecological characteristic of the yeasts tested as sensitives (i.e., the region, host plant, or habitat from which the yeasts were collected). We further tested the hypothesis that ecological factors influence killer phenotype by analyzing within—community patterns of killing. Killer strains from two different cacti were tested for their ability to kill a subset of the yeasts actually collected from the same pockets of rotting cactus tissue from which the killer strains were collected or from rot pockets in neighboring cacti. When the yeasts tested for sensitivity were classified a priori by their probability of interacting with each killer strain, it was shown that the sensitivity to killer toxin was correlated with the probability of interaction. Further, the yeast community composition of a rot pocket depended on the presence or absence of a killer strain in that pocket. We demonstrated that killer toxins could change community composition by growing strains of two sensitive yeasts in the presence of a killer or a non—killer strain of P. kluyveri. The killer strain reached a higher cell density than the non—killer strain in the presence of sensitive yeasts. The density of sensitive yeasts grown with a killer strain was lower than that of a strain grown with a non—killer strain of P. kluyveri.

ISSN:0012-9658    

DOI:10.2307/1938720

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The presence of leaf litter of different depths within a tropical forest creates many different microsites for plant establishment. The amount and distribution of leaf litter within a forest can influence patterns of plant establishment. In this study, we determined the spatial variability in leaf litter in the forest understory, and investigated how different litter depths (bare, 1, 6, and 12 cm) affected the establishment of several tropical tree species in both growth house (sun and shade) and field (gap and understory) experiments in the semideciduous tropical forest of Barro Colorado Island, Panama. The tree species used in this study (Aspidospermum cruenta, Ceiba pentandra, Cordia alliodora, Gustavia Superba, Luehea semmannii, Ochroma pyrimidale) were chosen to represent a range of seed masses and a gradient in the light requirement for establishment of the species. The spatial distribution of leaf litter was not correlated between adjacent sampling points within the forest understory, suggesting that the establishment environment for seedlings, with respect to litter, is highly variable at scales of 1—20 m. The presence of litter affected five of the six species, but the nature and the magnitude of the effect were species specific. The smaller seeded shade—intolerant species had fewer seedlings establishing under leaf litter than on bare ground. The species ranged from strongly nagatively affected (Luehae) to moderately negatively affected (Cordia, Ochroma) to affected only by extreme amounts of litter (Ceiba). The presence of litter influenced Gustavia, one of the larger seeded shade—tolerant species, but did not affect Aspidospermum, the other large seeded species. The effect of litter on Gustavia depended on the light environment. Gustavia had more seedlings establishing under litter in the sun, but the presence of litter had no effect in the shade. Differences among the smaller seeded shade—intolerant species in the amount they were negatively influenced by litter were not correlated with seed mass. Data from our field study were consistent with our growth house results for the shade—intolerant species. Additional data from the field study indicated that these species with similar habitat requirements differed in the developmental stage at which they were affected by the presence of litter. Luehea had fewer seeds germinating under litter while the other two species, Ochroma and Cordia, were affected only after germination. Interspecific comparisons done for each light level and litter depth indicated that the presence of litter caused reversals in the relative ranking of species success. For example, Gustavia preferentially established under relatively deep litter depths in the sun where Luehea could not establish. In conclusion, the presence of litter can potentially increase seedling diversity within the forest by creating heterogeneity in the establishment environment and by causing reversals in species' rankings.

ISSN:0012-9658    

DOI:10.2307/1938721

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

In a montane rain forest a °2500 m in Venezuela we fertilized plots with N, with P, or N and P for 4.5 yr. We measured trunk growth in circumference in control plots and in those fertilized with N and P, and measured litterfall in all four treatments. There were five 12 x 12 m plots per treatment. N was applied at 225 kg°ha—1°yr—1and P at 75 kg°ha—1°yr—1for the first 2 yr, and subsequently N was applied at 150 kg°ha—1°yr—1and P at 50 kg°ha—1°yr—1. Rates of trunk growth in N and P fertilized plots were roughly doubled those in control plots (1.4 vs. 0.6 mm/yr). Total litterfall averaged 4.3 Mg°ha—1°yr11in the control plots. In the 4th yr after fertilization litterfall was significantly higher in plots fertilized with N + P (1.50 g°m—2°d—1) than in control plots (1.00 g°m—2°d—1). There were no significant differences in rates of litterfall between treatments during the first 3 yr after the first fertilization. The changes of rates of litterfall with time were significantly different between treatments. Litterfall in control plots fell by 9% per year, that in P—fertilized plots fell by 5% per year, and litterfall in N— and in N + P—fertilized plots remained approximately constant. Nitrogen concentrations in litterfall were unaffected by fertilization with N or with P. Phosphorus concentrations were significantly increased by P fertilization. We conclude that N is more limiting that P in this forest.

ISSN:0012-9658    

DOI:10.2307/1938722

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

We measured the timing and magnitude of nitrogenase activity and N2fixation by lupines colonizing early successional volcanic sites at Mount St. Helens. Nitrogenase activity (measured by acetylene reduction) in Lupinus lepidus growing at a pyroclastic site exhibited significant diurnal trends, with lowest ethylene evolution rates at night. Nitrogenase activity also followed seasonal trends, with high rates in June, very low levels in August, the dry warm part of the season, and a partial recovery of nitrogenase activity in September after precipitation resumed. A comparison of typical nitrogenase activities measured at several sites suggested that rates of N2fixation were highest in L. lepidus growing at disturbed low N sites. Adult lupine C and N composition also varied during the growing season, with trends correlated with seasonal patterns of nitrogenase activity. Seasonal N2fixation in L. lepidus and L. Latifolius was measured using15N isotope. Both species fixed °60% of their N during the first season of growth with some evidence of preferential allocation to aboveground biomass. N fixation by Lupinus lepidus individuals was ° 18.1 mg/g biomass or an average of 15.4 mg per plant, while L. latifolius fixed an average of 16.3 mg/g biomass, equivalent to 22.9 mg per plant. Average net C fixation during the same period was 355 and 589 mg per plant for L. lepidus and L. latifolius, respectively. Despite these rates, the current distribution of L. lepidus into a few, small patches that occupy<1% of the surface area indicates that annual N inputs by lupines are<0.05 kg/ha and thus probably not the primary source of N input into developing Mount St. Helens pyroclastic sites except at a local scale.

ISSN:0012-9658    

DOI:10.2307/1938723

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

To understand the factors that structure ectomycorrhizal—basidiomycete communities at a local scale, we measured the strength of the relations among the fungal communities, the tree communities, and the environment of a series of forest ecosystems in southern Quebec. We collected fruit bodies belonging to ectomycorrhizal—basidiomycete families and genera, sampled the woody vegetation, and described soils and landforms at 11 permanent sampling stations. We first calculated similarity matrices among stations, one for each descriptor (fungal species abundance, woody species abundance, and abiotic variables). We then explored the dependence among these matrices using Mantel and partial Mantel tests, path analysis, and comparisons of ordinations and classifications. Similarity among ectomycorrhizal fungus communities was strongly and significantly correlated with tree community similarity, even when controlling for the effect of environmental similarity. When the tests were made with a similarity matrix based on those tree species that are known to be hosts of ectomycorrhizal fungi, abiotic similarity explained a significant portion of the residual variation in the similarity among fungus communities. To explore this complex relationship further, we analyzed species associations. The preference of fungus associations for different sets of abiotic conditions showed that some factors affecting fungal species distribution were different from those affecting the distribution of their tree hosts. Direct and indirect gradient analyses showed that humus characteristics seemed to be important niche dimensions of ectomycorrhizal fungi. The continuum concept was useful to interpret the complex relations among symbiotic species. Trees were the main component of the realized niche of ectomycorrhizal Basidiomycetes, but the fungal symbionts of a particular tree species followed that tree species for only a part of the abiotic gradients over which the host tree was found. This type of distribution predicts that beta diversity of fungi would be generally higher than beta diversity of ectomycorrhizae—forming trees. It also means that the ratio of fungus species richness to woody species richness would be high for most community gradients. Our results and those of previous mycosynecological studies agree with these predictions. The results have implications for the conservation of biodiversity: site selection for conservation based on vegetation classification or mapping, or on distribution of tree species, may miss important fungal species.

ISSN:0012-9658    

DOI:10.2307/1938724

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Assimilation of soil organic matter and fresh plant debris by Millsonia anomala, a tropical geophagous earthworm, was investigated by measuring changes in the13C/12C ratio of their tissues when fed on organic matter naturally labelled by13C. Individuals collected from a soil colonized by C4plants (C4soil) that were fed on C3soil had>50% of tissue C derived from the C3soil after 33 d. Assimilation of organic matter associated with different particle size fractions was investigated with individuals fed for 25 d on C4soil with C4particle size fractions substituted in turn for the equivalent C3soil particle fraction. A significant labelling of earthworms was observed with both the 250—2000 μm and the 0—20 μm particle size fractions. Addition of fresh C3plant debris to the C4soil also resulted in a significant change in earthworm C isotope ratio. Assimilation of fresh plant material was greater than that of soil organic matter. This shows that young M. anomala are able to assimilate young organic matter (fresh plant debris, coarse soil organic matter) as well as fine soil organic matter, both of which classically have been regarded as strongly resistant to decomposition in models of soil organic matter dynamics.

ISSN:0012-9658    

DOI:10.2307/1938725

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY