摘要:

We studied how an enriched CO2atmosphere, in a fully crossed design of light nutrients, influenced 1st—yr seedling growth in six New England deciduous forest tree species. The species, in the order of increasing shade tolerance, were gray birch (Betula populifolia), ash (Fraxinus americana L.), red maple (Acer rubrum L.), Red Oak (Quercus rubra L.), yellow birch (Betula alleghaniensis Britton), and striped maple (Acer pensylvanicum). Elevated CO2environments significantly stimulated the seedling growth of all six species. Generally this was more pronounced in low light. The greatest stimulation was found under the condition of low light and high nutrients. However, individual species responded differently to elevated CO2levels. Among the three early—successional species, gray birch, ash, and red maple, a significant increase in seedling growth under elevated CO2conditions was found only with high nutrients. The three late—successional species grown under elevated CO2conditions (red oak, yellow birch, and striped maple) showed a greater percentage increase in seedling growth in low light than in high light. Thus, for the early—successional species, the degree of enhancement of seedling growth by elevated CO2levels was more sensitive to nutrient levels, while in the late—successional species the enhancement was more sensitive to the level of light. Moreover, species with large seeds (e.g., red oak) exhibited a greater response to elevated CO2levels under low light than species with small seeds (e.g., gray birch). The results emphasize the importance of plant species as well as other environmental resources in modifying the response of plants to elevated CO2. Considering the light and nutrient environment observed in forest gaps of various sizes, the results of the present experiment suggest seedling regeneration in New England deciduous forests may be altered in a future high CO2environment.

ISSN:0012-9658    

DOI:10.2307/1939505

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

In full sunlight (photosynthetic photon flux density, 0.4—0.7. μm, PPFD), stomatal conductance to water vapor (g) and transpiration rates (E) are usually higher, and water use efficiency lower, in C3than C4species. In the C3grass Bromus inermis, the difference in conductance between full sun (photosynthetic photon flux [PPFD]>1500 μmol°m—2°S—1) and shade (PPFD ° 400 μmol°m—2°s—1) was twice as large as in the C4species Sorghastrum nutans (226.7 vs. 97.8 mmol.m—2.S—1, respectively). Similar differences were measured in four additional C3and C4grasses. Variability in PPFD incident on leaves is very common in grasslands, and stomata are known to respond relatively rapidly to fluctuations in this resource. This study evaluated the consequences of different responses in stomatal conductance between C3and C4grasses during periods of sunlight variability. Photosynthesis (A, measured as CO2uptake) and g varied substantially in both B. inermis and S. nutans as PPFD fluctuated between sun and shade levels (4—10 min intervals). Time constants derived form first—order equations describing responses in g after abrupt changes in PPFD indicated that g approached steady—state levels more rapidly in S. nutans than in B. inermis. However, because of the greater magnitude of change in g between sun and shade PPFD in B. inermis, the absolute rate of change in g in this C3grass was twice as rapid compared to S. nutans (25.7 vs. 12.4 mmol°m—2°s—1°min—1for reductions in g, respectively). New steady—state levels in g were achieved in 12—13 min after a change in PPFD in S. nutans and 15—19 min in B. inermis. A statistically based two—state (sun/shade) simulation model was used to quantify costs in A and E of the nonsteady—state responses measured for both species as PPFD varied. Costs were calculated as the difference between model output of A or E based on field responses and model output based on hypothetical gas exchange responses in which A and g changed instantaneously. Results suggested that even though the absolute rate of change in g changed was greater in the C3grass, costs in E during periods of short—term (2—6 min) variations in PPFD were as much as sixfold higher than in the C4species. Lower cost in E in C4species during periods of variable PPFD (due to inherently smaller alterations in g) is a previously unrecognized consequence of the C4photosynthetic pathway. C4species, and grasses in particular, often are most successful in high—PPFD, water—limited environments in which leaf—level variability in PPFD occurs daily from cloud cover and within—canopy shading. In these environments, stomatal responses that conserve water under nonsteady—state conditions may contribute significantly to the success of C4species.

ISSN:0012-9658    

DOI:10.2307/1939506

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Nutrient resorption from senescent leaves, by minimizing nutrient losses, could be just as important as uptake in determining nutrient balance, especially in low—nutrient environments. To examine the relative importance of phenotypic and genotypic factors in controlling nutrient resorption, we sampled several meditterranean trees and shrubs growing in serpentine and nonserpentine soils in northern California. Plants growing in serpentine soils had smaller, thicker leaves than those growing in richer sites and had lower N and P concentrations and pools on a unit—area basis. Pools of nutrients resorbed and nutrients left in litter in rich—site leaves were larger than pools in poor—site leaves, but N resorption efficiency (percent of total leaf N resorbed) was higher in less fertile sites (65 vs. 46%). Differences in efficiency may be due to the ratio of soluble/nonsoluble proteins, which is higher at low—N sites. Trends for phosphorus resorption efficiency were not significant (47 vs. 46% in poor and rich sites, respectively). There were no significant differences among species adapted to poor vs. rich soils, nor between needle vs. broad—leaved species. We conclude that a high nutrient resorption efficiency is a phenotypic response to low—nutrient environments.

ISSN:0012-9658    

DOI:10.2307/1939507

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

While much is known about control of production of NO and N2O by nitrifying and denitrifying bacteria at the cellular level, application of this knowledge to field studies has not yielded unifying concepts that are widely applicable and that foster understanding of global sources of these atmospheric trace gases. We applied a simple conceptual model to the investigation of sources of NO and N2O and the environmental factors affecting fluxes in a drought—deciduous forest of Mexico. Fluxes of NO and N2O were higher in the wet season than the dry season, but addition of water to dry soil caused large pulses of CO2, NO, and N2O emissions. Immediate increases of extractable soil NH4+and high rates of gross N mineralization and gross nitrification also were observed following wetting of dry soil NO2—had accumulated during the dry season, and that NO2—plus the pulse of increased soil NH4+were mostly consumed within 24 hours of wetting. This dynamic microbial processing of soil inorganic N coincided with the pulses of NO and N2O production following wetting of dry soil. Acetylene inhibition experiments indicated that NO production was dependent on nitrification, that nitrification was the dominant source of N2O when the soil was wetted at the end of the dry season, and that dentrification might be an important source of N2O during the wet season. Post—wetting soil moisture was correlated negatively with NO fluxes and positively with N2O fluxes. These results support a conceptual model in which N trace gas production is generally constrained by the rates of N mineralization and nitrification, while the specific ratios of NO and N2O fluxes and the contributions from nitrifying and denitrifying bacteria are controlled largely by soil moisture.

ISSN:0012-9658    

DOI:10.2307/1939508

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Unprecedenteded rates of deforestation and biomass burning in tropical dry forests are dramatically influencing biogeochemical cycles, resulting in resource depletion, declines in biodiversity, and atmospheric pollution. We quantified the effects of deforestation and varying levels of slash—fire severity on nutrient losses and redistribution in a second—growth tropical dry forest ("Caatinga") near Serra Talhada, Pernambuco, Brazil. Total aboveground biomass prior to burning was °74 Mg/ha. Nitrogen and phosphorous concentrations were highest in litter, leaves attached to slash, and fine wood debris (<0.64 cm diameter). While these components comprised only 30% of the prefire aboveground biomass, they accounted for °60% of the aboveground pools of N and P. Three experimental fires were conducted during the 1989 burning season. In these treatments consumption was 78, 88, and 95% of the total aboveground biomass. As much as 96% of the prefire aboveground N and C pools and 56% of the prefire aboveground P pool was lost during combustion processes. Nitrogen losses exceeded 500 kg/ha and P losses exceeded 20 kg/ha in the fires of the greatest severity. With increasing fire severity, the concentrations of N and P in ash decreased while the concentration of Ca increased. This indicates greater amounts of these nutrients were volatilized (i.e., greater ecosystem losses occurred) with increasing fire severity. Following fire, up to 47% of the residual aboveground N and 84% of the residual aboveground P were in the form of ash, which was quickly lost from the site via wind erosion. Fires appeared to have a minor immediate effect on total N, C, or P in the soils. However, soils in forests with no history of cultivation had significantly higher concentrations of C and P than second—growth forests. Based upon the measured losses of nutrients from these single slash—burning events, it would likely require a century or more of fallow for reaccumulation to occur. However, current fallow periods in this region are 15 yr or less.

ISSN:0012-9658    

DOI:10.2307/1939509

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We studied the influence of litter quality on the decomposition rate of leaves from nine Mediterranean shrubs and trees using litter bag methods for a 2—yr period at two ecosystems in southwest Spain. Linear and nonlinear regressions were calculated between mass loss and the concentrations of the major organic and inorganic constituents of leaves (lignin, cutin, cellulose, tannins, crude fat, soluble carbohydrates, nitrogen, and phosphorus) and between mass loss and leaf toughness in order to determine the best predictor of leaf litter decomposition in these ecosystems. In addition, ratios between some of the litter quality parameters were examined as mass loss predictors. Loss of soluble components from leaves was used to define two phases of decomposition: a leaching phase that lasted 2—4 mo and a postleaching phase. Leaf toughness and the ratio of toughness: phosphorus concentration were the best indicators of mass loss during the leaching phase in both ecosystems, and cutin: nitrogen or cutin: phosphorus ratios were the best predictors of mass loss in the postleaching phase, but only in the drier and more nutrient—poor ecosystem. When the two phases were combined, leaf toughness, toughness: nitrogen, and/or cutin: nitrogen significantly explained the mass loss in both ecosystems.

ISSN:0012-9658    

DOI:10.2307/1939510

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Long—term studies are required for an understanding of how temporal variation and space—time interactions affect the structure of communities. Here we report on a long—term study of the independent effects of, and the interactions among, two sources of temporal variation (seasonal and annual) and two sources of spatial variation for a rocky intertidal community in the northern Gulf of California. The sources of spatial variation were: (1) microspatial effects due to the foraging patterns of a common predatory snail (Acanthina angelica) and (2) macrospatial effects due to differences among sites. The results from semiannual samples of 100—cm2quadrats showed highly significant temporal and spatial effects for all members of the sessile community (barnacles, mussles, algae) and for limpets over the 8—yr study period. There were also highly significant season X space interactions for all sessile members of the community, which probably resulted from seasonal settlement by the sessile members of the community, and aestivation by the predator. Finally, we observed highly significant year X space effects as well year X season X space effects for most species. These latter interactions can be understood as an amplification of seasonal and spatial effects due to the largely unpredictable differences among years. An Analysis of the variance components showed that most of the variation in percentage cover of barnacles and a brown encrusting alga was due to microspatial effects, while most of the variation in mussels, limpets, and a green alga was due to year and season effects. This combination of results suggests that competition and predation by Acanthina are relatively more important in controlling the distributions and local abundances of barnacles and encrusting algae, and that unpredictable differences among years in settlement are more important in controlling the local population densities of mussels and limpets. The importance of these differences are discussed in relation to interpretation of short—term experimental studies in population and community studies.

ISSN:0012-9658    

DOI:10.2307/1939511

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Interspecific competitors often colonize communities at different times, but few studies have experimentally tested whether the strength of interspecific competition hinges on the order or temporal separation of species' arrivals. We added hatchlings of two sympatric anuran species (Hyla[=Pseudacris] crucifer and Bufo woodhousii) to artificial ponds on three different dates to manipulate the order and temporal separation of their arrival. Measurement of the growth and survival of each species in ponds where the second species arrived simultaneously (day 0), or after a delay of 7 or 14 d, indicated whether differences in the history of introductions affected interspecific competition. Other ponds contained H. crucifer alone, introduced on the same three dates, of B. woodhousii alone, introduced on the first and second date. These provided controls for seasonal differences in performance in the absence of competition from the other anuran species. Introductions of tadpoles at different times produced small differences in growth rates and larval periods when each anuran species occurred alone. In ponds containing both species, differences in the order and temporal separation of introductions had complex effects on the intensity of interspecific competition. When Bufo preceded Hyla, Hyla had prolonged larval periods and reduced mass and growth. When Hyla preceded Bufo, it was unaffected by Bufo. Hyla had no effects on Bufo, whether its introduction preceded or followed the introduction of Bufo. Asymmetric priority effects were not easily explained as a simple size—dependent competitive advantage of earlier arrivals. Early arrival increased the competitive impact of Bufo on Hyla, but failed to generate a competitive effect of Hyla on Bufo. Bufo tadpoles are more active than Hyla, and so may consume resources at higher rates that cannot be offset by a temporal advantage. Competition from Bufo was strongest when Hyla arrived 7 d later, suggesting that Hyla arriving 14 d after Bufo benefited from a longer period of reduced competition after Bufo metamorphosed from the ponds. The natural pattern, where Hyla breeds before or simultaneously with Bufo, permits Hyla to minimize competition from Bufo, while Bufo suffers no measurable cost from the size advantage obtained by competitively weaker Hyla tadpoles. Bufo may be prevented from breeding any earlier in the season by physiological constraints acting on eggs or breeding adults.

ISSN:0012-9658    

DOI:10.2307/1939512

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

When species differ in their ability to cope with varying abiotic conditions, a change in an abiotic factor can potentially alter the outcome of biotic interactions. The purpose of this study was to examine this phenomenon. We tested the effects of an abiotic factor (pH) and two biotic factors (intraspecific and interspecific competition) on survival and growth to metamorphosis by larvae of two anurans (Hyla gratiosa and Hyla femoralis) in outdoor tanks. Two levels of pH (4.5 and 6.0) and three levels of density (0, 30, or 60 embryos per 580 L tank) of each species were arranged in a fractional factorial design and replicated threes times. Larvae of H. gratiosa exhibited typical patterns of intraspecific density—dependent responses: as density increased, survival and size at metamorphosis decreased and larval period increased. The interactions of the abiotic factor and the biotic factors were complex in this experiment. The addition of H. femoralis at high pH caused decreased survival and increased larval period of H. gratiosa, but at low pH these negative effects were not detected. Variation in pH had no effect on the size at metamorphosis of H. gratiosa. Interspecific competition from H. femoralis caused decreased size at metamorphosis of H. gratiosa only at the high initial densities. Some of the responses of H. femoralis were different than those of H. gratiosa. In this species, intraspecific competition affected only the size at metamorphosis. Low pH increased the survival of H. femoralis, but had no effect by itself on any other trait. Increased conspecific density and pH interacted only in their effect on the size at metamorphosis of H. femoralis. Interspecific competition resulted in decreased size at metamorphosis but only increased larval period at the high initial densities. Interspecific competition at the high density interacted with pH to increase survival and size at metamorphosis at low pH in contrast to those results at high pH. Overall, the larval period was the response most strongly affected in H. gratiosa while for H. femoralis the size at metamorphosis was the most strongly affected. Hyla gratiosa had more negative impact on H. femoralis than vice versa. This may be due to the larger body size of H. gratiosa. Low pH enabled H. femoralis to compete more effectively, because the negative impact of H. gratiosa was lessened. Even in a relatively simple simulation, complex interactions occurred between biotic and abiotic factors. This finding demonstrates the importance of multi—factor analysis in furthering our understanding of the structure of assemblages.

ISSN:0012-9658    

DOI:10.2307/1939513

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

I manipulated relative body size of mussels (Mytilus californianus) and gull (Larus glaucescens) predation pressure within gaps in a mussel bed to determine (1) whether size differences could cause shifts in competitive dominance between goose barnacles (Pollicipes polymerus) and Mytilus, species with indeterminate growth, and (2) if the late recruiting Mytilus required gull predation on established adult Pollicipes to attain dominance. Increasing relative body size enhanced the area and the rate of change in area covered by both Pollicipes and Mytilus. Preventing gull feeding enhanced Pollicipes and reduced Mytilus cover, slowing the rates of succession and reducing the frequency of disturbance. However, even in the absence of predators on Pollicipes and with an initial size disadvantage, Mytilus maintained positive rates of change in cover, probably because adult recruitment into the gap altered the initial size hierarchy of competitors. These results indicate that bird predation and initial size asymmetries can alter the dynamics, but not the eventual end point, of intertidal succession. They also highlight the importance of choosing the right comparison (differences between treatments vs. deviations in rates from 0) when testing alternative modes of succession (i.e., facilitation, tolerance, or inhibition). Although gull predation does not change the end point of succession locally, it may have important regional effects by altering the dynamics of disturbance and recovery. Such regional effects cannot be determined by experiments on a local scale.

ISSN:0012-9658    

DOI:10.2307/1939514

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY