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AbstractWe examine the influence of overstory trees and abiotic environmental factors on the patterns of spatial variation and species composition in the sapling community of an old-growth Fagus-Acerforest in southwestern Québec, Canada. Our main focus was to identify differences in the sapling distribution patterns of Fagus grandifoliaand Acer saccharum, the two codominant species in the overstory, as well as the factors that determine the differences. Using canonical correspondence analysis (CCA), we show that soil moisture has the strongest influence on the spatial variation and species composition of the sapling community. Acer saccharumoccurred on a wide range of soil moisture conditions, while Fagus grandifoliasaplings were absent in dry habitats. Another factor that differentiates the distribution patterns of Fagus grandifoliaand Acer saccharumsaplings is the relative dominance of Fagus grandifoliatrees in the overstory, which correlates negatively with pH and Ca availability in the forest floor. Acer saccharumsaplings were not found on sites where Fagus grandifoliatrees dominate the overstory, while Fagus grandifoliasaplings are mostly limited to sites where Fagus grandifoliatrees have high representation in the overstory. These findings are discussed in light of previous hypotheses on canopy tree replacement patterns in Fagus-Acerforests.

AbstractWe grew 40 commonly co-occurring species of wetland herbs from eastern North America under uniform conditions to evaluate the overall pattern of interspecific variation in specific leaf mass (SLM), foliar nitrogen content, stomatal conductance (g), and internal leaf CO2concentration (ci). While the relationship between any two of these traits that influence net photosynthetic rate is constrained to some degree, there is sufficient flexibility to allow the evolution of different but more or less equally effective interrelationships among these central elements of leaf form and function. We use contemporary techniques of structural equation modelling to describe the general nature of such evolutionary diversification in leaf form and function among these wetland plants. Our model essentially extends the Cowan-Farquhar model of stomatal regulation to include relationships between SLM and foliar nitrogen. The model can take two forms, with variables expressed as either per unit leaf area or per unit leaf mass. When variables are expressed on an areal basis, the model predicts that a species with a higher SLM will have a higher foliar nitrogen level. The foliar nitrogen level, in accordance with the Cowan-Farquhar model, in turn determines the dynamics of stomatal regulation in relation to the marginal cost of water loss relative to carbon gain. The dependence of stomatal regulation on foliar nitrogen also determines the maximal rates of stomatal conductance and net photosynthesis. Internal CO2concentrations within the leaf follow as a necessary consequence of these interrelationships. This areal-based model describes the data for the 35 C3wetland species well; the same basic model applies to the five C4species in our sample, except for shifts in the quantitative effects of net photosynthetic rate and stomatal conductance on internal CO2levels. When the variables are expressed on a mass basis, a slightly different model results, as net photosynthetic rate decreases directly with SLM and is not related to species level variation in either leaf nitrogen concentration or maximal stomatal conductance. Both forms of the model indicate the need to advance our understanding of the ecological and evolutionary basis for variation in SLM, including its association with traits such as leaf demography and canopy architecture as well as environmental characteristics of the habitats where particular species predominate.

AbstractWe assessed the robustness of a standard classification of foliage suitability of North American tree species to the gypsy moth (Lymantria disparL.) for tree species of southern Quebec grown under contrasting water regimes. Dispersal trials for 1st-instar larvae and feeding trials for 4th-instar larvae were conducted to determine whether water regime and (or) maternal effects influenced the preference or avoidance of gypsy moth larvae for foliage from saplings of native tree species of Quebec. We found no effect of water regime on the suitability of foliage to 1st-instar larvae and no effect of water regime and maternal effects for 4th-instar larvae. The standard classification scheme used by U.S. Department of Agriculture researchers in the United States was generally robust to the above factors, and thus suitable for use in southern Quebec.

Electivity indices measure the utilization of food types (r) in relation to their abundance or availability in the environment (p). Foods that constitute a larger proportion of the diet than of the available foods are considered preferred; conversely those proportionately underrepresented in the diet are avoided. A food is eaten at random if its proportion in the diet equals its proportion in the environment. A family of electivity indices stemming from Ivlev's (1961) classic monograph exist and differ only in the particular algorithm used to calculate electivity from r and p.

The climatic control of productivity for two populations of the lichen Cetraria cucullata (Bell.) Ach. growing in the arctic tundra of northern Alaska (70°28'N, 157°23'W) was examined. Respiratory losses of carbon vary with tissue temperature, tissue water content, and time since wetting. Potential net photosynthetic gains of carbon are affected by photon flux density, tissue temperature, and water content. The net CO2 exchange responses of populations growing on ridge tundra and on upland tundra differ and these differences reflect possible adaptation to the normal environmental regimes in the two habitats. Simulation of the lichen's net carbon balance using continuous hourly records of photon flux density, temperature, and water content for the unusually dry period June 28 through July 17, 1977 show that lichen biomass is actually lost during climatic regimes leading to frequent but short periods of lichen metabolic activity. This result is confirmed by the negative relative growth rates measured for C. cucullata over the same monitoring period. This observed loss of biomass may be attributable to depletion of carbon reserves to reactivate dormant metabolism without sufficiently long periods favorable for net photosynthetic activity to replenish the lost reserves. These results illustrate that environmental limits exist on the success of the dormancy strategy characteristic of lichen and moss carbon metabolism.

Plant growth depends on the coordinated acquisition and allocation of carbon, water, and nutrient resources to the major plant organs (root, stem, leaf, flower, and fruit) and to the major classes of metabolic function (vegetative growth, maintenance, defense, and reproduction). Air pollutants like SO2, NO2, and O3can directly damage plant tissues and disrupt normal patterns of resource acquisition and allocation. These disruptions in turn potentially will influence the plant’s ability to defend itself against pests and pathogens. This review summarizes the quantitative and qualitative changes that have been observed when plants are exposed to low levels of SO2, NO2, and O3; the following generalizations emerge:Root biomass is reduced more than shoot biomass in plants exposed to SO2or O3, but NO2does not appear to induce the differential suppression of above-versus below-ground organs.Quantitative allocation to leaves increases and to stem decreases under SO2pollution regimes; too few data are available to generalize about O3or NO2effects on leaf: stem ratio.Root carbohydrate concentrations sometimes increase and sometimes decrease after SO2or O3fumigations. Leaf nitrogen concentrations tend to decrease after exposure to air pollutants, and leaf carbohydrate concentrations can increase or decrease. Too few data on leaf concentrations of lipids and secondary chemicals are available to justify any generalizations on pollutant responses.Reproduction is suppressed by O3, SO2, and NO2, with O3appearing to have the most marked effects. Seed lipid and protein composition can be altered by exposure to pollutants.While both quantitative and qualitative changes in plant resource allocation after exposure to pollutants are common, the importance of these diverse changes for plant-pest and plant-pathogen interaction requires more comprehensive study. Ideally, the time course of plant growth and of metabolite pools critical to particular pests or pathogens should be followed in plants exposed to realistic pollutant regimes and related to pest or pathogen performance on the treated plants.

The survival potential of lichens in a given habitat is determined by the response of CO2 exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO2 exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO2 exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.

We assessed the mechanisms underlying the ability of cocklebur to spread from its natural riverside habitats and establish weedy populations in urban waste areas. We collected fruits from plants growing in natural and urban ruderal habitats and planted 2 maternal families from each of 9 habitat populations in 3 experimental gardens. The gardens were all in full sunlight but differed in the availability of water and nutrient resources. Plant performance in the gardens was measured by numbers and size of fruits produced. Traits known to be associated with cocklebur reproductive success were also measured: times to emergence and anthesis, photosynthetic capacity, mean stomatal conductance, relative growth rate, and biomass allocation to leaves and stems. Although there were significant differences among populations in the tradeoff between rate of growth in height and timing of anthesis, these population differences were not associated with habitat. Apart from a tendency to produce larger fruits in ruderal populations, there were no detectable differences in the characteristics of plants from natural vs. ruderal habitats. Plants from both habitats did have substantial and significant plastic responses to growth environment. In the three experimental gardens, fruit numbers increased with resource availability but fruit size did not differ significantly. As resource availability increased, plants from both habitats sustained growth longer and became more branchy. Canonical discriminant analysis of all the somatic and reproductive traits together supported the idea that natural and ruderal populations do differ in their overallplastic response to resource availability. The subtly different plastic responses of plants from the two habitats do not arise by substantial adjustments in a few dominant traits, but instead by relatively minor adjustments in a host of functionally interrelated phenological, morphological, and physiological traits. It is these small but coordinated differences in the plastic responses of many traits that appear to differentiate cocklebur from natural vs. weedy urban habitats.

Chasmogamous (CH) and cleistogamous (CL) flower production was investigated in natural populations of the perennial herb Oxalis montanain southern Quebec, Canada. Every 10–12 days during two flowering seasons, we recorded the reproductive output of approximately 2,000 plants distributed among five forest sites. The percentage of plants flowering, proportion of flowering plants producing CH and CL flowers, CH and CL flower number per flowering plant, and the proportion of flowers that are CL differed significantly among sites and corresponded to site differences in forest type and habitat quality. Unlike patterns in most CL species, however, reproduction by cleistogamy increased in sites with habitat conditions favorable to plant growth and sexual reproduction, and decreased in less productive sites. Chasmogamous and CL flower production increased with increasing plant size but plant size explained a significantly greater proportion of the variation in CL flower numbers. The percentage of flowering plants producing CH flowers decreased between years while the proportion of CL flowers increased at all sites during the second flowering season. The somewhat unusual aspects of reproduction in Oxalismay stem from its perennial habit which allows use of stored resources in adjusting the balance of cleistogamy versus chasmogamy in different environmental regimes.

AbstractWe used a diurnally and seasonally realistic simulation of seedbed conditions in a controlled environment chamber to test the dependence of seedling growth during the first year on the timing of spring germination in jack pine (Pinus banksianaLamb.) and black spruce (Picea mariana[Mill.] BSP.). Jack pine germinated poorly under cold, early season conditions compared to black spruce. Survival in seedlings that did emerge was high and fairly constant through the early season in jack pine, and increased steadily as the season advanced in black spruce. Subsequent seedling development, measured as time to shedding of the seed coat, was slowed more by early season conditions in jack pine. Earlier emerging seedlings in both species achieved greater total biomass growth through the simulated season; jack pine seedlings consistently grew more than black spruce for all sowing dates. Height growth changed little with time of seedling emergence for jack pine, but increased greatly in earlier emerging black spruce. These results suggest that early season events can be important determinants of conifer seedling establishment and may help to explain the geographic limits of these two widespread boreal trees.

Canonical analysis, a multivariate statistical technique, is described in simple terms and is suggested as a useful method for interpreting data in physiological ecology. An example illustrates how the technique simplifies an unwieldy mass of data by eliminating error variation and by concentrating only on differences among groups. Using canonical analysis to compare graphs of photo synthesis as a function of temperature for three lichens in the genus Cladoniashows that Cladonia uncialisphotosynthesizes at a moderate rate over a wide range of temperatures and with little change in its optimal temperature over the season, while C. rangiferinaand C. mitisphotosynthesize at higher rates over narrower ranges of temperature and with optimal temperatures that vary over the season. A graph of canonical variates is very helpful in isolating from response curves the lichens' different adaptive strategies, including patterns that subjective analysis may miss.

The ephemerality of high quality foliage in spring may act as a defense for trees against early season folivores, but only if the duration of high quality is so short that it is difficult for insects to synchronize their eclosion with the period of high quality foliage that follows budbreak. The rate of change in foliage quality on a day to day basis through the spring was determined for 9 species of hardwood trees in 2–3 years. Measurement of physical and chemical parameters and a bioassay with gypsy moth larvae both showed decreasing quality during the three to five weeks of canopy development in all species. Rates of decline differed among species but the patterns were similar from year to year on a degree-day scale. Growth rates of larvae raised through the first stadium on foliage of differing ages reflected these changes in foliage acceptability. Increasing toughness and declining nitrogen and water contents of leaves were correlated with changes in acceptability to larvae but explained only a small part of the variation in acceptability. The host-seeking period of gypsy moth larvae over-lapped with the availability of highly acceptable foliage of the most preferred host species. Less preferred species had more rapid declines in foliage acceptability, and hence narrower overlaps with the host-seeking period, which may provide defense against use by this generalist forest pest.

Modified reciprocal transplant experiments were conducted with the annual plant species Impatients capensis and I. pallida to partition the influence of environment on fitness into two components; that of (1) conspecific neighbours occupying each transplant site, and (2) the abiotic and biotic features of each site exclusive of the conspecific neighbours. In the within-species reciprocal transplant series, differences in survivorship and fruit production by cleistogamous flowers were attributable primarily to the effect of conspecific neighbours. In addition, plants surrounded by neighbours from the site of origin produced significantly more fruit from chasmogamous flowers compared with plants surrounded by neighbours from the alien site. In the between-species reciprocal transplant series, one transplant site was consistently associated with greater survivorship and fruit production regardless of the identity of neighbouring plants. The results suggest that different aspects of the environment in these reciprocal transplant experiments (conspecific neighbours, other species, physical factors) determine fitness in different situations.

Drawing on data for 42 fruticose and foliose lichen species, four aspects of photosynthetic response have been ex&mined in relation to latitude and climatic variables: 1) the maximal rate of net photosynthesis (Pmax), 2) the tissue temperature at which Pmax occurs (Topt), 3) the photon flux density at which Pmax occurs (PhARsat), and 4) the tissue water status at which Pmax occurs. Tissue water status was measured as either relative water content (RWCopt) or percentage dry weight (DWopt). The Topt decreased significantly with increasing latitude; Topt also increased significantly with increasing July solar radiation, July air temperature, daily hours of bright sunshine, daily solar radiation, annual precipitation, and annual evaporation. Both RWCopt and DWopt increased significantly with annual precipitation. The Topt and RWCopt for a lichen could both be more accurately predicted from multiple regressions on macroclimatic variables describing the lichen's habitat. Both the PhARsat and the Pmax, despite their lack of simple univariate relationships to either latitude or climatic variables, could also be predicted by multiple regression. The limitations on the generality and precision of these ecological trends in lichen photosynthesis are discussed in relation to microclimatic considerations, morphological adaptations, and poikilohydric metabolism.

Male and female plants of Rumex acetosella were grown on a moisture gradient to measure possible differences in the drought tolerance of the sexes. The growth of both sexes declined under water stress but males were significantly more drought tolerant. This could not be explained by greater water use efficiency in the male plants; measured rates of both photosynthesis and leaf conductance did not differ significantly between the sexes. Multiple discriminant analysis showed that the sexes differed at all moisture regimes in their overall patterns of biomass allocation. Males had proportionately greater investment in root and leaf tissue which could explain their growth advantage over females under water stress. Despite essentially equal water use efficiencies, on a per plant basis males, with more leaf and root biomass, could fix more carbon and more rapidly exploit the local water resource than females. Thus the pattern of biomass allocation rather than intrinsic physiological differences appears to explain the greater drought tolerance of male plants of Rumex acetosella.

The reproductive success of sibling cocklebur plants (Compositae: Xanthium strumarium) was monitored after growth at different levels of availability of water and nutrient resources. Variation in reproductive success among individual plants was related to physiological, structural, and phenological characteristics. Reproductive success increased with increased availability of resources, but the relative contribution of particular traits to reproductive success varied with resource availability. Allocation of biomass to different vegetative tissues, time to seedling emergence, degree of branching, transpiration rates, water use efficiency, the rate of decline in height growth after seedling emergence and final plant size all varied significantly with resource availability. However, the changes in each of these phenotypic traits across three garden environments did not always correlate with reproductive success. The shifts across environments in the apparent importance of somatic traits for reproductive success were attributed to plastic changes in the traits but also to changes in the phenotypic correlations of the traits with reproductive success.

Genetic and environmental variances were estimated for a number of characters in the annual plant Impatiens pallidaby planting seed obtained through controlled crosses into their native field site or pots maintained in the greenhouse. Significant additive genetic variance was detected for three of 11 characters studied—germination date, cotyledon area, and date of first flower production. Significant dominance and/or maternal variance was found for seed weight, proportion of seeds germinating, cotyledon area, plant height, and number of leaves produced. Environmental variance was greater in the field compared with the greenhouse. Characters found to be under strong directional selection in a previous study showed no detectable additive genetic variance. While these populations exhibit conditions that in theory could contribute to the maintenance of genetic variation (limited pollen and seed dispersal distances and small‐scale variation for edaphic characteristics influencing plant growth), levels of additive genetic variance for most characters were not significantly different from zero.

AbstractIn the summer of 1980, gypsy moth populations were monitored in 13 sparsely infested forests in southwestern Quebec; counts of living and dead larvae and pupae were made on 1,870 trees representing 28 deciduous and one coniferous species. Contrary to our null expectations, the proportionate numbers of pupae compared with larvae on the various host species were not equal. Hosts more preferred by larvae were less preferred by pupae, and vice versa. For example, pupae were disproportionately abundant on host species like Acer pensylvanicumL., Carya ovata(Mill.) K. Koch, and Juglans cinereaL. which are not generally favored larval hosts. Conversely, favored larval hosts like Quercus rubraL. and Ostrya virginiana(Mill.) K. Koch carried lower numbers of pupae than expected from the numbers of larvae feeding on them. Such differential utilization of host trees by larvae versus pupae, which can arise either from host-dependent differences in larval mortality or from late instar migration between hosts, may contribute to maintaining the broad polyphagy of gypsy moth larvae.