The Northern Mockingbird (Mimus polyglottos), the state bird of Texas, Florida, Mississippi, Tennessee, and Arkansas, is noted for the highly complex and melodious springtime song of males. It is ideal for studying mate choice and breeding success because the flamboyant male song is surprising in a socially monogamous bird. I conducted a three year study of mating and breeding success to test the hypothesis that male song, measured as mean bout length and versatility, could be explained by high rates of extra-pair matings, something which has been found in many other apparently monogamous birds. However, I found that extra-pair fertilization accounted for only 3.1% of all young, so my initial hypothesis was disproved. Because song could not be explained by extra-pair fertilization, I investigated the role of this and other variables in mockingbird mate choice and breeding success. The additional variables included: four initial dates in the breeding season, prior-year experience, age, singing frequency, territory quality measured as amounts of shrubbery, three measures of size, three measures of white plumage areas, condition measured as subdermal fat amounts, and blood and gut parasitemia. I looked at the role of these characters in mate choice by comparing mated and unmated males in two years and found that amounts of white plumage, weight, wing lengths, first date sighted on territory, first date heard to sing, blood parasitemia, age, and territory quality were different (P $<$ 0.10) in one or both years. In 1993, 26.1% of mated males were bigamists (1992: 6.5%) who differed from monogamous males in first date heard singing and blood parasitemia. Females prefer older, larger, healthier males with good territories. The following were predictors (P $<$ 0.10) of breeding success in one or both years, measured as total numbers of broods or offspring, for either or both males and females: song variety, first date sighted on territory, first date heard singing, and first date building the first nest, prior-year breeding experience, prior-year territory occupancy, singing frequency, weight, wing length, fat levels, and territory quality. Experienced, healthier individuals with better territories and males with more song variety who sing less frequently have greater reproductive success.

The good genes hypothesis predicts that females discriminate among potential mates on the basis of their genetic quality. We measured the indirect benefits received by females with different levels of choice---from no choice to choosing among 5 males---in the housefly, Musca domestica , at high and low density. Secondly, we tested how the degree of choice affected the courtship behavior of both sexes and whether this behavior was correlated to female reproductive success. Opportunity for choice did not affect mating propensity or offspring survivorship, but did affect male courtship rate and the number of eggs females laid in their first clutch. Females at low density were more likely to mate, laid more eggs in their first clutches, and had greater egg-to-adult viability than females mated at high density. Overall, the degree of choice affected some aspects of mating behavior and fitness, but the effects were primarily density dependent.

The evolution of increased competitive ability (EICA) hypothesis posits that release from natural enemies favors exotic plants evolving traits associated with faster-growth and lower herbivore-resistance in the introduced range. Given a trade-off between resistance and tolerance, decreased resistance may translate into increased tolerance to herbivory by invasive plants compared to conspecifics in the native range. These genetic shifts in traits of invasive plants may also have effects on soil and ecosystem processes. To test the EICA hypothesis, we conducted experiments in the native range of Sapium sebiferum using seeds from populations in the native Chinese range (native ecotype) and the invasive North American populations (invasive ecotype). Plants from invasive populations showed greater competitive ability, reduced resistance but increased tolerance to herbivory than those from native populations of Sapium sebiferum when they were competing against each other in the presence of native herbivores. Despite that native specialist beetles Aphthonomorpha collaris preferred invasive populations over native populations of S. sebiferum when beetles had a choice between them in a bioassay trial, plants from invasive populations compensated for leaf damage more efficiently than those from native populations. These results suggest that invasive S. sebiferum has become a faster-growing, less herbivore-resistant and more herbivore-tolerant plant in the introduced range compared to conspecifics in the native range. A greenhouse study showed that invasive North American populations of S. sebiferum differed genetically from native Chinese populations in most plant functional traits. Of 13 measured plant variables, the root to shoot ratio (RSR), total leaf area (TLA) and net CO2 assimilation (A) were identified as powerful traits that contributed the most to the genetic difference between native Chinese and invasive North American population types. These shifts in functional traits resulted in higher soil-plant system CO 2 and N2O emissions for invasive ecotypes than for native ecotypes of Sapium in an outdoor pot experiment. Compared with native ecotypes, invasive ecotypes accelerated soil carbon and nitrogen processes and promoted more nitrogen uptake through soil-plant direct interactions. The results of this study suggest that shift in traits of invasive plants and their effects on ecosystem processes may have implications for their invasiveness.

Patterns of vegetation in fire-prone habitats were analyzed by ordination. The results showed that vegetation was highly related to soil texture, but was not obviously related to fire history. I concluded that either fire and soil effects are confounded or fire effects have been reduced by long-time fire suppression. Indicators of site productivity--height and volume increment of loblolly pine, and stand basal area--were weakly related to vegetation type, but not related to soil texture. The lack of a relationship between these indicators and soil texture suggests that site productivity may not be related simply to soil texture and that interactions with other species may obscure the response of loblolly pine to site productivity. The flammable portion of fine fuel was higher in dry types than in wet types which is consistent with the hypothesis that fire effects will be greater in dry types than in wet types.

Aluminum spring-band dendrometers were used to measure within-season tree growth at three sites (dry, mesic, and wet) in the Big Thicket National Preserve, Texas to determine what affects within-season tree growth and if patterns of growth vary among species, sites, and years. Growth patterns were found to differ among sites when the variation among species was removed, and among years and species. Significant interactions implied that trees were responding differentially to weather variation. Temperature, rain, soil moisture, and deficit were found to affect the within-season growth of trees. However, site patterns seemed to mediate the effects of weather, and intrinsic seasonal growth patterns defined how different species responded to weather variation. My results suggest that climate change will differentially affect tree growth, depending on species and how within-season weather patterns change.

Invasive species can cause significant changes to their introduced environments. A set of experiments was conducted to determine whether Sapium sebiferum exhibits allelopathic effects on neighboring plants in its introduced range. Bioassays were performed on Sapium, Liquidambar styraciflua, Schizachyrium scoparium, Andropogon ischaemum, and Lolium perenne seeds using extracts from Sapium, Acer saccharinum, Platanus occidentalis and Liquidambar leaves. Germination and early growth comparisons were made using: (1) solutions treated with activated carbon to remove allelochemicals or left untreated, (2) solutions in a dilution series, and (3) solution mixtures prepared from rinsed leaves or soaked leaves using activated carbon to negate potential allelopathic effects of all but one leaf species in each mixture. In almost every analysis there were few significant predictors. The effects of Sapium extracts on germination or growth never differed from those of the three native tree species' extracts. Allelopathy does not appear to contribute to Sapium's invasive success.

I investigated the role of shade tolerance in the dynamics of three forests of Big Thicket National Preserve, southeast Texas. In a mature mesic forest, shade-intolerant species had higher high-light growth and lower low-light growth than tolerant species. Results suggested that there was a tradeoff between high-light growth and low-light growth across species. Moreover, low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests where survival in low light may be achieved at the expense of growth, my results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. I conclude that both juvenile growth and survival are important components of shade tolerance and their relationships may be system-specific. In a floodplain forest, growth responses to light were consistent with the expectation that shade-intolerant species grow faster than shade-tolerant species in high light and vice versa. But mortality risks of some shade-tolerant species were unexpectedly high. The increased flooding during one of the study time periods may be responsible for the high mortality risks of shade-tolerant species. The results further supported that the success of shade-tolerant species in this forest may be limited by flooding as previous studies suggested. Compared with the mesic site, common species showed little intraspecific differences in shade tolerance. In a sandy upland pine-oak forest, low-light growth responses of saplings corresponded to shade tolerance expectation. However, shade-intolerant species did not show faster growth than shade-tolerant species in high light possibly because these species are drought-tolerant. The correspondence between increasing stem density and increasing death rates of shade-intolerant xeric dominants suggested that recent increases in canopy shading may be responsible for high death rates of these species. Cross-site comparison showed that shade-tolerant species had better performance at the dry site than at the mesic site. Consistent with the facilitative effect found at dry sites, these species may benefit from the less competition from surrounding vegetation.

The bluegreen algal community of a Texas coastal marsh was studied in order to estimate annual productivity and extracellular release of dissolved organic compounds. The effects of various environmental factors on these processes were also investigated. Productivity of the algal mat was highest in the summer, with the lowest rates of carbon fixation occurring in December. Annual productivity was estimated to be 71.04 g C m('-2), approximately 10% of the net aerial productivity of the grass canopy. While this may appear to be a minor contribution in terms of magnitude, the algal material is directly available to consumers while the grasses must decay before assimilation is possible. Of the environmental parameters studied, temperature had the greatest effect on productivity. The rate of carbon fixation was also affected by light intensity and salinity, however factors which modify the amount of biomass present such as grazing and flooding are more important in determining annual productivity. The pattern of ('14)C incorporation into various intracellular fractions (low molecular weight metabolites, polysaccharide, and protein) was useful for further assessment of environmental effects. For example, changes in the relative incorporation of label into the protein fraction were evident during moderately unfavorable conditions when the rate of carbon fixation was apparently unaffected. The filamentous bluegreen algae studied released an average of 3.7% of their photoassimilated carbon into the environment. Annual release was estimated to be 2.25 g C m('-2). Increasing extracellular release accompanied greater rates of carbon fixation and those environmental factors which influenced productivity had a similar effect on excretion. However, fluctuations in the water potential of the external environment were very important in determining the rate of extracellular release; high salinity or low soil moisture were responsible for high rates of excretion. Mannitol and trehalose were identified as the most commonly released compounds, with occasional evidence for the release of glucose and sucrose. The presence of mannitol, in particular, suggests that in natural situations extracellular release of organic compounds can function in osmotic adjustment.

Successional change following logging and major natural disturbance was investigated through the use of stand history reconstruction and population monitoring in an east Texas mixed pine-hardwood forest. Establishment of intolerant loblolly pine (Pinus taeda), white oak (Quercus alba) and water oak (Q. nigra) was limited to a relatively brief (10-20 yrs) period after a selective pine cut in 1910-1930. Establishment of shade tolerant beech (Fagus grandifolia) and southern magnolia (Magnolia grandiflora), initially suppressed after logging, began to increase as reproduction of less tolerant species declined, and is relatively abundant in the current forest. Timing of red maple (Acer rubrum) establishment was intermediate between these extremes. Current establishment is mostly occurring beneath patches of water oak, white oak and pine, rather than beneath beech and magnolia. Beech and magnolia trees and saplings are growing faster and mortality of young trees and saplings is lower than for other hardwood species. Thus, through a variety of mechanisms, the importance of beech and magnolia in the current forest appears to be increasing relative to pine and less tolerant hardwoods. Though the data are less complete, similar processes have apparently occurred after major natural disturbance.

I investigated spatial and temporal patterns of seed deposition, seedling appearance, survivorship, and growth in an East Texas river floodplain forest from 1980 through 1984, and estimated the importance of several factors including flooding, drought, shading, insect herbivory, and fungal attack in causing seedling mortality. Tree species fell into two groups on the basis of their demographic characteristics and responses to unfavorable conditions. For heavy-seeded species, of which water oak was the primary example, few seeds were produced, but seedling survival was high. Seedlings appeared late in the summer, thereby avoiding peak periods of flooding and damping-off mortality. Seedling survival was little affected by drought, insect herbivory, or proximity to a conspecific adult. Most of the common tree species were light-seeded, produced large seed crops, and saturated the study site with seeds due either to long dispersal distances (e.g. red maple, sweetgum, and elm) or high adult abundance (e.g. ironwood). However, these species often survived poorly as seedlings. Flooding, drought, damping-off, proximity to a conspecific adult, insect herbivory, and shade were important causes of mortality. Mortality for the light-seeded species was not constant, but was concentrated in peaks associated with particular events (e.g. a drought in 1980, and flooding and damping-off in 1981 and 1982). Seedlings germinating earlier in the spring were better able to survive these periods of stress. Conditions unfavorable to the survival of the light-seeded species resulted in increases in the proportion of water oak in the seedling layer, while favorable conditions allowed the more prolific, light-seeded species to increase in importance. I hypothesized that spatial and temporal variation in seedling composition leads to differential species success in canopy openings by favoring species currently important as seedlings. As a test of this hypothesis, seedling survival and abundance in the understory were compared to sapling abundance in nearby canopy gaps. Results for both sweetgum and water oak support the hypothesis that seedling abundance prior to gap formation is an important determinant of successful canopy recruitment. However, for red maple, success in gaps appeared more closely related to proximity of a seed source rather than advance regeneration of seedlings.