Every species has a limit to its geographic distribution. The problem is that we don’t really know why. Classical explanations propose that range limits occur where the ecological conditions overwhelm adaptation and populations are no longer self-sustaining. It is generally expected that population abundance and fitness decline towards range margins across a gradient of declining habitat quality. Current evolutionary and theoretical explanations of range limits predict that this geographic pattern of demography will result in genetic constraints in marginal populations, such that range expansion is thwarted by reduced evolutionary potential. In this thesis, I tested the key assumptions and predictions of range limit theory through an empirical evaluation of two coastal dune, endemic plants; Camissonia cheiranthifolia (Spreng.) Raim. (Onagraceae) and Abronia umbellata L. (Nyctaginaceae). In geographic wide surveys of a large proportion of populations across both species’ ranges, neither species exhibited declining abundance or performance towards its range limits. Central populations of C. cheiranthifolia tended to have a higher production of seeds per unit area than marginal populations. Although this pattern demonstrated the potential for gene flow from central sites to swamp selection in marginal sites, results from a transplant experiment suggested that this was unlikely. Experimental populations of C. cheiranthifolia originating from ≥ 675 km south of the northern range limit exhibited similar levels of fitness when planted at the range margin. Along a 200 km transect across the limit, and in contrast to expectations, fitness of all populations increased towards the limit and generally remained high beyond the limit. Individuals from all populations reproduced and matured fruit beyond the limit, suggesting that if individuals dispersed beyond the limit that populations would establish. The species’ abrupt distributional limit also did not correspond to an abrupt shift in ecological conditions, despite the association of fitness with plant community and microhabitat variables. Overall, ecological tolerances to fitness or niche-based explanations to range limits do not adequately describe the distributions of either species. Constraints on dispersal rate, the influence of anthropogenic factors on habitat dispersion and limited genetic variability for fitness related traits are addressed. / Thesis (Ph.D, Biology) -- Queen's University, 2007-05-13 16:45:44.456

All species on Earth occupy limited geographic space. More than a century of observational, experimental, and theoretical work investigating the factors that drive species distributions have demonstrated the importance of the interactions between abiotic, biotic, and demographic factors in determining why species are found where they are. However, it is still unclear when and where these factors interact to set species range limits. Filling the existing knowledge gaps is imperative for the accurate predictions of how species will respond to global change, and particularly for invasive species, many of which are expected to benefit from global change. Here, I sought to investigate the mechanisms that enable, as well as limit, the range expansion of the globally invasive plant Sorghum halepense (L.) Pers. (Johnsongrass). I performed a series of field and laboratory experiments to study population and range dynamics throughout Johnsongrass's North American distribution, and test for the effects of climate, local habitat, and competition on multiple functional traits. I found Johnsongrass consistently demonstrated impressive performance across varying environments, often growing more than 3 m tall, producing hundreds of flowering culms within a single growing season, and maintaining positive population growth rates, even under intense competition with resident weeds. I also found evidence that seed germination has adapted to varying climates encountered during Johnsongrass's range expansion resulting in a shift in the germination temperature niche from warmer to cooler as Johnsongrass spread from warmer climates in the south to more temperate climates in higher latitudes. This shift in the germination temperature niche may have been an important contributing factor in the range expansion of Johnsongrass by enabling the optimization of seed germination in varying climates. On the other hand, results from a field study suggested a possible trade-off between flowering time and growth in populations originating from the range periphery (i.e., range boundary) which may be limiting, or slowing, continued range expansion of Johnsongrass. Together, the outcomes of this work contribute to our understanding of the factors involved in the distribution of species, which is a fundamental goal of Ecology, and essential to accurately predict how invasive species will respond to global change. / Doctor of Philosophy / Invasive species threaten our natural ecosystems, our agricultural systems, and even our infrastructure, and we spend billions of dollars each year attempting to control them and reduce their negative impacts. Climate change, habitat destruction, and other forms of global change, will benefit many of these species, magnifying their impacts and promoting their invasion into new territories. Because of the damaging effects of invasive species, and the costs to control them, it is imperative that we are able to predict how they will respond to global change so that we can improve plans to reduce their impact and spread. First, we need to understand the processes that promote their invasion across large swaths of land. Just as importantly, we must study the processes that prevent their invasion of certain areas. Here, I investigated some of the processes that have facilitated, as well as hampered, the spread of the invasive plant Johnsongrass. For this work, I used Johnsongrass plants originating from different habitats, including regions where Johnsongrass is highly invasive and those where Johnsongrass is very rare. I found Johnsongrass originating from regions where it is highly invasive were able to grow very large and produce thousands of seeds that were able to germinate under a range of conditions. These traits may have contributed to the invasion success of this species. However, I found a different pattern for plants that originated from regions where Johnsongrass is rare. These plants reached reproductive age earlier and grew smaller across all environmental conditions, potentially due to the less hospitable climates of these range edges. These findings allow us to project into future climate change scenarios, because it is likely that, as temperatures warm, invasive species will be able to invade new regions, where they will impact the work of conservationists, natural resource professionals, agricultural produces, and other land managers.

Biological invasions provide opportunities to study evolutionary processes occurring during contemporary time scales. Here, I combine a literature review of common garden studies of invasive plant species, with field and glasshouse experiments on populations of the outcrossing, perennial, wetland invader Lythrum salicaria (purple loosestrife - Lythraceae), to investigate the evolutionary genetics of local adaptation in reproductive and life-history traits. A review of 32 common garden studies of 28 introduced species identified previously unrecognized latitudinal clines in phenotypic traits in both native and introduced populations. To obtain direct evidence for clinal variation and local adaptation, I investigated populations of L. salicaria sampled along a latitudinal gradient of growing season length in eastern North America. Controlled pollinations of plants from 12 populations provided no evidence for the breakdown of self-incompatibility to self-compatibility towards the northern range limit. However, a quantitative genetic experiment involving 20 populations revealed latitudinal clines in population mean, variance and skew for days to flower and vegetative size. Broad-sense estimates of genetic variance were significant for most traits; however, strong inter-correlations among traits suggested that fitness trade-offs have constrained population divergence. The observed clines supported a model of selection for early flowering in northern populations constrained by a trade-off between age and size at flowering. A comparison of variance-covariance matrices of family and population means (G and D, respectively) of life-history traits demonstrated that populations have evolved in response to selection under genetic constraints, rather than through neutral processes. A reciprocal transplant experiment involving six populations and three common gardens spanning the latitudinal range provided direct evidence for local adaptation in flowering phenology. Populations maintained the same rank-order for time to flowering and vegetative size at each site, and southern populations had the highest fecundity at the southern site but the lowest at the northern site. Finally, a phenotypic selection analysis in each common garden involving 61 F2 families of crosses between a northern × southern populations confirmed that selection favours earlier flowering in northern populations. These results demonstrate that natural selection on reproductive phenology has accompanied the invasive spread of L. salicaria in eastern N. America.

Biological invasions provide opportunities to study evolutionary processes occurring during contemporary time scales. Here, I combine a literature review of common garden studies of invasive plant species, with field and glasshouse experiments on populations of the outcrossing, perennial, wetland invader Lythrum salicaria (purple loosestrife - Lythraceae), to investigate the evolutionary genetics of local adaptation in reproductive and life-history traits. A review of 32 common garden studies of 28 introduced species identified previously unrecognized latitudinal clines in phenotypic traits in both native and introduced populations. To obtain direct evidence for clinal variation and local adaptation, I investigated populations of L. salicaria sampled along a latitudinal gradient of growing season length in eastern North America. Controlled pollinations of plants from 12 populations provided no evidence for the breakdown of self-incompatibility to self-compatibility towards the northern range limit. However, a quantitative genetic experiment involving 20 populations revealed latitudinal clines in population mean, variance and skew for days to flower and vegetative size. Broad-sense estimates of genetic variance were significant for most traits; however, strong inter-correlations among traits suggested that fitness trade-offs have constrained population divergence. The observed clines supported a model of selection for early flowering in northern populations constrained by a trade-off between age and size at flowering. A comparison of variance-covariance matrices of family and population means (G and D, respectively) of life-history traits demonstrated that populations have evolved in response to selection under genetic constraints, rather than through neutral processes. A reciprocal transplant experiment involving six populations and three common gardens spanning the latitudinal range provided direct evidence for local adaptation in flowering phenology. Populations maintained the same rank-order for time to flowering and vegetative size at each site, and southern populations had the highest fecundity at the southern site but the lowest at the northern site. Finally, a phenotypic selection analysis in each common garden involving 61 F2 families of crosses between a northern × southern populations confirmed that selection favours earlier flowering in northern populations. These results demonstrate that natural selection on reproductive phenology has accompanied the invasive spread of L. salicaria in eastern N. America.

Understanding how climate determines species’ geographic distributions is an important question in ecology with direct implications for predicting climate change-driven range shifts. For Lepidoptera, growing degree-days, a measure of growing season length, has been shown to be an important predictor of species’ distributions in some cases. Most studies use a standardized estimate of base development temperature in their calculations of growing degree-days instead of a species-specific threshold so past investigations of the influence of growing degree-days on Lepidoptera distributions may not have been optimal. Species distribution models (SDMs) are a commonly used approach in ecology that typically only implicitly capture the underlying processes that limit a species’ distribution. A species-specific estimate of growing degree-days should better characterize these processes than standard thermal thresholds and thus improve the accuracy of species distribution models. In this thesis, I use species distribution modelling to model the geographic distribution of 30 moth species native to North America. I ask whether a) growing degree-days are the best climatic predictor of these moth species distributions at broad scales; b) a lab-estimated biological threshold (i.e., BDT) can scale up and improve the predictive ability of SDMs; and c) the quality of experiments used to estimate species-specific BDT influences the predictive accuracy of SDMs. To do so, I compare the predictive accuracy of a correlative model based on a commonly-used thermal threshold to define growing degree-days to a hybrid model with degree-days defined based on a species-specific thermal threshold. I found that the predictive performance of the hybrid models was indistinguishable from the correlative models likely because growing degree-days was not the best climatic predictor of the geographic distributions of the majority of these moth species. I also found that there was no link between the quality of the lab experiments and the difference in performance of the hybrid and correlative models. My findings suggest that lab-estimated thermal thresholds may not always scale up to be predictive at a broad scale and that more work is needed to leverage the data from lab experiments into broad scale SDMs. Determining the ultimate factors that limit species’ distributions will be critical in accurately predicting species’ range shifts response to future climate change.

Invasive non-indigenous species are among the principle drivers of global change, altering nutrient cycles, changing disturbance regimes, and generally threatening biodiversity. Climate change is widely expected to exacerbate invasions by relaxing abiotic barriers, such as low temperature, but the mechanistic evidence supporting this is limited. Here, I evaluate the hypothesis that low temperatures determine the northern range limit of kudzu (Pueraria montana var. lobata), an invasive Asiatic vine in North America, by assessing freezing and chilling tolerance of kudzu plants in winter, spring, summer, and fall. Kudzu was widely planted throughout the southeastern U.S. in the early 20th Century to prevent erosion. It is winter-deciduous and reproduces primarily from buds on stem nodes. In the last 40 years, kudzu has migrated northward in concert with a northward shift in the -20oC minimum winter temperature isocline, indicating that less severe winter cold is permitting northward migration. Freezing mortality during winter does not explain this correlation. Electrolyte leakage assays demonstrate that above- and belowground kudzu stems can survive to -27oC and -17oC. Insulation provided by soil and snow protects belowground stems from lethal temperatures to well north of kudzu's current range limit. Severe spring chill stops growth and photosynthesis and causes some shoot mortality, but both growth and photosynthesis recover quickly following the chill. Summer growth rates are rapid (up to 22 cm/d), responding within hours to temperature changes, and are unimpaired by nighttime lows. Photosynthesis is reduced at cool temperatures, but on cool days, kudzu leaves tend to be warmer than air temperature, so photosynthesis rates generally remain close to optimal values. In autumn, growth stops below 15oC, but leaves are retained and maintain modest photosynthetic competence until killed by frost in November. In colder climates that occur far north of kudzu's current range, reductions in the length and quality of the growing season could accumulate over time to reduce kudzu's success. There is, however, no strong evidence that low temperatures in any season will prevent kudzu from migrating throughout southern Ontario, making kudzu a good candidate for invasive species regulations.

Invasive non-indigenous species are among the principle drivers of global change, altering nutrient cycles, changing disturbance regimes, and generally threatening biodiversity. Climate change is widely expected to exacerbate invasions by relaxing abiotic barriers, such as low temperature, but the mechanistic evidence supporting this is limited. Here, I evaluate the hypothesis that low temperatures determine the northern range limit of kudzu (Pueraria montana var. lobata), an invasive Asiatic vine in North America, by assessing freezing and chilling tolerance of kudzu plants in winter, spring, summer, and fall. Kudzu was widely planted throughout the southeastern U.S. in the early 20th Century to prevent erosion. It is winter-deciduous and reproduces primarily from buds on stem nodes. In the last 40 years, kudzu has migrated northward in concert with a northward shift in the -20oC minimum winter temperature isocline, indicating that less severe winter cold is permitting northward migration. Freezing mortality during winter does not explain this correlation. Electrolyte leakage assays demonstrate that above- and belowground kudzu stems can survive to -27oC and -17oC. Insulation provided by soil and snow protects belowground stems from lethal temperatures to well north of kudzu's current range limit. Severe spring chill stops growth and photosynthesis and causes some shoot mortality, but both growth and photosynthesis recover quickly following the chill. Summer growth rates are rapid (up to 22 cm/d), responding within hours to temperature changes, and are unimpaired by nighttime lows. Photosynthesis is reduced at cool temperatures, but on cool days, kudzu leaves tend to be warmer than air temperature, so photosynthesis rates generally remain close to optimal values. In autumn, growth stops below 15oC, but leaves are retained and maintain modest photosynthetic competence until killed by frost in November. In colder climates that occur far north of kudzu's current range, reductions in the length and quality of the growing season could accumulate over time to reduce kudzu's success. There is, however, no strong evidence that low temperatures in any season will prevent kudzu from migrating throughout southern Ontario, making kudzu a good candidate for invasive species regulations.

The accurate assessment of forest persistence under environmental change is dependent on the fundamental understanding of the genetic consequences of human intervention and its comparison to that of natural processes, as declines in genetic diversity and changes in its structuring can compromise the adaptive ability of a population. The European beech, Fagus sylvatica, has experienced prolonged human impact over its 14 million ha range with contemporary forests harbouring high ecological, economic, and cultural value. Historical traditional management practices, such as coppicing and pollarding, have impacted a large portion of Europe’s forests. This form of management encouraged vegetative regeneration, prolonging the longevity of individual trees. In several cases, the structure and function of managed trees and their associated ecosystems were significantly altered. Specifically, coppiced beech forests in Europe displayed significantly larger extents of spatial genetic structuring compared to their natural counterparts, revealing a change in the genetic composition of the population due to decades of management. Humans have also aided in the dispersal of beech within and outside of its natural range. In Great Britain, the putative native range retained signals of past colonisation dynamics. However, these signals were obscured by the wide-spread translocation of the species throughout the country. Evidence of post-glacial colonisation dynamics can be found in Sweden as well. In contrast to Britain, the structure of this natural leading range edge displays a gradual reduction in population size where isolation was found to have acted as an effective barrier to gene flow reducing the genetic diversity of populations.

Des changements de croissance ont été documentés pour le XXe siècle dans de nombreuses régions en Europe. Cependant une évaluation exhaustive des changements de productivité, à une large échelle géographique, avec une analyse de leur hétérogénéité spatiale et de la diversité interspécifique de la réponse fait encore défaut. L'objectif de cette thèse est d'évaluer les changements récents de la productivité forestière aux échelles nationale, régionale et locale en France, et de rechercher leurs causes environnementales, à partir d'une approche de modélisation statistique de l'accroissement en surface terrière du peuplement (∆G) et d'indicateurs environnementaux. Nous avons utilisé les données de l'inventaire forestier national français pour 8 espèces dont la niche écologique et la distribution diffèrent (Fagus sylvatica, Quercus robur, Quercus petraea, Quercus pubescens, Picea abies, Abies alba, Pinus sylvestris and Pinus halepensis), observées en peuplements purs et réguliers. Nous présentons les facteurs ayant un effet sur la productivité des différentes espèces à l'échelle nationale, ainsi que des cartes de productivité suggérant que l'aire de distribution des espèces n'est pas toujours limitée par les conditions environnementales. Entre 1980 et 2005, nous observons des tendances positives, modales ou non significatives de la productivité pour toutes les espèces à l'exception des deux espèces méditerranéennes dont la productivité a diminué, soulignant ainsi la variabilité interspécifique de ces changements. Nous observons également de fortes variations spatiales des changements de productivité, autant dans leur intensité que dans leur signe, aux échelles régionale et intra-régionale. Ces résultats remettent en question la pertinence d'une évaluation moyenne à large échelle et soulignent le caractère contextuel des estimations. Nous mettons en évidence le rôle du réchauffement climatique récent dans les tendances observées. Ce travail fournit une évaluation plurispécifique et multi-échelle de la réaction de la productivité des espèces arborées à un environnement changeant. Nous avons souligné le caractère spécifique des changements de productivité et leur caractère contextuel, du fait de différences dans l'autécologie des espèces et de variations spatiales des facteurs limitants. Une étude approfondie de l'effet des facteurs environnementaux et de leurs interactions complexes est nécessaire pour la prédiction de la productivité future des espèces. / Growth trends have been reported in many regions of Europe over the twentieth century. However, an integrated assessment of productivity changes, including focus on a wide geographical scale, analysis of spatial heterogeneity, and the inter-specific diversity of growth responses is still lacking. The aim of this Ph.D work was to assess recent changes in forest productivity on a national, regional and local scale in France, and to investigate their potential environmental causes, based on statistical modeling approaches of stand basal area increment (BAI), and environmental indicators. We used the French NFI data for 8 species of contrasted ecological niches and distributions (Fagus sylvatica, Quercus robur, Quercus petraea, Quercus pubescens, Picea abies, Abies alba, Pinus sylvestris and Pinus halepensis), taken in pure and even-aged stands. We identified the main factors influencing tree species productivity on a national scale, and produced productivity maps suggesting that species distribution ranges are not always limited by environmental conditions. Between 1980 and 2005, the productivity trends reported were positive, modal or non-significant for all species, except the two Mediterranean species for which productivity decreased, highlighting inter-specific differences in these changes. We observed strong variations of productivity changes, both in intensity and sign, on a regional and intra-regional scale. These results question the relevance of wide-scale average assessments and highlight their context-dependence. The role of recent climatic warming in featuring the BAI trends was highlighted. This work provides an enriched scale- and species-dependent assessment of tree species reaction to a changing environment. We emphasized the species- and context dependence of productivity changes, due to differences in species autecology and spatial variations in the limiting factors. Further focus on the effect of environmental factors and their complex interactions is needed for the prediction of species future productivity.