Effects of temperature and pollinator availability on plant reproductive success in the Indiana spring ephemeral community.

摘要

Temperature is a key driver of phenology in both plants and insects, and even small changes in temperature can impact the reproductive success of insect-pollinated plants by influencing access to pollinator services. While it is well-established that temperature variation directly impacts the flowering phenology of many plant species, the mechanisms by which it ultimately influences seed production via the pollinator community are not well understood. Climate change has the potential to disrupt the temporal synchrony between plants and their primary pollinators, especially if the two rely on different seasonal cues for the timing of their life history events. If the plants and their primary pollinators respond differently to environmental change, the plants may switch pollinators (host switching), obtain services or resources from other sources (e.g., self-pollination), or face extinction.;The goal of this dissertation was to experimentally test the hypothesis that temperature-mediated variation in flowering time influences seedset by determining access to a pool of active pollinators, and the extent to which a flower experiences competitive or facilitative interactions for pollinators with co-flowering plant species.;Three separate field studies were conducted using Claytonia virginica and Anemone acutiloba---members of the spring ephemeral community of deciduous forests in the eastern United States. This plant community is characterized by a brief 4 -- 6 week reproductive period in the late winter and early spring, when the temperatures are rising but the tree canopy has not yet developed, so substantial sunlight still reaches the forest floor. Only a few pollinating insects are tolerant of the low temperatures that characterize early spring in this ecosystem, potentially generating competition for pollinators among co-flowering plants in the community.;The first study evaluated the impacts of temperature, phenology, and co-flowering community diversity on the extent of pollen limitation experienced by the perennial herb C. virginica, a relatively abundant member of the spring ephemeral community. The study was conducted over two consecutive years that had markedly different spring temperature patterns. I observed a major advancement in the timing of flowering throughout the plant community in response to the warmer spring temperatures during the second year. However, seedset in C. virginica was not limited by pollen availability and was not influenced by the abundance and diversity of the co-flowering community. The second field experiment investigated temperature as a driver of plant phenology and the effects of shifted phenology on the reproductive output of A. acutiloba---the earliest flowering member of the spring ephemeral community. Individuals of A. acutiloba were transplanted into artificially warmed plots and monitored over two growing seasons. Increasing soil temperature resulted in advanced flowering time by at least 5 days in both years. However, despite this relatively large shift in flowering phenology, seed production in A. acutiloba was not affected by the temperature treatment. The third study evaluated the degree to which C. virginica functions as a "generalist" in the plant-pollinator network. This was done by characterizing the insect pollinator community that occurs in flowering C. virginica populations, and evaluating the relative abundance and diversity of the pollen from different co-flowering species on the bodies of the insects. I found that C. virginica pollen was present on all species of pollinators collected during its flowering period, and its pollen comprised the largest proportion of the total pollen abundance on all species of insect pollinators.;Collectively, the results of this dissertation suggest that both C. virginica and A. acutiloba maintain access to sufficient pollinator services across high levels of temperature-mediated variation in flowering phenology and co-flowering community structure. Furthermore, high relative abundances of C. virginica pollen on all of the pollinating insects in the community reflect the generalist strategy of this species within the plant-pollinator network of the spring ephemeral community. While successful pollination in these two species is robust across dramatic shifts in temperature and phenology, I suspect that other species---especially those that are relatively rare, that rely on specialist insect pollinators, or that have narrower flowering windows---could be more likely to be subject to phenological mismatching across years with different temperature profiles. Additional experiments that explicitly compare plant species with different levels of relative abundance, different pollinators, and different phenological windows could make it possible to generate a predictive framework for anticipating which species in a community are most likely to be vulnerable to phenological mismatching with their pollinators in response to climate change.