Abiotic and biotic drivers of grassland structure: from individual to community responses

Abstract

Climate has long been recognized as an important factor in determining the spatial-temporal distribution and abundance of species, consequently influencing global biological diversity. Model projections point to changes in precipitation regimes, with some geographic regions experiencing increases and others decline in rainfall; but it is also predicted increase in rainfall variability with lower frequency but higher intensity of precipitation events. Such changes in precipitation regimes will likely have large effects on plant responses. In addition to climate, disturbances can alter the structure and functioning of local systems through disruption in biota, consequently altering resources and conditions. In turn, local biota and their associated species interactions play an important role influencing the response of ecosystems to changes in precipitation and disturbance. Grasslands represent a large proportion of the terrestrial land surface, and provide valuable ecosystem services (e.g., forage production, soil C storage). Thus, it is especially important to understand the magnitude and direction of ecological responses since grasslands are strongly water-limited and experience disturbance by human management. In my dissertation I explore the effects of changing environments on plant communities and how these factors shape plant individual to community responses. In chapter one, 1 explore how organization levels (species-level, functional group level and community level) of the temperate tallgrass prairie are influenced by changes in precipitation and hay harvest (a proxy for human management). I do so by addressing how seven precipitation levels, along with clipping, affect an existing mixed-grass prairie ecosystem. I demonstrated that initial shifts in abundance were detected by examining species- to community-level changes over time. Across years, in dry conditions there was an increase in evenness that was related to the decline of the dominant species and increase in subdominants; whereas mesic conditions mildly promoted plant richness. Hay harvest enhanced plant richness not only over time through species gains, but also in each year. When combining altered precipitation with hay harvest, specifically under mild drought, I observed a decline in evenness that was related to the reduced abundance of C3 species and increase in C4 species. However, in extreme dry levels, clipping muted the effects of precipitation on the dominant plant species, plant evenness, functional groups (C3 and C4 species) and subdominants. These findings could potentially indicate species reordering in abundance of species within a community with experimental climate change and human management. In chapter two, I investigate precipitation and hay harvest effects by incorporating the relative contribution of biotic vs. abiotic factors and the role of species identity in influencing plant performance (measured by cover and height). I was able to provide new insights that acute hay harvest reduces the strength of the precipitation gradient on plant performance. I found that plant performance responds directly to abiotic change with hay harvest, but indirectly without hay harvest through increased precipitation. Hay harvest reduced the strength of precipitation effects on plant performance through changes in bare-ground cover. Conversely, altered precipitation without hay harvest promoted plant species performance through abiotic factors change first, followed by biotic. Most grassland species, including the dominant grass Schizachyrium scoparium, increased their performance with greater canopy structure. These findings provide evidence for hindering positive effects of biotic factors when hay harvest co-occurs with increasing precipitation. In chapter three, I focus on the effects chronic altered precipitation levels to understand the impact of changes in precipitation on plant phenology and reproductive success. Most studies examining the effects of climate change on plant phenology have focused on climate warming, but in grasslands, precipitation is a dominant factor given their water-limited nature. Furthermore, species with different seasonality (especially late-season species) across species of varying origin, growth form, and life cycle have been underrepresented in phenological studies. I, therefore, report the results of precipitation gradient manipulation on plant phenology (flowering/fruiting dates, duration and flower/fruit count) and reproductive success (seed viability) by dividing responses into community-level and its trait factors (bloom time, functional group and life span), and species responses. I found that traits factors are critical for driving different responses of early and late-flowering species, C3 and C4 species, annuals and perennials to drought. Early-blooming plants minimally advanced their flowering date and produced a lower proportion of viable seeds, whereas late-blooming plants responded in the opposite direction by delaying flowering date at a larger magnitude and producing a higher proportion of viable seeds than annuals. Differential drought tolerance also seemed to play a role in the way plant phenology responded to decreasing precipitation, as indicated by functional group (C3 vs. C4 plants), suggesting that water-use strategies may be related to phenological variation among plants growing in grasslands. When grouping species by life span (annual vs. perennial), C3 perennial plants exhibited stronger advances in flowering and fruiting dates than annuals in response to decreasing precipitation. Community-level analysis showed no response to the precipitation gradient, whereas species not only responded in different magnitudes, but also in different directions within the same community. Hence effects of precipitation on plant phenology might be overlooked if trait factors are not considered. This study adds to a growing body of literature showing that precipitation affects phenology, but the mechanism by which precipitation affects phenology is not understood.