Phenology as an integrator of climate change in the shortgrass steppe

Ecosystem response to climate change is one of the most important areas of research in ecology today. Phenology, the study of the timing of biological events, is considered a sensitive indictor of climate change. Most phenological research has focused on north temperate systems where climate change signals are amplified or in mesic forests where temperature is the primary controller of seasonal phenology. Phenological research in semiarid grasslands is uncommon, yet has great potential to answer questions about how these dynamic systems may respond to changing climatic factors.;In an exclosure at the Shortgrass Steppe Long-Term Ecological Research Site (SGS), a semiarid ecosystem on the western portion of the Great Plains in the USA I used 12 years of measurements of near-surface reflected radiation and derived vegetation indices combined with on site or nearby soil moisture and temperature observations to gain a detailed understanding of how the timing of plant growth is influenced by soil temperature and soil water. I show that neither adequate degree-day accumulation nor adequate soil water alone suffice to initiate canopy growth; only the combination of both, adequate temperature and soil moisture, can initiate canopy green-up. I determined that soil water content must be close to field capacity and soil temperature must be above 0°C to initiate a phenological response. I show for the first time that in the SGS, the timing of the onset of spring, i.e., initiation of plant growth, and subsequent temporal patterns of plant growth reflect a response to a combination of temperature and precipitation.;More complex are the end of season dynamics of grasslands, where the mechanism of dormancy is a gradual response to climatic variables. These complications are magnified in dry grasslands, where the effects of temperature on phenology are modulated by the availability of soil water. My objectives were to identify the primary drivers influencing the timing of end of season on the shortgrass steppe and determine if the timing of start of season, end of season, or both influences the growing season length of the shortgrass steppe. The results of my study suggest that temperature and soil water interact to influence the timing of end of season in the shortgrass steppe and growing season length is strongly related to the date of start of season and less so to the date of end of season.;Shifts in flowering dates are associated with climate change and warming temperatures. However, the influence of temperature and precipitation on annual phenology patterns in semiarid ecosystems is not well understood. I compared the first, last, and peak flowering dates of 21 shortgrass steppe species from 1995-2013. I used onsite and nearby climate data to elucidate relationships between the timing of flowering and temperature and precipitation. The probability of flowering success for the majority of the study species was greater than 0.80. The overall trend for first flowering dates of 21 species advanced 8.4 days over the 19-year period. I found that the advance of the first flowering date of 4 taxa was significantly related to increased spring temperatures and the delay of the last flowering date of 3 taxa was significantly related to increases in fall precipitation. In a water-limited ecosystem such as the shortgrass steppe advances in flowering phenology are occurring primarily in response to warming spring temperatures for early taxa and delays primarily in response to precipitation for later taxa.;My findings bear important implications for understanding semiarid ecosystems under climate change. My research illustrates how flowering phenology responses may differ across ecosystems. Because future precipitation and temperature tend to diverge, understanding responses in seasonality of greenness as well as productivity in general must take both precipitation and temperature into accoun.