Soil water and temperature explain canopy phenology in the shortgrass steppe

Background/Question/Methods

In the US Great Plains, biomass seasonality is variable and depends on either temperature or precipitation, depending on which remains limiting as spring progresses. Primary production in the shortgrass steppe is positively related to precipitation, but the influence of temperature is dependent on precipitation and the combined effects determine water availability. In water-limited ecosystems such as the shortgrass steppe, the timing of the onset of spring very likely reflects a response to both temperature and precipitation.  In the shortgrass steppe, surface reflectance measurements and derived vegetation indices combined with on-site or nearby climate data can be used to gain a detailed understanding of how the timing of plant growth is influenced by temperature and precipitation. The overall goal of this study is to address questions about relationships between the timing of plant growth, soil temperature, and soil water using surface reflectance measurements.  Specifically, how do soil water and soil temperature individually and jointly influence the timing of the onset of spring and peak greenness on the shortgrass steppe?

In this paper we compare 7 years of plant canopy development. We used two-channel radiometers and measured reflected radiation in the red and near infrared wavelengths on ungrazed shortgrass steppe. Reflectance measurements were polled daily, averaged and stored.  We used these data to calculate a response variable, the greenness index.  Precipitation, air temperature, soil water and soil temperature were measured daily on-site and nearby.  We calculated soil growing degree-days and accumulated soil water days as explanatory variables.

Results/Conclusions

We found that accumulated soil water and accumulated soil temperature individually influence the timing of green up and peak growth.  We determined that a matric potential close to field capacity is necessary to initiate the onset of spring on the shortgrass steppe.  We found that the onset of spring was delayed 60 days during an extreme drought year when compared to average precipitation years.  Additionally, regression models showed that the combined effects of accumulated soil temperature and accumulated soil water explained from 70 to 92 percent of the variability of the seasonal phenological patterns of the shortgrass steppe.  Multiple regression models were able to elucidate pulses in greenness, but only when considering the combined influence of soil water and soil temperature. The results of this research illustrate how important precipitation is when estimating phenological metrics such as growing season length and peak growth in arid and semiarid systems.