Using high frequency digital repeat photography to quantify the sensitivity of a semi-arid grassland ecosystem to the temporal repackaging of precipitation

Authors: ZHANG, F. - University Of Arizona; Biederman, Joel; DEVINE, C.J. - University Of Arizona; PIERCE, N.A. - University Of Arizona; YAN, D - University Of Arizona; JAVADIAN, M. - University Of Arizona; POTTS, D.L. - Buffalo State College; SMITH, W.K. - University Of Arizona

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2023
Publication Date: 7/15/2023
Citation: Zhang, F., Biederman, J.A., Devine, C., Pierce, N., Yan, D., Javadian, M., Potts, D., Smith, W. 2023. Using high frequency digital repeat photography to quantify the sensitivity of a semi-arid grassland ecosystem to the temporal repackaging of precipitation. Agricultural and Forest Meteorology. 338(15). Article 109539. https://doi.org/10.1016/j.agrformet.2023.109539.
DOI: https://doi.org/10.1016/j.agrformet.2023.109539

Interpretive Summary: arge portions of the western United States are experiencing larger and less frequent rainfalls, driving water more deeply into the soil while the near-surface is often dry. This will likely affect the productivity of rangeland plants differently based on the depth of their roots, affecting decisions made in ranching, land management and carbon cycle science. We conducted a field experiment which excluded natural rainfall and replaced it with irrigation in fewer, larger simulated rainstorms, while keeping the total amount the same over the summer growing season. We found that automated daily photographs compared favorably to labor-intensive measurements of photosynthesis, the main process driving plant productivity. Using the “greenness” of each photo, we quantified how fewer, larger rainfalls 1) produce a time lag of photosynthesis following rainfalls that is greater than under many/small rainfalls by up to a week and 2) result in grasslands where the deeper-rooted perennial plants respond more strongly to few/large rainfalls and the shallow-rooted annual plants are sensitive to many/small rainfalls. These results imply that expected future changes in rainfall will favor deeper-rooted species.

Technical Abstract: The acceleration of global climate warming has intensified the hydrologic cycle and increased the frequency and intensity of climate extremes. Across large portions of the western United States, drought intervals are increasing, often accompanied by larger-magnitude rainfalls. Semi-arid ecosystems are expected to be especially responsive to such temporal repackaging of rainfall because of their high sensitivity to variation in soil moisture. Since the effects of rainfall timing and magnitude on soil moisture vary with depth in the soil profile, rainfall repackaging will likely have differing impacts on vegetation that depend upon rooting depth. We conducted a field manipulation experiment to evaluate the impacts of summer rainfall repackaging (small/many events or large/few events with a fixed total seasonal amount) on a semi-arid mixed annual/perennial bunchgrass ecosystem. The aim of our analysis is to assess the impact of seasonal rainfall repackaging of a fixed total rainfall amount on the sensitivity of plant greenness to soil moisture under each rainfall pulse. We do so by combining automated, high-frequency repeat digital images at both canopy- and species- level, plot-level CO2 uptake measurements, and continuous in-situ moisture data. We found that canopy greenness was closely correlated with gross primary productivity (GPP) across rainfall repackaging treatments (R=0.84). Interestingly, canopy greenness was weakly correlated to soil moisture at the beginning of the growing season but showed a significant positive correlation during peak and late growing season. Notably, we found a significant time lag of ~5 days between changes in soil moisture and canopy greenness under large/few events. We posit that the deeper soil water infiltration observed with large/few rainfall events resulted in: 1) increased sensitivity of deeper-rooted perennial plans to soil moisture during the peak and late growing season compared with annual forbs; and 2) increased perennial grass sensitivity to soil moisture changes and its productivity compared with the small/many and climatic normal treatments. In a semi-arid ecosystem, we show that the canopy greenness, which extracted from high-frequency repeat digital images in our study, could be a good indicator of GPP. We provide additional evidence that differential effects of rainfall repackaging on the moisture distribution in the soil profile mediates plant productivity and greenness. Our results further highlight the differential sensitivity of plant functional-type responses to soil moisture variability. Predicting semi-arid grassland responses to soil moisture dynamics under ongoing rainfall repackaging, and subsequent impacts on the regional to global carbon budget, should consider plant communities’ phenological characteristics and functional trait composition.