Using phenology to unravel differential soil water use and productivity in a semiarid savanna

Authors: STEINER, B. - Old Dominion University; Scott, Russell - Russ; HU, J. - University Of Arizona; MACBEAN, N. - Western University; RICHARDSON, A. - Northern Arizona University; MOORE, D.P. - University Of Arizona

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/5/2023
Publication Date: 2/27/2024
Citation: Steiner, B., Scott, R.L., Hu, J., MacBean, N., Richardson, A., Moore, D. 2024. Using phenology to unravel differential soil water use and productivity in a semiarid savanna. Ecosphere. 15(2). Article e4762. https://doi.org/10.1002/ecs2.4762.
DOI: https://doi.org/10.1002/ecs2.4762

Interpretive Summary: Plant growth in savannas in the western U.S. are limited by water and characterized by two dominant plant types: trees and grasses. Different growth patterns and root structures of these plant types complicate how savanna plant productivity responds to changes in water availability. We tested the hypothesis that productivity in savannas is controlled by the timing and spatial distribution of soil water changes and differences in growing season length of grasses and trees. We used automated cameras and satellite observations to measure changes in plant greenness and meteorological techniques to measure carbon dioxide exchange between the land and atmosphere at a semiarid savanna site in Arizona. We also measured soil water at shallow and deeper depths. We found that tree greenness increased with soil water at both depths while grass was only sensitive to the shallower soil water. The date that plants became dormant in fall was most related to the amount of annual savanna carbon dioxide exchange. Higher soil water led to an extended growing season, caused by delayed dormancy in trees but, the grasses showed no evidence of delayed dormancy in wetter periods. These findings suggest that savanna plant productivity depends on different responses to soil water by savanna grasses and trees.

Technical Abstract: Savannas are water-limited ecosystems characterized by two dominant plant types: trees and an understory primarily made up of grass. Different phenology and root structures of these plant types complicate how savanna primary productivity responds to changes in water availability. We tested the hypothesis that productivity in savannas is controlled by the temporal and vertical distribution of soil water content (SWC) and differences in growing season length of understory and tree plant functional types. To quantify the relationship between tree, understory, and savanna-wide phenology and productivity we used phenocam and satellite observations surrounding an eddy covariance tower at a semiarid savanna site in Arizona, USA. We distinguished between SWC across two different depth intervals (shallow, < 0-30 cm and deep, > 30-100 cm). We found that tree greenness increased with SWC at both depths while understory greenness was only sensitive to the shallower SWC measurements. Ecosystem dormancy, estimated from satellite observations close to the eddy covariance tower, explained more variability in annual GPP than other phenometrics. Higher SWC led to an extended growing season, caused by delayed dormancy in trees but, the understory showed no evidence of delayed dormancy in wetter periods. We infer that the timing of ecosystem scale dormancy, driven by trees, is important in understanding changes in a savanna’s GPP. These findings highlight the important effects of rainfall during the winter and considering nuanced phenological responses different savanna plants have to water. These findings suggest that savanna GPP is conditional on differential responses to moisture available by each of the dominant vegetation components.