Soil-Plant-Atmosphere Coupling during Interstorm Periods

Abstract

The future trajectory of net terrestrial carbon uptake and agricultural yields are dependent on how vegetation responds to climate forcings. However, characterizing vegetation responses to water stress and other environmental drivers is challenging because these forcing factors are inter-related, especially on seasonal timescales. Here, newly available global mapping measurements from microwave satellite sensors are used to characterize water exchange in the soil-plant-atmosphere continuum. These satellites enable evaluation of time evolution of landscape-scale plant water content during interstorm periods, providing insights into underlying mechanisms and allowing disentangling of their drivers. Here, I ask: what are the fundamental landscape-scale plant responses to rainfall events and interstorm drying? What does interstorm land surface behavior reveal about landscape responsiveness to climate forcing and vulnerability to change?

Using satellite and field tower observations, plant water and carbon uptake responses to rain pulses are characterized across global ecosystems. Responses depend on pulse characteristics: smaller pulses on initially dry soils produce slow rehydration responses; larger pulses on initially wet soils trigger rapid growth. Though more pronounced in drier environments, these responses occur across global ecosystems, which is more widespread than previously thought. Furthermore, the soil moisture threshold is estimated below which waterlimitation occurs and land-atmosphere interactions strengthen. With sufficient drying below this threshold, soil and plant water loss become highly linked to surface warming and drying. This enhances plant water stress. Finally, regions spending more time in this water-limited evaporative regime are most responsive to surface forcings.

Several findings emerge here: (1) rapid plant growth and slow rehydration responses to surface rewetting are more widespread than previously thought. Given their additional water stress responses to interstorm drying, global vegetation and consequently the carbon cycle are vulnerable to shifting rainfall intensity and interstorm length under climate change. (2) Landscapes most vulnerable to environmental variability and change are those that progressively spend more time in the water-limited regime, not necessarily those that receive the greatest forcing variability. (3) Satellite plant water content observations contain information on subweekly timescales consistent with plant hydraulic theory and field measurements, which reveals new avenues for vegetation remote sensing and ecological modeling efforts.