How big is big enough? Surprising responses of a semiarid grassland to increasing deluge size

Abstract

Climate change has intensified the hydrologic cycle globally, increasing the magnitude and frequency of large precipitation events, or deluges. Dryland ecosystems are expected to be particularly responsive to increases in deluge size, as their ecological processes are largely dependent on distinct soil moisture pulses. To better understand how increasing deluge size will affect ecosystem function, we conducted a field experiment in a native semiarid shortgrass steppe (Colorado, USA). We quantified ecological responses to a range of deluge sizes, from moderate to extreme, with the goal of identifying response patterns and thresholds beyond which ecological processes would not increase further (saturate). Using a replicated regression approach, we imposed single deluges that ranged in size from 20 to 120 mm (82.3rd to >99.9th percentile of historical event size) on undisturbed grassland plots. We quantified pre‐ and postdeluge responses in soil moisture, soil respiration, and canopy greenness, as well as leaf water potential, growth, and flowering of the dominant grass species (Bouteloua gracilis). We also measured end of season above‐ and belowground net primary production (ANPP, BNPP). As expected, this water‐limited ecosystem responded strongly to the applied deluges, but surprisingly, most variables increased linearly with deluge size. We found little evidence for response thresholds within the range of deluge sizes imposed, at least during this dry year. Instead, response patterns reflected the linear increase in the duration of elevated soil moisture (2–22 days) with increasing event size. Flowering of B. gracilis and soil respiration responded particularly strongly to deluge size (14‐ and 4‐fold increases, respectively), as did ANPP and BNPP (~60% increase for both). Overall, our results suggest that this semiarid grassland will respond positively and linearly to predicted increases in deluge size, and that event sizes may need to exceed historical magnitudes, or occur during wet years, before responses saturate.

Semiarid grasslands and extreme precipitation events: do experimental results scale to the landscape?

“…We used the above dataset to identify isolated summer (June-September) deluge events. We define a deluge as a statistically large event based on the long-term rainfall record (Post & Knapp, 2021). The average monthly deluge size included in this study ranged from 47-70 mm (Table 1), representing event sizes that exceed the 90 th percentile based on the long-term precipitation record (Post & Knapp, 2021).…”

“…This article is protected by copyright. All rights reserved Knapp, 2021). This is because rain days tend to be clustered in the SGS and function as a single soil moisture pulse (Noy-Meir, 1973;Loik et al, 2004).…”

“…For example, in the semiarid shortgrass steppe (SGS) of Colorado, productivity was stimulated most by a deluge at mid-growing season relative to deluges of the same size occurring earlier or later in the season (Post & Knapp, 2020). Further, although a threshold event size might be expected (beyond which ecosystem responses saturate), productivity responses to experimental deluges increased strongly and linearly even as deluge size reached historical maxima (Post & Knapp, 2021).…”

“…Here, we assessed responses to naturally occurring deluges across the 62.7 km 2 Central Plains Experimental Range (CPER) in NE Colorado, a SGS ecosystem with considerable spatial variation in soils and grazing management, and where several small-scale deluge experiments have been conducted (Post & Knapp 2019, 2020, 2021. This allowed us to directly compare responses to simulated deluges at the plot scale to natural deluges at the landscape scale.…”

“…We then related spatial variation in precipitation to subsequent responses in canopy greenness (via the normalized difference vegetation index, NDVI) as a surrogate for aboveground net primary production (ANPP, Hermance et al, 2015). Our goal was to test two general predictions from past experiments (Post & Knapp 2020, 2021: (1) that ANPP responses to deluges would vary Accepted Article seasonally, with the greatest sensitivity at mid-growing season, and (2) that ANPP would increase linearly as precipitation event/deluge size increased.…”