Wet Events Split the World: Excess Water Boosts Some Ecosystems, Weakens Others

Too much water, it turns out, is not always a gift to the world’s plants. A new study published in Ecological Processes maps how terrestrial ecosystems around the globe respond to wet events—periods of persistent precipitation surplus—and finds that the same excess moisture that revives dryland grasslands can quietly suppress the productivity of boreal forests and tundra.

The team focused on gross primary productivity (GPP), a key vegetation function used to represent terrestrial vegetation productivity. Using the MODIS Terra GPP product at 500-meter resolution, aggregated to monthly values, they compared vegetation productivity during wet events against a climatological baseline built from months with normal moisture conditions. Wet events themselves were identified with the three-month Standardized Precipitation-Evapotranspiration Index, or SPEI-03, a widely used measure of land water balance that accounts for both rainfall and atmospheric evaporative demand. Wet conditions were defined as SPEI values of 1.0 or higher and sorted into moderate, severe, and extreme categories.

What makes the study distinctive is its use of three complementary metrics drawn from ecological stability theory. Sensitivity captures the direction and magnitude of the immediate GPP response per unit of wet stress, retaining the sign so that enhancement and suppression can be distinguished. Resistance measures how far GPP deviates from its normal state during an event, with larger values indicating stronger buffering. Resilience quantifies the rate at which GPP anomalies shrink in the first month after an event ends. Together, the three metrics describe the full arc of a disturbance: the initial shock, the capacity to hold function steady, and the speed of recovery.

The historical picture that emerges is one of pronounced hemispheric divergence. Regions with negative sensitivity—where GPP actually declines during wet events—cluster across the mid- to high latitudes of the Northern Hemisphere, including most of Canada, the northern United States, much of Europe, and the mid- to high-latitude stretches of Eurasia such as Siberia and the Mongolian Plateau. Positive sensitivity, by contrast, concentrates in the mid- to low latitudes of the Southern Hemisphere: much of Australia, eastern and southern South America, southern Africa, and the Sahel. Area-based statistics show that weakly negative sensitivity values dominate globally, meaning that for most of the land surface, wet events either suppress productivity or fail to enhance it.

The biome-level analysis sharpens this contrast. Tropical forests and other high-biomass ecosystems, including tropical moist, dry, and coniferous forests, montane grasslands, and mangroves, show median sensitivity values close to zero across all wetness intensities, indicating that their productivity barely budges during wet events. Temperate and boreal forests and tundra, however, display predominantly negative sensitivity, reflecting a tendency for excess water to suppress carbon assimilation. Grasslands and sparsely vegetated ecosystems in semi-arid regions—tropical and temperate grasslands, flooded grasslands, Mediterranean vegetation, and deserts and xeric shrublands—generally show positive sensitivity, gaining productivity when water arrives. Notably, sensitivity varies more with biome type than with event intensity, pointing to ecosystem structure as the dominant modulator of the immediate response.

Resistance and resilience tell a more troubling story, because both decline as wet events intensify. Most ecosystems maintain relatively high resistance under moderate wet conditions, but buffering capacity erodes under severe and extreme events, with inter-biome differences widening. Tundra and desert ecosystems exhibit the lowest resistance at every intensity level. On the recovery side, regions with high resilience—where GPP anomalies shrink within a month of an event ending—account for 57.5 percent of affected land area, concentrated in South America, tropical Africa, and Australia. But 42.5 percent of affected ecosystems fail to recover effectively, with low resilience widespread across mid- to high-latitude North America and Eurasia. Boreal forests and tundra show negative median resilience across all wetness intensities, meaning their GPP anomalies persist or even intensify after events end. Semi-arid grasslands, though they benefit from moderate wetting, lose both resistance and resilience under intense conditions, likely because shallow-rooted herbaceous species exceed their physiological tolerance under prolonged saturation.

The mechanistic explanation follows the classic water-energy framework. In energy-limited northern regions, where temperature and radiation constrain growth, surplus water cannot be converted into additional carbon uptake; instead, persistent soil saturation induces root-zone oxygen deficiency and nutrient leaching, while cloud cover reduces the photosynthetically active radiation available to plants. In water-limited Southern Hemisphere regions, by contrast, wet events relieve the constraint that normally limits photosynthesis, so productivity rises. The latitudinal patterns of sensitivity were highly consistent across moderate, severe, and extreme events, indicating that event intensity is not the primary driver of the spatial differentiation—the hydroclimatic background is.

Looking forward, the researchers turned to five CMIP6 earth system models under four Shared Socioeconomic Pathways, comparing the future period 2071–2100 against a 1981–2010 baseline. The projections show these divergent responses amplifying under higher emissions. Sensitivity increases in high-latitude countries but decreases across most tropical and Southern Hemisphere countries, a north–south contrast that grows more pronounced from SSP1-2.6 through SSP5-8.5. In some regions the direction of change itself reverses with emissions: Central Asia shows enhanced sensitivity under the low-emission scenario but weakened sensitivity under SSP3-7.0 and SSP5-8.5, with similar reversals in parts of Europe and the southern tip of South America. Resistance generally improves in mid- to high-latitude regions under all scenarios, but declines markedly in central South America, central Africa, and island Southeast Asia under higher emissions—meaning tropical rainforests, historically the most stable ecosystems, lose buffering capacity precisely where warming intensifies extreme precipitation.

Resilience projections are perhaps the most scenario-sensitive of all. Under SSP1-2.6, most countries show enhanced recovery, with significant increases in Russia, Central Africa, and maritime Southeast Asia. But as emissions rise, the extent of enhancement shrinks and widespread reductions emerge across parts of North America, South America, and most of Europe, signaling a slowdown in post-event recovery. The authors also examined country-level trajectories: ecosystems in parts of Central America, such as Mexico, are projected to lose both short-term productivity gains and long-term stability, while high-latitude countries like Canada and Russia may shift from historically negative sensitivity toward near-neutral responses, with the main challenge being growing uncertainty rather than immediate degradation. In tropical countries such as Brazil and Indonesia, sensitivity changes little, but substantial declines in resistance and resilience point to a persistent erosion of recovery capacity.

The study’s conclusions carry weight for the global carbon cycle. Wet events, often perceived as benign or beneficial water inputs, emerge as genuine disturbances capable of eroding ecosystem stability, particularly in the high latitudes where carbon-dense boreal forests and tundra dominate. Because resistance and resilience respond more sharply to event intensity than sensitivity does, extreme wet events pose a disproportionate threat to functional stability even where the immediate productivity response is modest. The authors caution that their resilience metric is an event-scale measure of immediate recovery rather than a gauge of long-term ecosystem restoration, and their threshold sensitivity analysis using alternative SPEI cutoffs of 0.8 and 1.2 confirmed that the main spatial patterns are robust. As climate change increases both the frequency and severity of wet extremes, the findings argue for incorporating wet-event risks into climate adaptation and ecosystem management strategies—treating water surplus not as a guaranteed blessing, but as a disturbance whose consequences depend on the vulnerability of the ecosystems it reaches.

Subject of Research: Global responses of vegetation gross primary productivity to wet events across biomes and future climate scenarios

Article Title: Spatially divergent responses of global vegetation productivity to wet events

Article References: Zhou, X., Xin, Q., Tian, Y., Gui, H., Zhang, Z., Sun, Y., & Dai, Y. (2026). Spatially divergent responses of global vegetation productivity to wet events. Ecological Processes, 15(1), Article 96. https://doi.org/10.1186/s13717-026-00751-z

Image Credits: AI Generated

DOI: 10.1186/s13717-026-00751-z

Keywords: gross primary productivity, wet events, vegetation, climate change, SPEI, CMIP6, ecosystem resilience, boreal forests, tundra, semi-arid grasslands, remote sensing, carbon cycle