Consequences of Climate Change for Rangelands: Unexpected Consequences of More Extreme Rainfall Patterns Alan K. Knapp Colorado State University Jana Heisler-White University of Wyoming Melinda D. Smith Melinda D. Smith Yale University John M. Blair John M. Blair Kansas State University

Why should we care about climate change? – The Grassland Ecologist’s Perspective We may be witnessing one of the most profound climatic changes in the Earth's history. Predictions for the 21st century show warming of the troposphere especially in the Arctic, an increase in precipitation extremes, a decrease in snow and ice cover, and an increase in sea level. The danger facing society today is that anthropogenic global warming may be too fast to allow humans, and other species, to adapt to its detrimental impacts.

T. Riley and J.A. Klein Grasslands, rangelands, steppe, tundra, savanna and shrub- grasslands cover ~ 40% of the Earth’s land surface

Ecosystem Services Grasslands Provide: Forage for domestic livestock Forage for domestic livestock Habitat for native ungulates Habitat for native ungulates Habitat for grassland plants/animals Habitat for grassland plants/animals Migratory birds – breeding/forage Migratory birds – breeding/forage Soil erosion control Soil erosion control Carbon sequestration Carbon sequestration

Sala et al Grasslands (most) are water limited systems… Rainfall vs. Production across the Central US Setting the stage…

The world is getting warmer…. (thermometer evidence)

Dave Theobald Plant evidence…

Dave Theobald

Overpeck et al History provides few analogs for the type of climate change forecast…we are in uncharted waters… Little Ice Age: ’s - bitter winters, famine in Europe Younger Dryas – 1300 yr cold period (11,000 BP), then rapid warming IPCC predictions

Rainfall regimes CO 2 effects Temperature effects Soil moisture Plant water relations Water, temperature (and CO 2 ) are linked in grasslands and most ecosystems…

Global Climate Model (GCM) predictions for changes in mean annual rainfall changes are inconsistent with each other in magnitude and sign…But more extreme weather as the atmosphere warms is consistently predicted.

Dust Bowl of 1930’s in US Drought in Senegal More intense hurricanes and floods Are grasslands vulnerable to this aspect of climate change? Focus on “subtle” increases in “extreme” events – or shifts in rainfall patterns…not the obvious…but the pervasive… Proportion of total rainfall in the US from large (>5 cm) rainfall events Proportion (%) of rainfall in “extreme events”

Proportion of total rainfall in the US from large (>5 cm) rainfall events Proportion (%) of rainfall in “extreme events” Abundant evidence that rainfall regimes are changing Oklahoma

 12 rainfall manipulation plots  Collect, store, and reapply natural rainfall on intact grassland plots The Rainfall Manipulation Plot (RaMP) Experiment Address the impact of changes in size and timing of growing season rainfall To what extent will altered rainfall patterns affect ecological processes in grasslands?

Experimental manipulation: 50% longer inter-rainfall periods, larger individual rain events No change in total precipitation quantity Fewer events Greater storm size Increased dry periods

Ambient ppt timing: “typical” seasonal pattern Altered ppt timing: repeated deficits more extreme wetting and drying cycles Key driver of responses: Soil moisture dynamics Average soil water content in top 30 cm: - reduced by 12% Variability in soil moisture: - increased by 27%

All rainfall treatments

What about the Shortgrass steppe – & semi-arid and arid grasslands? Like all grasslands, a system of “extremes”

Frequent small events (10 day intervals, 15 mm events) Infrequent big events (30 day intervals, 47 mm events) All received “average” MAP amounts Shortgrass steppe

(9 events) - Heisler et al Semi-arid grassland Extreme rainfall events Long dry intervals Increases ANPP

Ojima and Lackett 2002 Tall-grass Mid-grass Short-grass Precipitation gradient Regional perspective

Semi-arid grassland Extreme rainfall events Long dry intervals Increases ANPP? With the same experimental design, how will these different grasslands respond? Mesic grassland Extreme rainfall events Long dry intervals Reduces ANPP?

SGS Experimental alteration of rainfall patterns across the Great Plains of North America Frequent, small events Fewer, infrequent large events VS. A = 12 events B = 6 events C = 4 events Total = 191 mm A = 12 events B = 6 events C = 4 events Total = 450 mm KNZ Semi-arid Mesic Jana Heisler

Mesic Grassland A shift from 12 to 4 events results in a 15% decrease in productivity. More “extreme” rainfall regime Shift to more extreme (large) rainfall events  reduced productivity

Semi-arid Grassland A shift from 12 to 4 events resulted in a 30% increase in productivity. More “extreme” rainfall regime Shift to more extreme (large) rainfall events  increased productivity

Sevilleta LTER – Central New Mexico Monsoon rainfall addition experiment: 20 mm events vs. four 5 mm events

Seasonally averaged soil water content, plant water status, photosynthesis, and production were highest in plots receiving a small number of large events Collins, Thomey, Johnson, Brown, Natvig, Friggens 20 mm events 5 mm events 20 mm events 5 mm events

Why do dry vs. mesic grasslands respond differently? Two aspects of more extreme rainfall regimes: 1. Larger rain events 2. Longer dry periods between events When soils are usually moist, longer dry periods are most important… When soil are already dry, larger rain events cause the greater response…

Wet relative to ambient Dry relative to ambient

Sala et al Responses to climate change depends on location even in the same grassland biome – this make generalizations a challenge… Prediction based on wet grassland responses Prediction based on dry grassland responses The future?

Goldewijk and Battjes (1997) Reid & Miller (1989) Vitousek (1994) NOA A Ecological Systems Climate Change is but one facet of “Global Change” Grazing J.A. Klein

…we are changing Earth more rapidly than we are understanding it. -Vitousek et al We have moved beyond questions such as “Is climate change happening?” There is little uncertainty about this. The uncertainty is with the consequences of climate change…

Key Points: Research indicates that subtle climate change can have surprising consequences. 1. Even “subtle” changes toward more extreme rainfall patterns can alter production and the sensitivity of the grasslands to changes in rainfall amounts 2. Grasslands can respond in opposing ways to the same form of climate change