Grassland ecosystems and climate change: Insights from experiments. Alan K. Knapp Department of Biology Graduate Degree Program in Ecology

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1 Grassland ecosystems and climate change: Insights from experiments Alan K. Knapp Department of Biology Graduate Degree Program in Ecology

2 we are changing Earth more rapidly than we are understanding it. -Vitousek et al Global population (millions) Temperature 4 Population CO 2 Nitrogen 2 Carbon Year We have moved beyond What questions do we such know? as Is climate change happening? and What is the cause? The uncertainty is with the consequences of climate change We know that climate change is chronic and directional, but also that variability will be increased Carbon emissions (Gt) Total reactive N (Tg N y -1 ) Atmospheric CO 2 (ppm) Global temperature anomaly (C)

3 Global changes in rainfall, temperature and CO 2 can be linked in most ecosystems through their indirect effects Rainfall regimes CO 2 effects Soil moisture Temperature effects - Organism Water relations Ecosystem Process & Function

4 NA Grasslands (most) are water limited systems and precipitation amount predicts spatial patterns of ANPP well Rainfall vs. Production across the Central US Sala et al. 1988

5 What do we need to know? How will climate change affect earth s ecosystems? Can we forecast the future? "I would not say that the future is necessarily less predictable than the past. I think the past was not predictable when it started." ~Former Defense Secretary Donald Rumsfeld

6 How we think - A resource perspective A. Disturbance B. Global Change Discrete disturbance Biota Biota Resources Resources Chronic resource alteration - Smith, Knapp and Collins - Ecology 2009 Moving from a disturbance centric world

7 Mesic Grasslands of North America The Flint Hills of Kansas The Konza Prairie Diverse C 3 forbs KPBS A dynamic & tractable system C 4 grass dominance Tallgrass prairie is the most mesic of Great Plains grasslands sufficient precipitation to support forest Flint Hills are located at the dry edge of the original range of tallgrass prairie Mean annual precipitation = 835 mm

8 Chronic (water) resource addition What is the degree of water limitation in this mesic grassland? If water is limiting this system, what are the consequences of a chronic change in resource abundance (soil moisture)?

9 Long-term Irrigation Study Treatments initiated in 1991 Supplemental water added during the growing season to replicate 140 m transects (paired with control transects) Designed to meet plant water demand and minimize intra-annual variability in soil water deficits

10 Summary of the first eight years Water availability limited ANPP 6 out of 8 years Irrigation increased ANPP by ~25% (physiological response) Total ANPP (g/m 2 ) * Control Irrigated * * * * * r 2 = PPT (mm) ANPP (g/m 2 ) Year Annual ppt + irrigation (mm)

11 The value of long-term experiments (+15 years) Mean increase in ANPP for the next nine years was 65% 70% (vs. 25%) An 8 year lagged response ANPP (g/m 2 ) r 2 =0.52 Control irrigated irrigated Annual ppt + irrigation (mm)

12 Stimulation in ANPP (g/m 2 ) = r 2 = 0.66 = Irrigation water added (mm) What would change the relationship between ANPP and precipitation? Resource alteration Community change Response Community re-ordering Physiological response 8 yrs 9 yrs Time Smith, Knapp and Collins 2009

13 Difference in Absolute cover between Irrigation and Control transects Panicum virgatum Absolute Cover (%) Δ ANPP = g/m 2 Δ ANPP = g/m Date

14 Sala et al relationship Control plots Control relationship 17-yr mean control plots Initial 8-yr mean, irrigated lowlands Next 9-yr mean, irrigated lowlands ANPP (g/m 2 ) Precipitation (mm)

15 Proportion (%) of rainfall in extreme events Abundant evidence that rainfall regimes are changing Question: b. Given increased variability and frequency 12.0 of 10.0 precipitation patterns affect ecological processes Rain Intensity (mm/event) extreme events, to what extent will altered in mesic grasslands? a. c. Oklahoma Year r 2 =0.534 r 2 = r 2 =0.638 Proportion of total rainfall in the US from large (>5 cm) rainfall events Year

16 The Rainfall Manipulation Plot (RaMP) Experiment Address the impact of changes in size and timing of growing season rainfall 12 rainfall manipulation plots (RaMPs) + non-sheltered controls Collect, store, and reapply natural rainfall on intact prairie plots Treatments include ambient and altered rainfall patterns

17 Target altered rainfall pattern: 50% longer inter-rainfall periods, larger individual rain events Current Precipitation amount Greater storm size Predicted Increased dry periods No change in total precipitation quantity Day of of year Fewer events

18 Key driver of responses: Soil moisture dynamics Ambient ppt timing: typical seasonal pattern 50 A - Ambient rainfall treatment Mean SWC = 29.5 ± 1.2% Depth (cm) Altered ppt timing: repeated deficits more extreme wetting and drying cycles Average soil water content in top 30 cm: - reduced by 12% Variability in soil moisture: - increased by 27% Soil water content 0-30 cm (%) B - Altered rainfall treatment Mean SWC = 24.7 ± 1.4% Ambient Jun Jul Aug Sep Oct Change in SWC (%) per sampling interval Root mass (% of total) P = 0.01 Altered Precipitation (mm) Precipitation (mm) Date

19 ANPP responses to altered rainfall timing ANPP (g m -2 ) 1000 ambient timing altered timing * pre-treatment * * Year * * 13-22% reduction in years with significant responses

20 Across all years and all rainfall treatments Ambient timing Altered timing ANPP (g m -2 ) r 2 = CV of soil water content (0-15 cm)

21 A regional assessment Short-grass Tall-grass Ojima and Lackett 2002 Mid-grass Precipitation gradient Jana Heisler-White et al Contingent productivity responses to more extreme rainfall regimes across a grassland biome. Global Change Biology 15:

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

23 Production (g/m 2 ) Mesic Grassland Shift to more extreme KNZ 2006 (large) rainfall events reduced productivity a b More extreme rainfall regime c c A shift from 12 to 4 events results in a 15% decrease in productivity. Ambient 12 events 6 events 4 events # of events

24 Semi-arid Grassland Production (g/m 2 ) Shift to more extreme (large) rainfall events increased productivity a a b More extreme rainfall regime b A shift from 12 to 4 events resulted in a 30% increase in productivity. 20 Ambient 12 events 6 events 4 events # of rain events

25 Why do arid vs. mesic grasslands respond differently? Two aspects of more extreme rainfall regimes: Precipitation amount Current Predicted Increased dry periods Greater storm size 1. Larger rain events 2. Longer dry periods between events Fewer events Day of year When soils are usually moist, longer dry periods are most important When soil are already dry, larger rain events cause the greater response

26 Where do we need to go with future field experiments? We need more long-term experiments where we push ecosystems We need comparative analyses of ecosystem responses to climate change at large spatial scales

27 Acknowledgements: John Briggs, Scott Collins, John Blair, Patrick O Neal, Melinda Smith, James Koelliker, Jana Heisler-White Colorado State University Kansas State University Konza Prairie Biological Station USDA Managed Ecosystems Program NSF LTER & Ecosystem Studies Program Department of Energy Program in Ecosystem Research