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Vulnerability of Prairie Grasslands to Climate Change

Jeff ThorpeSaskatchewan Research Council

March, 2011

Modeling future climates:•Use 1961-90 normals as baseline (PRISM model)•Select future climate scenarios based on Global Climate Models:•Future climates projected for the 2020s, 2050s, and 2080s.

Scenario Manitoba Sask. Alberta

ECHAM4 A2 (cool scenario) X X X

GFCM20 B1 X

CGCM3_T47_2 A1B X

MIMR B1 X

HADCM3 B2 X

HADCM3 A2 (warm scenario) X X X

CSIRO2 B2 X

1960 1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000

3500

Growing Degree-Days (average over Prairie Ecozone)

AB, warm scenario

AB, cool scenario

AB, baseline

SK, warm scenario

SK, cool scenario

SK, baseline

MB, warm scenario

MB, cool scenario

MB, baseline

1960 1980 2000 2020 2040 2060 2080 2100300

400

500

600

700

800

900

Potential Evapotranspiration (mm) (average over Prairie Ecozone)

AB, warm scenario

AB, cool scenario

AB, baseline

SK, warm scenario

SK, cool scenario

SK, baseline

MB, warm scenario

MB, cool scenario

MB, baseline

1960 1980 2000 2020 2040 2060 2080 2100300

400

500

600

700

800

900

Annual Precipitation (mm) (average over Prairie Ecozone)

AB, warm scenario

AB, cool scenario

AB, baseline

SK, warm scenario

SK, cool scenario

SK, baseline

MB, warm scenario

MB, cool scenario

MB, baseline

1960 1980 2000 2020 2040 2060 2080 21000.50

0.55

0.60

0.65

0.70

0.75

Proportion of Precipitation in Summer (average over Prairie Ecozone)

AB, warm scenario

AB, cool scenario

AB, baseline

SK, warm scenario

SK, cool scenario

SK, baseline

MB, warm scenario

MB, cool scenario

MB, baseline

Modeling of vegetation zones in relation to climate

•Most existing studies deal with the forest/grassland boundary, but not with vegetation zones within the grassland.•SRC model based on vegetation zones from Canadian Prairies to Colorado and Nebraska •Use the U.S. as an analogue for a warmer future Canada.•Three climatic variables (1961-90 normals):

• annual precipitation• proportion of precipitation in May-Sep• annual potential evapotranspiration

• Statistical relationships between climate and vegetation zone.

Ecoregions of Canada

Vegetation types in the U.S.

• Standard tool used in range management for representing vegetation dynamics.

• Boxes represent vegetation states and community phases within each state.

• Arrows represent types of transition between them.

State and transition diagrams

Crested Wheatgrass –

native grasses

←exotic invasion

Northern Wheatgrass – Needle-and-

thread

↓heavy grazing

recovery ↑

Needle-and-thread –

Wheatgrass – June Grass – Blue

Grama

↓heavy grazing

recovery ↑

Blue Grama – Needle-and-

thread – June Grass – Western

Wheatgrass

Dry Mixed Grassland, SW Saskatchewan

Crested Wheatgrass –

native grasses

←exotic invasion

Northern Wheatgrass – Needle-and-

thread

climate change (cool scenario)

→

Blue Grama – Needle-and-

thread – Western

Wheatgrass

climate change (warm scenario)

→

Blue Grama – Needle-and-

thread – Western

Wheatgrass

climate change (warm

scenario) →

Blue Grama - Buffalogass

↓heavy grazing

recovery ↑

Needle-and-thread –

Wheatgrass – June Grass – Blue

Grama

↓heavy grazing

recovery ↑

Blue Grama – Needle-and-

thread – June Grass – Western

Wheatgrass

Dry Mixed Grassland, SW Saskatchewan

Snowberry – native grasses

↓fire no fire↑

Smooth Brome – native

grasses

←exotic invasion

Plains Rough Fescue – Northern Wheatgrass –

Western Porcupine GrassKentucky

Bluegrass – native

grasses

←exotic invasion

↓heavy grazing recovery ↑

Western Porcupine Grass – Northern Wheatgrass – Sedge – Pasture Sage

↓heavy grazing recovery ↑

Sedge – Pasture Sage – Western Porcupine Grass –

Northern Wheatgrass

Aspen Parkland, North Battleford

Snowberry – native grasses

↓fire no fire↑

Smooth Brome – native

grasses

←exotic invasion

Plains Rough Fescue – Northern Wheatgrass –

Western Porcupine Grass

climate change

(cool scenario)

→

Western Porcupine Grass – Northern

Wheatgrass

Kentucky Bluegrass –

native grasses

←exotic invasion

climate change (warm

scenario) →

Northern Wheatgrass – Needle-and-

thread

climate change (warm

scenario) →

Blue Grama – Needle-

and-thread – Western

Wheatgrass

↓heavy grazing recovery ↑

Western Porcupine Grass – Northern Wheatgrass – Sedge – Pasture Sage

↓heavy grazing recovery ↑

Sedge – Pasture Sage – Western Porcupine Grass –

Northern Wheatgrass

Aspen Parkland, North Battleford

Implications of zonation model:

• Zonation model does not give exact prediction of future vegetation:• shifts in species may lag behind changes in climate.• new combinations of species could result from

differences in migration rate.

But zonation model shows probable future trends:

• gradual reduction in tree and tall shrub cover.• regeneration failure after disturbance in the boreal fringe• shrinking aspen groves• reduced woody encroachment on grasslands

• shifts in structure of grasslands: decrease of midgrasses, increase of shortgrasses.

• decrease in cool-season grasses, increase in warm-season grasses (other literature supports this).

• gradual introduction of plant and animal species currently found only in the U.S. (e.g. buffalograss, big sagebrush).

•Same three climatic variables (1961-90 normals):• annual precipitation• proportion of precipitation in May-Sep• annual potential evapotranspiration

•Measured grassland production at various locations in Canada and the United States.•Restrict to loamy upland soils (Loam Ecosite).•Statistical relationship between climate and production.

Modelling of grassland production

1960 1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000

3500

Grassland Production on Loam (kg/ha)

Dauphin, baseline

Dauphin, cool scenario

Dauphin, warm scenario

SE Manitoba, baseline

SE Manitoba, cool scenario

SE Manitoba, warm scenario

Lloydminster, baseline

Lloydminster, cool scenario

Lloydminster, warm scenario

Estevan, baseline

Estevan, cool scenario

Estevan, warm scenario

Cardston, baseline

Cardston, cool scenario

Cardston, warm scenario

49°N/110°W, baseline

49°N/110°W, cool scenario

49°N/110°W, warm scenario

• Large increase in potential evapotranspiration (PET) suggests much lower moisture indices (“desertification”).

• However, precipitation has a bigger effect than PET.• Campbell et al. (1997): 90% of the variance in

productivity in grasslands can be accounted for by annual precipitation.

• Rise in temperature has a secondary negative effect, probably because of direct evaporation from the soil.

• Decrease in proportion of precipitation in summer also has a secondary negative effect.

• Other literature supports conclusion of modest productivity changes.

Why not larger decreases in productivity?

Carbon fertilization effect

• These models do not account for possible carbon fertilization effect:• rate of photosynthesis increases with ambient CO2

concentration.• stomatal conductance decreases, meaning improved water use

efficiency.

• Field experiments with CO2 enrichment chambers:• average global increase in grassland production of 17%.• greatest response when moisture supply is limited.

How important is carbon fertilization?

• Is carbon fertilization effect large enough to compensate for the effect of a drier climate?

• Other factors such as heavy grazing or nutrient deficiency could reduce the ability of plants to take advantage of carbon fertilization.

• Some research shows that forage quality declines under carbon fertilization, so cattle would have to consume more to achieve a given weight gain.

• Overall, effect of carbon fertilization in our grasslands is uncertain, but it may help to reduce the impact of a drier climate.

Effects of Drought

• These models represent the average climate (30 year normals) – what about year-to-year variation?

• Droughts are a characteristic feature of grassland climates.• immediate response – reduced grassland productivity• multi-year response – shift in species composition from taller

to shorter species.

• Some studies indicate that climate change will increase variability in precipitation, possibly resulting in more frequent and more intense droughts.

• This could be more important than the changes in average productivity.

Year-to-year variation in measured production at Manyberries, Alberta

0

100

200

300

400

500

600

700

800

900

1000

1930 1940 1950 1960 1970 1980 1990 2000 2010

yearly

average

Yearly Production at Manyberries, and Effect of Climate Change on Average Production

1920 1940 1960 1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600

700

800

900

1000

measured

cool scenario

warm scenario

pro

du

ctio

n (

kg/h

a)

• SRC Forage Calculator• use “forage-year precipitation” – sum from

previous September to current August.• calculate percent deviation of current forage-

year precipitation from long-term average.• predict percent deviation of current forage

yield from the long-term average.• apply to average forage yield to estimate

current forage yield.

Modeling effect of drought on production

• based on variability of precipitation from historic data.

• use statistical distribution to estimate “dry threshold” – precipitation level below which the driest 20% of years fall.

• calculate drought threshold for precipitation in future scenarios (warm scenario shown).

1. assume same variability as in historic data

2. assume increasing variability

Climate change and effect of drought

Drought and grassland composition• Changes during the drought of the 1930s were well

documented. • General U.S. trends – taller grasses decreased,

shorter grasses increased; elimination of most forbs, increase of cactus.

• In northern mixed prairie (including Canada), the main change was increase of early-growing species: Sandberg’s bluegrass, June grass, sedges.

• Impacts of drought were made worse by the heavy grazing practiced at that time.

Drought and woodlands

• During the drought of 2001-2002, average growth of aspen stands declined to near zero.

• Drought interacts with outbreaks of forest tent caterpillar in reducing growth.

• Substantial mortality of aspen (“aspen dieback”) was observed, especially in the Aspen Parkland.

• Tree mortality during droughts could be one of the major processes in shifts in vegetation zonation.

Drought and animals

• Most grassland birds are less abundant in dry years than wet years.

• Shifts in species: e.g. drought of 1988 in North Dakota:• Grasshopper Sparrow, Sprague’s Pipit, Clay-colored

Sparrow, and Baird’s Sparrow decreased.• Horned Lark and Western Meadowlark became more

dominant.

• A wide range of insects declined during the drought of the late 1980s.

• However, outbreaks of plant-eating insects (grasshoppers, forest tent-caterpillar) are often preceded by warm, dry weather.

Climate change and wetlands• It is well known that the number of wetlands and number

of ducks depend on weather cycles, declining in dry years.

• In the long term, models predict decreasing pond numbers and duck populations with climate change.

• The most productive area for ducks, in southeastern Saskatchewan, southern Manitoba, and the Dakotas, will become a more episodic, less reliable source of waterfowl production, similar to the drier areas further west.

• Favourable water and cover conditions will be found further north and east.

• Interaction with land use: drainage of wetlands exacerbates impact of climate change.

Classification of wetlands (Stewart and Kantrud 1971):

• One way species can adjust to climate change is by moving their ranges.

• Globally, average range shift 6.1 km northward per decade over 20th Century (Parmesan and Yohe 2003).

• Species vary in migration rate, so there will be sorting of species along the migrational front, led by the most invasive and trailed by the least invasive.

• Impacts of fragmentation - habitat specialists with poor dispersal ability will be the least able to keep pace with climate change.

General impacts on biodiversity – geographic shifts

• Another way in which species can adjust to climate change is by phenological change.

• Globally, average shift toward earlier spring timing of 2.3 days per decade through the 20th Century.

• At Edmonton, first-flowering date of trembling aspen advanced by 26 days.

• At Delta Marsh, 25 of 27 bird species showed earlier arrival dates over a 63 year period.

• Phenological change can lead to mismatches in timing between predators and prey, pollinator and plant, etc.

• Snowshoe hare – colour change is driven by daylength, so may be mismatched with snowmelt.

Phenological shifts:

• Another form of adaptation in place is evolutionary change.

• Contemporary evolution in thermal tolerance has been observed in frogs, insects, and plants.

• But probably less important than range shifts.• e.g. following Pleistocene glaciation, there was northward

movement of existing species rather than evolution of new species.

• Weedy species are likely to show fastest evolution in response to climate change, because of large population size and short generation time.

Evolutionary change

• Faster evolution (adaptation in place).• Efficient dispersal allowing faster range shifts.• Large native ranges, indicating broad climatic

tolerances.• High habitat connectivity because of use of

disturbed habitats.

Advantages of invasive species under climate change

Increasing susceptibility to invasion:

• Climate change could be a stress that makes communities more susceptible to invasion.

• Existing late-successional plant species could become increasingly ill-adapted to the climate, so more likely to be out-competed by newly arriving species.

• However, invasion also depends on resources: temporary surplus of water or nutrients favours invasion.

• In dry environments, invasion increases with water availability, so increasing drought would actually reduce the risk of invasion.

• Grassland birds – impacts depend on the species.• shift to shorter, more open grassland will reduce

habitat for Sprague’s Pipit but increase habitat for Burrowing Owl.

• loss of tall shrubs will reduce habitat for Loggerhead Shrike.

• Many of our species at risk are northern fringe populations of species that are common in the U.S. • examples: Buffalograss, Western Spiderwort, Hairy Prairie-

clover.• climate change should increase the area of suitable climate for

these species.

Species at Risk

Mixed Prairie – largest area of remaining grassland.

• Mix of growth-forms and photosynthetic types, so shifts in composition can occur initially by increase in species already present.

• Northward migration of southern species is more likely to be successful because of larger grassland area and relatively lower level of fragmentation.

• Dry climate and incidence of drought will reduce risk of exotic invasion.

• However, prolonged, severe droughts leading to soil erosion are more likely in the Mixed Prairie.

• Livestock producers already practice conservative stocking and plan for drought so may be better equipped to deal with climate change.

Impacts on specific grassland types

• Lower grassland area and higher fragmentation compared to Mixed Prairie.

• Shrinking aspen groves and reduced woody encroachment could benefit grasslands and livestock grazing.

• Existing fescue grasslands will shift toward mixed prairie composition – loss of a unique community.

• Shifting of fescue northward will be impeded by high fragmentation of native habitats. Native habitat to north is boreal forest – unknown successional pathways as trees decline.

• Greater risk of exotic invasion – already a bigger problem in Northern Fescue Prairie.

Northern Fescue Prairie

• Zonation models indicate that this type will persist in its current location.

• May shift towards characteristics of Montana foothills grasslands.

Foothills Fescue Grassland

• Part of Aspen Parkland climatically, but with shifts in composition eastward (tallgrass species, Kentucky bluegrass), and with extensive poorly drained soils.

• Risk of in exotic invasion with climate change will be greatest in Manitoba because of moist climate and moist soils. However, shift toward somewhat drier Mixed Prairie may reduce exotic invasion and woody encroachment.

• Tallgrass species are generally warm-season (C4) grasses, which are expected to benefit from climate change. However, future climate may not provide enough moisture over much of southern Manitoba.

• Problems of excess moisture in extreme wet years are greater concern in Manitoba.

Manitoba grasslands

• Decline of forests will create new grasslands – could significantly increase the area of grazing land. But what kind of grasslands?

• With initial canopy opening, forest understory species will grow more vigorously.

• With complete loss of tree cover, some forest shrubs could persist, but most forest herbs will eventually be replaced by sun-adapted species.

• Best-case scenario is forest surrounded by native grassland; propagules will gradually spread into the opening forest, and increasingly out-compete the forest herbs as light levels increase.

New grasslands on former forest land

• But in many situations the propagules that are most available and most aggressive will be exotics such as smooth brome and Kentucky bluegrass.• may already be in the grassland• may have already invaded the forest• nearest source of sun-adapted species may

be roadsides or fields seeded to tame forage.

Former forests:

The resilience of native grassland

• Prairie grasslands have evolved in a highly variable climate – higher drought tolerance than forests or annual crops.

• Prairie grasslands are made up of a mix of species: • taller and shorter species• warm-season and cool-season species• drought-tolerant and moisture-requiring species

• The grasslands of the Great Plains show continuous variation over a huge area (Canada to Mexico).

Resilience continued:

• The drought of the 1930s has shown that native grasslands can adjust by shifts in the proportions of species, while maintaining a grassland ecosystem.

• So there is a large potential for prairie grasslands to adjust to changes in climate.

Barriers to resilience

• Species that are already here can shift in abundance (e.g. increase in native warm-season species).

• But habitat fragmentation may slow the movement of southern species into Canada.

• Maintaining connections between habitats will facilitate range shifts.

• Idea of “assisted migration” – human intervention to help southern species to migrate northward.

Maintaining range health as an adaptation to climate change

• Vigorous plants are better able to cope with climatic stress.

• Litter cover helps to “drought-proof” grassland.• Increased efforts to control exotic invasion may be

needed under climate change.• Good range management practices will become

increasingly important.

• Predict production in the coming year based on fall and winter precipitation.

• Heavier culling of herd than in normal year.• Keep part of herd in yearlings – reduce number in dry

years.• Rent alternative grazing land in moister regions.• Keep portion of grazing land as reserve for dry years.• Sow annual crops for grazing.• During years of poor crop production, turn cattle out on

crops rather than harvest them.• Buy more feed.

Producers already adapt to drought

• Producers in drier regions experience more frequent drought, so tend to have emergency plans in place.• making contacts for alternative grazing• maintaining larger feed reserves• grazing more conservatively

• With climate change, producers in moister regions may have to shift their management in this direction.

Producers and drought

• Rangeland monitoring networks can provide the information needed to assess the impact of climate change.• shifts in species composition• long-term trends in productivity – use to adjust

stocking rate recommendations.• Examples:

• Alberta Rangeland Reference Area Program (183 plots)

• Manitoba Forage Benchmarking Project (63 plots)

Need for monitoring