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Vulnerability of Prairie Grasslands to Climate Change. Jeff Thorpe Saskatchewan Research Council March, 2011. Modeling future climates:. Use 1961-90 normals as baseline (PRISM model) Select future climate scenarios based on Global Climate Models:
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Vulnerability of Prairie Grasslands to Climate Change Jeff Thorpe Saskatchewan 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.
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.
State and transition diagrams • 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.
Implications of zonation model: • Zonationmodel 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).
Modelling of grassland production • 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.
Why not larger decreases in productivity? • 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.
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
Yearly Production at Manyberries, and Effect of Climate Change on Average Production
Modeling effect of drought on production • 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.
Climate change and effect of drought • 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). • assume same variability as in historic data • assume increasing variability
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.
General impacts on biodiversity – geographic shifts • 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.
Phenological 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.
Evolutionary change • 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.
Advantages of invasive species under climate 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.
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.
Species at Risk • 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.
Impacts on specific grassland types 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.
Northern Fescue Prairie • 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.
