Response of the mean global vegetation distribution to interannual climate variability Michael Notaro Associate Scientist Center for Climatic Research University of Wisconsin-Madison

Potential Natural Vegetation Data by Ramankutty and Foley (1999) Deciduous Evergreen Grass Tree

Lund-Potsdam-Jena Dynamic Global Vegetation Model LPJ - DGVM Process-based representation of land-atmosphere coupling. Explicit inclusion of major processes of vegetation dynamics, including natural fires, growth, and competition. The fundamental entity is the average individual of a plant functional type (PFT). Grid cells contain any combination of PFTs and bare ground. 10 PFTs: 2 tropical trees, 3 temperate trees, 3 boreal trees, 2 grasses Woody PFTs include evergreens, summergreens (temperature-limited), and raingreens (water-limited) 2 soil layers (0.5 & 1.0 m): Grass roots are primarily limited to the top layer, but tree roots extend substantially into deeper soil. References: Sitch (2000); Sitch et al. (2003)

Input Data - CRU TS2.1 (Mitchell and Jones 2005) -Monthly data for : Temperature Precipitation Cloud cover fraction # wet days per month  x0.5  resolution

Simulations SimulationTemperature Interannual Variability? Precipitation Interannual Variability? VarYes NoVarNo VarNotTmpNoYes VarNotPcpYesNo SimulationT Var? DJF Pcp Var? MAM Pcp Var? JJA Pcp Var? SON Pcp Var? VarNotPcpDJFYesNoYes VarNotPcpMAMYes NoYes VarNotPcpJJAYes NoYes VarNotPcpSONYes No [CO2]=375 ppmv 1000 year spinup

Difference in Vegetation Cover Fraction (Var-NoVar)

Histograms of annual vegetation cover fraction for the Southwest United States

Difference in Vegetation Carbon (kgC) (Var-NoVar) Difference in Fire Burn Fraction (Var-NoVar)

Difference in Tree Cover Fraction (Var-NoVar)

Difference in Grass Cover Fraction (Var-NoVar)

Grass vs. Tree Fire: Interannual precipitation variability allows for prolonged wet periods, during which above-ground litter accumulates, and prolonged droughts that dry the soil, both contributing to extensive / frequent fires. The simulated increase in fire intensity / duration / frequency favors grassland expansion at the expense of forests, since the latter are slower to recover following fire. Drought: The ability of grass to quickly reestablish, compared to trees, gives it an advantage following intense droughts, associated with climate variability. Grass can survive droughts through dormancy, while trees either require continual water supply or must rely on deciduousness. Interception: Since grasses intercept substantially less falling precipitation than trees, a grassland is more likely to maintain sufficient soil water during periods of high rainfall variability than a forest, giving grasses an advantage under climate variability.

Difference in Evergreen Tree cover Fraction (Var-NoVar) Difference in Deciduous Tree cover Fraction (Var-NoVar)

Deciduous vs. Evergreen Cold season: One bioclimatic limit that differs between summergreen and evergreen trees, in temperate / boreal zones, is minimum coldest month T. Summergreen trees can survive at colder winter T’s, so an increase in variability in mean coldest month T favors summergreens over evergreens. During extreme cold, evergreen trees suffer winter desiccation, since they continue to maintain needles and respire but cannot retrieve water from frozen soil, while water in leaves freezes. T variability allows such low T thresholds to be exceeded, leading to evergreen tree mortality while summergreen trees are unaffected. Interception: Evergreen trees are parameterized with a higher canopy interception of rainfall, thereby diminishing soil water reserves during variable rainfall periods and making growing conditions less favorable. Respiration: Deciduous trees shed leaves during droughts or dry season, limiting respiration costs, while evergreen trees maintain year-round leaves / needles, resulting in year-round respiration costs and sensitivity to precipitation in all seasons. Deciduous and evergreen trees have leaf longevities of 0.5 and 4 years, respectively, and leaf respiration is a function of leaf cover. Summergreen trees exhibit leaf senescence when daily T 5  C. The wintertime loss of leaves eliminates its respiration costs then.

Area

Area

Distribution of Evergreen Forests, Deciduous Forests, and Grasslands Across the United States X = SW Missouri (37.25N, 93.25W)

Focus on Southwest Missouri

CONCLUSIONS Interannual climate variability reduces net global vegetation cover, particularly over semi-arid regions. Interannual climate variability favors the expansion of grass cover at the expense of tree cover, due to differences in growth rates, fire impacts, and interception. The area burnt by global fires is substantially enhanced by interannual precipitation variability.

CONCLUSIONS The current position of the central US ecotone (forests to east and grasslands to west) is largely attributed to climate variability. Among woody vegetation, climate variability supports expanded deciduous forest growth and diminished evergreen forest growth, due to differences in bioclimatic limits, leaf longevity, interception rates, and rooting depth. These results offer insight into future ecosystem distributions since climate models generally predict an increase in climate variability and extremes.