California rangeland status, structure and func2on Valerie Eviner, UC Davis
Million acres Rangeland 57 Area Available for 41 Grazing Area Grazed 34 FRAP 2003
Grasslands (including within hardwood woodlands) provide over 2/3 of CA livestock forage FRAP 2003
States Oak woodland Oak savanna Shrubs “New” exo2cs ‐ Medusahead Exo2c annual grassland ‐ Goatgrass (grasses and forbs) ‐ Yellow starthistle Na2ve grassland (grasses and forbs)
a Precipitation Major driver of Temperature Wet-up func?on/ Climate composi?on • Timing of precipita?on and warm Summer Fall Winter Early Late Summer temperatures b Spring Spring Aboveground • Variability in Wet-up Belowground precipita?on Plant growth rate • Past 20 years, 6‐8 fold annual varia?on in precipita?on • At least 8 mul?‐ year droughts since 1900 Summer Fall Winter Early Late Summer Spring Spring Biswell 1956, Eviner & Firestone 2007
Factors influencing produc?on and states and transi?ons • Precipita2on‐ amount, seasonality • Soil‐ nutrients (N,P, S), water • Grazing‐ ?ming, intensity, frequency, type, dura?on – Also herbivory by: grasshoppers, elk, deer • Fire‐ ?ming, intensity, frequency, type • Soil disturbance by animals (gophers, ground squirrels, feral pigs, earthworms) e.g. pocket gophers turn over en?re soil surface every 3‐15 years • Granivory‐ voles, mice, ants • Pathogens‐ barley yellow dwarf virus, sudden oak death • Compe??on between plant groups
Plant community Nitrogen Other Key determinants availability of impacts Fire Spring burning o[en Short‐term Short‐term Timing increases forbs in increase in erosion Frequency short‐term nitrogen availability Soil O[en increases forbs Short‐term Short‐term Timing disturbance by in short‐term increase in erosion Frequency mammals poten?al to increase nitrogen annual exo?cs availability, soil carbon release Grazing Generally increase No impact to Compac?on Timing na?ve annual forbs slight increase in N Alter small Frequency Varied impacts on: pools, cycling mammal ac?vity Dura?on exo?c grasses, na?ve Redistribu?on Trampling of Intensity grasses, produc?vity thatch Grazer Type Increase root Weather alloca?on Vegeta?on type Grazing Increased thatch can: exclusion ‐ Decrease produc?on ‐ Decrease forbs (2‐ fold) ‐ Increase fire ‐ Increase goatgrass
States alter func?on Oak woodland Oak savanna Shrubs “New” exo2cs ‐ Medusahead Exo2c annual grassland ‐ Goatgrass (grasses and forbs) ‐ Yellow starthistle Na2ve grassland (grasses and forbs)
Annual state is “weird” • Mediterranean climate‐ cool season grasses dominate • Invaders are early successional in their na?ve range, stable state in CA (this is gedng increasingly common throughout W. US rangelands) • Standard rangeland assessments don’t work in CA‐ annual is considered “degraded” state Westoby et al. 1989
Need different frameworks to assess annual func?on Oak woodland Oak savanna Shrubs “New” exo2cs ‐ Medusahead Exo2c annual grassland ‐ Goatgrass (grasses and forbs) ‐ Yellow starthistle Na2ve grassland (grasses and forbs)
Future challenges Invasive species • Temperature Change • – increased temperature, 2‐4 fold increases in frequency of heat stress days Precipita?on Change • – Mean annual precipita?on predic?ons • Northern California‐ 0‐18% decrease ( Greatest decrease in the Central Valley and North Coast) • Southern California‐ 26% decrease to 8% increase – Seasonality • Shorter growing seasons (most decreases in late spring/summer) • Moderate winter decreases‐ but less frequent, more intense storms • Spring/fall precipita?on not well‐modeled (but likely most important for vegeta?on composi?on) – Annual variability will increase • 1.5‐2.5 fold increase in frequency of cri?cally dry years • Increased El Nino frequency Nitrogen deposi?on‐ increase invasion • Land use change • – Projected to lose 750,000 acres of rangeland by 2040‐2100) Increased expecta?ons of what rangelands should deliver •
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