Oak trees on California rangeland ameliorate microclimate and may reduce cattle heat stress in a warmer future

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Amber Kerr, Energy and Resources Group, UC Berkeley, Berkeley, CA

Background/Question/Methods
Rangelands, defined as a variety of vegetation types on which livestock can graze, cover half of California's land area. The most economically important rangeland types in California are annual grassland and oak savannah, which together produce the majority of cattle in California's $3 billion per year beef industry. Although California rangelands have undergone major ecological changes since European arrival, they continue to provide crucial ecosystem services, such as C storage, water infiltration, and habitat for native plants and animals.
In recent years, California’s livestock industry has been challenged by historic droughts and high temperatures, which decrease forage availability and cause physiological stress to cattle. Under such conditions, cattle can only thrive with adequate food, water, and shelter. California range managers are well aware that cattle seek out shade under oaks during hot weather, but few efforts have been made to quantify the trees’ microclimatic effects and potential benefits to cattle.
This is a preliminary effort to obtain management-relevant data on oak microclimate effects in California rangelands. Radiation-shielded Hobo Pendant Temp dataloggers and black globe thermometers were deployed in summer 2018, 2019, and 2020 to quantify sun/shade temperature differences at four inland East Bay Regional Parks with actively grazing cattle on grasslands and oak savannah. At two parks (Brushy Peak and Pleasanton Ridge), plots used coast live oak, Quercus agrifolia. At the other two parks (Del Valle and Sunol), plots used blue oak, Quercus douglasii.
Results/Conclusions
Ambient air temperature during the warmest part of the day (10 AM – 4 PM), as measured by the radiation-shielded Hobo loggers at 20-minute intervals, was 4-12° F warmer in open fields than under the shade of single isolated trees (average: 8.2° F cooler). Black globe temperature difference, measured by one sun and one shade black globe 60’ apart, was on average 11.2° F, with spikes of well over 30° F difference during extremely hot and sunny days. Diameter of tree canopy, and species of tree (Q. agrifolia vs. Q. douglassii), did not seem to have major effects.
The trees in this study reduced “Danger” heat stress temperatures for cattle (>91° F) from an average of 2.5 hours per day to about 15 minutes per day. As climate change continues, microclimatic refuges for cattle and for wild animals on California rangelands will become increasingly important – especially in the Central Valley, already prone to high summer temperatures and predicted to see above-average warming by 2050.