Summary
In California hardwood rangelands (south-west USA), recruitment of blue oak Quercus douglasii and valley oak Q. lobata is poor. These rangelands comprise grasslands with scattered trees and are often of considerable wildlife value. Strategies to enhance oak recruitment are required. Experiments were undertaken in six north-central Californian counties (between latitudes 35º15’-39º15’N) on representative sites to assess the effect of herbaceous plant control and protection from small mammal and insect herbivory on acorn emergence, and seedling survival and growth.
Acorns were collected locally in late summer-autumn. In November-December 1985-1988 on eight sites (five blue oak, three valley oak), acorns were sown within fenced exclosures to exclude deer, feral pigs Sus scrofa and livestock (know to browse seedlings/eat acorns).
At each site, treatments were herbaceous plant control by herbicide application (glyphosate, atrazine and/or oxyfluorfen) and hoeing, vs. naturally occurring vegetation as main plots: each main plot (4 replicates) comprised 100 acorns planted at 5 cm depth in four rows of 25 each at 30 cm spacing.
Seedlings were randomly split into: protected (against small mammal and insects) and unprotected. Protection comprised ‘window screen cages’ (close-topped cylinders 15 cm diameter x 40 cm high) or rigid plastic seedling protectors (mesh screens) 5 cm diameter x 30 cm high (these replaced cages when threat of grasshopper Melanoplus devastatorherbivory was minimal, as these might otherwise pass through the mesh). Hairnets (as used in the food processing industry) were used to protect seedlings against grasshoppers in 1986 at one site.
Rainfall and soil factors were recorded to help interpret seedling emergence, survival and growth.
Some acorns and seedlings were eaten by (larger) small mammals at all locations. Pocket gophers Thomomys bottae were common and control was undertaken where considered a significant threat, but losses still occurred. Black-tailed jackrabbits Lepus californicus, rabbits Sylvilagus spp. and ground squirrels Spermophilus beecheyi were suspected as major causes of browsing damage.
Herbicide application (reducing competition for moisture by other plants) was the most important factor in enhancing seedling emergence: blue oak - 45% with herbicide, 28% no herbicide; valley oak 64% with herbicide, 50% no herbicide. Highest emergence of blue oak occurred when there was above average rainfall.
Average seedling first year survival was significantly higher in herbicide plots: blue oak (33% vs. 18% of acorns sown), valley oak (45% vs. 21%). Screening significantly improved survival: blue oak (average across 3 sites) - screen (no herbicide) 34%, screen (+ herbicide) 46%, no screen (no herbicide) 26%, no screen (herbicide) 41%; valley oak (average across 4 sites) - screen (no herbicide) 28%; screen (+ herbicide) 54%; no screen (no herbicide) 18%; no screen (herbicide) 46%.
Growth was also significantly improved by screening. Window screen cages proved best, also producing some shade which may have enhanced survival and growth further (although this possible beneficial effect was not measured).
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