Interactions of factors affecting seedling recruitment of blue oak (Quercus douglasii) in California

• Published in: Ecology (Durham) 1989-04, Vol.70 (2), p.389-404
• Main Authors: Borchert, Mark I., Davis, Frank W., Michaelsen, Joel, Oyler, Lyn Dee
• Format: Article
• Language: English
• Subjects: ACORNS; Animal and plant ecology; Animal, plant and microbial ecology; BELLOTAS; Biological and medical sciences; CALIFORNIA; CALIFORNIE; Canyons; Deer; DEPREDACION; Ecology; Forest ecology; Fundamental and applied biological sciences. Psychology; GERMINACION; GERMINATION; GLAND; Mice; MICROENVIRONMENTS; MORTALIDAD; MORTALITE; MORTALITY; Planting; PLANTULAS; PLANTULE; PREDATION; Predators; QUERCUS; Quercus douglasii; Rodents; SEEDLINGS; Synecology; Terrestrial ecosystems; Trees
• ISSN: 0012-9658; 1939-9170
• DOI: 10.2307/1937544

Acorn germination and seedling recruitment of blue oak (Quercus douglasii) were studied in relation to postdispersal predators, planting depth, oak canopy cover, slope angle and aspect, and herb layer. Removal of acorns and seedling recruitment from surface-sown and buried acorns were measured for 2 yr at two sites, a north-slope forest and a ridgetop savanna. Nested exclosures were used to measure acorn predation by birds, mice, gophers, deer, and cattle. The rate of seedling recruitment (P"s) was related to treatment variables using cluster analysis combined with stepwise logistic regression. Average acorn fall ranged from 3.5 to 58.7 acorns/m^2, with germination rates varying from 28 to 85% in different years. 8172 sown acorns yielded 2922 (35.76%) seedlings, but P"s varied from 0.09 to 0.71 among the different treatments. Year, site, and rodents interacted strongly to affect seedling recruitment. P"s averaged 0.48 during a cool wet year compared to 0.23 in drier years. Low correlations between seedling recruitment rates for the same plot in different years were due to annual changes in the distribution of favorable microsites and patchy, unpredictable acorn predation. Recruitment rates for buried acorns were twice those of surface-sown acorns due to improved germination and reduced predation. Pocket gophers (Thomomys bottae) were the major predator of buried acorns. Surface acorns suffered high mortality from dying and overheating, as well as from predation by mice, gophers, and cattle. Seedling recruitment was positively associated with increasing canopy cover and more mesic microsites at the low elevation site, but was negatively associated with these factors at the cooler, high elevation site. Hierarchichal classification combined with stratum-specific logistic regression models was important in revealing strong interactions among seedling recruitment factors.