ESTIMATING ACORN CROPS USING VISUAL SURVEYS

We describe a visual survey technique for evaluating acorn production. In contrast with previously proposed methods, our technique yields ra tio-level data on annual productivity that are analyzable with standar d statistics and, by sampling the same trees each year, data on the re productive patterns of individual trees. We compared this technique wi th two independent sets of acorn-trap data acquired on oaks of three s pecies at Hastings Reservation in central coastal California. Correlat ions between acorns counted by the visual surveys and collected from a corn traps under the same trees were significant for all three species . Most scatter in the data appeared to be attributable to three causes : (1) sampling error, especially among trees with very small crops, (2 ) finite counting speed, leading to a lack of discrimination among tre es with very large crops by the visual surveys, and (3) arboreal acorn removal by animals. This latter factor can be particularly large, ren dering visual surveys more reliable than the use of traps. Furthermore , only the high efficiency of visual surveys allows for the practical assessment of samples large enough to accommodate high within-populati on variation and detect widespread geographic variation in acorn produ ction. Visual surveys offer a method of assessing the fruit or cone cr ops of many hardwood and conifer species that is not only more efficie nt but also more accurate than the use of traps.