Long-term trends in annual reproductive output of the scrub hickory: factors influencing variation in size of nut crop.

Reproductive output by the Florida-endemic scrub hickory (Carya floridana Sargent) was studied over a 28-yr period in three south-central Florida vegetation associations: southern ridge sandhill, sand pine scrub, and scrubby flatwoods. The objectives were to describe multi-annual patterns of variation in nut production, identify factors involved in this variation, and investigate differences in patterns among associations. Peaks (higher values bracketed by lower values) in nut production occurred in 7 yr in sandhill and scrub and 8 yr in scrubby flatwoods during the 22-yr period for which we had continuous data. Of the total of 22 peaks in the three associations combined, 17 occurred at intervals of 2 or 3 yr, and peaks occurred in the same years in 18 of the 22 cases. Periodicities of nut production generated by spectral analyses (Fourier transforms) generally agreed with the observed peaks. Numbers of nuts per bearing ramet, proportion of ramets bearing nuts, ramet height, and light availability were positively correlated with nut production. Weather variables, specifically winter rainfall and minimum spring temperatures, accounted for a total of about one-quarter to one-half of the variance in nut production depending on the vegetation association. Following a prescribed fire in a sandhill plot, scrub hickory quickly regained fruit production, but over a 5-yr period following the fire nut production by ramets in the largest size class was reduced compared with the unburned control plot.