REPRODUCTIVE ECOLOGY OF CARYA (JUGLANDACEAE): PHENOLOGY, POLLINATION, AND BREEDING SYSTEM OF TWO SYMPATRIC TREE SPECIES.

Observations and experiments on both shagbark (Carya ovata; 2n = 32) and mockernut (C. tomentosa; 2n = 64) hickories in a New Jersey forest were performed over a 2-year period to determine the mechanisms operating both to maintain reproductive fidelity and to regulate fruit initiation. Phenological observations indicated significant differences between the species for almost all vegetative and reproductive characters examined; however, a large amount of overlap was also observed. Pollination experiments indicated that pollen limitation is not a significant factor in reducing fruit set in these anemophilous species. Pollen-ovule ratios ranged from 1 to 25 × 106 . Pollen was 80-90% viable at time of shedding. The large surface area of vegetative structures (e.g., leaves) at the time of pollination may retard the flow of pollen into and out of the canopy and thus increase self-pollination. A full diallel cross, replicated 5 times, was made among 8 trees (4 trees of each species) to characterize combining ability and compatibility relationships. All trees were found to be both geitonogamous and xenogamous to varying degrees. A high frequency of putative interploidal cross-compatibility was also observed. Fruits from these interspecific crosses were found to be germinable. Observations on individual trees suggested that dichogamy may help to reduce selfing and promote outcrossing in this breeding system. While most trees exhibited a temporal separation of genders, the trait was found to be quite plastic (i.e., trees switched between protogyny and protandry between years). Pistillate flowers were receptive for all trees within a species during the same general time period; however, pollen shedding varied considerably among trees and between years. Both species appear to be utilizing a broad array of mechanisms in order to maintain reproductive fidelity and adequate fruit set.

Fruit maturation patterns of Carya spp. (Juglandaceae): an intra-crown analysis of growth and reproduction.

Fruit survival patterns, from fertilization to maturation, were examined for Carya ovata and C. tomentosa in a New Jersey USA forest. We observed fruiting and shoot growth characteristics over a 3-yr period to determine: (1) the patterns of fruit survivorship (from initiation to maturity) within and among years, (2) the relationships between shoot growth, fruit initiation, and fruit survival to maturity, and (3) the influence of phytophagous insects on fruit survival. We found that within years, smaller infructescences (1-2 fruits) exhibited greater relative survivorship than larger ones (3-4 fruits); however, absolute nut production was greatest for mid-sized infructescences (2-3 fruits). Among years, fruit survivorship varied considerably within populations. Across the 3-yr period we observed average fruit survivorship to be convex, linear, and concave, respectively. Likewise, shoot characteristics (length, width, number of leaves) varied concomitantly (decreasing fruit survivorship was accompanied by decreasing shoot length and number of leaves). Within years, we found no strong relationship between shoot characteristics and infructescence size and survival. The patterns of tree-to-tree variation suggested a strong genetic basis to shoot growth and fruit maturation. However, patterns of variation within and among years also indicated a strong environmental influence on these traits as well. Natural phytophagy by insects was observed to be low (<5%); however, shoot defoliations of 10-25% were not uncommon. Experimental defoliations (ambient, 10-15%, 20-40%, and 75-100%) did not result in reduced survival to maturity. Collectively, the data suggest that year-to-year variability in shoot growth has a greater influence on fruit maturation patterns than within year fruit-shoot relations.