Summary1. Mast seeding is a reproductive strategy in some perennial plants defined as synchronous production of large seed crops at irregular intervals. One widely accepted theory to explain this behaviour is the predator satiation hypothesis, which states that the synchronous and variable production of seeds within a population will maximize the probability of seed survival through satiation of seed predators. 2. Although some short-term studies have documented the influence of variable and synchronized production of seeds on herbivore attack rate during one or few mast years, long-term data including multiple mast seeding years and patterns of cone production and herbivore attack on individual trees are needed to assess (i) how cone production, variability and synchrony affect individual plant fitness and (ii) the functional responses of seed predators to mast seeding events. 3. We tested these objectives, collecting long-term (29 years) data on female seed cone production and rates of seed predator attack from 217 individual contiguous trees within a Pinus ponderosa population. 4. Our results support the predator satiation hypothesis. First, we found high interannual synchrony and variability in seed cone production and a type II functional response of seed predators to available cones. Secondly, years with high cone production (mast years) had markedly lower rates of seed predator attack than years of low production (i.e. a population-level satiation effect). Thirdly, within mast years, individuals with high cone production had markedly lower rates of attack than individuals with low cone production (i.e. an individual-level satiation effect). Finally, individual trees with greater synchrony and more variable cone production suffered lower rates of attack. 5. Synthesis: Our long-term data on individual trees lend strong support to the hypothesis that mast seeding in Pinus ponderosa has evolved in response to natural selection from insect seed predators.

Many plant species exhibit variable and synchronized reproduction, or masting, but less is known of the spatial scale of synchrony, effects of climate, or differences between patterns of pollen and seed production. We monitored pollen and seed cone production for seven Pinus ponderosa populations (607 trees) separated by up to 28 km and 1,350 m in elevation in Boulder County, Colorado, USA for periods of 4–31 years for a mean per site of 8.7 years for pollen and 12.1 for seed cone production. We also analyzed climate data and a published dataset on 21 years of seed production for an eighth population (Manitou) 100 km away. Individual trees showed high inter-annual variation in reproduction. Synchrony was high within populations, but quickly became asynchronous among populations with a combination of increasing distance and elevational difference. Inter-annual variation in temperature and precipitation had differing influences on seed production for Boulder County and Manitou. We speculate that geographically variable effects of climate on reproduction arise from environmental heterogeneity and population genetic differentiation, which in turn result in localized synchrony. Although individual pines produce pollen and seed, only one-third of the covariation within trees was shared. As compared to seed cones, pollen had lower inter-annual variation at the level of the individual tree and was more synchronous. However, pollen and seed production were similar with respect to inter-annual variation at the population level, spatial scales of synchrony and associations with climate. Our results show that strong masting can occur at a localized scale, and that reproductive patterns can differ between pollen and seed cone production in a hermaphroditic plant.

Abert's squirrels (Sciurus aberti) are restricted to stands of ponderosa pine (pinus ponderosa) and feed heavily on the inner bark tissues of these trees. Although preferences for inner bark of particular trees (target trees) have been well documented, the basis of the squirrels' selectivity has not been clearly demonstrated. Compared with matched nontarget (uneaten) trees, and with whole populations of ponderosa pine, target trees had significantly lower xylem oleoresin flow rates, and significantly lower levels of β—pinene and β—phellandrene in the xylem oleoresin. In feeding trials with free—ranging animals, food laced with ecologically realistic concentrations of either β—phellandrene were eaten significantly less than control food. The phloem of target trees had significantly higher concentrations of nonstructural carbohydrates and sodium, and significantly lower concentrations of iron and mercury, than the phloem of matched nontarget trees. Oleoresin characteristics associated with selective herbivory wee unaffected by simulated herbivory over 2 yr, supporting existing evidence that these characteristics are under strong genetic control. Because herbivory is associated with these host traits and results in dramatic reductions in host fitness, Abert's squirrels are probably important agents of natural selection in host ponderosa pine populations.

No abstract

No abstract