Masting Mediated by Summer Drought Reduces Acorn Predation in Mediterranean Oak Forests

Abstract

Temporally variable production of seed crops by perennial plants (masting) has been hypothesized to be a valuable mechanism in the reduction of seed predation by satiating and starving seed consumers. To achieve these benefits, coexisting species subjected to the same predator would benefit from a similar pattern of seeding fluctuation over time that could lead to a reduction in predation at the within-species level. We tested for the existence of an environmental factor enforcing synchrony in acorn production in two sympatric Mediterranean oaks (Quercus ilex and Q. humilis) and the consequences on within-species and between-species acorn predation, by monitoring 15 mixed forests (450 trees) over seven years. Acorn production in Q. ilex and Q. humilis was highly variable among years, with high population variability (CVp) values. The two species exhibited a very different pattern across years in their initial acorn crop size (sum of aborted, depredated, and sound acorns). Nevertheless, interannual differences in summer water stress modified the likelihood of abortion during acorn ripening and enforced within- and, particularly, between-species synchrony and population variability in acorn production. The increase in CVp from initial to mature acorn crop (after summer) accounted for 33% in Q. ilex, 59% in Q. humilis, and 60% in the two species together. Mean yearly acorn pre-dispersal predation by invertebrates was considerably higher in Q. humilis than in Q. ilex. Satiation and starvation of predators was recorded for the two oaks, and this effect was increased by the year-to-year variability in the size of the acorn crop of the two species combined. Moreover, at a longer time scale (over seven years), we observed a significant reduction in the mean proportion of acorns depredated for each oak and the variability in both species' acorn production combined. Therefore, our results demonstrate that similar patterns of seeding fluctuation over time in coexisting species mediated by an environmental cue (summer drought) may contribute to the reduction of the impact of seed predation at a within-species level. Future research should be aimed at addressing whether this process could be a factor assisting in the coexistence of Q. ilex and Q. humilis.

Temporal trends in the enhanced vegetation index and spring weather predict seed production in Mediterranean oaks

“…for the current climatological standard normal period, 1991-2010). We selected forest stands of Q. ilex because it is the most abundant tree species in Collserola, has a strong masting behaviour (Espelta et al 2008;Fernández-Martínez et al 2012), and has evergreen leaves that allow continuous monitoring using remotely sensed vegetative indices. Regarding Q. ilex fruiting phenology, the pollination period normally starts during the first week of April and pollen release reaches its maximum during May (Fernández-Martínez et al 2012).…”

“…Fifteen trees per plot were tagged, and the number of branches per tree was estimated using a regression model between crown projection and the number of branches previously determined for a subsample of trees (see Espelta et al 2008). Acorn production on four branches per tree was recorded each September, i.e.…”

“…at the peak of seed production in holm oaks before acorns were ripe enough to fall or to be harvested in appreciable number by birds (see Abrahamson and Layne 2002). The total number of acorns produced per tree was estimated by multiplying the mean acorn production per branch by the number of branches per tree (see Espelta et al 2008 for further details). Since aborted and infested seeds usually fall much before maturation happens, insect-infested or aborted acorns do only consume a small amount of the tree resources, mostly insignificant in comparison with those needed in a mast year.…”

“…regulation of the populations of seed consumers (McShea 2000;Espelta et al 2008), and the propagation of diseases [e.g. lyme disease (Ostfeld 1997)].…”

“…more favourable weather conditions for photosynthesis would lead to larger harvests (e.g. Espelta et al 2008Espelta et al , 2009). On the other hand, the resource budget model, or the resource accumulation hypothesis (Sork et al 1993;Isagi et al 1997), suggests that plants produce extraordinary seed crops when a resource threshold is reached after several years of progressive accumulation.…”