Magdalena Żywiec

Population and individual level of masting in a fleshy-fruited tree

Masting is usually considered as a population phenomenon but it results from individuals’ reproductive patterns. Studies of individual patterns of seed production and their synchrony are essential to an understanding of the mechanisms of masting. The aim of this study was to find the relationship between population and individual levels of masting. We examined individuals’ contribution to masting, considering their endogenous cycles, interannual variability and associated weather cues, as well as inter-individual synchrony of fruit production. We studied masting of Sorbus aucuparia L., which in Europe is one of the most common trees bearing fleshy fruits and is strongly affected by a specialized seed predator. The data are 11-year measurements of fruit production of 250 individuals distributed on a 27-ha area of subalpine forest in the Western Carpathians (Poland). Population- and individual-level interannual variability of fruit production was moderate. Synchrony among individuals was relatively high for all years, but the trees were much less synchronized in heavy crop years than in years of low fruit production. Weak synchrony among trees for heavy production years suggests that the predator satiation hypothesis does not explain the observed masting behavior. Fruit production, both at individual and at population level, was highly correlated with weather conditions. However, the presence of masting cannot be fully explained by the resource-matching hypothesis either. We suggest that adverse weather conditions effectively limit fruit production, causing high inter-individual synchrony in low crop years, whereas the unsynchronized heavy crop years seem to have been affected by individually available resources.

Microgeographical, inter-individual, and intra-individual variation in the flower characters of Iberian pear Pyrus bourgaeana (Rosaceae)

Flower characteristics have been traditionally considered relatively constant within species. However, there are an increasing number of examples of variation in flower characteristics. In this study, we examined the variation in attracting and rewarding flower characters at several ecological levels in a metapopulation of Pyrus bourgaeana in the Doñana area (SW Spain). We answered the following questions: what are the variances of morphological and nectar characters of flowers? How important are intra-individual and inter-individual variance in flower characters? Are there microgeographical differences in flower characters? And if so, are they consistent between years? In 2008 and 2009, we sampled flowers of 72 trees from five localities. For six flower morphological and two nectar characteristics, we calculated coefficients of variation (CV). The partitioning of total variation among-localities, among-individuals, and within-individuals was estimated. To analyze differences among localities and their consistency between years, we conducted generalized linear mixed models. The CVs of nectar characters were always higher than those of morphological characters. As expected, inter-individual variation was the main source of variation of flower morphology, but nectar characters had significant variation at both intra- and inter-individual levels. For most floral traits, there were no differences among localities. Our study documents that variation is a scale-dependent phenomenon and that it is essential to consider intra- and inter-individual variance when investigating the causes and consequences of variation. It also shows that single year studies of floral characters should be viewed with caution.

Does a heavy fruit crop reduce the tree ring increment? Results from a 12-year study in a subalpine zone

Life history theory posits that an increased investment in reproduction should result in decreased vegetative investment. Switching resources from growth to reproduction are also expected in species experiencing selection pressure for high variation of seed crops. In this study, we tested whether the reproductive effort of trees, measured as the number of fruits produced, is related to their radial growth increment. We examined a population of Sorbus aucuparia, a fleshy-fruited tree species with highly variable interannual individual fruit production growing in the subalpine zone and under strong selection pressure from pre-dispersal seed predators. We used 12-year data to test the relationship between fruit crop and radial growth increments in current, previous and subsequent years, and found no trade-off between growth and reproduction. For almost all trees, there was no correlation between fruit crop and radial growth increment in the same year or next year. Only a few trees showed a positive correlation between fruit crop and previous-year growth. In the statistics, we took advantage of the high variability of individual fruit crops in high production years. In four heavy crop years, we compared the radial growth increments of trees with heavy crops with those of trees with low fruit crops. Current and next-year radial growth did not differ between trees with low and high fruit crops. In all those years, however, trees having heavy fruit crops had higher previous-year growth increments. We suggest that the harsh subalpine weather conditions account for the lack of a trade-off between growth and reproduction in the studied population.

Seedling Survival Under Conspecific and Heterospecific Trees: the Initial Stages of Regeneration of Sorbus aucuparia, a Temperate Fleshy-Fruited Pioneer Tree

This study examined whether recruitment of Sorbus aucuparia under conspecifics and heterospecifics differs in: (i) seed-bank longevity, (ii) seedling emergence rate, (iii) seedling survival rate, and (iv) density-dependent mortality. I studied the first three cohorts of seedlings that emerged after the 2009 mast year using pairs of plots located under conspecific and heterospecific (Norway spruce, Picea abies) trees. The interannual pattern of seedling emergence did not follow the fruit production pattern. In 2010 and 2011, there was no difference in the density of emerged seedlings among the plots under rowans and under spruces. Survival of the 2010 and 2011 seedling cohorts was higher under spruces than under rowans. Under either tree species seedling survival was not correlated with the density of emerged seedlings. The survival pattern of rowan seedlings in subalpine spruce forest seems to follow the Janzen-Connell model in temperate forest.

Strong among population variation in frugivory strength by functional diverse frugivores: a ‘reciprocal translocation’ experiment

Fruit–frugivore interactions are critical for the dynamics and evolution of many plant communities. The strength of the interactions between a given plant species and different frugivore guilds (e.g., seed dispersers, seed predators) often vary in space due to changes in plant extrinsic factors (e.g., frugivore abundances) and plant traits (e.g., fruit size and reward). By reciprocally translocating Pyrus bourgaeana ripe fruits representative of five Mediterranean localities during 2 consecutive years, we experimentally quantified guild-specific variations among populations in frugivory strength, while accounted for plant-intrinsic and- extrinsic factors. Though overall fruit removal did not differ among localities, there were strong guild-specific differences in fruit removal strength. Fruit removal by pulp feeders, seed dispersers, and fruit predators varied among populations up to 8.5-, 5.6-, and 4.0-folds, respectively. These strong variations seemed mediated by changes in frugivore relative abundances rather than on availability of alternative fruits. As expected, all fruit traits considered (e.g., fruit size, pulp amount) markedly varied among tree populations. However, no frugivore guild showed preference for fruits from any locality, suggesting that fruit traits did not contribute much to differences in frugivory strength among populations. Since the functional diverse frugivore guilds played contrasting roles in P. bourgaeana dynamics (e.g., seed dispersal vs. seed predation), our study highlights the importance of accounting for functional diversity in frugivore guilds when estimating spatial variations in the strenght of seed dispersal. This investigation also illustrates a neglected but widely applicable experimental approach to identify the relative importance of extrinsic factors and fruit traits in mediating fruit–frugivore interactions.