Background and Aims In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, resource allocation strategies to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation. Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth, and germination potential), and on the production of vegetative and reproductive organs the year following defoliation. Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring, defoliated branches produced less shoots and compensated leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio. Conclusions Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects.
The key stones of resource budget models to explain mast seeding are that fruit production depletes tree stored resources, which become subsequently limiting to flower production the following year. These two hypotheses have, however, rarely been tested in forest trees. Using a fruit removal experiment, we tested whether preventing fruit development would increase nutrient and carbohydrates storage and modify allocation to reproduction and vegetative growth the following year. We removed all the fruits from nine adult Quercus ilex trees shortly after fruit set and compared, with nine control trees, the concentrations in nitrogen, phosphorus, zinc, potassium and starch in leaves, twigs and trunk before, during and after the development of female flowers and fruits. The following year, we measured the production of vegetative and reproductive organs as well as their location on the new spring shoots. Fruit removal prevented the depletion of nitrogen and zinc in leaves during fruit growth. It also modified the seasonal dynamics in zinc, potassium and starch in twigs, but had no effect on reserves stored in the trunk. Fruit removal increased the production of female flowers and leaves the following year, and decreased the production of male flowers. Our results show that resource depletion operates differently for male and female flowering, because the timing of organ formation and the positioning of flowers in shoot architecture differ between male and female flowers. Our results suggest that nitrogen and zinc availability constrain flower production in Q. ilex, but also that other regulatory pathways might be involved. They strongly encourage further experiments manipulating fruit development over multiple years to describe the causal relationships between variations in resource storage and/or uptake and male and female flower production in masting species.
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