Sex allocation in California oaks: trade-offs or resource tracking?

Holm oak fecundity does not acclimate to a drier world

Climate change might impact tree fecundity by altering the relative influences of meteorological and physiological drivers, and by modifying resource investment in reproduction. Using a 13-yr monitoring of Quercus ilex reproduction in a rainfall exclusion experiment, we analysed the interactive effects of long-term increased aridity and other environmental drivers on the inter-annual variation of fecundity (male flower biomass, number of initiated and mature fruits). Summer-autumn water stress was the main driver of fruit abortion during fruit growth. Rainfall exclusion treatment strongly reduced the number of initiated and mature fruits, even in masting years, and did not increase fruit tolerance to severe drought. Conversely, the relative contribution of the meteorological and physiological drivers, and the inter-annual variability of fruit production were not modified by rainfall exclusion. Rather than inducing an acclimation of tree fecundity to water limitation, increased aridity impacted it negatively through both lower fruit initiation due to changes in resource allocation, and more severe water and resource limitations during fruit growth. Long-term increased aridity affected tree reproduction beyond what is expected from the current response to inter-annual drought variations, suggesting that natural regeneration of holm oak forest could be jeopardised in the future.

Continent-wide tree fecundity driven by indirect climate effects

Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in North America that climate-condition interactions dominate responses through two pathways, i) effects of growth that depend on climate, and ii) effects of climate that depend on tree size. Because tree fecundity first increases and then declines with size, climate change that stimulates growth promotes a shift of small trees to more fecund sizes, but the opposite can be true for large sizes. Change the depresses growth also affects fecundity. We find a biogeographic divide, with these interactions reducing fecundity in the West and increasing it in the East. Continental-scale responses of these forests are thus driven largely by indirect effects, recommending management for climate change that considers multiple demographic rates.

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

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, strategies for resource allocation 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 was 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 fewer shoots and compensated for 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.

Drivers of tree fecundity in pedunculate oak (Quercus robur) refugial populations at the species' southwestern range margin

The current low latitudinal range margins of many extra-tropical plant species consist of small and scattered populations that persist locally in microrefugia. It remains poorly understood how their refugial distribution affects mating patterns and reproductive success. Here we examine flower and acorn production and their determinants in refugial populations of the widespread European forest tree pedunculate oak (Quercus robur). We monitored male flower, female flower and acorn production in 159 adult trees from 12 oak stands over 2 years. We related these and derived parameters to a series of ecological and genetic predictor variables extrinsic (stand size, density and isolation as well as elevation, topography and forest cover) or intrinsic (size, phenology and several genotypic measures) to the target tree. Tree fertility was unrelated to extrinsic factors but determined by tree size, although we detected size-independent variation in reproductive investment. Female flower number accurately predicted acorn crop size. Fruit set differed between years, evidencing the existence of pollen limitation at the landscape but not at the local scale. Fruit set also tended to increase with the number of mates of the target tree. We detected no other evidence for genetic constraints on mating. Reproduction was triggered by a combination of small-scale and landscape-scale drivers. Although short-distance mating prevailed, limited pollen flow did not appear to significantly constrain reproductive success. The high intrinsic ability of populations to maintain their reproductive capacity may help explain their successful long-term persistence in an adverse broader environment.

Why is seed production so variable among individuals? A ten-year study with oaks reveals the importance of soil environment

Mast-seeding species exhibit not only a large inter-annual variability in seed production but also considerable variability among individuals within the same year. However, very little is known about the causes and consequences for population dynamics of this potentially large between-individual variability. Here, we quantified seed production over ten consecutive years in two Mediterranean oak species - the deciduous Quercus canariensis and the evergreen Q. suber - that coexist in forests of southern Spain. First, we calibrated likelihood models to identify which abiotic and biotic variables best explain the magnitude (hereafter seed productivity) and temporal variation of seed production at the individual level (hereafter CVi), and infer whether reproductive effort results from the available soil resources for the plant or is primarily determined by selectively favoured strategies. Second, we explored the contribution of between-individual variability in seed production as a potential mechanism of satiation for predispersal seed predators. We found that Q. canariensis trees inhabiting moister and more fertile soils were more productive than those growing in more resource-limited sites. Regarding temporal variation, individuals of the two studied oak species inhabiting these resource-rich environments also exhibited larger values of CVi. Interestingly, we detected a satiating effect on granivorous insects at the tree level in Q. suber, which was evident in those years where between-individual variability in acorn production was higher. These findings suggest that individual seed production (both in terms of seed productivity and inter-annual variability) is strongly dependent on soil resource heterogeneity (at least for one of the two studied oak species) with potential repercussions for recruitment and population dynamics. However, other external factors (such as soil heterogeneity in pathogen abundance) or certain inherent characteristics of the tree might be also involved in this process.