Impact of climate change on forest reproduction and regeneration – FOREPRO

To address these questions, FOREPRO will develop a new integrative approach based on complementary investigations : (i) field experiments (tree density and fertilizer use, partial rain exclusion, fruit removal, hand pollination associated with DNA metabarcoding); (ii) the collection and analysis of consistent data over spatially extended network of field sites (individually-based monitoring of male and female flowering, fruiting, growth and phenology, the airborne pollen amount, insect community dynamics and regeneration success), (iii) biochemical assays (nitrogen and carbon content of flowers and fruits), and finally (iv) the development of four inter-connected models (two ecophysiological models, CASTANEA, PHENOFIT, one Resource Budget Model for simulating masting and one stochastic dynamic model for insect communities).

Linking flower phenology to pollen limitation and masting (WP1, Schermer et al. 2020) : From a large field monitoring network, we have shown that early flowering in temperate oak species results in pollen often being released when the weather is unfavorable to aerial pollen spreading and frequently results in reproductive failure. We propose that early flower phenology might be adaptive by making mast-seeding years rare and unpredictable, which would greatly help in controlling the dynamics of seed consumers.
Resource allocation strategies between growth and reproduction (WP1, Le Roncé et al. 2020) : Using a defoliation experiment, we studied how allocation to reproduction was impacted by resource limitation. We found that defoliation had greater legacy effects than immediate effects: it showed little impact on fruit development, while, in the following spring, defoliated branches overproduced leaves at the expense of flowers. Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale.
Holm oak fecundity does not acclimate to a drier world (WP1, Le Roncé et al. 2021). Using a 13-yr monitoring of Quercus ilex reproduction in a rainfall exclusion experiment, we analyzed the interactive effects of long-term increased aridity and other environmental drivers on the inter-annual variation of fecundity. Increased long-term aridity affected tree reproduction beyond what is expected from the current response to interannual drought variation. These results suggest that natural regeneration of holm oak forests may be jeopardized in the future.
Counter-gradient variation of the reproductive effort (WP1, Caignard et al. 2021). Using an elevation gradient experiment in the Pyrenees, we assessed the relative contributions of genetic and environment on variations in the reproductive effort along climatic gradients of sessile oak populations (Quercus petraea). In situ monitoring revealed a decline in reproductive effort with increasing elevation and decreasing temperature. In common garden conditions, significant genetic differentiation was observed between origins for reproduction and growth: trees originating from cold environments (high elevations) grew more slowly, and produced larger acorns and in larger numbers. Our observations show that genetic and phenotypic clines for reproductive traits have opposite signs (counter-gradient) along the environmental gradient. Such a counter-gradient reveals local adaptation of reproductive strategies and that genetic variation moderates the change in reproductive effort according to temperature. The evolutionary trade-off between reproduction and growth may be at the core of this local adaptation.

There is an ongoing work aimed at exploring pollen limitation, resource allocation, the effects of masting on seed consumer demography and forest regeneration success, using both field observations and modeling.

1. Schermer, E, Bel-Venner MC, Gaillard JM, Dray S., Boulanger V. , Le Roncé I, Oliver G., Chuine I, Delzon S, and Venner S. 2020. “Flower Phenology as a Disruptor of the Fruiting Dynamics in Temperate Oak Species.” New Phytologist 225(3):1181–92.
2. Touzot, L, Schermer E, Venner S, Delzon S, Rousset C, Baubet E, Gaillard JM, and Gamelon M. (2020a). “How Does Increasing Mast Seeding Frequency Affect Population Dynamics of Seed Consumers? Wild Boar as a Case Study.” Ecological Applications 30(6):e02134.
3. Le Roncé, I, Toïgo M, Dardevet E, Venner S, Limousin JM, and Chuine I. 2020. “Resource Manipulation through Experimental Defoliation Has Legacy Effects on Allocation to Reproductive and Vegetative Organs in Quercus Ilex.” Annals of Botany 126(7):1165–79.
4. Caignard, T, Kremer A, Bouteiller X, Parmentier J, Louvet JM, Venner S, and Delzon S. 2021. “Counter-gradient Variation of Reproductive Effort in a Widely Distributed Temperate Oak (Quercus Petraea).” Functional Ecology 35(8):1745–55.
5. Le Roncé I, Gavinet J, Ourcival JM, Mouillot F, Chuine I, and Limousin JM. 2021. “Holm Oak Fecundity Does Not Acclimate to a Drier World.” New Phytologist 231(2):631–45.

The lack of robust understanding of the determinants of tree reproduction and forest regeneration has restricted the effectiveness of forest management practices in many countries worldwide. Masting, a highly variable seed production over years and synchronized within populations, occurs in many forest tree species and has dramatic impact on the demographic of seed consumers and forest regeneration. Providing robust projections of forest dynamics in the future requires predicting how climate change and societal decisions will affect not only masting, but also the demography of seed-consumers and their effect on forest regeneration. In this project, we will study tree reproduction in oak species, that are of major ecological and economic importance. We will focus on three species in contrasted environments (the temperate deciduous pedunculate and sessile oaks and the Mediterranean holm oak) and on their interaction with acorn specialist insects (four weevil sibling species) that negatively impact seed germination and hence, early stages of forest regeneration. FOREPRO aims at quantifying the impact of societal decisions on seed production and forest regeneration and providing guidance to sustainable forest management in the context of climate change through acquiring sound knowledge and accurate tools for regeneration policies. The research program of FOREPRO is organized along three work packages:
(1) The physiological and climatic determinants of oak masting will be identified (i) by assessing the effects of tree resource availability on reproduction and growth, (ii) by measuring the relative weight of pollen limitation and weather vetoes on massive fruiting failure and fruiting synchrony and (iii) by assessing the environmental and genetic determinism of seed production. In addition, we will analyze the impact of forest management practices (i.e., plantation density, soil fertilization) on seed production.
(2) After evaluating the impact of seeding intensity on regeneration success, we will analyze how the different components of masting make oaks efficient at controlling the dynamics of acorn parasitic insect communities, including species with greatly diverse life history traits.
(3) We will develop new mechanistic models of masting that implement well-established ecophysiological processes, make projections about the future of masting and regeneration potential of oak forests, and provide avenues for forest management innovation.
This program will provide forest managers with knowledge about mechanisms of masting and decision support tools for optimizing oak forest regeneration strategies in the context of climate change. To address this challenge, FOREPRO will develop a new integrative approach based on complementary investigations : (i) field experiments (tree density and fertilizer use, partial rain exclusion, fruit removal, hand pollination associated with DNA metabarcoding); (ii) the analysis and collection of consistent data on spatially extended network of field sites (individually-based monitoring of male and female flowering, fruiting, growth and phenology, the airborne pollen amount, insect community dynamics and regeneration success), (iii) biochemical assays (nitrogen and carbon content of flowers and fruits), and finally (iv) the development of four inter-connected models (two ecophysiological models, CASTANEA, PHENOFIT, one Resource Budget Model for simulating masting and one community stochastic dynamic model for insects).
FOREPRO is at the interface between research and public action and should bring significant progress in our understanding and management of oak forest regeneration. It was built and will be developed thanks to the strong complementarity of partners from research and management fields (four research labs: LBBE -Lyon-, BioGeCo -Bordeaux-, CEFE -Montpellier-, ESE –Orsay-, and a management agency : ONF).