Adapting Forest Regeneration to Climate-Driven Masting Shifts in Beech and Oak Populations

We focus on masting in two european forest species of major ecological and economic importance: European beech (Fagus sylvatica) and sessile oak (Quercus petraea), which are central to temperate forests, and whose ability to regenerate naturally (the current practice in France and Europe) is intricately linked to their masting patterns. Our main objective is to equip forest managers with scientifically sound insights into masting and its effectiveness in regulating specialized seed consumers. In addition, we aim to provide operational guidelines for enhanced regeneration practices for beech and sessile oak forests in the context of climate change. 

Divergent Masting patterns between Beech and Oaks:

beech displays cyclical, almost two-year fruiting patterns, with a remarkably high degree of synchronization among trees within and between populations, on a regional or even continental scales3–5. In contrast, oaks exhibit more unpredictable fruiting patterns, with limited synchronicity across populations4. It is widely accepted that the mechanisms behind masting differ qualitatively between oaks and beech6. In oaks, masting is primarily attributed to significant inter-annual variation in fruiting rates, while in beech, it would be mainly due to inter-annual variations in flowering. This statement was recently challenged, however, as we found great inter-annual variation in oak flowering7. Additionally, fruiting rates, which are highly dependent on successful pollination, also contribute to interannual variation in fruiting for both species7–11. REGEMAST proposes that, contrary to this dichotomous view, the mechanisms of masting are not fundamentally different between oaks and beech. In both temperate species, floral induction and pollen limitation are major, critical phases affecting the fruiting process, and still deserve full consideration for understanding masting.

We hypothesize that the differences in masting between oak and beech are mainly due to differences in floral developmental phenology, including induction and maturation, as well as to the genetic variability associated with these phenological traits. Beech trees are expected to use a combination of photoperiod and temperature to stabilize their phenology and synchronize their response to weather conditions, which would foster synchronization among trees in reproduction and cyclic fruiting patterns. On the contrary, oak trees are expected to exhibit a high degree of genetic and floral phenology variability, which would increase stochasticity in masting.

The evolutionary causes highlight the differences in masting mechanisms.

The prevailing assumption is that masting has evolved as an efficient way to regulate specialist seed consumers by depriving them of resources in years of poor fruiting, rendering them incapable of exploiting the abundant resources in mast years (consumer satiation hypothesis12). We hypothesize that the differences in masting strategies between oaks and beech arise from distinct selective pressures exerted by specialist fruit-eating insects. Beech trees have to deal with a single specialist insect species, the moth Cydia fagiglandana, which has a one-year life cycle. Our hypothesis is that the evolution of characteristics favoring biennial mast years and highly synchronized trees would effectively control moth populations while minimizing the loss of reproductive opportunities for beech. In contrast, French oak trees are exposed to insect communities of five acorn-specialist species including one moth and four weevil species, with life cycles ranging from one to three years, making these diversified insect communities ready to take advantage of any cyclicity in oak fruiting. We hypothesize that this has favored the evolution of traits that generate highly variable but also stochastic seed production dynamics.

In the context of climate change, the future of forest regeneration dynamics is not only linked to the future of masting but also to its effectiveness in controlling specialist consumers.

A 40-year study of beech fruiting in the UK suggests a recent breakdown in masting driven by climate change; this breakdown is marked by reduced interannual variation in fruiting, weaker synchronization among trees, and a subsequent decline in the effectiveness of controlling specialist insects11,13,14. In the case of temperate oaks, the future of masting may hinge on the evolution of their floral phenology9, which could not follow the same pattern in various climatic regions. These variations could lead to more cyclical and frequent mast-year events in some climatic regions, which could have dramatically positive effects on consumer population dynamics15. Over the long term, masting in beeches and oaks could also become exceedingly sensitive to summer drought conditions, as already documented in evergreen oaks16. We anticipate that the trajectories of masting and regeneration success will diverge significantly in the next decades, depending on the species, the soil and climate conditions of the populations and their genetic diversity.