Effects of variations in masting on biodiversity

Fruit production in oaks (acorn production) varies considerably from year to year. At the population scale, this fruiting regime, known as "masting", often involves very heavy acorn production certain years, followed by low fruiting in subsequent years.

This could be a reproductive strategy designed to control acorn predator populations; heavy mast years would insure that a large proportion of the acorns would not be consumed and would therefore guarantee a strong regeneration potential. Masting should therefore strongly affect population and evolution dynamics in acorn eaters (insects, birds, rodents, ungulates) and, through a cascade effect, influence the dynamics of the whole animal community in the forest (for example, predators/parasites of the acorn eaters).

There may also be more or less long-term socio-economic repercussions related to forest regeneration success, the organisation of forest stands (relative abundance of different tree species) and their associated economic sectors, or certain disease dynamics (for example, Lyme disease depends on tick populations, which in turn depend on rodent populations).

However, the mechanisms that drive the strong variations in masting are still relatively unknown. This makes it difficult to:

• Anticipate acorn production in the short term in order to adjust management policy (for example, whether to optimise seed harvesting or forest regeneration);

• Predict the effect of climate change on oak reproductive potential (variations in the intensity or frequency of heavy masting years) and the ecological and socio-economic consequences of these changes on the ecosystems concerned.
  

The "PotenChêne" programme, financed by the Ministries of the Environment and of Agriculture (GIP ECOFOR 2014-2018), aims to better understand the mechanisms involved in oak masting and its impact on the population dynamics of acorn consumers (ungulates and insects) and on forest regeneration. The programme brings together eight partner organisations (see figure below).

The "PotenChêne" program is composed of 9 partners, including 5 research organizations - LBBE, BioGeCo, CNR, IRSTEA (National Institute of Research in Science and Technology for the Environment and Agriculture), ISPRA (Institute for Environmental Protection and Research, Roma, Italy) and 4 public bodies - NFB, ONCFS, FNC, RNSA. © Samuel Venner / Université Lyon 1

Thanks to litterfall samples collected from 1994 to 2007 on each plot in the network, RENECOFOR provides an exceptional data set, even on the international stage, to analyse variations in oak masting.

In addition, since 2012 the "PotenChêne" programme has equipped 15 sites in France (12 in the RENECOFOR network), covering a vast range of climatic conditions, to monitor flowering and acorn production on individual sessile oak trees. The diversity and abundance of acorn-parasite insect communities (5 species) are recorded. Wild boar populations are also monitored at several research sites included in the programme.

The results reveal that acorn production has increased on average over the last few years in France, concomitant with the increase in spring temperatures. A mathematical model was built to identify the key processes in masting, based on data collected tree by tree and on pollen measurements carried out by the National Network for Aerobiological Monitoring (RNSA).

The simulations produced by the model suggest that masting dynamics are closely related to the proportion of resources that the trees allocate to pollen production from one year to the next, and to spring weather conditions, which effect pollen dispersal. Mast intensity in turn affects the reproductive success of wild boar sows and boar population levels.

Masting intensity also affects the dynamics of parasitic insect communities; this parasitism usually prevents acorns from germinating. The oak masting regime seems to have favoured a diversification of strategies among the insect species competing for acorns (for example, diapause varies widely from species to species and can last from 1 to 4 years). These results illustrate the complexity of the challenge oak trees face as they try to withstand attacks from this broad diversity of acorn consumers and to maintain a high potential for recruitment and regeneration.

Future climate changes, in particular modifications in spring meteorological conditions, may affect pollen dispersal dynamics, fruiting dynamics (studies in progress), the dynamics of acorn-consumer communities and the consequent effects on regeneration potential in oak stands, the damage done by wild boar and even certain disease patterns (for example, Lyme disease).

To complement the monitoring network set up in the programme, we have developed a "light" monitoring method for masting; it is easy for forest managers to apply and will be implemented as of 2017 on a larger set of forest sites. These monitoring efforts, coupled with the development of a mast prediction tool, could improve both oak regeneration management and wild boar population control.