Etude et suivi de la biodiversité forestière

Summary

During the 20th century, floristic composition was the main tool used to define and identify forest habitat types. Since then, floristic monitoring has also become one of the pillars for monitoring biodiversity dynamics.

The RENECOFOR Network began collecting floristic data on its 102 plots in 1995, shortly after it was established. At each plot, eight sub-plots of 100 m2 each, four inside an ungulate-proof enclosure and four outside the enclosure, are inventoried every 5 years (excluding 2010). Each of the sub-plots is inventoried twice during the campaign year. Ten plots are inventoried on a yearly basis. This floristic monitoring network is the only one in France to combine the following features for such a large number of sites:

• A consistent protocol has been adhered to since the beginning of the observations;
• Plots are permanently located at clearly marked, fixed locations;
• Data on soil chemistry is recorded at the same time.

Before each inventory campaign, the botanists involved attend "inter-calibration" training sessions over several days to refine and test certain sensitive points in the protocol. Particular attention is given to standardising procedures in the protocol, which has been applied at a wide diversity of sites and by nearly 80 observers since the beginning of the Network's monitoring programme.

One of the Networks first contributions was to adapt and refine the floristic inventory protocols typically in use at the time for vegetation studies (phyto-sociology studies, habit-type identification manuals...) to make them more rigorous. Indeed, if certain points in a protocol are not precisely controlled, variability in the observations will increase, for example concerning the plants located at the edge of the inventory plot, divisions among vertical strata, precise definitions and rigorous application of abundance-dominance coefficients, inventory methods for species linked to the presence of micro-habitats (deadwood, localised soil compaction...), inventory time limits...

The second contribution, again in terms of methodology, came with the weighty realisation that no floristic inventory can ever be complete, even on a surface area of only 100m2 - considered quite small for forest environments. Exhaustiveness varies considerably among botanist-observers and two inventories per year are an absolute necessity, to cover seasonal variations (in particular for prevernal plants) or simply to capture species overlooked during the first round. The most important methodological implication is that observers progressively improve the exhaustiveness of their inventories during the first few years, or in other words, several years of inventorying are necessary to ensure results consistent with monitoring needs.

From an ecological point of view, RENECOFOR's large-scale data have brought to light the role ungulates play in the current floristic composition of our forests. Inside the enclosures, shrubs and other species that compete with grasses (for example, brambles) were no longer browsed and rapidly thrived, inducing a very significant decline in floristic richness (see figure below). To a certain degree, ungulates preserve higher floristic diversity in the forest, mainly by browsing shrubs and allowing more light to reach the forest floor. However, the plant species that take advantage of the more open environment created by ungulates are  mainly heliophiles (light-loving species) and nitrophiles (nitrogen-loving species), ruderal or "pioneer" species less associated to forest environments than the ones favoured by the protective enclosures. To sum up, though species richness increases in the presence of ungulates, this increase favours plants that are not typical forest-dwellers.

Was there a clear trend for changes in flora between 1995 and 2010? For now, the variations observed in the Network are small but significant. RENECOFOR's floristic monitoring work, supported by its robust methodology, globally confirms observations from other networks for previous periods: vegetation is becoming more nitrophilous and neutrophilic in the plots left untouched by the 1999 storms. This suggests that eutrophication is occurring due to atmospheric nitrogen deposits, which remain relatively high in France. However, these changes in flora do not correlate with the trends observed for soil chemistry, thereby weakening the findings. Contradictory trends in soil chemistry appear throughout the network: decreasing pH and an increase in soil C/N ratios seem to indicate a decrease in available nutrients, in particular nitrogen. Finally, analyses did not reveal any trend linked to climatic variations.

Several lessons have been learned from the floristic monitoring carried out in the RENECOFOR Network. Concerning methodology, it has become clear that the protocols required to provide rigorous monitoring over time cannot simply be borrowed from those used for the spatial monitoring of vegetation. A revision of the protocols used in the Network may therefore be necessary. Questions have also arisen concerning the functional links between changes in the soil and trends in plant communities. The Network's results also confirm that monitoring temporal dynamics in biodiversity is a very long-term venture, in particular for forest flora, which shows little variation over short time spans.

Concerning environmental changes, the results with the RENECOFOR data obtained partially question the validity of rapid variations in flora observed after resampling on plots that were not designed with resampling in mind, or inferred from databases that mix very heterogeneous samples.  

Summary

Fungi are major contributors to forest biodiversity. The group includes a large number of species and represents a considerable part of the living biomass. Their activities contribute to the healthy functioning of the whole ecosystem: they help decompose organic matter and recycle the nutrients it contains, they structure the soil, they facilitate mineral uptake for trees through a vast network of mycelium filaments (mycorrhizea), etc.

Yet, compared to other organisms (like flora), little is known about the taxonomic, phenotypic and ecological diversity of fungi or about how their communities function in ecological terms (community structure, organization and determining factors).

In 1996, the ONF initiated a partnership with the French Mycology Observatory and the French Mycological Society to complete the monitoring effort on RENECOFOR plots with an inventory of fungal communities.

A first trial was conducted on 12 plots from 1996 to 1998. Then the experiment was repeated on a larger number of plots from 2003 to 2007. In all, about 60 plots were inventoried based on the observation of fruiting bodies visible above ground (sporophores). Since these fruiting bodies are often ephemeral, the inventories were repeated 12 times (over 3 consecutive years with a minimum of 4 repetitions per year) to reflect the diversity of species present on each plot as exhaustively as possible. Then followed a long period of extensive data analysis; in particular to standardise species names according to a single reference list.

This study was designed to investigate the variations in fungal community composition on the inventoried plots with various analytical tools. The study focused on the Basidiomycota, the most recorded species by far (84% of the 1,604 identified species) and the ones the participating mycologists knew the best. Plots that had not undergone at least two consecutive years of inventorying with three repeated campaigns per year were excluded from the analyses. The remaining 51 plots still encompassed a large range of environmental conditions with differing soils, climates, altitudes and atmospheric deposits and with different tree species (pedunculate oak, sessile oak, beech, Douglas fir, spruce, maritime pine, Scots pine and silver fir).

In practice, recording the number of species present (species richness) seems insufficient to assess changes in community diversity since exhaustiveness was unattainable, despite repeated campaigns on each plot. Even on the plots where inventories were carried out most often (up to more than 30 times in three years), each new inventory revealed at least one new species that had not been previously recorded.

On the other hand, species communities did vary from stand to stand in a non-random way following a specific organisational pattern. Above all, the fungal community was influenced by the dominant tree species in the stand; this is in remarkable contrast with observed patterns for other groups of organisms such as flora, where bio-geographical factors (climate) play a major role. For fungi, bio-geographic factors (altitude, latitude, precipitation) do have an influence, but the role they play is of secondary importance, on a par with "more local" soil factors (pH, carbon/nitrogen ratio, percentage base saturation).

Furthermore, the study linked the patterns of variation in fungal community composition to the more or less generalist, or specialist, nature of the species related to their habitat and ecological niche. A species was considered to be more generalist if it appeared along with numerous other species of varying nature.

Conversely, a species was deemed more specialist if it was associated to few other species, which tend to be the same from one inventory to the next. The RENECOFOR campaign revealed that most ectomycorrhizal fungi (in symbiosis with tree roots) are more generalist than saprophytic fungi (organic matter decomposers), which tend to be more specialised.

Furthermore, the fungal communities found in resinous stands were mainly composed of generalist species, while those found under broadleaves included a larger proportion of specialists. In this variation pattern from resinous to broadleaf plots, Douglas fir stands were noticeably different; the fungal communities therein hosted an intermediate range of species with the lowest number of species (probably due to the exotic nature of Douglas fir in Europe).

To conclude, though certain methodological hurdles were encountered, this pioneering initiative to inventory the fungal community produced robust, original results and has made it possible to document variation patterns in these communities over a wide range of ecological conditions. In addition, the inventory results have raised new questions related to silvicultural management and soil biodiversity preservation and have brought to light the preponderant role local factors (tree species choice, physio-chemical soil properties) play in fungal communities, which are so important in forest ecosystem functioning.

Summary

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

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.