Tree harvesting in OPTMix experimental plots is now completed (related articles here and here). Most of the exploitation was performed in one month (from 2017-09-25 to 2017-11-03) and was realized in very good conditions.

A big thank you to the National Forest Office (ONF) who carried out and coordinated this operation and who was able to find solutions to take into account all the constraints of this experimental device.

An article dealing with stand composition effect on tree functional response during severe drought has just been published in Annals of Forest Science. This work was partly performed on OPTMix experimental plots in collaboration with Damien Bonal (INRA Nancy). This work was started during the internship of Mathilde Pau (Master 2, Irstea Nogent-sur-Vernisson, year 2016). Below are the key points of the article.

Bonal, D., M. Pau, M. Toigo, A. Granier and T. Perot (2017). “Mixing oak and pine trees does not improve the functional response to severe drought in central French forests.” Annals of Forest Science 74(4): 72. doi: 10.1007/s13595-017-0671-9

• Key message: Mixing sessile oak and Scots pine in central France to reduce intraspecific competition for water resources did not improve the ability of these two species to withstand severe drought during the summer.
• Context: In order to reduce the impact of increasingly extreme droughts on forests, managers must adapt their practices to future climate conditions. Maintaining a greater diversity of tree species in temperate forest ecosystems is one of the recommended options.
• Aims: We addressed how interactions between sessile oak and Scots pine in mixed forests in central France affect their functional response to drought.
• Methods: We characterized the carbon isotope composition (δ13C) in the tree growth rings formed during wet (2001, 2007) or dry (2003, 2004) summers for each of the two species growing both in pure and in mixed stands in order to compare the effect of stand composition on variations in carbon isotope discrimination (Δ13C) among contrasted years.
• Results: The severe drought in 2003 induced a strong decrease in Δ13C for all trees and in all stands as compared to 2001. This decrease was greater in pine than in oak. There was no significant difference between pure and mixed stands in the response of either species to drought.
• Conclusion: Mixing sessile oak and Scots pine in stands in central France does not improve the ability of either species to withstand severe drought during the summer.

Mean Δ13C values for each year according to tree species and stand composition. Vertical bars represent standard errors of the mean

OPTMix has recently received foreign researchers for soil sampling (more details here), but OPTMix members also visit experimental sites in foreign country in order to promote international exchanges between researchers. It is with this goal of scientific exchanges that Jordan Bello (PhD student on OPTMix) went to Sweden, and more specifically to Umeå at SLU, Swedish University of Agricultural Sciences for one month (november 2017). He is working with Niles Hasselquist, a researcher working on soil-plant relationships and mainly on how climate change and resource management are changing energy and nutrient cycles. Jordan will take advantage of this exchange to analyse data from isotopic measurements taken in the soil and trees of OPTMix at the end of summer 2016 (more details here). The objective of these analyzes is to determine the depth of water uptake by trees during a period of water stress and to see wether this depth depend on tree species (sessile oak or Scots pine), tree size, and stand composition (pure or mixed). Jordan also visited their experimental site where isotope measurements of sap water are made in real time (see picture below).

Tree harvesting began on September 26th (see also here). Nine management units are concerned for a total of about 40ha. This operation is carried out and coordinated by the National Forest Office (ONF).
When possible, tree harvesting is done mechanically, making it possible to better take into account the constraints associated with the presence of sensitive equipments installed in the plots (sensors, collectors, fences) and to concentrate the impact of the thinning in skid trails.

OPTMix is one of the European site chosen for the ERA-NET SUMFOREST REFORM (REsilience of FORest Mixtures) project supervised by Quentin Ponette (professor at the Catholic University of Louvain). The aim of this REFORM project is to develop silvicultural prescriptions for mixed species stands that are highly resistant and resilient to biotic and abiotic disturbances in the future. As part of WP4 dealing with the long term provision of ecosystem services, the present sub-project focuses on the impacts of tree species composition on soil organic carbon storage. Therefore, soil samples are collected, and for OPTMix this is done by Benjamin Willems and Richard Osei, from the Catholic University of Louvain at the end of September. They use a combined triplet – transect approach across Europe for a set of contrasting two species mixtures. The specific objectives of this research are:
(i) to compare the soil organic C stocks (forest floor and mineral soil down to 40 cm) under pure and mixed stands;
(ii) to quantifiy the effects of species identity and mixing on the soil organic C partitioning;
(iii) to test for a possible site effect or (site×mixing) interaction on the above processes.

This work is complementary to the post-doc of Céline Pradier starting on December 2017 at irstea Nogent-sur-Vernisson, who will study the carbon pool on OPTMix and compare above and below-ground carbon pools in pure and mixed oak-pine stands.

Soil sampling from litter down to 40 cm deep (Richard Osei, PhD at the Catholic University of Louvain)

Measurement of the soil bulk density using small glass marbles (Benjamin Willems, technician at Catholic University of Louvain)

Two stand densities are tested in the OPTMix device: a low density with a Relative Density Index (RDI) around 0.4 and a medium density with a RDI around 0.7. For the mixture there is also a third modality without thinings (for more details see here). A first thinning was carried out between 2013 and 2015 which makes it possible to reach the target densities for most of the plots. Nevertheless, for some plots this was not possible because the starting densities were too high. A new thinning should therefore take place this winter to reach the definitive densities of the experimental device.
In a first step a complete plot inventory was carried out to precisely determine the situation before thinning and to determine the number of stems to be removed to reach the target densities. In a second step, for all the plots, tree marking was carried out on cells of approximately 5000 m² so as to obtain a density as homogeneous as possible at the plot level.
The thinning will be carried out during this winter with the help of the National Forest Office.

As part of the Jordan Bello Ph.D. (Irstea Nogent-sur-Vernisson), sap flow sensors were installed in the OPTMix plots in collaboration with Damien Bonal (INRA Nancy).
The objective of this experiment is to better understand the water consumption of sessile oak and Scots pine when these species grow in a soil with a perched water table. For this purpose, the density of the flow of the individuals will be measured when water resource is not limiting as well as during periods of strong water stress. In addition, the sensors were installed in pure and mixed stands to determine if the water comsuption of these species change when they grow in a mixture.

Wild boars are considered as key elements of forest ecosystems by having direct or indirect effects on the environment through tillage, zoochory, or trampling, for example. These impacts can be observed in bird communities. This problem is well known in agricultural environments but what about forest bird species?

This question is the subject of a Master 2 internship (Amanda Cocquelet, University of Saint Etienne) which began on 27 February under the supervision of Frédéric Archaux and Anders Mårell for the COSTAUD project. Part of the internship was performed on OPTMix plots. The protocol consisted in placing artificial nests containing quail eggs in the fenced and unfenced plots. Some of these nests were accompanied by a photographic trap to know the identity of the predators and to estimate a frequentation of each plot by the different species present in the Orléans forest. At the same time, point counts were set up to define the bird communities on the OPTMix plots. Herbaceous and shrub cover were recorded to characterize the structure of the vegetation and the percentage of soil returned was recorded as an indicator of the presence of wild boars.

The aim of this study is to determine whether the presence of wild boar can directly influence forest bird communities by predation of nests, or indirectly by effects on vegetation cover.

According to the latest estimates, there are between 500 and 700 species of bramble in France (http://www.tela-botanica.org/). In France the inventory of bramble species is still very incomplete. The first floristic inventories carried out on OPTMix revealed the presence of several species of brambles without being able to determine them reliably.

An experiment has just begun in order to determine the species present on the OPTMix device. For this purpose, brambles will be multiplied in-situ by layering and then raised ex-situ under full light conditions allowing a better determination of species.

The determination at the species level of brambles will make it possible to better estimate the biodiversity on the OPTMix experimental plots. On the other hand, the species of brambles differ in their ecology and palatability. It is therefore important to be able to distinguish them in order to better understand the understorey vegetation dynamics in interaction with large herbivores.

Vincent Seigner has just left us this summer for other horizons (Quebec). A big thank you to Vincent for all he has contributed to the OPTMix project since the installation of the plots until the installation of all the sensors and dataloggers. Below is a small illustration made by his daughter showing Vincent in action and showing how much he put his heart in this job.
We are sure that he will showcase his multiple skills to the forestry community in Quebec and we are looking forward to working with him in the future.