As part of the seventh international day dedicated to plant sciences entitled “Fascination of Plants Day – 2024”, the EFNO unit of the INRAE Val de Loire center will welcome 70 students from the Bernard Palissy high school in Gien (Loiret). This welcome will take place on May 7 at the OPTMix experimental device located in the Orléans forest.
High school students from 1st year “Speciality: biology” will therefore participate in various work devoted to the study of the forest ecosystem. In the program :

• Spot 1: estimation of tree biomass in relation to the ecosystem service of climate regulation
• Spot 2: regulation service on climate and water
• Spot 3: forest and climate change: OPTMIX project: what silvicultural strategy to adapt the functioning of this ecosystem to climate change (INRAE animation)
• Spot 4: interactions between species in their diversity
• SPot 5: research profession (INRAE animation)
• Spot 6: Environmental metrology (INRAE animation)

As part of a project that aim to better define the leaf surface of mixed stands (InFoMix), we benefited from the scientific and technical support of the Zone Atelier Loire and Mathieu Bonnefond from LECAM in Tours, for a flight of a drone equipped with a LiDAR in February 2024. This flight will be repeated, coupled with multispectral measurements on two other dates: end of May, during the vegetation period but without water stress, and in August, during a more stressed period. We will compare the images between dates, as well as the results of the calculation chain between different methods used for estimating the LAI (Leaf Area Index) of oaks and pines

At the end of March, a new experiment was set up in the low-density plots in enclosures and exclosures. The project, named OSCAR (“How wild ungulates modify the cycle of nutrients and carbon in the forest”) aims at testing the effect of ungulates on the biogeochemical cycles. We installed ion exchange resins, buried at 8 cm depth, to capture soil available nutrients during a year, and we also installed tea bags to monitor the decomposition of organic matter for 6 months. Soil and humus samples were also taken.

At the beginning of March, a new manipulation was installed on 3 plots (a triplet pure oak, mixture, pure pine). This experiment is replicated on different sites of the AnaEE network. The project is conducted by collaborators : the eDNA platform and the CEREEP Ecotron IDF, the EcoGeno platform and Lucie Zinger from the IBENS laboratory, and proposes to compare the dynamics of degradation of environmental DNA (eDNA) and organic carbon (13C) in freshwater, lake sediment, and soil matrices.

On OPTMix, in addition to the composition, we test the effect of the exclusion of ungulates, by working inside and outside the enclosures.

The manipulation consists in placing 4 small bags of 100g of soil from the plot mixed with a few leaves of rice marked with 13C at 5 cm in the soil, repeted 3 times in a plot, and this in the different situations (enclosure and exclosure, pure oak, pure pine, mix). Four sampling dates are scheduled between March and the end of May. At the same time, and at the same locations, a few green tea bags will be installed at the end of March to monitor the loss of tea leaf mass on 2 dates by the end of May.

For two years, the OPTMix experiment has contributed with data on seed production to the MASTIF network. The MASTIF Network, Mast Inference and Prediction, (https://sites.nicholas.duke.edu/clarklab/projects/mastif-network/) is a long-term monitoring network with more than 500 plots and crop-count locations across the world, representing decades of intensive field and laboratory work.

In a recent article, Michal Bogdziewicz and collaborators link seed size and number with tree characteristics to shed new light on the role of fruiting in the various theories around plant ecological strategies (Bogdziewicz et al. 2023). The authors of the article found that high seed productivity (the combination of size and number of seeds) is associated with trees with high leaf area, low foliar nitrogen, low specific leaf area (SLA) and dense wood. The paper’s findings can help to better predict tree fertility across forests globally, which is particularly important in the context of climate change.

Map of raw data used to estimate the number of seeds produced by trees with the masting inference and forecasting (MASTIF) model. (Global Ecology and Biogeography, DOI: https://doi.org/10.1111/geb.13652)

Conditional relationships between traits after accounting for climate and phylogeny. Posterior distributions are shown as boxes that contain median vertical lines and are bounded by 68% credible intervals, with 95% credible interval whiskers. Coefficients are evaluated on a standardized scale. The inset plots in (a) highlight the relationships between species seed productivity (SSP; the product of seed size and seed number) and other traits after removing the effects of seed number and seed size that are part of SSP. Insets in (b) and (c) are analogous. Figure 3 summarizes the significant relationships. Both SSP and seed number are standardized to a tree basal area. N, nitrogen; SLA, specific leaf area; SSP, species seed productivity. (Global Ecology and Biogeography, DOI: https://doi.org/10.1111/geb.13652)

References

Bogdziewicz, M., Acuña, M.-C.A., Andrus, R., Ascoli, D., Bergeron, Y., Brveiller, D., Boivin, T., Bonal, R., Caignard, T., Cailleret, M., Calama, R., Calderon, S.D., Camarero, J.J., Chang-Yang, C.-H., Chave, J., Chianucci, F., Cleavitt, N.L., Courbaud, B., Cutini, A., Curt, T., Das, Adrian J., Davi, H., Delpierre, N., Delzon, S., Dietze, M., Dormont, L., Farfan-Rios, W., Gehring, C.A., Gilbert, G.S., Gratzer, G., Greenberg, C.H., Guignabert, A., Guo, Q., Hacket-Pain, A., Hampe, A., Han, Q., Hoshizaki, K., Ibanez, I., Johnstone, J.F., Journé, V., Kitzberger, T., Knops, J.M.H., Kunstler, G., Kobe, R., Lageard, J.G.A., LaMontagne, J.M., Ledwon, M., Leininger, T., Limousin, J.-M., Lutz, J.A., Macias, D., Marell, A., McIntire, E.J.B., Moran, E., Motta, R., Myers, Jonathan A., Nagel, T.A., Naoe, S., Noguchi, M., Oguro, M., Kurokawa, H., Ourcival, J.-M., Parmenter, R., Perez-Ramos, I.M., Piechnik, L., Podgórski, T., Poulsen, J., Qiu, T., Redmond, M.D., Reid, C.D., Rodman, K.C., Šamonil, P., Holik, J., Scher, C.L., Van Marle, H.S., Seget, B., Shibata, M., Sharma, S., Silman, M., Steele, M.A., Straub, J.N., Sun, I.-F., Sutton, S., Swenson, Jennifer J., Thomas, P.A., Uriarte, M., Vacchiano, G., Veblen, T.T., Wright, B., Wright, S.J., Whitham, T.G., Zhu, K., Zimmerman, J.K., Zywiec, M., Clark, J.S., 2023. Linking seed size and number to trait syndromes in trees. Global Ecology and Biogeography. https://doi.org/10.1111/geb.13652

Like last year, the OPTMix device participated in the Snapshot Europe initiative in 2022. Snapshot Europe (https://app.wildlifeinsights.org/initiatives/2000166/Snapshot-Europe) is a coordinated and standardized camera trap effort to collect data on mammals across Europe. The initiative is supported by Euromammals and the Max Planck Institute of Animal Behaviour, in partnership with Snapshot USA (https://app.wildlifeinsights.org/initiatives/2000156/Snapshot-USA).

Collaborators sample 10-50 sites during September-October of each year for at least 3 weeks per site and 400 camera trap-days across all sites. The study is designed to sample sites in all countries stratified across habitat types and development zones (suburban/rural/wild/urban).
This year brought together researchers around 69 monitoring sub-projects with over 900 monitored sites. The Snapshot initiative started in the United States in 2019. Since then, it has been renewed every year.

An article has just been published in the “Journal of Applied Ecology” journal dealing with the effect of mixing tree species on forest productivity. This article shows that two species mixed stands have a greater productivity stability over time than comparable monospecific stands. Mixed stands would buffer the destabilizing effect of temperatures on growth, which would lead to a greater stability in the productivity of mixed stands compared to monospecific stands.

The OPTMix plots contributed to the data of this article based on 261 plots across Europe (87 triplets) of monospecific and mixed stands (beech – Scots pine; sessile and pedunculate oak – Scots pine; spruce – Scots pine).

del Río, M., H. Pretzsch, R. Ruiz-Peinado, H. Jactel, L. Coll, M. Löf, J. Aldea, C. Ammer, A. Avdagić, I. Barbeito, K. Bielak, F. Bravo, G. Brazaitis, J. Cerný, C. Collet, S. Condés, L. Drössler, M. Fabrika, M. Heym, S.-O. Holm, G. Hylen, A. Jansons, V. Kurylyak, F. Lombardi, B. Matović, M. Metslaid, R. Motta, T. Nord-Larsen, A. Nothdurft, J. den Ouden, M. Pach, M. Pardos, C. Poeydebat, Q. Ponette, T. Pérot, D. O. J. Reventlow, R. Sitko, V. Sramek, M. Steckel, M. Svoboda, K. Verheyen, S. Vospernik, B. Wolff, T. Zlatanov and A. Bravo-Oviedo (2022). « Emerging stability of forest productivity by mixing two species buffers temperature destabilizing effect. » Journal of Applied Ecology n/a(n/a) DOI: https://doi.org/10.1111/1365-2664.14267.

Abstract:

1. The increasing disturbances in monocultures around the world are testimony to their instability under global change. Many studies have claimed that temporal stability of productivity increases with species richness, although the ecological fundamentals have mainly been investigated through diversity experiments. To adequately manage forest ecosystems, it is necessary to have a comprehensive understanding of the effect of mixing species on the temporal stability of productivity and the way in which it is influenced by climate conditions across large geographical areas.
2. Here, we used a unique dataset of 261 stands combining pure and two-species mixtures of four relevant tree species over a wide range of climate conditions in Europe to examine the effect of species mixing on the level and temporal stability of productivity. Structural equation modelling was employed to further explore the direct and indirect influence of climate, overyielding, species asynchrony and additive effect (i.e. temporal stability expected from the species growth in monospecific stands) on temporal stability in mixed forests.
3. We showed that by adding only one tree species to monocultures, the level (overyielding: +6%) and stability (temporal stability: +12%) of stand growth increased significantly. We identified the key effect of temperature on destabilizing stand growth, which may be mitigated by mixing species. We further confirmed asynchrony as the main driver of temporal stability in mixed stands, through both the additive effect and species interactions, which modify between-species asynchrony in mixtures in comparison to monocultures.
4. Synthesis and applications. This study highlights the emergent properties associated with mixing two species, which result in resource efficient and temporally stable production systems. We reveal the negative impact of mean temperature on temporal stability of forest productivity and how the stabilizing effect of mixing two species can counterbalance this impact. The overyielding and temporal stability of growth addressed in this paper are essential for ecosystem services closely linked with the level and rhythm of forest growth. Our results underline that mixing two species can be a realistic and effective nature-based climate solution, which could contribute towards meeting EU climate target policies.

An article has just been published in the “Forest Ecology and Management” journal dealing with the stand structuring of monospecific Scots pine forests. This article shows that the stand structuring decreases with site quality because of mortality and asymmetric competition processes. The OPTMix plots contributed to the data of this study based on 90 Scots pine stands.

Pretzsch, H., A. Bravo-Oviedo, T. Hilmers, R. Ruiz-Peinado, L. Coll, M. Löf, S. Ahmed, J. Aldea, C. Ammer, A. Avdagić, I. Barbeito, K. Bielak, F. Bravo, G. Brazaitis, J. Cerný, C. Collet, L. Drössler, M. Fabrika, M. Heym, S.-O. Holm, G. Hylen, A. Jansons, V. Kurylyak, F. Lombardi, B. Matović, M. Metslaid, R. Motta, T. Nord-Larsen, A. Nothdurft, C. Ordóñez, J. den Ouden, M. Pach, M. Pardos, Q. Ponette, T. Pérot, D. O. J. Reventlow, R. Sitko, V. Sramek, M. Steckel, M. Svoboda, E. Uhl, K. Verheyen, S. Vospernik, B. Wolff, T. Zlatanov and M. del Río (2022). « With increasing site quality asymmetric competition and mortality reduces Scots pine (Pinus sylvestris L.) stand structuring across Europe. » Forest Ecology and Management 520: 120365 DOI: https://doi.org/10.1016/j.foreco.2022.120365.

Highlights
• The study based on 90 mature Scots pine stands along a productivity gradient across Europe.

• Growth partitioning became more asymmetric and structuring with increasing site quality.

• Mortality eliminated predominantly small trees with increasing site quality.

• We found the highest size variation on poor sites and the lowest on rich sites.

• As a result stand structure became more homogeneous with increasing site quality.

Abstract
Heterogeneity of structure can increase mechanical stability, stress resistance and resilience, biodiversity and many other functions and services of forest stands. That is why many silvicultural measures aim at enhancing structural diversity. However, the effectiveness and potential of structuring may depend on the site conditions. Here, we revealed how the stand structure is determined by site quality and results from site-dependent partitioning of growth and mortality among the trees. We based our study on 90 mature, even-aged, fully stocked monocultures of Scots pine (Pinus sylvestris L.) sampled in 21 countries along a productivity gradient across Europe. A mini-simulation study further analyzed the site-dependency of the interplay between growth and mortality and the resulting stand structure. The overarching hypothesis was that the stand structure changes with site quality and results from the site-dependent asymmetry of competition and mortality.

First, we show that Scots pine stands structure across Europe become more homogeneous with increasing site quality. The coefficient of variation and Gini coefficient of stem diameter and tree height continuously decreased, whereas Stand Density Index and stand basal area increased with site index.

Second, we reveal a site-dependency of the growth distribution among the trees and the mortality. With increasing site index, the asymmetry of both competition and growth distribution increased and suggested, at first glance, an increase in stand heterogeneity. However, with increasing site index, mortality eliminates mainly small instead of all-sized trees, cancels the size variation and reduces the structural heterogeneity.

Third, we modelled the site-dependent interplay between growth partitioning and mortality. By scenario runs for different site conditions, we can show how the site-dependent structure at the stand level emerges from the asymmetric competition and mortality at the tree level and how the interplay changes with increasing site quality across Europe.

Our most interesting finding was that the growth partitioning became more asymmetric and structuring with increasing site quality, but that the mortality eliminated predominantly small trees, reduced their size variation and thus reversed the impact of site quality on the structure. Finally, the reverse effects of mode of growth partitioning and mortality on the stand structure resulted in the highest size variation on poor sites and decreased structural heterogeneity with increasing site quality. Since our results indicate where heterogeneous structures need silviculture interventions and where they emerge naturally, we conclude that these findings may improve system understanding and modelling and guide forest management aiming at structurally rich forests.

Matias Bentkowski is carrying out his internship at the EFNO unit of INRAE. The aim is to study the impact of summer drought on tree regeneration. The field experiment is done using the OPTMix research facility. It consists of measuring water stress and the annual growth of seedlings of Scots pine and Sessile oak under different natural conditions (competition with understory vegetation, level of lighting due to the forest cover, pressure of browsing and fraying by deer). The field measurements will take place during 3 months (June, July and August). The tree seedlings will be regularly measured throughout the summer period. The aim is to estimate the impact of different natural factors on forests regeneration.

“Where is the student?”. Photo taken by Guilhem Parmain during a field trip on the OPTMix device

water stress measurement using a porometer. Photo taken by Matias showing the device used for field measurements

In mixed forests, complementarity can be expressed by a differentiate use of space between species so that each species makes the best use of the light resource according to its photosynthetic capacities. This spatial differentiation between species is possible thanks to plasticity in crown architecture. Thus many articles report an increase in the extent and the volume of the crowns in the mixtures compared to the monospecific stands, but with a significant tree species and size tree effects. We wanted to know if the effect of the mixture on tree crown architecture was also observed on the OPTMix plots between sessile oak and Scots pine and to see if there was a relationship with the amount of litter and the nutritional status of the trees.

We therefore carried out measurements of the height of the first living branch and the radius of the crowns according to the “vertical sighting method” (Preuhsler, 1979), to calculate the projection surface and the volume of the crowns of 75 pines and 75 oaks. Several Master trainees did this measurement campaign. The operator effect on the crown measurements will also be studied. A comparison with previous crown data collected in 2017 will be made if possible.