The variability of leaf phenology among regional beech

A new paper entitled ‘Leafy season length is reduced by a prolonged soil water deficit but not by repeated defoliation in beech trees (Fagus sylvatica L.): comparison of response among regional populations grown in a common garden‘ has been published by Massonnet et al. (SILVA Department) has been published in Agricultural and Forest Meteorology. 

Abstract. Bud-burst and leaf-senescence determine the length of the growing season for deciduous trees and therefore the duration of potential carbon assimilation with consequences on biomass production. In Fagus sylvatica L., leaf phenology depends on both photoperiod and temperature. The future climate is expected to induce more frequent soil water deficits and biotic attacks (possibly resulting in severe defoliation).

The aim of the present study is to assess whether these constrains may alter leaf phenology. In a common garden, we sowed seeds collected from six beech forests along a small latitudinal gradient (140 km) in North-Eastern France. In 2014, after seven years growth, a rain exclusion was installed above the trees to test how recurrent soil water deficits impacted bud-burst (BB) and leaf-yellowing (LY) over three years. We also analyzed the response of leaf phenology to annual defoliation, aiming at affecting carbon and nitrogen availability in trees.

Delayed BB and early LY were observed, reducing the growing season (GS) until 14 days in response to soil water deficit whereas no influence of defoliation was detected. These time lags were not in relation with leaf nitrogen content. In the control treatment, BB occurred earlier and LY later in the northernmost populations than in the southernmost without clear relationships with local climate. A significant treatment x population interaction was observed revealing a plasticity in the leaf phenology response to soil water deficit among populations.

These results suggest that beech trees present a genetic differentiation of leaf phenology even within a small latitudinal gradient but that these differentiations could be disrupted by soil water deficit that is predicted to increase in the future.

The experiment and operating costs were funded by the LabEx ARBRE within the framework of the Mepib-Death project. Pierre-Antoine Chuste received a PhD grant from ARBRE.

Large-scale genomics sheds light on the evolutionary history of mutualistic forest-dwelling fungi

Mutualistic fungi, known as mycorrhizae, play a major role in terrestrial ecosystems because they help plants acquire nutrients. However, how these fungi became symbionts was a mystery until now. Answers to this question have been provided by an international research consortium coordinated by INRAE and the Joint Genome Institute (US Department of Energy) to which the University of Lorraine and CNRS also contributed. The group analysed the genomes of 135 species of forest-dwelling fungi. The study’s results clarify how fungi living as decomposers became plant symbionts over the course of evolution. These findings were published on October 12 in Nature Communications.

Fungi display a variety of modes of nutrition (i.e. lifestyles): some are pathogens that parasitize living organisms, some are saprotrophs that exploit decomposing organic matter, and yet others are mycorrhizae that form beneficial symbioses with plants. More specifically, the plant roots and fungi engage in a mutualistic relationship that benefits them both. The mycorrhizae help the plants absorb essential minerals, such as nitrogen and phosphorus, promoting growth. The plants provide the mycorrhizal fungi with sugars and favourable environmental conditions. This mutualism has allowed plants to colonise most terrestrial habitats. To understand the evolutionary emergence of symbiotic traits in forest-dwelling fungi, the research consortium compared the functional traits encoded by the genomes of 135 fungal species, including 62 species of mycorrhizal fungi. Notably, the researchers sequenced and analysed, for the first time, the genomes of 29 symbiotic fungal species belonging to taxonomic orders like Russulales and Cantharellales; these taxa are important keystone species in temperate forest ecosystems.

The study revealed that, over evolutionary time, forest fungi transitioned multiple times from sapotrophic to symbiotic lifestyles. These transitions were accompanied by a loss of the genes that encode plant-cell-wall-degrading enzymes (such as cellulases and lignin-modifying enzymes). Other genes (e.g., sugar and amino acid transporters) are conserved in saprotrophic ancestors, but have been recruited for new symbiotic functions. Finally, several symbiosis-specific genes involved in fungus-plant communication were created de novo.

These transitional patterns were observed in all the families of ectomycorrhizal fungi belonging to the phyla Basidiomycota and Ascomycota2  (which represent 20,000 species). This finding provides a striking example of recurrent convergent evolution3  taking place over a time span of more than 100 million years. The researchers also discovered a few “hybrid” fungi—species that are still capable of decomposing organic matter but that can also live in symbiosis within the roots of their host plants. In these species, the genes encoding plant cell-wall-degrading enzymes are still present. However, they are not expressed when the fungi live as symbionts. It is possible that these species illustrate the first steps towards strict symbiosis.

This study has considerably increased the genomic resources available for studying the mechanisms underlying the development and function of mycorrhizal symbioses. In addition to clarifying the evolutionary history of forest-dwelling fungi, these discoveries will facilitate future studies examining how fungal communities in forest ecosystems could deal with climate change.

[1]Ectomycorrhizal fungi colonise the roots of plants. They live in symbiosis with their hosts, helping the latter absorb minerals.

[2]Basidiomycota and Ascomycota are two fungus phyla. Species in Basidiomycota form spores on the outside of specialised cells called basidia; mushroom-forming fungi are the most emblematic members of this taxon. Species in Ascomycota form spores within specialised cells called asci; commonly recognised members of this taxon are the morels, truffles, and yeasts.

[3]Convergent evolution is a process whereby different species that experience the same environmental constraints evolve similar adaptive responses. In this case, ectomycorrhizal symbiosis is that convergent response.

Reference

Miyauchi, S., Kiss, E., Kuo, A. et al. Large-scale genome sequencing of mycorrhizal fungi provides insights into the early evolution of symbiotic traits. Nat Commun 11, 5125 (2020).

Le Génie des Arbres

Le génie des arbres, un documentaire diffusé lors de la soirée Science Grand Format du jeudi 15 mai à 20h50 sur France 5 – avec plusieurs interventions des équipes du LabEx ARBRE …

Soutenance HdR

Cyrille Rathgeber soutiendra son habilitation à diriger des recherches (HdR) intitulée :

“Écophysiologie de la Formation du Bois : Concepts, Méthodes et Applications”

jeudi 5 septembre 2019 à partir de 14h00

Salle de conférence du Centre Inra Grand Est-Nancy


Analyzing the growing stock expansion in French metropolitan forests

Ph.D. defense: Anaïs Denardou-Tisserand

“Changements du stock de bois sur pied des forêts françaises – Description, analyse et simulation sur des horizons temporels pluri-décennal (1975 – 2015) et séculaire à partir des données de l’inventaire forestier national et de statistiques anciennes”,

Friday 8 February 2019 at 9.00 am at Amphi Cuénot (Aquarium), Nancy.

Context. After centuries of decrease, the forest area of most developed countries increase, a phenomenon termed “forest transition”. While current increase in growing stock (GS) is greater than that in area, it remains far less studied. These changes are linked to major current issues. It is essential to assess these changes, to decipher their underlying causes, and to quantify them over the longer term in order to locate current forest resources on a broad trajectory and to anticipate their future dynamics. This thesis is dedicated to French metropolitan forests, which exhibit the most intensive changes in the growing stock in Europe, and relies on data from the French NFI program.

Objectives.

(1) Analyse forest areal, GS and GS density (GSD) changes and their spatio-temporal variations over 40 years (1975-2015). They were related to factors hypothesized to feature forest changes (geographical contexts, ownership and species composition). We screened for changes in the rate of expansion. The relationships between GS changes and some forest attributes (initial GS and GSD, recent forest area increase) were investigated.

(2) Uncover the processes of GS changes and to split the GS expansion magnitude across dynamically-homogeneous forest ensembles. The study was based on GS flux estimation (growth, ingrowth, mortality and harvest).

(3) Locate the actual GS expansion in a secular perspective. This analysis consisted in reconstructing the GS chronology since 1850. Levels of GS in 1892, 1908 and 1929 (associated to area of ancient statistics) were estimated using a conditional imputation approach for GSD estimation. Then, a holistic growing stock densification model was implemented to inquire the conditions required on densification patterns and magnitude to simulate the reconstituted GS chronology.

Results.

(1) Over 40 years, GS increases were three times faster than the areal ones, underlining the intensity of forest densification. No sign of saturation was found. Private forests, and mainly broadleaved ones, presented the greatest GS and GSD increases, suggesting the essential role of natural expansion and agricultural land abandonment. Regression models revealed the positive effect of initial GS and of recent areal increases on GS expansion.

(2) The analysis of GS expansion processes evidenced the low level of harvests in comparison to forest growth, and the contribution of recent forests to wood resource development. It led to identify four synthetic forest ensembles contributing to the expansion and of distinct dynamics, mainly composed of private forests.

(3) GS suggested a very low mean GSD at the beginning of the period (25 m3/ha) and a GS increase by almost +300% between 1892 and 2010, underlying the importance of this expansion. A convex growth model was required to simulate historical forest densification, attesting of a significant inertia in wood resource reconstitution after the forest transition, interpreted based on a gradual decrease in harvest rates for which indices were collected, or to a gradual recovery of site fertility. The analysis also suggested a distinct kinetics for GS densification in plantation forests.

Conclusions. These researches reveal the magnitude of GS expansion and the importance of its analysis across forest contexts. This ancient expansion does not present any current sign of saturation and constitute a persistent carbon sink which should not decrease in the next decades assuming similar contextual conditions. According to the process analysis of GS expansion, a significant fraction of the GS increases does not constitute readily available additional wood resources. Thus, future harvest intensification policies must be contextualized and evolving in time.

Keywords: forest expansion – forest transition – forest area – growing stock – volume – basal area – forest composition – forest management – land-use abandonment – plantations – harvest – national forest inventory