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

Seminar: Vanessa Haverd

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Abstract
Several lines of evidence point to an increase in the activity of the terrestrial biosphere over recent decades1-4, impacting the global net land carbon sink (NLS) and its control on the growth of atmospheric carbon dioxide (ca). Global terrestrial gross primary production (GPP) — the rate of carbon fixation by photosynthesis — has risen by (31 ± 5)% since 19004. This increase remains to be attributed. Here we show that this increase in GPP is predominantly driven by CO2.. We reconcile leaf-level and global atmospheric constraints on trends in modelled biospheric activity to reveal a global CO2 fertilisation effect on photosynthesis of 30% since 1900, or 47% for a doubling of ca above the pre-industrial level. Our historic value is nearly twice as high as current estimates3,5 (17 ± 4)% that do not use the full range of available constraints. Consequently, under a future low emissions scenario6, our projected natural land carbon sink (174 PgC, 2006 to 2099) is 57 PgC larger than if a lower CO2 fertilisation effect comparable with current estimates is assumed. These findings suggest a larger beneficial role of the land carbon sink in modulating future excess anthropogenic CO2 in lower emissions scenarios consistent with the target of the Paris agreement to stay below 2°C warming, and underscores the importance of preserving terrestrial carbon sinks.
Speaker
Vanessa Haverd, Climate Science Centre, CSIRO Oceans & Atmosphere, Canberra, Australia
Vanessa Haverd has a Ph.D. in physical chemistry (Oxford University, 2003), and joined the Continental Biogeochemical Cycles Team at CSIRO in 2007 as a terrestrial biosphere modeller. She has co-developed new descriptions of fundamental physical processes, including coupled transport of heat, water and stable isotopes in soil and litter, encapsulated in the SLI (Soil-Litter-Isotope) model; heat storage, and radiative transfer in plant canopies. In 2013, she published the first complete carbon budget for the Australian continent, quantifying the contribution of the Australian biosphere to the global carbon budget, using multiple observational constraints. More recently, she co-developed a novel approach to representing vegetation structural dynamics in Earth system modeling, introducing vegetation structure and disturbance patterns into land surface models. Vanessa contributes CABLE-POP simulations to the annual update of the Global Carbon Budget, as part of the TRENDY ensemble of global terrestrial biosphere models.

Workshop BENCHAFOR

On October 2nd & 3rd, scientists from the ONF, INRA and DSF will discuss the current knowledge on the biology and ecology of cockchafers to explore novel options to limit the insect outbreaks and their detrimental impact on forest regenerations.

The common and forest Cockchafer are two beetle species well known for the damages they can inflict to forest ecosystems. Whereas the adults may occasion spectacular defoliations to adult trees, the larvae also feed on the roots of young trees causing severe mortality in forest regenerations, further impacting the whole forest management process. Eastern European countries had been facing recurrent infestations since the 1960’s; in France, cockchafer populations seem to have turned from endemic to epidemic in the last 10 years, with dramatic damages reported in young forest stands of Picardie and Northern Alsace. In Germany, chemical treatments proved to be efficient in reducing populations and subsequent damages but their use is now forbidden. A wide diversity of alternatives had been investigated in Europe, mainly based on biological control methods. To date, these studies did not produce results that could be developed and ultimately applied in everyday forest management.

Forest managers now question the opportunity of modifying forest ecosystem parameters that are critical for the different phases of the cockchafer lifecycle. Such actions are likely to produce short-term efficient methods with limited environmental impacts. This project is based on a benchmarking approach of the acquired scientific knowledge of cockchafer biology and of thetechnical experiences accumulated in the European countries that have been subjected to outbreaks in the last decades. We aim at investigating whether changes in silvicultural management are able to limit cockchafer population dynamics.

Our first objective is to establish a state-of-the-art review of the knowledge concerning the environmental factors which determine cockchafer outbreaks. Then, our second objective is to propose a protocol to describe stand cover structure and ground vegetation characteristics which determine cockchafer larval density. These two objectives will initiate national and international networking on the research for solutions to cockchafer damages to forests.

Finally, the proposed project will provide a framework of hypothesis for the research of ecological solutions to prevent cockchafer outbreaks. This innovative project is likely to initiate a technical and scientific strategy for the management crisis caused by cockchafer outbreaks.

 

Dynamics of responses to canopy opening in beech trees

Ph.D. defense

Estelle NoyerRéponses des perches de hêtre (Fagus sylvativa L.) à l’ouverture de la canopée : approche multidisciplinaire et multi-échelle.

Friday 12 May at 9.30 am at AgroParisTech-Nancy (Amphithéâtre A).

Abstract: Opening of the canopy exhibits advantages (resources availability) but also new constraints (wind, higher evaporative demand). Rather well documented in saplings, response dynamics to canopy opening is less known in large trees. The thesis aims to identify the dynamics of responses to canopy opening in beech trees suppressed during long periods. Adopted approach is multi-disciplinary and multi-scale, based on a retrospective analysis of axial and radial growth, anatomy and biomechanical traits. For suppressed trees, the competition for light results in preferential allocation of biomass to axial growth in comparison with radial growth resulting in trees with high slenderness. Moreover, one third of suppressed trees are sagging. After the release, high slenderness presents a biomechanical risk: 15 from 36 trees are broken by the wind two years after the release. To increase their safety against the wind-break, trees reduce their axial growth during four years after the release and boost their radial growth reaching a stabilisation plateau after two years likely due to the size and resources limitations. Trees with lean angle higher than 6° up-right after the release. The tree ring hydraulic conductivity increases and stabilises after two year also. The dynamics of responses to canopy opening are therefore clearly trait dependent. Moreover, integrative approach highlighted the importance of size in the responses to canopy opening: while saplings adjust both wood tissue properties and tree geometry, large trees rely only on geometry adjustments.

Key-words : Fagus sylvatica, growth, biomechanics, anatomy, canopy opening, wood.