Underlying mechanisms linking forest productivity and diversity of tree species

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Within the framework of Alexandre Fruleux’s PhD project, a new paper, available online at Oecologia, investigates the underlying mechanisms linking forest productivity and diversity of tree species.

Abstract: Aboveground overyielding in a mixed temperate forest is not explained by belowground processes

The relationship between forest productivity and tree species diversity has been described in detail, but the underlying processes have yet to be identified. One important issue is to understand which processes are at the origin of observed aboveground overyielding in some mixed forests. We used a beech–maple plantation exhibiting aboveground overyielding to test whether belowground processes could explain this pattern. Soil cores were collected to determine fine root (FR) biomass and vertical distribution. Correlograms were used to detect spatial arrangement. Near-infrared reflectance spectroscopy was used to identify the tree species proportion in the FR samples and spatial root segregation. An isotopic approach was used to identify water acquisition patterns. The structure and the composition of the ectomycorrhizal fungal community were determined by high-throughput sequencing of DNA in the soil samples. We found no spatial pattern for FR biomass or for its vertical distribution along the gradients. No vertical root segregation was found, as FR density for both species decreased with depth in a similar way. The two species displayed similar vertical water acquisition profiles as well, mainly absorbing water from shallow soil layers; hence, niche differentiation for water acquisition was not highlighted here. Significant alterations in the fungal community compositions were detected in function of the percentage of maple in the vicinity of beech. Our findings do not support the commonly suggested drivers of aboveground overyielding in species-diverse forests and suggest that competition reduction or between-species facilitation of belowground resource acquisition may not explain the observed aboveground overyielding.

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

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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.