Recently, we highlighted some presentations from ICOS Science Conference 2024 that made extensive use of ICOS data, and/or benefited from collaboration with the ICOS community of researchers.
This time we’re highlighting a pair of studies presented in Versailles which make use of ICOS Ecosystem data and the ICOS Ecosystem station network. The two cases below demonstrate how ICOS data can contribute to studies of forest carbon assimilation.
Flux towers paired with dendrometers to help better understand dynamics of forest carbon sinks
Understanding the specific dynamics of tree growth due to carbon assimilated through photosynthesis was the focus of Mukund Rao’s presentation. Allocation of carbon to tree processes such as growth, respiration, and reproduction are highly dynamic due to many environmental factors. To try and better understand these dynamics – and thus, the potential of forests as long-term carbon sinks – Mukund and his colleagues analysed data from a network that combined flux tower measurements with data from dendrometers.
The dendrometers – precision sensors that monitor micrometre-scale changes in tree radius – were co-located within the footprint of eddy covariance towers and tower-based remote sensing measurements.
The station network comprised of 20 locations in 14 countries across North America and Europe, with ICOS Ecosystem stations part of the European network. “We are aiming to include all ICOS Ecosystem stations located in forests that collect dendrometer and phenological camera (PhenoCam) data in addition to the standard flux measurements,” Mukund explains.
“We are aiming to have a broad representation of sites from semi-arid Mediterranean oak forests to temperate and boreal forests in Europe,” Mukund continues. “ICOS data is really valuable for this work due to the guarantee of high-quality data.”
Preliminary results from a study based on a small subset of this larger network at four oak forests sites in North America indicate that in spring trees grow with low photosynthetic capacity, while in autumn tree growth halts and carbon assimilation continues. Tree growth occurs at lower temperatures and a lower vapor pressure deficit (VPD, the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated) than carbon assimilation.
“This preliminary works provides evidence that that carbon assimilation and allocation to growth might be temporally decoupled at least in temperate deciduous oak forests in North America,” says Mukund. “An important question is whether similar mechanisms occur across different forest types, including in the European boreal region. We hope to answer such questions as we progress with our study.”
In search of a holistic understanding of Northern European forest carbon balances
Despite a roughly three-decade tradition of forest carbon flux observations through eddy covariance measurements in Northern Europe, a holistic understanding of the region’s forest carbon balances remains elusive. Through a synthesis of annual carbon fluxes from 50 Northern European forests, Samuli Launiainen’s presentation at ICOS Science Conference 2024 highlighted what direct flux measurements can tell us about the role of forest age on net ecosystem exchange (NEE). He also showed what eddy covariance measurements have revealed regarding the impacts of forest management practices, such as clear-cutting and thinning, on forest carbon balances.
“We used data from 10 ICOS L1, L2 or Associate sites: Hyytiälä (FI-Hyy), Lettosuo (FI-Let), Sodankylä (FI-Sod), Värriö (FI-Var), Kenttärova (FI-Ken), Svartberget (SE-Svb), Norunda (SE-Nor), Hyltemossa (SE-Htm), Soroe (DE-Sor),” Samuli outlines. “And some other ‘non-ICOS’ sites were available from ICOS Warm Winter data release.”
Samuli continues, “ICOS data offers excellent harmonised data source, with quite nice spatial coverage in terms of latitudinal climate gradient.” Since the current network of Ecosystem sites consist of middle-to-late rotation forests, ICOS data is limited in what it can reveal regarding management impacts. “Overall, studies with reference measurements before clear-cutting mature forests are limited,” say Samuli. “The exceptions are Norunda (SE-Nor) that was recently clearcut, and Lettosuo drained peatland where a satellite site measures development of carbon balance after clear-cutting in 2016.”
The preliminary findings suggest gross primary productivity (GPP) and respiration are highly variable on a site-by-site basis, with differences owing to factors including temperature, site fertility and species composition. Forest management practices have significant impacts on a site’s capacity to act as a carbon sink. Clear-cutting turns the forest from a sink to a strong carbon source in the short term, but recovery of GPP is rapid and sites typically revert back to carbon sinks after around 8-12 years. Also, partial harvest (thinning) decreases GPP and carbon sink strength, but data suggests the decrease is only short-lived and recovery takes place in a few years (Aslan et al., 2024).
According to Samuli, syntheses such as what was presented at ICOS Science Conference 2024 could contribute to future forest policies. “The insights offered by long-term efforts to measure forest-atmosphere carbon fluxes can, via this synthesis, contribute evidence on means and variability of carbon balance, how it varies with stand age, and how carbon sinks recover after management disturbances,” explains Samuli.
“This is highly relevant for developing forest policies in the Northern European countries and EU and has more long term potential to help benchmarking the forest land carbon sink estimated in LULUCF sector National Greenhouse Gas Inventory reporting, for example.”
Presentations featured in this story:
- Northern European Forests’ carbon balance and management disturbances: the tale of the direct flux measurements, Samuli Launiainen et al
- A paired flux tower-dendrometer network to investigate forest carbon from assimilation to allocation to tree growth, Mukund Rao et al
Read the abstracts here. View the presentations featured in the story here. All presentations from authors that have allowed the public sharing of their slides can be found here.