EU’s climate policy aims to reach carbon neutrality by 2050. In this, forests have a key role as they remove CO2 from the atmosphere. Nevertheless, climate change and the emergence of new pests are leading to reduction in trees’ increments and carbon sequestration capacity. Those additional challenges that the Republic of Croatia will have to face in achieving the goal of greenhouse gas emissions reduction require special attention. This project aims to build the capacity of the Croatian Forest Research Institute by establishing a dendrochronological laboratory, upgrading the existing monitoring system of trees’ increments and carbon cycle in Jastrebarski lugovi and continuing long term work on the improvement and validation of the process-based model Biome-BGCMuSo. The establishment of a dendrochronological laboratory will enable a recent tree increment analysis and the implementation of an analysis of the effects of the oak lace bug, a new invasive species, on the pedunculate oak tree increment and carbon storage in the forest. The analysis of the δ13C and δ18O isotopes should provide further insights into the mechanism of the influence of this pest. The improvement of monitoring tree increment on permanent plots and the fourth soil measurement in a five-year cycle in Jastrebarski lugovi will contribute to a better understanding of the carbon cycle in forests. Data will be used for the validation of the Biome-BGCMuSo model and the assessment of the climate change impact based on different climate scenarios, as well as potential adaptation measures on the growth of pedunculate oak forests. The obtained results have the potential to contribute to achieving the goals in the LULUCF sector of the Republic of Croatia.
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Establishment of a Laboratory for Dendrochronology
Within the scientific research project Dendro-Carbon, a laboratory for dendrochronology (Dendro-lab) is being established to enable systematic and standardized determination of tree growth based on the tree ring widths measured from wood core samples.
Dendrochronological analysis is a standard method used in ecological research to assess the impact of drought, late or early frost, fires, defoliator attacks, and other pests, etc., on tree growth (Speer 1971). In recent times, it has been complemented by stable isotope analyses, particularly δ13C and δ18O (Gagen et al. 2022).
In 2024, equipment was acquired for the Dendro-lab, and staff training was conducted. The equipment includes a microtome for long cores (WSL, Gärtner & Nievergelt 2010), a microscope and camera for analysing core samples, a laboratory mill for wood samples, and a professional wood sander. At the beginning of 2025, the first tree core samples from various tree species were processed using the microtome for long cores, scanned, and analysed with a microscope and camera in the Dendro-lab.
Processing of wood core samples using a microtome for long cores (top), analysis of tree rings with a microscope and camera (middle), and scanned core samples of pedunculate oak and red oak (bottom).
Quantitative Assessment of the Oak Lace Bug Impact on Growth and Carbon Storage
The oak lace bug (Corythuca arcuata, Say 1832) is a newly invasive forest pest in the Republic of Croatia (Hrašovec et al., 2013), first detected in Spačva in 2013. This pest is spreading rapidly and is causing great concern as its infestation does not pass or considerably decrease.
Within Dendro-Carbon project, in the newly established dendrochronology laboratory an analysis of tree core samples is being conducted to quantitatively assess the growth reduction of pedunculate oak (Quercus robur L.) that can be attributed to the impact of the oak lace bug.
For this purpose, in collaboration with G. Kovač (Croatian Forests Ltd.), a study site was selected based on spatial forest data of the Republic of Croatia – compartment Haljevo – Kozaračke šume, where red oak (Quercus rubra L.), a comparable species unaffected by the oak lace bug, grows alongside pedunculate oak. Tree ring analyses from red oak will serve as a control in comparison to the corresponding rings of pedunculate oak, which is negatively affected by the pest.
A portion of the wood core samples will be analysed for δ13C and δ18O to further investigate the mechanism of impact of the oak lace bug. Additionally, monitoring NDVI through satellite imagery provides an additional assessment of infestation intensity.
Phenological NDVI curves for the period 2016–2023 reveal a significant decline in NDVI as the vegetation season progresses in pedunculate oak stands from 2017 onwards (top), indicating that the damage caused by the oak lace bug became substantial during that time. In red oak stands (bottom), no such decline is observed, as this species is not affected by the oak lace bug.
Visualization of NDVI derived from drone imagery captured using a camera that records red, green, blue, and near-infrared spectral channels.
Monitoring Tree Growth
Long-term monitoring of tree growth and carbon fluxes in a pedunculate oak forest in management unit Jastrebarski lugovi has provided insights into the dynamics of CO₂ exchange between the forest and the atmosphere, as well as tree growth trends (Anić et al., 2018). It has also allowed investigation of the temporal evolution of carbon stocks throughout the forest rotation period, based on chronosequence data (Ostrogović Sever et al., 2019; Bitunjac, 2024). The experimental plot system established in Jastrebarski lugovi is unique in Croatia and among the few of its kind worldwide and its scientific value increases with the length of the monitoring period, which has been ongoing since 2008.
Within the Dendro-Carbon project, the equipment in the existing experimental plot system in Jastrebarski lugovi has been upgraded with point dendrometers enabling high-frequency tree growth monitoring at 15-minute intervals. These measurements allow for the reconstruction of radial growth and its correlation with daily meteorological conditions, which is crucial for process-based modelling using models such as Biome-BGCMuSo (Hidy et al., 2012, 2016, 2022).
In addition to the oak chronosequence, point dendrometers have also been installed in Haljevo – Kozaračke šume on pedunculate oak and red oak trees in order to determine differences in growth rates in relation to the increasing intensity of oak lace bug infestations and the resulting leaf damage as the vegetation season progresses.
Installation of point dendrometers on pedunculate oak trees in the chronosequence experiment.
