Climatic stress events, such as extreme temperatures and prolonged droughts, are increasingly affecting tree growth and phenology—the timing of developmental stages like leaf burst and senescence.
To better understand these processes, researchers from University of Helsinki and the Finnish Geospatial Institute of the National Land Survey of Finland set up a long-term experiment with a permanent laser scanning station located at SMEAR II research station in Hyytiälä, Southern Finland. The scanner uses laser light to create centimeter-precise 3D models of individual trees, enabling scientists to track growth and structural changes with unprecedented detail. A similar research setup has not been attempted anywhere before.
‘Laser scanning time series enable the observation of tree changes over time without interfering with their natural growth. For the first time, we were able to accurately measure day-level differences in the phenology of trees in an automated manner. Subsequently, we could study the factors influencing and the effects of these phenological variations within one growth season,’ explains Senior Research Scientist Mariana Campos from the National Land Survey of Finland.
Several days' difference in leaf budding
The study focused on silver birch trees and found that species richness and competitive pressure for light in the immediate vicinity influenced the timing of spring leaf burst, while water availability shaped the timing of fall leaf senescence. Additionally, the timing of growth proved critical; for example, early leaf burst was linked to increased crown area growth later in the season. There was a difference of up to 12 days in the time when leaf senescence occurred in the observed trees.
‘This research highlights how individual trees differ in the timing and duration of their growth period due to the local growth environment, even in a relatively small and homogenous forest area. These insights, like the impact of local water availability on leaf senescence also helps to understand how changing climate impacts tree phenology and growth within a forest stand,’ comments Associate Professor Anna Lintunen from the University of Helsinki.
The experiment provides a better understanding of how local factors drive tree growth. Until now, frequent and detailed measurements of individual tree phenology across a forest stand have been nearly impossible to achieve.
The Hyytiälä laser scanning monitoring experiment continues alongside other long-term measurements at the SMEAR II station, contributing to a broader understanding of forest-atmosphere interactions. This work was carried out as part of two Research Council of Finland Flagships: the Atmosphere and Climate Competence Center (ACCC) and the Forest-Human-Machine Interplay (UNITE).
Further information
Mariana Campos, Senior Research Scientist, Finnish Geospatial Research Institute of the National Land Survey of Finland, firstname.lastname@nls.fi, +358 29 531 4133
Anna Lintunen, Associate Professor, University of Helsinki, firstname.lastname@helsinki.fi, +358 40 751 0341
Read more about the study (published in the journal Agricultural and Forest Meteorology).
Silver birch point clouds used in the study have been published as open data in the National Land Survey of Finland open scientific LiPheStream dataset. The dataset is available in ETSIN service.