What factors determine trends in tree growth in a changing climate?
Forests play a key role in our ecosystem. Their impact on the carbon cycle and the Earth’s climate is crucial. Approximately 20% of all CO2 emissions are absorbed by forest ecosystems. Loss of forest areas and/or disruption of their vitality has negative effects on carbon sequestration in biomass. The influence of individual factors on tree growth is thus a topical and extremely important scientific topic. A group of experts led by Jiří Mašek from the Department of Physical Geography and Geoecology, Faculty of Science, Charles University, have investigated the influence of factors (other than climate) on tree growth trends.
Every tree can tell a story based on changes in tree-ring growth. This is influenced by external factors that affect the tree. The degree of variability of growth is theoretically determined by the location of each tree in the relevant climatic niche of the species. In practice, however, we note in forest stands that individual trees naturally differ in their response to the external environment. In temperate coniferous forests, approximately 30–60% of growth variability can be attributed to the growth responses of individual trees.
A team of dendrochronologists (experts in the analysis of tree-rings) investigated two forest types typical for our landscape that colonise the contrasting environment. The first type of forest consisted of Norway spruces (Picea abies) from the upper forest limit of the Krkonoše Mountains, which are limited mainly by low temperatures during the growing season. The second type was Scots pine (Pinus sylvestris) from low altitudes in the Kokořín region, where the primary limiting factor is drought. Two areas of interest with a northern and southern orientation were selected for both tree species.
The researchers tagged and sampled all trees with a diameter at breast height greater than 10 cm on each plot. Using a special hollow drill, they extracted two increment cores perpendicular to each other that captured the full sequence of tree-rings. Subsequently, they measured other important parameters such as tree height, trunk, and crown size.
Individual tree-ring samples were further analysed in the dendrochronological laboratory. For each sample, a sequence of tree-ring widths was measured that represented the annual rate of wood growth of each tree. Subsequently, the natural age trend was statistically removed from the measured widths of the tree-rings. Due to the growth of the tree, the increase in thickness alters due to the increasing size of the trunk. For the following analyses, topographic parameters describing the microrelief and climatic data for all habitats were determined.
Using Principal Component Analyses (PCA) and linear models, the researchers quantified the relative influence of age, size, and topographic conditions on the variability of growth and climate response, taking into account year-on-year variability and medium-term trends.
A great deal of the within-stand growth variability and climate-growth responses in two Central European forest types was explained by tree age and size, but not by topographical differences. When only high-frequency variability was considered (detrended by the 30-year spline method), tree age and size continued to exert the strongest influence, but the importance of topographic variables increased. At the same time, the stands of Norway spruce exhibited less individual variability in growth than the stands of Scots pine. This could be due to the existence of a more heterogeneous age structure in pine stands. The impact of acid deposition in the Krkonoše Mountains may also have played a role, as it was a common limiting factor for spruce growth in the 1980s and 1990s and caused a greater degree of growth coherence.
The results of the current study have illuminated the influence of individual factors on tree growth. The authors highlight the importance of carefully selecting individual trees and groups of trees to determine growth responses to changing climatic conditions. Trees are individuals which are significantly affected by the age, size, and topographic conditions of their immediate surroundings.
In conclusion, in the words of Jiří Mašek: “The forest occupies a third of the Earth’s land. But that figure is decreasing rapidly…” And what is important? “To find out what bothers them when they are old, when they are young, how it all is, how it all works, can we stop today’s devastation and preferably remediate it?” Thanks to Jiří Mašek and the entire team, we now have information about the influence of surrounding factors on the forest ecosystems and are one step closer to achieving this aim.
Kateřina Fraindová
Published:
Jan 12, 2022 09:00 PM
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