When trees aren´t growing
Extreme tree growth reductions are visible in the width of tree rings, thanks to which scientists are able to study various trends in growth and even make predictions. These reductions can be climate-driven or caused by pests or other disturbances and are often the main cause of high forest mortality. During these extreme reductions, carbon sequestration declines to 20-40% of normal values. Pinus sylvestris and Picea abies are two conifer species occurring in Central Europe. Václav Treml from the Faculty of Science at Charles University and his colleagues analyzed extreme tree growth reductions in high- and low-elevation forests, their recurrence, and how they are linked to climatic drivers. They looked at long-term trends in extreme growth reductions since they are still largely unknown.
Extreme climate events can be abrupt cooling, drought, or other types of disturbance, due to which tree growth is extremely decreased. How big this decline is, depends on tree growth history, vigor, and interactions within the location. Trees with different life strategies are also influenced by such climate events differently. Slow-growing trees invest in defense mechanisms and root biomass more than fast-growing trees. As a result, they aren’t as affected by extreme events. On the other hand, even big and dominant trees can be advantaged when compared to small ones, because they are more effective in water obtaining thanks to wide root systems.
These different strategies and life preferences can be observed in Picea abies and Pinus sylvestris. While Picea is more sensitive to drought than Pinus, Pinus tolerates a bigger climate range than Picea. As a result, extreme growth reductions aren’t as obvious as in Picea.
The study compared these two species and their extreme growth reductions, but it also considered another factor – their placement. Each tree species was divided into high elevation trees, where warm and dry events have mostly positive effects, and low elevation trees, where such events have a negative effect. As a result, there were four tree categories, that were analyzed, and the differences between high and lowlands could be therefore precisely studied.
What did the combination of these factors reveal in the research? Extreme growth reductions of low elevation Picea abies were the biggest and their frequency has increased in the last decades. On the contrary, the number of extreme growth reductions of high elevation Piceas has been decreasing and these trees are profiting from warm and dry events. The main climate factors causing extreme growth reductions of both high and low elevation Pinus and Picea have changed since the 1990s. It used to be the combination of big warm and dry events or cold summers, but recently the main cause for growth reductions has been only the warm and dry events. Also, the trend of increasing growth (the trees are bigger) has been observed in Piceas. There are two possible outcomes this can have on the extreme growth reductions. The increased growth can partially compensate for the past growth reductions by creating more aboveground biomass. On the other hand, such well-grown individuals are more sensitive to future extreme droughts, because they have a bigger leaf area for evaporative loss, and they need more water.
In contrast, Pinus sylvestris has very ambiguous trends. The slope of extreme growth reductions was very weak, which can be explained by the species’ adaptability to hostile conditions, where it often grows. The strongest trend of extreme growth reductions of both Picea abies and Pinus sylvestris is in low elevation stands, which are stronger and stronger. Especially in low elevation Piceas, the extreme growth reductions are often bigger than what was observed in the 20. century.
Eliška Leštinová
Published:
Aug 23, 2022 09:25 AM
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