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News Article

Wood density decreasing despite increases in wood volume

Faster volume growth does not translate into denser biomass for Central European forest trees

As climate change continues to steadily impact life on earth we are only slowly discovering its effects on many facets of plant growth and distribution. A recent example of this is the discovery that despite increases in wood volume, forest trees in Central Europe are showing a reduction in overall density that was found to have been occurring for over 100 years.

Researchers at the Technical University of Munich carried out their study in Central European forests covering some of the oldest surviving experimental plots in Europe. This area contains the dominant species: Norway spruce (Picea abies (L.) H.KARST.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), and sessile oak (Quercus petraea (MATTUSCHKA) LIEBL.) In total 41 monospecific stands were sampled containing 13 Norway Spruce, 11 Scots pine, 9 Sessile oak and 8 European Beech, covering 13 ecoregions and 17 sub-ecoregions. Ages of sampled trees ranged from 31-194 years old. For each tree sampling involved taking an increment core at breast height designed to capture as many growth rings as possible. A LIGNOSTATION™ high frequency densitometer was used to calculate wood density along the sample by analysing the level to which a 10 MHz signal is propagated through the wood. Density was determined for each ring in a given core in three values: mean wood density (MWD), earlywood density (EWD) and latewood density (LWD).

The results showed that all dominant species have been decreasing in wood density for over 100 years. Reductions in tree density for each species were as follows; Norway spruce (7.7%), European beech (11.8%), sessile oak (11.2%) and scots pine (5.4%). Despite the decrease in wood density, overall biomass still increased for all species studied due to an increase in volume growth rate; the result of longer growing seasons, warmer temperatures and higher atmospheric CO2 concentrations.

The reduction in density was thought to have been caused by higher N-deposition, temperature increases and longer growing seasons. The authors also reported that decreased latewood strongly contributed to a reduction in overall wood density, suggesting an increased role for temperature as it has previously been linked to changes in average and latewood density. Wood density was also found to be impacted by ring width; the authors propose that by reducing stand density, the subsequent increase in growth ring size may offset the decrease in density.

The reduction in overall and late wood density naturally translates into a lower stiffness and strength for affected trees. For a controlled stem diameter and annual ring width, tree stability against windthrow, woodstrength, energy content and C sequestration are all reduced leaving the trees at an increased risk of snow and storm damage. The plots sampled covered roughly 7,000,000 ha making the results of this study more widely applicable for the species studied across all central European conditions. The sample areas chosen mean the authors were able to rule out silvicultural interventions as the cause for this loss of density, suggesting it to be the effect of environmental variables. In summary, the key message of this study is that it is inaccurate to use historical statistical values for wood density – similar to growth rate, density calculations need to be monitored and updated to produce accurate estimates of carbon sequestration and biomass production both for science and management.


Read the original article here:

Pretzsch, H., Biber, P., Schütze, G., Kemmerer, J. and Uhl, E., 2018. Wood density reduced while wood volume growth accelerated in Central European forests since 1870. Forest Ecology and Management429, pp.589-616. DOI: 10.1016/j.foreco.2018.07.045


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("density" OR "strength" OR "anatomy" OR "growth effects" OR "hardness" OR "vibrational properties" OR "dendroecology" OR "dendroclimatology" OR "dendrochronology" OR "earlywood" OR "latewood" OR "growth rings" OR "increment boring" OR "seasonal growth") AND ("temperate") AND ("forest*" OR "tree*" OR "wood") AND ("climate change" OR "global warming")