Climate influences on the maximum size-density relationship in Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) stands

Condés, S., Vallet, P., Bielak, K., Bravo-Oviedo, A., Coll, L., Ducey, M.J., Pach, H., Pretzsch, H., Sterba, H., Vayreda, J., Río, M. (2017) - Climate influences on the maximum size-density relationship in Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) stands - Forest Ecology and Management

The maximum size-density relationship (MSDR) reflects the boundary site occupancy and the selfthinning
line for a given species, being a useful tool in forestry. Studies focusing on the MSDR often do
not cover the whole distribution of the studied species, which results in different boundaries for a given
species in different regions. A common MSDR is lacking for the increasingly demanded large-scale
studies. However, this information is important where silvicultural responses must be prioritized among
monospecific stands or where comparisons among maximum and relative stand densities between and
within species are required.
For the purposes of this study, we used data from 9911 sample plots located in Scots pine and European
beech monospecific stands. Both of these species are of considerable importance and widely distributed
throughout Europe. The data came from National or Regional Forest Inventories of five European countries
(Austria, Germany, France, Spain and Poland) and therefore were distributed across a wide range
of climatic conditions.
The main aim of this study was to determine whether the MSDR of these species depends on environmental
variables and to develop a MSDR model for each species that explain this variability along a
climate gradient.
The resulting models showed that both parameters of species boundary lines were climate-dependent,
but that the pattern of variation differed between species. Hence, the higher the humidity, the steeper the
MSDR (more negative exponent) and the higher the intercept for beech, while in the case of pine, the
higher the humidity, the straighter the MSDR and the lower the intercept. According to these models,
the stand density indices, for a reference diameter of 25 cm, varied with the humidity in a different
way for each species. Consequently, the ratio between the two species increases with humidity, although
it also depends on stand diameter.
These results are in accordance with the yield level theory and could contribute to the development of
more precise silvicultural guidelines and growth models based on the self-thinning line. Moreover, they
are of particular importance in the discussion of growth and the effects of mixing on mixed species
stands.

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