Single tree selection forests (plenter forests)

Single tree selection forests are a particular type of uneven-aged forest composed of many size or age classes with a negative exponential shaped diameter distribution. Although negative exponential distributions are relatively common at larger scales (e.g. > 1 ha), we are referring to stands where negative exponential shapes occur within a very small area (e.g. as small as 0.1 ha).

The structure and growth of single tree selection forests is often considered to be relatively stable in comparison to even-aged forests. However, given temporal variability in climate or markets for wood products, it is likely that the structure and functioning of single tree selection forests can also change in the long term.

Based on 19 Swiss plots measured for an average of 94 years, it was found that over the past 100 years common long-term trends included increases in growth, maximum tree sizes, species diversity, and increasingly convex diameter distributions with fewer medium sized trees and more very large trees (5). Differences in growth between species compositions were generally consistent with their light-use efficiency, transpiration, and water-use efficiency, which were calculated using the process-based model 3-PG.

We assumed that structural differences of single tree selection forests with different species compositions would be related to the shade tolerance or crown allometry of the component species e.g. forests with many shade intolerant species would have lower stand basal areas or volumes. Stand volume of each forest type was negatively correlated related with the shade tolerance (but not crown architectures) of the species it contained, but no such correlations existed for stand basal area (5). This reflects the high structural variability of these forests (through time and between sites) as well as the high tree neighbourhood variability within plots (6), which appear to buffer the effects of stand density on regeneration and recruitment.

Comparisons with even-aged forests suggested that while climatic conditions influence the growth of both structures (even-aged vs. single tree selection), the effects are not the same with single tree selection forests benefiting from higher temperatures and even aged forests suffering (1). Both forests types also showed greater productivity when composed of a mixtures of species compared with monocultures (1).

Importantly, this study showed that despite significant temporal and spatial variability in structure, the negative exponentially shaped diameter distributions were maintained in the long term by applying simple silvicultural principles rather than requiring a very specific stand structure i.e. a certain stand density, or that all diameter classes strictly lie on the curve of a negative exponentially shaped diameter distribution (5).

More information about the Swiss single tree selection plots can be found here.

Journal articles related to this project:

1. Forrester, D.I., 2019. Linking forest growth with stand structure: Tree size inequality, tree growth or resource partitioning and the asymmetry of competition. Forest Ecology and Management 447, 139-157. doi:10.1016/j.foreco.2019.05.053

2. Gillerot, L., Forrester, D.I., Bottero, A., Rigling, A., Lévesque, M., (2021). Tree neighbourhood diversity has negligible effects on drought resilience of European beech, silver fir and Norway spruce. Ecosystems, 24: 20–36. doi:10.1007/s10021-020-00501-y

3. Bottero, A., Forrester, D.I., Cailleret, M., Kohnle, U., Gessler, A., Michel, D., Bose, A.K., Bauhus, J., Bugmann, H., Cuntz, M., Gillerot, L., Hanewinkel, M., Lévesque, M., Ryder, J., Sainte-Marie, J., Schwarz, J., Yousefpour, R., Zamora-Pereira, J.C., Rigling, A., (2021). Growth resistance and resilience of mixed silver fir and Norway spruce forests in central Europe: Contrasting responses to mild and severe droughts. Global Change Biology 27, 4403-4419 doi:10.1111/gcb.15737

4. Brüllhardt, M., Rotach, P., Forrester, D.I., Bugmann, H., (in press). Sustainable regeneration in uneven-aged deciduous forests managed by selection silviculture: the role of demographic structure. Forestry

5. Forrester, D.I., Schmid, H., Nitzsche, J., (in press). Growth and structural changes in Swiss uneven-aged forests over 100 years, and comparisons between 15 uneven-aged forest types of Europe, North America and Australia. Forestry. doi:10.1093/forestry/cpab042

6. Forrester, D.I., (in press). Within-stand temporal and spatial dynamics of tree neighbourhood density and species composition: Even-aged vs. single-tree selection forests. Forestry. doi:10.1093/forestry/cpab016