This study is the result of an international collaboration between Dr. Keith Moser and Prof. Hans Pretzsch (Technical University of Munich). Over the years, Moser and Pretzsch discussed possibilities for joint research in density management and ecophysiology. The opportunity to expand Prof. Pretzsch’s work into the southwest came when the Eva Mayr-Stihl Stiftung (www.eva-mayr-stihl-stiftung.de) funded a research visit to the Fort Valley Experimental Forest as part of a joint effort by the Chair of Growth and Yield Science at the Technical University of Munich (TUM) and the Rocky Mountain Research Station.
This collaboration analyzed the effect of reduced stand density on tree growth, growth sensitivity (annual variability in growth resulting from environmental conditions), and short-term drought responses (resistance, recovery, and resilience) of Scots pine and sessile oak in Bavaria and ponderosa pine in Arizona. Tree ring series from 409 trees, growing in stands of varying stand density, were analyzed at sites with different water availability.
For all species, trees grew more rapidly in stands of low density compared with stands at maximum density. The growth sensitivity of Scots pine was higher under low stand density compared with moderate and maximum density, while growth sensitivity of ponderosa pine peaked under maximum stand density. Recovery and resilience of Scots pine, as well as recovery of sessile oak and ponderosa pine, decreased in denser stands. In contrast, resistance and resilience of ponderosa pine increased with increasing stand density. Generally, the drought response of Scots pine and sessile oak was worse at sites with greater water availability. In ponderosa pine, higher site water availability lessened recovery. Higher water availability at a site moderated the positive effect of reduced stand density on drought response in all species.
Pretzsch, H., 2020. Density and growth of forest stands revisited. Effect of the temporal scale of observation, site quality, and thinning. For. Ecol. Manag. 460, 117879. https://doi.org/10.1016/j.foreco.2020.117879.
Pretzsch, H., Schütze, G., Uhl, E., 2013. Resistance of European tree species to drought stress in mixed versus pure forests: Evidence of stress release by inter-specific facilitation. Plant biology (Stuttgart, Germany) 15, 483–495. https://doi.org/10.1111/j.1438-8677.2012.00670.x.
Steckel, M., del Río, M., Heym, M., Aldea, J., Bielak, K., Brazaitis, G. et al, 2020. Species mixing reduces drought susceptibility of Scots pine (Pinus sylvestris L.) and oak (Quercus robur L., Quercus petraea (Matt.) Liebl.) – Site water supply and fertility modify the mixing effect. For. Ecol. Manag. 461, 117908. https://doi.org/10.1016/j.foreco.2020.117908.