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Is three a crowd? The effect of stand density reduction on drought response

Date: July 13, 2020

Historical climate may set the stage for how tree species respond to drought after stand density reduction.


A thinned ponderosa pine forest.
Ponderosa pine stand density trial FV U2 (Fort Valley Experimental Forest) in Arizona. Moderately thinned stand (~40% stand density reduction compared with control).

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. 

Looking up at the bare crowns of oak trees, with sky in the background.
Sessile oak stand density trial GEI 649 in Bavaria. Photo shows the crowns of an untreated control plot.

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. 

Key Findings

  • Short-term tree growth responses to drought stress are significantly influenced by stand density, water availability, and, to a lesser degree, stand age.
    A researcher measuring the diameter of a pine tree with a measuring tape.
    Measuring the diameter of a ponderosa pine in the Fort Valley Experimental Forest.
  • Treatments that reduce stand density can improve drought responses for Scots pine and sessile oak, so they may be a good choice for adapting stands of these species to climate change.
  • Beneficial effects of reduced stand density are stronger for more arid sites.
  • In contrast with Scots pine and oak, chronically water-stressed ponderosa pine showed an improved response to drought under higher stand densities.
  • The results indicate that reduced competition may improve resource availability over a wide range of growing conditions.
  • However, under extremely harsh growing conditions like those for the Arizona ponderosa pines, the benefits of facilitative interactions may override competition between individuals. As one possible example, shade in higher-density stands could reduce evapotranspiration and improve water retention.

Publications

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.

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Principal Investigators: 
Principal Investigators - External: 
Mathias Steckel - Technical University of Munich
Hans Pretzsch - Technical University of Munich
External Partners: 
Miren del Río, Forest Research Center, INIA and Sustainable Forest Research Institute, University of Valladolid and INIA