Climate has a larger effect than stand basal area on wood density in Pinus ponderosa var. scopulorum in the southwestern USA
Stand basal area of ponderosa pine ( Pinus ponderosa var. scopulorum Engelm.) in the US Southwest has little effect on the density of the wood produced, but climatic fluctuations have a strong effect. Wood density increases during drought, particularly if the drought occurs in late winter/early spring. Future droughts, as are predicted to increase in the US Southwest, may lead to production of smaller radial increments of higher density wood in ponderosa pine.
Forest restoration treatments in the US Southwest are generating large quantities of small-diameter logs. Due to negative perceptions about ponderosa pine wood quality, this material is often seen as a “waste disposal” problem rather than a high-value resource.
Our objective was to understand more about variation in southwestern US ponderosa pine wood density, an important indicator of wood quality. Specifically, we investigated the effect of stand basal area on wood density, and the effect of annual and quarterly climatic variation on wood density.
We collected samples from 54 trees grown at six different basal area levels from a replicated stand density experiment. Pith-to-bark strips were used in an X-ray densitometer to obtain annual density and growth measurements from 1919 to the present.
Stand density had a strong effect on growth rate, but little effect on wood density. However, climatic variation did influence wood density, which increased in drought years before quickly returning to pre-drought levels.
Stand basal area is not a good indicator of wood density for foresters planning to utilize material from timber harvests in the southwestern USA. Future droughts, as are predicted to increase in the region, will likely reduce wood volume production but may increase wood density in ponderosa pine.
KeywordsX-ray densitometry Forest restoration Wood density Ponderosa pine Growing stock level Dendroecology
We would like to thank the ARCS Foundation, the Kay and Irene Hafen Forestry Scholarship, the Wally Covington Travel award, and the NAU Graduate Student Government International Travel Award for generous contributions. We thank the US Department of Agriculture, Forest Service, Rocky Mountain Research Station, for permitting access to their long-term level of growing stock study on the Fort Valley Experimental Forest. We have many individuals to thank: our 2017 field crew (James Forst and Scarlet Jackson) for their help collecting samples, Donald P. Normandin of the Ecological Restoration Institute for help with sample preparation, and Dr. Kristen Waring for sharing historical Taylor Woods tree growth data. Finally, thanks to two anonymous reviewers who provided detailed and constructive comments on an earlier draft.
Funding for the project was provided by McIntire-Stennis appropriations to Northern Arizona University and the State of Arizona. Additional funding was provided by the Ecological Restoration Institute.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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