The density effect: Red/far red signaling and douglas-fir seedling growth in a variable density field test

Abstract

Coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings, when planted in a reforestation setting, exhibit early height and diameter growth that is inversely proportional to planting density. One hypothesis to explain this observation is that they are able to detect the presence of nearby trees using phytochrome by sensing the ratio of red to far red light (R/FR) reflected from their crowns. This provides an “early warning signal” of future competition. This signal then triggers a change in growth rate or allometry, which leads to greater height and diameter growth as trees attempt to avoid being overtopped. This is called the “Density Effect.” As a partial test of this hypothesis, we established a variable density field test to determine whether changes in R/FR could be detected at different growing densities, and if this was associated with variations in height and diameter growth. The predicted reduction in R/FR, and concomitant increase in height growth at higher plant densities, were observed beginning early in the second growing season. These observations lend support to the R/FR signaling hypothesis.

Citation

Ritchie, Gary A.; Keeley, James; Bond, Barbara J. 2007. The density effect: Red/far red signaling and douglas-fir seedling growth in a variable density field test. In: Riley, L.E.; Dumroese, R.K.; Landis, T.D. National Proceedings: Forest and Conservation Nursery Associations - 2006. Proceedings RMRS-P-50. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. p. 38-45