What causes the density effect in young forest plantations?
In young forest plantations, trees planted at high densities frequently show more rapid height and diameter growth than those plants at lower densities. This positive growth response to density (the ''density effect'') often manifests long before seedlings are tall enough to shade one another, so it is not a simple response to shade. The mechanism(s) which trigger and sustain this growth enhancement are unknown. Our objectives were to document the temporal dynamics of positive growth response to increasing density in Douglas-fir plantations and to test two hypotheses as potential mechanisms for this response. The hypotheses are (1) a canopy boundary layer effect, and (2) alterations in the quality of light reflected from neighboring trees. The ''boundary layer'' hypotheses proposes that changes in atmospheric mixing occur in high-density plantations, promoting increased concentrations of CO{sub 2} and H{sub 2}O vapor during early morning hours, which in turn would enhance carbon assimilation. The ''light quality'' hypothesis proposes that the presence of neighbors alters the ratio of red to far red light in the canopy environment. Plant sensors detect this change in light quality, and growth and development is altered in response. We found that boundary layer conductance was higher, as we predicted, in low-density Douglas-fir stands than in high-density stands five years after planting. The changes in boundary conductance were accompanied by higher CO{sub 2} and H{sub 2}O vapor during early morning hours. However, we also found that the primary manifestation of the density effect in Douglas-fir occurs two to four years after planting, and we were not able to measure differences in boundary conductance in different densities at that time. Also, we found no difference in carbon isotope composition of wood cellulose formed in high- vs. low-density stands two to three years after planting. We conclude that although stand density may have a significant impact on boundary layer conductance in young stands, it does not account for the ''density effect''. Our tests of the light quality hypothesis were slowed due to poor plantation establishment in the early phase of this study. In a variable density experiment we detected significant changes in R:FR related to density. Also in that study we measured a significant enhancement of tree height at high density. However, after three years of growth, the study trees did not show significant differences in stem diameter related to density. Experimenters at Weyerhaeuser therefore decided not to harvest the trees at the end of the 3rd growth year, as originally planned. In a 1-year study of seedlings planted in raised beds subjected to different light quality treatments using transparent plastic film, we found that tree height but not diameter increased in response to decreased R:FR. At this point, we conclude that R:FR remains a viable hypothesis for the ''density effect'', but evidence is not conclusive. We expect that continued measurements in the variable density test plot at Weyerhaeuser will add more evidence in the future.
- Research Organization:
- Barbara J. Bond (US)
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- FC07-97ID13530
- OSTI ID:
- 797321
- Report Number(s):
- DOE/ID/13530; TRN: US200215%%468
- Resource Relation:
- Other Information: PBD: 21 Jul 2002
- Country of Publication:
- United States
- Language:
- English
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