Apparent Photosynthesis of Douglas-Fir in Relation to Silvicultural Treatment

Publisher： Society of American Foresters

ISSN： 0015-749X

Source： Forest Science, Vol.10, Iss.4, 1964-12, pp. : 432-442

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Abstract

A study of apparent photosynthesis of Pseudotsuga menziesii (Mirb.) Franco in its natural environment was carried out over a period of 2 1/2 years in western Washington using a Hartmann-Braun U.R.A.S. infra-red gas analyzer. For the latter part of the study, a continuous automatic recording was made of air temperature, light intensity and relative humidity inside each of the sampling cuvettes by means of a 24-line 2-millivolt recorder in conjunction with thermocouples and photo-cells as sensing elements. After a 1 1/2-year "calibration period" during which the characteristic seasonal net assimilation patterns were determined for five trees in each of the dominant, co-dominant and suppressed crown classes, severe treatments were carried out to determine their effects on the characteristic patterns previously observed. Thinning, pruning and decapitation increased photosynthetic efficiency of the foliage on a unit dry weight basis during the first year after treatment. Fertilization had no appreciable effect on the rates of apparent photosynthesis over this period. To determine whether the considerable differences in net assimilation rates observed between naturally growing trees were due to genetic variability, similar treatments were carried out on ramets of a 4-year old clone; however, no reduction in inter-tree variability in net assimilation rate was observed.