© 1996 Heron Publishing—Victoria, Canada
Evaluating a simple radiation/dry matter conversion model using data from Eucalyptus globulus plantations in Western Australia
J. J. Landsberg (1, 2) and F. J. Hingston (3)
1. CSIRO Centre for Environmental Mechanics, Canberra, ACT 2601, Australia / 2. Author to whom correspondence should be addressed / 3. CSIRO Division of Forestry, Wembley, WA 6014, Australia / Received February 13, 1996
Summary
A simple model that describes growth in terms of physical and physiological processes is needed to predict growth rates and
hence the productivity of trees at particular sites. The linear relationship expected between absorbed photosynthetically
active radiation (φpa, MJ m–2) and dry mass production (G(t)); i.e., G(t) = εφpa, where ε is the radiation utilization coefficient, was fitted to three years’ data from five Western Australian Eucalyptus globulus Labill. plantations for which monthly growth measurements, leaf area indices, weather data and soil water measurements were
available. Reductions in growth efficiency relative to absorbed photosynthetically active radiation were associated with high
vapor pressure deficits (D, kPa) so the relationship between monthly aboveground biomass increments and D was used to calculate utilizable φpa. Plotting cumulative aboveground growth against utilizable φpa gave strong linear relationships with slope ε. Values of ε ranged from 0.93 to 2.23 g dry mass MJ–1 φpa. The variation could not be explained either in terms of soil water content in the root zones, because all plantations appeared
to have access to groundwater, or in terms of soil chemistry. A value of ε ≈ 2.2 is considered near the maximum likely to
be applicable to Eucalyptus plantations. An interesting peripheral finding was a strong relationship between allometric ratios and soil phosphorus; this,
if confirmed elsewhere, will be of considerable value in converting biomass increments to wood production. There was also
a strong negative relationship between the average ratio of leaf/total aboveground biomass and soil nitrogen content.
Keywords:
growth, productivity, radiation utilization efficiency, sustainable yield.
