A Comparison of the Seasonal Rates of Dry Matter Production of Three Coniferous Species with Contrasting Patterns of Growth

Seasonal rates of growth and dry-matter production were examined in second-year seedlings of Larix leptolepis, Pinus contorta, and Pinus radiata grown in an unheated glasshouse. The deciduous Larix had a higher rate of production of dry matter than either of the two species of Pinus until the time of leaf fall, and this was accompanied by a greater height and diameter increment. However, between the time of leaf fall in Larix and the end of the growing season, the species of Pinus increased in dry weight by more than 25 per cent, and in consequence Larix, because of its deciduous habit, lost much of the advantage of its fast growth-rate. Comparison of the two pine species showed that P. radiata, while making nearly 3.5 times as much height increment as P. contorta, had only a 45 per cent higher dry weight than that species at the end of the experiment. The dry-weight differences were due to differences in relative growth-rate (RGR). The RGR differences between the two pine species resulted from differences in net assimilation rate (NAR) rather than differences in the ratio of photosynthetic to non-photosynthetic tissue, and P. radiata gained most of its dry-weight advantage during the two months of October and November. It seems possible that the higher NAR of P. radiata at this time was a reflection of the difference in growth habit between the two pine species, and possible mechanisms by which growth may have affected NAR in this experiment are briefly considered.