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 examinedin second-year seedlings of Larix leptolepis, Pinus contorta,and Pinus radiata grown in an unheated glasshouse. The deciduousLarix had a higher rate of production of dry matter than eitherof the two species of Pinus until the time of leaf fall, andthis was accompanied by a greater height and diameter increment.However, between the time of leaf fall in Larix and the endof the growing season, the species of Pinus increased in dryweight by more than 25 per cent, and in consequence Larix, becauseof its deciduous habit, lost much of the advantage of its fastgrowth-rate. Comparison of the two pine species showed thatP. radiata, while making nearly 3.5 times as much height incrementas P. contorta, had only a 45 per cent higher dry weight thanthat species at the end of the experiment. The dry-weight differenceswere due to differences in relative growth-rate (RGR). The RGR differences between the two pine species resulted fromdifferences in net assimilation rate (NAR) rather than differencesin the ratio of photosynthetic to non-photosynthetic tissue,and P. radiata gained most of its dry-weight advantage duringthe two months of October and November. It seems possible thatthe higher NAR of P. radiata at this time was a reflection ofthe difference in growth habit between the two pine species,and possible mechanisms by which growth may have affected NARin this experiment are briefly considered.