Comparative Physiology and Development of Trembling Aspen (Populus Tremuloides Michx.) And Bigtooth Aspen (Populus Grandidentata Michx.) From Michigan Populations

Trembling and bigtooth aspens are taxonomically very closely related and morphologically similar species of the genus Populus. However, they differ significantly in their ecological requirements which probably partially accounts for trembling aspen having a far greater natural range than bigtooth aspen. Studies of the comparative physiology of both species were undertaken in order to determine further possible reasons for their differences in distribution. The ability to produce adventitious shoots (suckers) from excised root segments by trembling and bigtooth aspens was studied over two successive years. Growth patterns of shoots and roots of container-grown seedlings were examined for both species under greenhouse conditions. A study of comparative development of container-grown seedlings and sucker-derived vegetative propagules in the greenhouse was conducted. Rates of photosynthesis and respiration in a controlled environment were also compared for both species. Juvenile shoot development showed very similar patterns for both species; cumulative height growth, dry matter accumulation and leaf area were similar in spite of significant differences in the number of leaves. Trembling aspen developed many more leaves than bigtooth aspen but the latter compensated by producing much larger leaves. Trembling aspen also significantly outgrew bigtooth aspen in height. Root development was very similar for both species. Excised root segments of either species suckered equally well. Trembling aspen was consistent in its sprouting ability be tween the two years of study, but bigtooth aspen varied signi ficantly. Seedlings of both species produced lateral roots at the same rate, the roots showed similar growth rates, and their total dry weights were similar. Developmental comparisons of seedlings and sucker-derived cuttings revealed that seedlings grew at least three times as fast as cuttings during the 15 week observation period. Seed lings were superior in all the developmental measurements made, except in leaf number, which was similar in both propagule types, and net assimilation rate, in which values for cuttings were higher for both species. Signifcant geographic variation in height growth was observed in both species between clones from different areas of Michigan. The results also showed that seedlings of northern Michigan origin are superior in height growth to southern Michigan sources in this early stage of evaluation. Rates of photosynthesis and respiration were deter mined in the laboratory with an infra-red gas analyzer in a closed system. Seedlings of both aspen species showed similar rates of photosynthesis and respiration. However, under light-saturating conditions, leaves of trembling aspen had significantly higher photosynthetic, and respiration rates, but their CO2 compensation points were similar. Distinct differences were observed in the survival and first growing season performance between outplanted containerized and bare-rooted seedlings. Containerized planting stock survived and grew better than bare-rooted seedlings by the end of the first growing season after transplanting