Introduction The results of my investigations have been discussed in detail within individual chapters of this dissertation; specific recommendations have been made regarding continuation of several research projects. This final chapter will be devoted to a summary of my results, general conclusions, and broad recommendations concerning future investigation in aspen physiology. Summary and Conclusions In the initial portion of the dissertation, several aspects of aspen root sucker formation were examined. First, suckers were described as developing from buds originating in the root pericycle. Populus tremuloides suckers were found on smaller and shallower parent roots than those of P. grandidentata. Next, a study of sucker occurrence in uncut stands indicated that while suckers did not normally arise annually in healthy clones, they were frequently found in clones heavily infected with hypoxylon canker. Considerable clonal variation in sucker occurrence was observed in clear-cut areas. Light was shown to have no direct relationship to sucker formation. Roots maintained at a temperature of 80 F suckered more rapidly than those at 65 F. The second phase of investigation dealt with the physiology of sucker formation as explained in. terms of auxin-controlled apical dominance. In one study, lateral roots severed from the parent tree gave rise to numerous suckers. Girdling of stem phloem, on the other hand, failed to stimulate sucker formation. Indoleacetic acid applied to root cuttings inhibited suckering, but had no effect when applied indirectly to roots "in situ" under field conditions. It was concluded that although root suckering appears to be closely related to the apical dominance phenomenon, the exact nature of the controlling mechanism cannot be determined on the basis of my experimental evidence. The effect of light intensity upon sucker height growth was tested under field conditions and in a controlled environment. In the field experiment full shade (less than 100 foot candles) caused a reduction in occurrence and growth rate of P. tremuloides suckers, but had no significant efect upon P. grandidentata sucker formation and growth. Partial shade (4000 to 6000 foot candles) had no significant effect upon height growth in either species. Since P. tremuloides parent root diameter (0.2 inch) was smaller than that of P. grandidentata (0.4 inch ), the species difference in carbohydrate storage capacity of roots may have been responsible for the difference in growth response. Because of their smaller parent root diameter, P. tremuloides suckers were probably more dependent upon photosynthate produced. Both species exhibited wide clonal variation in growth rate which is believed to be primarily due to genetically controlled variation in physiological response. In controlled environment experiments, reduced light intensity was not associated with a decrease in height growth under a 65 : 70 F (night:day) temperature regime. Growth rates were the same for plants growing under 72 percent shade (500 foot candles) and full light (1700 foot candles) even when a daily illumination period as short as six hours was utilized. On the other hand, when temperature was elevated to 72 : 76 F shaded plants exhibited reduced height growth relative to plants under full light. This response is believed to be related to a temperature-induced increase in respiration rate relative to rate of photosynthesis. At 70 F plants under both light regimes probably produced sufficient photosynthate to support both respiration and maximum growth at that temperature. However, at the higher temperature the presumed decrease in total photosynthesis associated with reduced light intensity was directly reflected in growth rate. A chlorosis-like discoloration was characteristic of aspen leaves growing in the controlled environment room. The aspens provide suitable material for several aspects of physiological research because of their rapid early growth and quick response to environmental stimuli. These features make P. tremuloides ideal material for use in mining temperature and light effects upon the energy relationship between photosynthesis, respiration and growth. My single series of experiments in a controlled environnent is an initial step in this line of investigation. These same characteristics also make aspen a suitable material for studying regulatory mechanisms. Adaptability to vegetative propagation is a distinct asset in physiological investigation with aspen. Through the use of single clones, experiments can be easily freed from error due to genetic variation. Furthermore, studies of clonal variation in physiological characteristics are of immediate practical value to the forest geneticist. Aspen+s major disadvantage as experimental material is its susceptibility to insects and pathogenic fungi. Red spider mite and powdery mildew were the major pests encountered in my studies. Constant surveiliance and frequent applications of pesticides were necessary to prevent serious damage to experiments. The discoloration of leaves in my controlled environment room must also be considered a disadvantage at present. Future research into the effect of light quality upon aspen development may remedy this problem. Recommendations The following general recommendations are based upon my three years+ experience in (aspenology?. They are supplemented by specific recommendations in the text and are intended to serve as guides to future research. (1) More extensive use of the aspens could be made in tree physiology research. P. tremuloides is particularly suitable as experimental material in the following areas of physiology: (a) A study of shoot and root growth as affected by mineral nutrition (b) Further study of root suckering as related to regulatory mechanisms. Although root suckering is characteristic of relatively few species, its regulation probably involves a mechanism that is common to many other taxa of plants. Consequently, I feel that data resulting from such experimentation would contribute significantly to our knowledge of shoot-growth regulation. (c) A detailed investigation of growth and related processes as affected by environment. Work of this nature has as its final goal a mathematical expression of the energy relationships between photosynthesis, respiration and growth. It will ultimately require controlled environment chambers, elaborate instrumentation, and advanced mathematical methods. However, initial steps in the direction of this goal may be achieved with modest equipment and techniques. (d) Studies of intraspecific variation in photosynthesis, respiration, and growth rates. (2) Continuation of vegetative propagation research is necessary to more extensive use of the clonal approach. Full scale physiological experimentation will require large numbers of plants, uniform in size and genetically identical. (3) Basic investigation with aspen should remain "process oriented". Because of the species’ commercial importance, there is a marked tendency to define even basic research goals in terms of potential silvicultural benefits. Such "product orientation" at early stages of an investigation sometimes restricts the researcher at a time when the broadest type of analytical thinking is needed. As a result, potentially important avenues of inquiry may be overlooked because they have no obvious relationship to rather narrow goals. To date, the basic aspen research carried on at The University of Michigan has been process oriented and has had as its goal simply a better understanding of aspen silvics. It is my belief that the most significant contribution to aspen silviculture can be made through a continuation of this research policy.