Species: Populus balsamifera

SEXUAL REPRODUCTION: Flowers appear before leaf expansion, usually in April or May. Phenology varies between clones, with air temperature, and geographic locations (Maini 1972). Following wind pollination, fruits ripen in May or June and are dispersed by wind or water May through July. The light seeds have a silky hair aiding in dispersal. <br><br>Most aspens are capable of flowering at ten years (Maini 1972). P. balsamifera seed is viable for a few days (Fowells 1965). <br><br>ASEXUAL REPRODUCTION: In established aspen clones, root suckering accounts for most reproduction. Stump sprouting is less frequent. Suckering sometimes occurs within undisturbed clones, but survival is low. Suckering is most profuse following top removal, and removal of other cover species by cutting, fire, windthrow, and disease. <br><br>Plants as young as two years old, and both male and female plants are capable of suckering (Fowells 1965). <br><br>Suckers arise from adventitious buds produced on an extensive lateral root system that rises and falls just below the soil surface. Some suckers arise from dormant buds; most suckers are from buds initiated the same season. These buds form where the parent root is closest to the soil surface; usually in the top 5.1 to 7.6 cm of soil, and often above mineral soil (Sandberg 1951). <br><br>Sucker initiation and growth is influenced by growth regulator levels, carbohydrate levels, light, and temperature. Root suckering of Populus species is inhibited by the auxin effect of apical dominance (Farmer 1962, Eliasson 1971, Schier 1973). Auxin production is highest in apical buds during maximum spring shoot elongation, usually in June, and translocation of auxin to the roots inhibits suckering. <br><br>Removal of the above-ground plant portion in June or July after maximum auxin production results in fewer suckers than top-removal during the dormant season. Suckers formed early in the season also exhibit apical dominance by reducing the number and success of suckers formed later in the same season (Schier 1972). Seasonal variation in suckering is probably also influenced by other growth regulators including cytokinins and gibberellins (Schier 1972). Cytokinins produced in the roots stimulate suckering (Schier 1981). <br><br>Parental root carbohydrate reserves do not affect the number of suckers initiated, but do influence early sucker success. Following initiation, suckers are dependent on parental root reserves until they emerge above the soil and produce their own photosynthates (Schier 1971). Root carbohydrates are lowest in aspen clones after leaf flush (Tew 1970) and remain low until late July (Schier 1971). Both root carbohydrate reserves and early photosynthates are used during this time, primarily for shoot elongation and cambial activity. <br><br>Amount of sucker initiation increases with abundant light (Zehngraff 1949) probably because of increases in soil temperature. <br><br>SEEDLING ESTABLISHMENT: Seeds germinate readily within a day or two of dispersal if they reach an exposed moist site. Alluvium or the exposed mineral soils following fire are appropriate sites for establishment (Weigle 1911). <br><br>SUCKER ESTABLISHMENT: Early sucker growth and survival depends primarily on time of sucker initiation, root connections to parent roots, and ample light. The most numerous, tallest and competitive suckers are produced when above-ground portions are removed during the dormant season. Summer top-removal results in short suckers that compete poorly with shrubs, herbaceous species (e.g. Pteridium aquilinum (L.) Kuhn.) and overstory species (Perala 1972). They are also susceptible to winter injury (Zehngraff 1949). However, top removal in the summer results in continued suckering the next season, so that by the end of the second season, the effects of top-removal are sometimes negligible (Graham 1963). <br><br>Suckers depend primarily on parent roots during the first season of growth (De Byle 1964) when adventitious root production by suckers is low. The number of root interconnections between suckers and parents decreases with age. Populus spp. require abundant light for early growth. <br><br>Growth rate of suckers is rapid for about five years after clearcutting. Then rates slow as suckers compete for light and moisture (Graham 1963). An aspen stand initially producing 40,000 suckers/A can be reduced to 1,000 to 1,500/A in 30 years (Maini 1972). <br><br>CLONES: Asexual reproduction results in a group or clone of suckers that has its origin in a seedling established tree (the ortet). The clone is composed of genetically identical stems (ramets) that often remain interconnected by roots to form a single functional unit (Blake 1963). <br><br>Intraclonal ramets are of the same sex, have similar bark color, leaf forms, branching habits, and disease and insect pest susceptibility (Barnes 1966). Phenological patterns including time of leaf flush and fall coloring are also similar, with slight variations from the center to the edge of a clone (Barnes 1969). This intraclonal similarity of ramets is useful in distinguishing one clone from another where clones intermix. <br><br>Interclonal differences include ramet density, ability to sucker, and growth rate (Barnes 1969). <br><br>Clone profiles indicate ramet origin or topographical variations. Truncated clone profiles usually indicate simultaneous ramet origin (fire, windthrow, clearcutting). Dome-shaped clones result when suckering occurs at the periphery, especially into open sites such as grasslands. Wavy or notched clones usually indicate specific limits to expansion (severe slope, soil texture changes, fluctuating water levels, blowouts, etc.) (Maini 1960). <br><br>Expansion at the clone periphery is encouraged by favorable moisture, abundant light, and lack of competition by other ramets (Barnes 1966). Ramets on the periphery usually lack taproots or "sinker roots". In prairies the superficial aspen roots are mixed in the upper soil layers with grass and forb roots. Clones can function opportunistically by expanding under optimal growing conditions, and contracting under stress. <br><br>ALLELOPATHY: There is some evidence of aspen allelopathy. Jobidon (1981) showed that extracts of various parts of P. balsamifera (leaf litter, fresh leaves, buds) variously inhibited germination, radicle and hypocotyl growth of Alnus crispa.