Habitat
Climatic conditions vary throughout the ranges, but are often characterized by low seasonal temperature provided by high altitudes or northern latitudes, and short growing seasons. P. grandidentata is usually found on dry sites, at elevations from 500 to 2,000 feet (152 to 660 m). Soil textures include sand, loamy sand, light sandy loams, and, less frequently, heavier textured soils. High water tables closer than 18" (45.7 cm) to the surface reduce aeration, and increase chances of windfall (Fowells 1965). It is most commonly associated with quaking aspen (P. tremuloides), gray birch, paper birch and red maple.
Ecology
PESTS AND DISEASE: Populus spp. have many natural enemies. The forest tent caterpillar, Malacosoma disstria, is a major insect that attacks P. grandidentata.
The most serious diseases of P. grandidentata are the wood-rotting fungi and cankers. Fomes ignarius is a wood-rotting fungus that attacks this species throughout its range, causing decay of heartwood and sapwood (Fowells 1965). Hypoxylon canker is widely distributed in the Northeast and Lake States and causes heavy losses of aspen in these regions. French (pers. comm.) stated that perhaps 15-20% of all trees in the Lake States are infected with Hypoxylon. The Hypoxylon fungus attacks the phloem, and kills the tree within 2-4 years of initial infection (French pers. comm.). P. grandidentata is less frequently attacked than P. tremuloides, and infection in P. balsamifera has so far been negligible (Fowells 1965). More information on the numerous pests and diseases of aspen is available in forestry literature (e.g. Graham 1963, Fowells 1965, USDA 1972).
Moderate browsing by mammals such as deer causes little permanent damage to suckers. Mice, voles, and rabbits can girdle suckers, and beaver frequently cut larger trees.
Reproduction
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.
Most aspens are capable of flowering at ten years (Maini 1972) and 20 year old trees of P. grandidentata produce good seed crops every four or five years (Fowells 1965). Under favorable natural conditions, seeds maintain viability up to two or three weeks.
ASEXUAL REPRODUCTION: In established aspen clones, root suckering accounts for most reproduction, while 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. Tens of thousands of suckers per hectare follow clearcutting of P. grandidentata (Perala 1981).
Plants as young as two years old, and both male and female plants are capable of suckering (Fowells 1965). In Michigan, P. grandidentata produces a maximum number of suckers at age 40. (Graham 1963). Number of suckers and length of time required for their initiation varies between clones in P. grandidentata (Garrett 1964).
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). Parent roots range from .51 to 11.43 cm in P. grandidentata (Maini 1972).
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.
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).
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.
Amount of sucker initiation increases with abundant light (Zehngraff 1949) probably because of increases in soil temperature.
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). P. grandidentata is capable of germinating while submerged in water, and at temperatures between 32 degrees F (0 degrees C) and 95 degrees F (35 degrees C) (Fowells 1965). Under controlled conditions, 80-95% germination is possible, but germination is less under natural conditions. Maini (1972) gives the following reasons for seedling failure:
1. Short seed viability
2. Presence of a water-soluble germination and growth inhibitor in the seed hair
3. Inadequate moisture conditions on upland sites
4. Susceptibility of seedlings to high temperatures possible on soil surfaces blackened by fire
5. Fungal pathogens
6. Diurnal temperature fluctuations hindering early seedling growth
7. Unfavorable chemical composition of substrate (e.g. high pH, high salt concentrations)
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).
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. P. grandidentata depends partially on parent root functions for 25 years but relies more heavily on adventitiously produced roots after the first six years of sucker establishment (Barnes 1966, Zahner 1965).
P. grandidentata requires abundant light for early growth. Under full sunlight, dominant suckers can grow 4 feet to 8 feet (1.22 m to 2.43 m) the first season.
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). 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).
Clones of P. grandidentata are usually small. In the Great Lakes states, clones of .04 to .08 ha are common (Perala 1981, Barnes 1966). Larger clones have been recorded in Utah (10.1 and 43.3 ha) and the southern Rocky Mountains (81 ha) (Perala 1981).
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.
Interclonal differences include ramet density, ability to sucker, and growth rate (Barnes 1969).
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).
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.
ALLELOPATHY: There is some evidence of aspen allelopathy.