Populus tremuloides

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. tremuloides tolerates a wide range of soil conditions from rocky soils or loamy sands, to clay soils. The most favorable soils are porous, and loamy soils that have abundant lime and humus. Growth in clay soils is reduced because of poor aeration; growth in sand is poor because of low moisture and nutrient levels. Rocky soils can hinder the spread of lateral roots. P. tremuloides grows at elevations up to 5,800 feet (1768 m) in the north, and rarely below 8,000 feet (2438 m) in lower California, growing at sea level only as far south as Maine and Washington. In the southwest United States, it often grows in cool shaded mountain slopes, canyons, and on stream banks, at about 6,500 to 10,000 feet (1981 to 3048 m) in elevation. P. tremuloides grows with other aspens and often in the following forest types: Jack pine-aspen, white spruce, balsam fir-aspen, black spruce-aspen, aspen-paper birch (Fowells 1965).

PESTS AND DISEASE: Populus spp. have many natural enemies. The forest tent caterpillar, Malacosoma disstria, is one of the most important insects to attack P. tremuloides. The fungus Marssonia populi induces a leaf and twig blight of P. tremuloides that periodically becomes epidemic over extensive areas of the northwestern U.S. Clones vary in their susceptibility to the disease, and may become anywhere from only slightly damaged to entirely defoliated. The last reported epidemic occurred over large areas of northeastern Utah, southeastern Idaho, and western Wyoming (Harniss 1984).

The most serious diseases affecting P. tremuloides are the wood-rotting fungi and cankers. Fomes ignarius is a wood-rotting fungus that attacks the 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 Populus spp. 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.).

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.

SEXUAL REPRODUCTION: A new discovery of bisexual flowers in P. tremuloides has been reported by Dr. Tashneem Khaleel of Montana State University- Billings, who found the group of plants in 1992. Further research is being conducted of the species, previously known to be monoecious. (JHH 1995) 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. tremuloides produce good seed crops every four or five years (Fowells 1965). A 23 year old P. tremuloides tree 33 feet tall in Ontario produced 1.6 million seeds (Maini 1972). Under favorable natural conditions, seeds of P. tremuloides 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 (Perala 1981).

Plants as young as two years old, and both male and female plants are capable of suckering (Fowells 1965). In Michigan, P. tremuloides produces a maximum number of suckers at age 35 (Graham 1963). Number of suckers and length of time required for their initiation varies between clones (Barnes 1966).

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 usually are .76 to 1.78 cm in diameter, and suckers most often arise at intersections of lateral branch roots, in underdeveloped branch roots, in irregularities in roots, and as an injury response (Sandberg 1951).

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. In Wisconsin, maximum cambial activity is in late May and early June. Rates decrease rapidly in July until, in late July and early August, no cambial cell division occurs (Davis 1968). In a study of P. tremuloides cuttings in Alaska and Utah, root carbohydrates were found to significantly increase in late July to a maximum level about September first, and then decreased slightly with leaf senescence (Schier and Zasada 1973). Because of this seasonal growth pattern, suckers that initiate growth before maximum shoot growth probably benefit most from carbohydrate reserves in parental roots.

Amount of sucker initiation increases with abundant light (Zehngraff 1949) probably because of increases in soil temperature. In one study, root cuttings of P. tremuloides formed roots most readily at 74 degrees F (23 degrees C), and burning and clear-cutting resulted in increased soil temperature and sucker formation (Maini 1966b). The heat absorption by blackened soil following fire may increase the number and height of suckers (Shirley 1931, 1932). In Alberta, drought and high soil temperature in one season increased sucker invasion the next season (Bailey 1974). Diurnal temperature fluctuations possibly increase sucker initiation (Maini 1966b).

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. tremuloides 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. After two to five years P. tremuloides has self-sustaining roots and root connections to parents either may be destroyed by rotting or remain indefinitely (Sandberg 1951). Requiring abundant light for early growth, P. tremuloides cuttings in green-house tests showed that the maximum light level tested stimulated the most root development, secondary growth, and consistent height growth of suckers (Sandberg 1951). 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).

Growth of P. tremuloides is influenced by its capacity for bark photosynthesis. Schaedle (1971) found that 5 and 6 year old suckers in a clone provided 5 to 10% of total plant photosynthetic activity by bark photosynthesis.

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).

Clones 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. Freshly fallen leaves of P. tremuloides have been shown to decrease early growth of Festuca elatior, F. rubra and Poa pratensis (Younger 1980).