Search

Gap dynamics theory suggests that spatial and temporal patterns of species recruitment within canopy gaps may be due to differences in shade tolerance. Although this theory has been developed in lo...

In the fall of 1984, a sugar maple ( Acer saccharum ), white ash ( Fraxinus americana ), trembling aspen ( Populus tremuloides ) and black cherry ( Prunus serotina ) woodlot was clear-cut for the construction of a transmission right-of-way. The first year's re-growth was dominated by seedlings of black cherry and white ash (total density of 4.5 stems m −2 ). Rapid proliferation of trembling aspen suckers, and growth of choke cherry ( Prunus virginiana ) and raspberry ( Rubus idaeus ) seedlings increased the density to 31.8 stems m −2 by the end of the second year. By the sixth year, the trembling aspen and choke cherry had declined, white ash had increased, and the overall density declined to 22.1 stems m −2 . The relative densities of tree seedlings and sprouts were similar; the sprouts were considerably taller, however, and represented the major revegetation source. Therefore, a second phase of the study was undertaken to investigate the growth and survival characteristics of sprouts and suckers originating from the stumps of 317 trees. The most prolific sprouters, black cherry and white ash, exhibited the highest stem mortality, while the poorest sprouter (sugar maple) experienced better survival. After 6 years, the sprout numbers were converging on a range of five to seven stems per stump, irrespective of species. The height growth was greatest in the first year and has since declined. Black cherry and trembling aspen had the highest growth rates and white ash the lowest. Growth was correlated to parent tree height and diameter in the maple and aspen; the ability to predict growth from these parameters, however, is very poor.

A thin layer chromatography bioassay was used to detect antifungal compounds in tissue culture plantlets and potted seedlings of aspen. Catechol and the phenolic glycosides salicin and salicortin were identified as inhibitory compounds; a fourth compound was observed but was not identified. Inhibitory compound levels were estimated in eight tissue culture clones grown on unamended media and media amended with 0.22 M mannitol to induce water stress. Aspen tissue culture plantlets grown under water stress conditions had significantly lower levels of catechol, salicortin, and salicin. Significant clonal variation in levels of catechol, salicin, and the unidentified compound was also observed. Catechol, salicortin, and salicin were inhibitory to Hypoxylon mammatum when tested at levels similar to those employed in the thin layer chromatography bioassay. These results suggest that a reduction in the levels of inhibitory compounds in water-stressed aspen may be a factor in the water stress induced susceptibility of aspen to H. mammatum. Key words: Populus tremuloides, Hypoxylon mammatum, water stress, tissue culture.

Decay resulting from increment borings varies with tree species. Trembling aspen (Populus tremuloidcs Michx.) in Newfoundland is seriously affected by this particular wound. Three years after 20 trees had been bored, none had healed probably because of a pronounced split of the bark. The wood discoloration extended horizontally to about 2 cm on each side and vertically half metre above and below the wound. Peniophora polygonia (Pers. ex Fr.) Bourd. et Galz. was the most common fungus isolated while Polyporus adustus Willd. ex Fr. and several imperfect fungi and bacteria were also present.

Freshly wounded stem sections of P. tremuloides Michx. produce phytoalexin when inoculated with mycelium of Hypoxylon mammatum (Wahl.) Mill., and Alternaria sp. These exudates are inhibitory against spore germination of these two fungi and, although inhibitory against mycelial growth of Alternaria sp., have no effect on the mycelial growth of H. mammatum. It was possible to correlate the amount of phytoalexin elicited by different strains with the inhibitory activity.

The decline in carbohydrates in roots of aspen (Populustremuloides Michx.) as a consequence of shoot production and the contribution of new shoots to root carbohydrate levels was investigated as part of a study of the factors controlling aspen regrowth following top removal. The level of non structural carbohydrates in the roots of young aspen shoots which had been clipped during the growing season was estimated at the commencement of the dormant season. Segments of the roots were then placed in growth cabinets and kept either in darkness or in one of two light regimes having equal quantum flux densities, but with quality akin to either normal sunlight or shade. Shoot growth in darkness was greater from roots taken from plots where clipping had been conducted in midseason rather than later in the season, suggesting that some carbohydrate replenishment had taken place in those plots. Carbohydrate concentration in roots placed in the three light regimes declined to approximately the same extent over the 8 weeks of the experiment, but total weight of shoots was greater in light than in darkness, which suggests that newly synthesized carbohydrates were retained in the tops for growth. While shoots in normal and shade quality light were not phenologically different, shoots in normal light had greater leaf development than shoots in shade light and were heavier, presumably as a consequence of the greater leaf development leading to a greater net photosynthesis.

Annual early spring burning has markedly altered the physiognomy and species composition of aspen parkland vegetation of east-central Alberta. Burning was conducted in April for at least 24 years when soil moisture was normally high. The number of herbaceous species per quadrat doubled while a number of woody species declined slightly. Forest cover declined while grassland increased. In the grass and shrublands, cover of the shrub Symphoricarpos occidentalis and the cool season grass Festuca scabrella declined greatly. Major increases in Calamovilfa longifolia, Carex obtusata, C. heliophylla, and Solidago missouriensis were noted. While the cover of Festuca scabrella and Stipa spartea var. curtiseta decreased; frequency of these species increased. Cover of all shrubs decreased, Populus tremuloides suckers excepted, while density of the fire adapted Elaeagnus commutata, Amelanchier alnifolia, Prunus virginiana, and Populus tremuloides suckers increased. Burning reduced the leaf blade length of all major grasses and sedges. Effect of burning on inflorescence production varied from species to species. Annual herbage production was reduced by burning with the grass and sedge component experiencing about a 50% reduction. Burning caused an increase in organic matter and phosphorus content of the Ah horizon. Burning did not appear to detrimentally affect the nitrogen status of the Ah horizon.

Vegetation and soils were sampled in adjacent 40-year-old stands of red pine (Pinusresinosa Ait.), jack pine (Pinusbanksiana Lamb.), white spruce (Piceaglauca (Moench.) Voss), and aspen (Populustremuloides Michx., P. grandidentata Michx.) on a very fine sandy loam soil in north-central Minnesota. Total tree biomass was greatest for red pine followed by aspen, spruce, and jack pine. Nutrient weights (N, P, K, Ca, Mg) in the trees were greatest in aspen followed generally by spruce, red pine, and jack pine. Particularly large proportions of biomass and nutrients were found in aspen bark and spruce foliage and branches. Understory biomass contributed less than 1.2% of the total organic matter in the vegetation–soil complex but contributed up to 5.0% of the nutrients. Exchangeable Ca in the surface soil was much lower under aspen and spruce than under the pines. No significant soil differences between species were detected below 36 cm. Harvesting the entire aboveground portion of the tree would remove up to three times more nutrients from the site than would harvesting only the bole.

Fifteen lactating Holsteins were used to test processed aspen added to corn silage at the levels of 0, 10 and 20% (dry matter basis). The experiment was designed as a latin square with three experimental periods each 42 days in length. The forage mixtures were fed free choice to the cows, which were housed in a free-stall barn, and their individual feed intakes were recorded by using electronic doors. The processed aspen contained 45.4% dry matter, 73.7% acid detergent fiber and 0.54% protein. There was no evidence of heating or mold growth in the aspen during the 5 mo of the trial. Silage dry matter intake and milk yield were 11.8, 13.4 and 13.4 and 25.7, 27.0 and 26.8 kg per day, respectively, for the silage mixtures containing 0, 10 and 20% processed aspen. Fat test was decreased slightly and milk protein content significantly (P 0.05) by the inclusion of aspen chips in the silage. Neither the molar proportion of acetic acid nor the ratio of acetic acid to propionic acid in the rumen fluid was changed when the cows were fed silage containing processed aspen chips. The apparent digestibilities of total ration dry matter and protein were lowered significantly by the addition of 10% aspen chips to the forage mixture. It was concluded from the results of this trial that steam-processed aspen chips had little nutritive value when fed to lactating cows as a partial substitute for corn silage.

Soil moisture and potential evapotranspiration were monitored in four stands of Populustremuloides Michx. during two growing seasons. Measured soil moisture was compared with soil moisture predicted by four models: THIRSTY, SOGGY, Zahner's, and a simple water budget. THIRSTY, an original model incorporating many soil layers, root density, and a variable resistance to moisture flux, generally gave the best fit. Water uptake was related to relative root density.

Fire has been a natural component of the aspen ecosystem. Any fire in an established aspen stand will cause injury. Aspen is easily top-killed, but the roots remain viable. A fire's heat can stimulate sprout growth from these roots, aiding natural regeneration.