Long term prediction of vegetation performance on mined sands

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  • This project on the \"Long Term Prediction of Vegetation Performance On Mined Sands\" (V.E.6.1) was undertaken to provide management with answers on the predictive ability to maintain different kinds of vegetation on raw sands. The research was designed as an integrated, multi-disciplinary program that would concentrate on the role of water stress in a dynamic soil-plant-atmosphere system of a planted grass cover and a natural Jack pine forest. To date only the latter project has been initiated because of the lack of funding and approval to work on the GCOS dike in 1975. This and the Syncrude dyke represent the worst (driest) environmental situation and therefore revegetation of other sand deposits should be more easily accomplished. The Richardson Fire Tower site was chosen because of the representativeness of its Jack pine - lichen woodland on deep sands, a forest type so characteristic of northeastern Alberta. The results of the first full year show that climatically this southwest-facing sand slope warms more rapidly in spring than do level sites at Mildred Lake and Fort McMurray and that the 1976 summer was above normal for temperature. Precipitation was near normal based upon the 1941 - 1970 period. Of the >60 days of precipitation, over 60% were 4 mm or less and thus little if any water entered the soil due to tree, lichen, and litter interception. Both needle duff and lichens provide a significant barrier to surface evaporation compared with open sand. Resistance to evaporation is 2 to 3 times greater with a lichen cover than with litter. The soils are very porous which is advantageous for water entrance, thus preventing erosion but porosity is a disadvantage in maintaining higher water levels near the soil surface for plant growth. These soils recharge during snowmelt in late March - early April; little runoff occurs and over the summer soil water drawdown takes place. Soil moisture content (volume basis) is generally 8 - 15% near the surface in spring, but by late September is 1 - 3% at all depths. Xylem water potentials, a measure of tree water content, were never very low (mean maximum at dawn -5 to -7 atm. and mean minimum at midday -11 to -14 atm.) which reflect a year of average precipitation with frequent light rains and periodic heavier storms. Transpiration and stomatal closure were controlled largely by vapour pressure deficits. Jack pine avoided spring drought by remaining dormant when air and needle temperatures were above freezing, yet when soils were still frozen. Although Jack pine did not show indications of severe drought in a relatively moist summer, it did develop xylem water potentials of -16 to -18 atm., values which are probably detrimental to many of the species being used in revegetation trials on the dike (Bromus inermis, Phleum pratense, and species of Agropyron). This means that potential species must be drought hardy and tested under laboratory rather than only under field conditions to determine their survival under severe drought conditions that may occur but once in 30 to 50 years. The studies of mycorrhizae show that a large number of species of fungi infect the roots of Jack pine and that the infecting flora from disturbed soils (old burns) is quite different from that of undisturbed forests. Since mycorrhizae are critical for the proper growth and survival of pines, care in innoculating tree seedlings with the proper species is essential. The energy and water balance mathematical model predicts the heat and water status of the Jack pine forest. Examination of the model outputs suggests that late season resistance to water uptake occurs because of increased root resistance in autumn. If this is confirmed with further experimental data, and model runs, it means that fall droughts may be especially critical because of the reduced ability of the trees to absorb water through their roots. A second field season coupled with the laboratory studies to determine lethal and sublethal levels of water stress in Jack pine will provide the added inputs to the models necessary for predicting tree response to severe climatic stress. These data, gathered in a highly integrated manner, will permit the calculation of tree survival on sands, be they dikes or other kinds of mined sand, in terms of soil water content and tree density (including crown extent) in relation to the exceptional dry year that may occur once in 30 to 50 years. Data from field trials of grasses or woody species, without supporting measurements of plant physiological responses to environmental conditions cannot provide this essential predictive tool for management unless the one in 30 to 50 year drought cycle is encountered. It is for this reason that modelling of the data in order to predict plant response to unusual environmental conditions becomes so useful. In summary, this study should be able to provide sufficient data to determine whether or not an open stand of Jack pine or similar conifer is the desired end point in maintaining vegetation at a low maintenance cost on sands, the result of open pit mining of the oil sands.

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