Performance of vegetation on mined sands

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  • This reports covering the results of the 1977 research, adds significantly to the earlier findings. It is always difficult to predict long term plant responses on only a few years of data; in this case, only two years. Climatically the two summers (May - August) were very similar, with the mean monthly temperatures averaging 0.2°C lower in 1977. Precipitation was ~20% higher in 1977, cloud cover was also greater, and as a result, diffuse radiation was higher. Short wave and net radiation were similar to the previous year. Net radiation increased from 70 W m-2 in early March to a maximum 207 W m-2 in mid-June. Photosynthetically active radiation (PHAR) was about 50% of incoming radiation, similar to other temperate region studies. Summer climate of the Richardson lookout Station is similar to the long term summer climate at Fort McMurray. Temperatures for May through August were slightly warmer (0.4 to 1.3°C) at Richardson and precipitation is ~20 mm less than at Fort McMurray. There are also fewer summer days with precipitation at the research site (44 vs. 55 days). As found the previous year, little precipitation runs off these porous sands. During spring thaw. water runs down slope within the soil above the frozen soil layer. About 50% of total precipitation moves out of the rooting zone to greater soil depths and is therefore unavailable for plant growth. The Jack Pine forest and its limited understory use only 170-200 mm of water per year, yet soil water potentials seem to not drop below 1.0 MPa. Microclimate within a forest canopy is difficult to measure and this is further complicated by working on a slope. As in 1976, gradients of temperature, water vapour and wind were small within the forest canopy. The exchange surfaces for heat and water vapour are diffuse due to openness of the canopy and its slope position. In spite of this, the canopy acts as a \"surface layer\" for radiation. On nights with little cloud cover and light winds (0.5 to 0.8 m s-l) cold air drainage was greatest, resulting in temperature differences of 3.5 to 7.0oC between the mid-slope and bottom-slope sites. Soil heat flux amounted to 7% of net radiation measured above the canopy. Soils were naturally warmest near the surface, decreasing in heat with depth. At 200 cm soils were generally 8 to 10 oC cooler than at -2 cm and the time lag for heat transfer to the deepest level measured was 3-4 days. As observed in 1976, interception of precipitation by the tree canopy and trunk is high in storms with little precipitation and rains of low intensity predominate. On average, 53% of all precipitation was absorbed by the trees. Interception was greatest (44%) near the tree base (25 cm) and significantly less (17%) at a distance of 1 m. As a result of increased turbulent exchange within the forest canopy a more homogeneous environment of temperature, water vapour, and air movement results. This, however, makes it more difficult to establish the actual sources and sinks within a Jack Pine forest. The field and laboratory data on young and mature Jack Pine show how well this tree species is adapted to high water stress environments. As with other conifers, this species has relatively low maximum net assimilation rates (6 to 8 mg dm-2 h-1). After two drying cycles, maximum rates were 4.5 mg dm-2 h-1. This indicates some supression of photosynthetic capacity following severe drought yet an ability for considerable recovery. Net assimilation reached zero at 2.2 MPa leaf water potential, again indicating a considerable adaptation to stressful environments. Trees avoid winter and spring drought stress by maintaining high leaf resistances; the stomates remain closed when the soils are frozen. In winter cavitation does not occur and as a result, water columns are maintained in the xylem and therefore a better water balance. In summer the stomates are sensitive to atmospheric VPD and close prior to xylem tension or ~ leaf triggered closure. Considerable osmotic adjustment also occurs which permits further water uptake as ~ soil increases. The lethal limit for  leaf is at least -3.5 to -4.0 MPa. Under extreme drought conditions the more photosynthetically active needles survive at the expense of the young ones. All of these data contribute to a more complete understanding of why Jack Pine is so well adapted to these cold winter and warm droughty summer conditions that dominate these porous sands. The mycorrhizal studies showed that 14 species of fungi formed mycorrhizae with Jack Pine under synthetic conditions. From this work and the field studies, it appears that no small group of fungi dominate the below ground symbiotic system. In the field 57 species of fungi were found associated with Jack Pine within a limited area. The synthesis test showed that a variety of fungi produce the non-descript associations found in the field. Mycorrhizal fungi were studied in relation to Arctostaphylos uva-ursi and all of the infections were of the ectendomycorrhizal type. Fifteen species of mycorrhizal fungi were grown at different osmotic potentials using NaCl and sucrose. All species grew the most at high osmotic potentials (-0.16 and -0.61 MPa). This differs from other studies for the fungi grew most at lower potentials (-1.5 to 5.8 MaPa). In summary this second year of limited field work but intensive laboratory and greenhouse studies contribute to a much fuller understanding of the factors related to the very successful growth of Jack Pine and its associated lichen understory on these deep sands.

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