Ninety hectares in a treed fen in north central Alberta were drained to improve growth of stagnant black spruce (Piceamariana (Mill.) B.S.P.) and tamarack (Larixlaricina (Du Roi) K. Koch) stands. Installation of 30-, 40-, 50-, and 60-m ditch spacings resulted in a lowering of the average water table by 79, 66, 56, and 73 cm, respectively. The results and the groundwater level criteria used (drainage norm, 40 cm; flood duration limit, 14 days) indicated that the 50-m ditch spacing was hydrologically the most appropriate one for this area. Given the relatively high hydraulic conductivity of the area, it is believed the 30-m spacing was too narrow and resulted in an excessively low average water table. The 60-m spacing was also overeffective, but in this case, overeffectiveness was attributed more to "edge effects" i.e., to site factors such as the proximity to uplands and the small size of upstream source areas, than to the distance between ditches. The results illustrate the importance, for ditch network design purposes, of taking into account hydrologic conditions both within and well beyond the boundaries of an area proposed to be drained. Peat subsidence after drainage appeared to be related to the average drop in water table level and amounted to about 5 cm•a−1.
Hydrologic drainage criteria that describe the position of the water table between adjacent ditches are commonly used to assess the initial effectiveness of peatland drainage projects for tree growth improvement. However, these criteria do not reflect the soil conditions that regulate tree growth and performance after drainage. The effect of three drainage ditch spacings (30, 40, and 50 m) on the spatial variability of soil water conditions at three drained boreal Alberta peatlands was studied. Soil water content (0–30 cm depth) was found to be insensitive (p = 0.686) to drainage ditch spacing several years after drainage. Greater variation was observed between different sampling locations (p = 0.024) relative to the position of the ditch edge within similar ditch spacings. Spatial distribution of soil water in the unsaturated zone was found to be inversely related to the degree of water table lowering. Spatial patterns of soil water content were strongly associated with changes to substrate bulk density resulting from post-drainage peatland subsidence.
A drainage and fertilization experiment was established in 1989 in a tamarack (Larixlaricina (Du Roi) K. Koch) and black spruce (Piceamariana (Mill.) B.S.P.) mixed natural stand growing on a minerotrophic peatland. The drained site was ditched in fall 1987. Fertilization treatments of N–P–K at 0:0:0, 0:80:120, 200:80:120, and 400:80:120 kg•ha−1 (N as urea) were applied in late May 1989 Tamarack short-shoot needles and black spruce current-year needles, sampled on 31 August 1989 and 2 September 1990, were analysed for unit needle mass and N, P, and K. Drainage without fertilization increased foliar N, P, and K concentration of tamarack and black spruce, with larger responses occurring in tamarack. However, drainage decreased needle mass of unfertilized black spruce, although net assimilation rate increased, suggesting that an increased proportion of photosynthate was allocated to root development. Needle mass of tamarack was not affected in 1989 but was increased in 1990 by drainage, indicating different acclimation mechanisms to drainage by the two species. Responses in unit needle mass and foliar N and P concentration of both species to P–K alone, and (or) N–P–K fertilization, were higher on the drained than on the undrained site. Increased availability and uptake of N and P with a lowered water table were a likely consequence of increased near-surface soil temperature, improved soil water–soil aeration relations, and accelerated N and P mineralization and uptake rates. Foliar N, P, and K of tamarack were more responsive to fertilization than those of black spruce. N–P–K elevated foliar N concentrations of tamarack and black spruce and resulted in concomitant increases in needle mass, suggesting that N was limiting both species. Addition of P–K alone increased foliar P concentration and foliar mass of tamarack, indicating a P limitation to tamarack growth.
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