Early nitrogen fertilization effects on 13 years of growth of 4 hybrid poplars in Saskatchewan, Canada

Published literature shows contrasting effects of fertilization in forest plantations. The objective of this study was to determine the effects of early Nitrogen (N) fertilization on the growth of four hybrid poplar clones (Katepwa, Hill, Okanese, Walker) at two contrasting sites in the Boreal Plains ecozone of Canada. Using ammonium nitrate fertilizer, three N fertilizer rates were applied annually for the first three growing seasons, 0 kg ha−1 of N (0N, control), 150 kg ha−1 of N (150 N) and 300 kg ha−1 of N (300 N). Tree growth measurements, soil nutrients, and foliar nutrients data were analyzed. Fertilization resulted in tree growth remaining unchanged, or decreasing significantly for some of the poplar clones, compared to the control. Mean stand volume (300 N plots across the four clones) at age 3 years was significantly lower than the control. However, there were no significant fertilization effects on stand volume at age 8 and 13 years, suggesting a short-term effect of fertilization. Stand volume yields (0N) at age 13 years were 104-128 (Katepwa), 58-83 (Hill), 57-82 (Okanese), and 56-85 m3 ha−1 (Walker). On average, stand survival was >90% for the Katepwa, Okanese, and Hill clones, but was much lower for the Walker clone, ranging from 44 to 56%. Fertilization resulted in significant increases of soil exchangeable NO3-N, but there were no significant changes in soil exchangeable NH4-N, extractable Phosphorus (P), or pH. A foliar nutrient vector analysis showed that fertilization resulted in excess amounts of all major foliar macronutrients (Ca, Mg, N, P, K), compared to the control, which could have a possible toxic or antagonistic effect on tree growth. Fertilization also had an effect on micronutrient uptake, by nearly doubling the foliar Manganese (Mn) concentrations at both sites, averaging 387 (Alfalfa site) and 127 mg kg−1 (Pasture site). The increased foliar Mn had a toxic effect on tree growth (Alfalfa) for three of the clones, Okanese and Walker being the most affected. The top yielding poplar clone was Katepwa (8.00-9.84 m3 ha−1 year−1, age 13 years), which was not affected by the increased Mn uptake, regardless of soil pH levels. This study showed that hybrid poplar clones can be used for successful plantation establishment in the Boreal Plains without the need for any fertilization, which is very promising for the forest products industry of Canada.