Tree growth and nutrient cycling in dense plantings of hybrid poplar and black alder

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Abstract
The cycling of N, P, K, Ca, and Mg was quantified during the third growing season in plantings (33 × 33 cm) of black alder (Alnusglutinosa (L.) Gaertn.) and hybrid poplar cv. Roxbury (Populusnigra L. × Populustrichocarpa Torr and Gray). First-year nutrient uptake, tree growth, and nutrient status of poplar were also assessed. During the 1st year, height and above-ground biomass growth of poplar were positively correlated with the proportion of alder in a plot. Poplar was twice as tall as alder in a mixed treatment and produced three times the aboveground biomass of alder in mixed plantings. On an individual tree basis, 1st year soil N uptake of alder averaged 46% of poplar N uptake. First-year winter dieback of poplar in this study prevented alder from being completely shaded by the poplars. Biomass growth and N status of poplar in the 2nd year were improved in mixed culture. After 3 years, accumulation of N and P in trees increased with the proportion of alder in a plot (maxima of 219 and 21 kg ha−1, respectively), but the greatest accumulations of N derived from the soil and K, Ca, and Mg were in mixed plantings (140, 88, 69, and 22.4 kg ha−1). Except for P, soil nutrient uptake during the third growing season was highest in plots with one alder for two poplars (maxima of 108, 9.1, 50, 60, and 19 kg ha−1 for N, P, K, Ca, and Mg). Throughfall nutrient content was not affected by species mixture. Except for Ca, nutrient content of total leaf litter increased with the proportion of alder (maxima of 80, 3.1, 13, 35, and 6.9 kg ha−1 for N, P, K, Ca, and Mg). After 3 years, no accretion of total N was detected in the soil, but exchangeable K increased 93% in the top 5 cm. Because stimulatory effect of interplanted alder on poplar growth decreased with time, reduced competition for soil N and light from the smaller alder during the first growing season were considered the most important factors in increasing individual poplar growth in our plantation.