Nutrient cycling in hybrid poplar stands in Saskatchewan : implications for long-term productivity
Intensive management of short rotation hybrid poplar (HP) plantations on agriculture land has demonstrated good early yields and promise as an alternative crop for farmers selling fibre to the forest industry. However, multiple rotations of HP may impact the future productivity of plantations through nutrient removals. The objectives, therefore, of this study were to determine the nutrient stores and fluxes for two HP plantations with differing site quality, fertilizer applications and past land management practices and to construct a 20-year nutrient budget to examine impacts of harvesting short rotation HP on long-term productivity.Heights and biomass were measured by harvesting above- and below-ground and separating biomass into tree components; measurement of atmospheric deposition, mineral weathering, litterfall, litter decomposition, and leaching for HP plantations on an Alfalfa (HPA) and Pasture (HPP) sites in 2004-05. The budget was developed by averaging fluxes over 2 years and scaling up to a 20-year rotation. Unfertilized treatments in the HPA plantation showed greater tree growth than all other treatments. Fertilized and unfertilized treatments had greater biomass production and nutrient pools than treatments at the HPP plantation. The fertilizer treatments did not affect on biomass production and nutrient accumulation.Nutrient additions to the HPA were greater than the HPP plantations for leaf litterfall and leaching. Nutrient resorption from senescing leaves was greater at the HPP plantation suggesting that nutrient pools were smaller and that trees responded by keeping nutrients in the biomass. Fertilization at both plantations increased nutrient flow for inputs and outputs in 2004-05. Water leachate and leaf litterfall showed increased nutrient contents in fertilized treatments at both plantations.A high fertility plantation that used fertilizer and practiced whole-tree harvesting exported more nutrients (and fibre) than a plantation with marginal site quality practicing stem-only harvesting. Time to replenish nutrients from atmospheric deposition and mineral weathering would range from 6 to 50 years for Ca and N, respectively, suggesting that subsequent plantations would require fertilizers to replenish soil nutrient reserves.While HP plantations in Saskatchewan can produce high yields, they require large nutrient inputs and are inefficient (sequester a large amount) in nutrient use. High site quality is important to obtain high yields but conservational techniques, such as stem-only harvesting, are important in maintaining site quality over the long-term.
Long-term productivity, Hybrid poplar
Master of Science (M.Sc.)