Mature Hybrid Poplar Riparian Buffers along Farm Streams Produce High Yields in Response to Soil Fertility Assessed Using Three Methods

This study had three main objectives: (1) to evaluate the aboveground biomass and volume yield of three unrelated hybrid poplar clones in 9 year-old riparian buffer strips located on four farms of southern Québec, Canada; (2) to compare yield data at 9 years with previous data (at 6 years); (3) to evaluate how soil fertility, measured using three different soil testing methods (soil nutrient stocks, soil nutrient concentrations, soil nutrient supply rates), is related to yield. Across the four sites, hybrid poplar productivity after 9 years ranged from 116 to 450 m³ha⁻¹, for stem wood volume, and from 51 to 193 megagrams per hectare (Mg ha⁻¹), for woody dry biomass. High volume and woody dry biomass yields (26.3 to 49.9 m³ha⁻¹yr^-1, and 11.4 to 21.4 Mg ha⁻¹yr^-1) were observed at the three most productive sites. From year 6 to 9, relatively high yield increases (8.9−15.1 m³ha⁻¹yr⁻¹) were observed at all sites, but the productivity gap between the less fertile site and the three other sites was widened. Clone MxB-915311 was the most productive across the four sites, while clone DxN-3570 was the least productive. However, at the most productive site, clone MxB-915311 experienced severe stem and branch breakages. Independently of the soil testing method used, available soil P was always the first soil factor explaining volume yield.