Planting Density and Site Effects on Stem Dimensions, Stand Productivity, Biomass Partitioning, Carbon Stocks and Soil Nutrient Supply in Hybrid Poplar Plantations

Benoit Truax,France Lambert,Julien Fortier,Daniel Gagnon

doi:10.3390/f9060293

Benoit Truax, France Lambert + Show 2 more

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https://doi.org/10.3390/f9060293

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Abstract

In this study, planting density and site effects on hybrid poplar productivity and stem dimensions were evaluated on the mid-term and longer term (8 and 14 years) in southern Québec (Canada). We also evaluated the effects of planting density and site on biomass accumulation and carbon stocks in different plantation compartments, on biomass partitioning at the stand-level, on soil carbon stocks and on soil nutrient supply rate after 14 years. The experimental design consisted of three replicate poplar stands located along a site fertility gradient. Each stand contained six planting densities (ranging from 494 to 1975 trees/ha) and a single genotype (Populus canadensis × P. maximowiczii hybrid). Planting density had a large effect on stem dimensions, but a minor effect on stand volume, aboveground woody biomass production, and aboveground biomass carbon stocks. Site selection and tree survival were more important factors affecting these variables. At all sites, and independent of planting density, mean annual volume increments were also higher after 14 vs. 8 years. On fertile sites, strong correlations between area per tree at planting and biomass partitioning, carbon allocation belowground, soil nutrient supply rate and soil carbon stocks were observed. Aboveground, higher competition for light with increasing planting density resulted in an increase in the stem to branch ratio. Belowground, higher competition for soil resources with increasing planting density reduced soil macronutrient availability (except for potassium), which likely stimulated carbon allocation belowground and carbon accumulation in the soil. Over the longer-term, higher density plantations of poplars could provide greater benefits in terms of carbon storage belowground (soil and roots).

Highlights

In temperate ecosystems, fast-growing poplars and their hybrids (Populus × spp.) are among the most widely used species for afforestation and agroforestry [1,2,3]
Larger trees were produced at the fertile sites of Brompton and Mégantic, while lower planting density treatments produced trees with greater diameter at breast height (DBH) and stem volume
Planting density treatments significantly affected the gain in DBH or in stem volume between the end of the 8th and 14th growing seasons, with greater gains being observed at lower planting density (Table 3)

Summary

Introduction

Fast-growing poplars and their hybrids (Populus × spp.) are among the most widely used species for afforestation and agroforestry [1,2,3]. Marginal agricultural land are often characterized by sub-optimal growth conditions [9], and appropriate management decisions and silvicultural practices are needed to maximize stand productivity. These include intensive site preparation, vegetation management, irrigation, fertilization and the selection of genotypes (clones) adapted to local environmental conditions [10]. The selection of planting density has important economic consequences because it influences the rotation length, and plantation establishment costs, weed control strategies, and the dimensions of stems at harvest [12]

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