Response of Aspen Fine Roots to Soil Compaction on Two Soil Types in the Upper Great Lakes Region

Abstract

Soil compaction caused by timber harvesting can reduce regeneration and productivity in many forested situations. As part of the national Long-term Soil Productivity project (LTSP), this study examined the effects of specific soil compaction treatments on the fine-root ( < 2mm) biomass and nutritional status of aspen (Populus spp.) regeneration in the Upper Great Lakes Region. The three levels of soil compaction included no compaction (C0), light compaction (C1), and heavy compaction (C2). Unharvested areas served as controls (NH). The study sites were located at the Ottawa and Huron-Manistee National Forests in upper and lower Michigan, respectively. The soil at the Ottawa site is an Ewen clay (very fine, mixed Glossic Eutroboralf); at the Huron-Manistee site the soil is transitional between a Rubicon (sandy; mixed, frigid Entic Haplorthod) and a Grayling sand (mixed, frigid Typic Udipsamment). The LTSP study design also incorporates three levels of organic matter removal (merchantable bole removal, whole tree harvest, and whole tree harvest plus removal of the forest floor). This study examined the full array of soil compaction within the whole tree harvest level of organic matter removal. During the growing-season and fall of 1999, soil cores and foliage samples were collected from each treatment level. Aspen fine roots were separated from the fine roots of the other species based on external morphology. The aspen root identification was verified using random amplified polymorphic DNA analysis; 90% of the root fragments were correctly identified. During the growing season and the fall, over 60% of the aspen fine-root biomass was in the top 10-cm of soil. Growing season aspen fine-root biomass was significantly less (p < 0.05) in the compacted plots than in the unharvested plots for all sampling depths (NH= 1,129 g m^-3, C0 = 328 g m^-3, C1 = 650 g m^-3, C2 = 547 g m^-3; for the 0-10 cm depth). Growing season aspen fine-root biomass was significantly (p < 0.05) greater at the Huron site than on the Ottawa site for all sampling depths. Fall aspen fine-root biomass had a significant soil X treatment interaction. Soil compaction decreased the fine root biomass on the Ottawa site. However, soil compaction tended to increase aspen fine-root biomass on the Huron site. The fine-root data from the Ottawa site indicate that the effects of soil compaction were still noticeable seven years after its application. Significant differences (p <0.05) among the mean growing-season foliar nitrogen concentrations within the treatments were observed at the Ottawa site. This was not noticeable in the foliage collected just prior to senescence. The results indicate that any effects the compaction treatments have had on tree nutrition have begun to disappear.