Humus in northern forests: friend or foe? Cindy E. Prescott a,* , Doug G. Maynard b , Raija Laiho c a Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 b Canadian Forest Service, 506 West Burnside Road, Victoria, BC, Canada c Department of Forest Ecology, University of Helsinki, P.O.B. 24, 00014 Helsinki University, Helsinki, Finland Accepted 6 October 1999 Abstract Organic matter is of primary importance to the sustainability of long-term site productivity in forest ecosystems. In boreal forests, organic matter accumulates at the surface as mor humus. This may represent a substantial portion of the total nutrient capital of a site, and its decomposition is essential for the short-term availability of nutrients for tree growth and long-term site fertility. However, organic matter accumulation at the soil surface can also effect the forest ecosystem by immobilizing nutrients making them unavailable for plant uptake, and by creating physical and environmental conditions that can impede seedling establishment and survival. Therefore, it is necessary to understand the processes of humus formation and decomposition in order to manage these soils in a manner that will maintain or improve site productivity. This paper provides an overview of (i) the composition of humus, (ii) the conditions in the boreal forest that result in the surface accumulation of humus, (iii) decomposition processes and (iv) the effects of humus on nutrient (especially nitrogen) availability. Questions relating to the detrimental role of surface organic matter accumulation, the effects of natural disturbances (e.g., ®re) and harvesting disturbances on humus loss and accumulation and management practices that can maintain long-term site productivity will also be discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Humus; Decomposition; Disturbance; Peat; Harvesting; Site preparation 1. Introduction A notable characteristic of northern forest soils is surface accumulations of organic matter, often referred to as humus. This material consists mostly of plant remains, either in relatively fresh forms or partly decomposed, as well as by-products of decom- position. Although the importance of humus to the nutrition of northern forests has been recognized for at least a century, management of this resource has been impeded by con¯icting ideas about its role in site nutrition. On one hand, humus is a key source of nutrients, and also contributes to moisture retention and soil structure. A substantial proportion of nutrient capital of the site may be located in humus, especially nitrogen. This nutrient reserve is critical to long-term site fertility, and helps to buffer the site against dis- turbances that might lead to nutrient depletion. Humus may also be viewed as a nutrient sink that essentially competes with trees for this growth-limiting resource. Especially in the case of surface accumulations, the progressive immobilization of nutrients into humus may over time deplete the supply of available nutrients Forest Ecology and Management 133 (2000) 23±36 * Corresponding author. Tel.: 1-604-822-4701; fax: 1-604-822-9102. E-mail address: cpres@interchg.ubc.ca (C.E. Prescott) 0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0378-1127(99)00295-9