Ground-based skidder traffic changes chemical soil properties in a mountainous Oriental beech (Fagus orientalis Lipsky) forest in Iran Abolfazl Jaafari a , Akbar Najafi a,⇑ , Eric K. Zenner b a Department of Forestry, Faculty of Natural Resources, Tarbiat Modares University, Iran b School of Forest Resources, The Pennsylvania State University, Forest Resources Building, University Park, PA 16802, USA Received 5 February 2014; received in revised form 27 April 2014; accepted 8 June 2014 Abstract Increased soil compaction following harvest traffic is an inroad for adverse changes in soil biological properties and processes, soil microbial activity, bacterial communities, and growth and development of plant roots. This study investigated the impacts of three levels of traffic intensity (3–6, 7–14, and 15+ passes) of a Timberjack 450C skidder on changes in several chemical soil properties at two levels of skid trail slope gradients (gentle =<20% and steep >20% inclination) in the Hyrcanian forest. Skidding increased soil bulk density between 19% and 39% and, averaged over both slope gradients, reduced amounts of soil organic carbon (33–67%), concentrations of nitrogen (51–80%), phosphorous (0–17%), potassium (11–36%), and hydrogen ions (78–98%) compared to undisturbed areas. Most soil damage occurred after a few skidder passes, particularly on steep slopes that generally experienced the highest level of soil deterioration. The primary mechanism that induced immediate chemical soil changes was an uplift and exposure of deeper soil layers in response to compression and displacement of the former soil surface. Skidding can jeopardize the sustainability of forest ecosystems by creating unfavorable changes in soil characteristics and nutrient status. Ó 2014 ISTVS. Published by Elsevier Ltd. All rights reserved. Keywords: Ground-based operations; Skidding; Soil disturbance; Timberjack 450C skidder; Hyrcanian forest 1. Introduction Forest soils are an important component of the natural environment and a primary medium for many biological activities for flora and fauna [1,2]. Answers to questions about the sustainability of forest operations often depend on the impacts of forest management activities on soil properties [3]. In timber harvesting, there is a growing interest in improving the efficiency of logging machines by increasing their size, power and load. Owing to their heavy weight and vibration, skidder traffic modifies impor- tant soil structural properties and leads to physical soil disturbances in forest ecosystems [4]. Increased soil com- paction that increases bulk density [5,6] and resistance to penetration [7], reduces mineral and organic matter and porosity [4], and increases soil temperatures and their diur- nal fluctuations [8]. Soil compaction is only the first of many adverse impacts to the soil profile caused by traffic of heavy machinery [9], however. Soil compaction is an inroad for adverse changes in ground flora [10], soil biolog- ical properties and processes, soil microbial activity, bacte- rial communities, and growth and development of plant roots [11]. Finally, increased soil compaction may lead to the loss of soil nutrients and thus an overall long-term reduction of soil fertility and site productivity [8,12]. Bulk density increases above a threshold of 15% are considered http://dx.doi.org/10.1016/j.jterra.2014.06.001 0022-4898/Ó 2014 ISTVS. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +98 1226253101; fax: +98 1226253499. E-mail address: a.najafi@modares.ac.ir (A. Najafi). www.elsevier.com/locate/jterra Available online at www.sciencedirect.com ScienceDirect Journal of Terramechanics 55 (2014) 39–46 Journal of Terramechanics