Topic 9.3: Effects on Soils and Belowground

Logging causes a multitude of changes in the soil, particularly during ground-based yarding operations.  Both physical (e.g., porosity and infiltration rates) and chemical (e.g., nutrient availability and redox potential.) can be dramatically influenced.  Although the amount and types of impacts vary with the care with which logging operations are carried out, some changes are inevitable.

One of the principal impacts of logging on soils is compaction resulting from the passage of heavy vehicles.  Soils in most mature forests in the tropics are characterized by good internal drainage, which is why puddles are uncommon even after exceedingly heavy rain.  Continuous series of large pores penetrating from the soil surface down to the subsoil allow large volumes of water to pass through (causing little leaching of nutrients in the process).  These macropores are created by the activities of soil animals (e.g., earthworms and termites) and plant roots, along with purely physical processes (e.g., shrinking and swelling of 2:1 clay minerals with drying and wetting).  Unfortunately, this superb structure is easily disrupted by logging vehicles and may require decades to recover.

When a bulldozer or a skidder with pneumatic rubber tires drives over soil, compression occurs and many pores are sealed in the process.  If surface roots and the litter layer are intact prior to compaction, damage to soil structure is reduced. This damage is likewise lessened if the soil is dry. The presence of rocks on the surface also protects the soil from compaction (but can damage the under-carriages of the machines used for timber yarding).  Soil compaction is therefore most severe in wet soils with few rocks near the surface from which the liter layer and surface roots were removed.  In general, the first passes of heavy machinery do most of the damage; after the first 2-3 passes, little additional damage occurs unless the surface soil is scraped away exposing deeper and deeper materials.

In addition to decreasing water infiltration rates and thereby increasing surface flow and the likelihood of erosion, soil compaction stresses plants in several complimentary ways.  In soils compacted to bulk densities of >1.2-1.4 Mg/m^-3 (vs. 0.5>1.0 Mg/m^-3 when not compacted), extension growth of roots is often impeded.  Root growth rates can be further slowed by the anaerobic conditions (i.e., low redox potential) that often develop in compacted soils when they are wet.  Poor conditions for root growth in compacted soils often result in nutrient impoverishment and hence stunting.  Where surface soil is removed by bulldozers, thereby exposing the generally more nutrient-poor subsoil which is then compacted, nutrient-relations for colonizing plants can be extremely strained.

Poor growth of plants in compacted soil is part of the reason for the extremely slow rate at which natural porosity recovers on skid trails, log landings, and other soils over which heavy machinery has passed.  Unfortunately, recovery of soil structure (i.e., increases in porosity and decreases in bulk density) can requires several decades. This means that forest areas used as skid trails, log landings, and roads, are out of production for at least one harvest cycle, sometimes two. Plowing compacted soils may improve conditions for plant growth for a short time, but most damaged soils rapidly settle back into their compacted condition.  Encouraging plant growth by fertilizing, mulching the soil surface, or plowing in organic matter may all help somewhat, but the best solution is to avoid soil compaction in the first place so as to keep as much of the forest productive as possible.

Logging undoubtedly influences soil nutrient availability, but the effects are apparently subtle and difficult to detect.  By removing large trees that prior to logging used substantial quantities of nutrients, and by releasing the nutrients held by their leaves, branches, and roots, soil nutrient status should increase after logging.  Increased temperatures in the litter layer and in surface soils after logging should likewise result in release of a pulse of nutrients from decaying organic matter due to increased mineralization rates.  These expected changes may be difficult to detect in selectively logged forest because they are transient and any nutrients that become available are rapidly taken up by the residual stand.  In clearcut areas, in contrast, especially if the area is burned, herbicided or plowed after logging to prepare the site for planting, the pulse of nutrients can be large and some nutrients lost due to limited uptake.