Topic 8.3: Harvesting Schedules

The environmental impacts of different logging methods vary greatly with the season during which they are carried out and with the specific conditions at the time of logging.  In particular, forest phenology and soil moisture content contribute to determining whether logging helps towards achieving the goal of sustainable forest management or simply results in damage.

Not surprisingly, felling trees before they release their seeds can result in regeneration failures that are expensive to rectify.  Unfortunately, where fruiting of commercial trees happens late in the dry season when logging operations are carried out, this is exactly what happens. This problem is not readily solved because waiting until seeds are dispersed before felling might require unacceptable curtailment of the harvesting period.  Trees that produce substantial fruit crops at intervals of more than one year (=mast-fruiting species) are even more problematic unless it is feasible to delay harvesting until after a mast year.

Weather conditions during logging influence greatly the consequent environmental damage mostly because soil strength decreases rapidly with increasing moisture content.  One way engineers describe soil strength is on the basis of measures of force needed to push a metal cone of known dimensions through the soil.  Soil strength determines trafficability, that is, the resistance of soil to deformation under the wheels or tracks of vehicles.  Anyone who has been stuck in the mud understands the potential consequences of driving across wet soil but, unfortunately, logging operations often continue regardless of soil moisture content.  To improve traction in areas over which bulldozers have passed, breaking the matrix of fine roots near the surface in the process, an all-too-common practice is blading off the surface soil after every pass.  The resultant skid trails, dug down to subsurface soil, may facilitate bulldozer passage, but are extremely difficult to drain during closure operations.  If the “box cut” skid trail is at all sloped, the result is gully erosion that may stop only when bedrock is reached.

Water reduces the mechanical integrity of soil and makes it more prone to erosion. Where the soil is saturated and infiltration rates are thereby reduced, additional water must either pond or flow on the surface.  Due to the abundance of freely draining macropores of several millimeters in diameter in many uncompacted forest soils, internal drainage continues even in wet soils.  Surface crusts of algae and litter layer removal that exposes mineral soil to the direct impact of raindrops, as well as compaction due to vehicular traffic, can effectively seal the soil surface and made overland flow and erosion likely.

Some clay-rich soils, particularly those dominated by montmorillonite, illite, and other clay minerals that shrink and swell with changes in water content, can have extremely low trafficablility when wet.  On such soils (e.g., vertisols), ground-based yarding operations should be carried out only during dry weather, or aerial extraction methods employed.  Fortunately, logging on such soils when they are wet often makes little sense either ecologically or economically; recovering bemired vehicles is time consuming and expensive.