Nutrient Management
A second possible consequence of intensive management and whole-tree harvesting includes nutrient depletion. Factors that affect nutrient depletion include crop species nutrient demand and composition, orginal site condition, harvesting operations, and age of the stand. Site-specific assessments are required to evaluate the consequences of management system design on site nutrient capital and availability. However, the following generalizations appear applicable to most sites:
- Deciduous trees contain more nutrients in biomass than conifers, but can be harvested without leaves by either harvesting during winter or leaving felled trees on-site until foliage drops off.
- Younger stands harvested using the whole-tree method are more likely to have site nutrients depleted, due to nutrient accumulation patterns which result in a greater percentage of nutrients in crowns than boles of young trees.
- Stem-only extraction removes fewer nutrients than whole-tree harvesting.
- Soil tillage increases the rate of soil organic matter decomposition and mineralization.
- Harvesting and site preparation desynchronize nutrient supply and demand. Nutrient availability is very high immediately after harvesting and site preparation operations, but tree uptake demand is typically low and less than available nutrient supply until crown closure is achieved in young plantations.
- Site preparation that removes harvesting residues associated with branches and foliage reduces the size of active nutrient pools.
The consequences of these practices on site fertility and stand productivity vary by site.
Several comprehensive reviews of the literature on the long-term effects of intensive forest management of site productivity, including harvest machine traffic, site preparation operations, and intensified biomass utilization have been published (Leaf 1979; Morris and Miller 1994; Burger 2002; Powers and others 2005; papers following Vance and Sanchez 2006). These reviews have reported that, to date, few long-term studies have supported the hypothesis that soil productivity would decline with prolonged periods of intensive harvesting practices. However, trials conducted on sites with previously diagnosed nutrient deficiences (i.e. N and P) indicate that plantation productivity in succeeding rotations will decline if excessive organic matter removals (i.e. forest floor, foliage and tops of harvested trees) result in reduced nutrient availability to the developing crop. For example, loblolly pine productivity has been reduced by whole-tree harvesting on phosphorous deficient sites in the South that previously required fertilizer additions to maintain acceptable growth rates in the rotation before whole-tree harvesting (Scott and others 2004). In New Zealand, removal of the forest floor and whole-tree harvesting reduced productivity in the second rotation of radiata pine (Pinus radiata) planted on recent coastal sand dunes (Smith and others 2000).
Forest managers and landowners should practice site-specific nutrient management. This approach will allow identification of the relative level of risk associated with intensive management, and ensure that any proposed management practices and utilization levels are sustainable. Managing nutrient availability to trees may require fertilization (at right) of soils with deficient nutrients, just as done by farmers to maintain and improve the fertility of agricultural production systems.
Fertilizer (at right), wastewater biosolids (below left), and wood ash (below right) can be added to nutrient deficient forest soils to replenish nutrients removed during intensive harvesting operations.
A balanced nutrient management regime that includes nutrient replenishment, monitoring, and selection of appropriate harvesting and site preparation techniques (as described by Peter Clinton and others with the New Zealand Forest Research Institute, at right) will ensure the long-term sustainability of forest soil productivity.
Encyclopedia ID: p1277
