The problem of soil compaction in forestry differs from that in agriculture because of differences in the nature of the crop, in particular the weight and size of the plant members and the length of time that they persist. The roots compact the soil as they increase in size, but they also transmit the weight of the tree and forces generated by the wind onto the soil. There are important differences in management practices; in forestry modern harvesting machines apply heavy loads and, for reasons of cost, tend to be kept in operation throughout the year. As a consequence the structure of the soil suffers some damage, often manifested as compaction. Compaction arising from such sources may reduce the growth of the current trees or trees subsequently planted on the site. But it is difficult to predict the extent of such reduction, if any, because of the complex of interactions involved. Important factors concerned, namely, the soil water regime and the organic matter content, are emphasized. A conceptual model is proposed as a predictive tool. The mechanics of soil compaction, the effects of compaction on the physical properties of the soil, and techniques for the prevention and amelioration of compaction of forest soils, are discussed.
Competition for water and nutrients between trees and other vegetation is discussed using examples from the interactions between tree and weeds in production forests, and trees and pasture in agroforestry systems. Production and economic viability of plantation forests are dependent upon sound weed management practices. Competition for water and nutrients by a plant is registered as a water deficit or a nutrient deficit. Plant responses to competition are similar to those for coping with water and nutrient deficiency in the soil. One species may have a competitive advantage over another for water and nutrients by (i) acquiring a greater proportion of available soil water and (or) nutrients, (ii) using water and nutrients more efficiently in producing biomass, and (or) (iii) allocating assimilate in ways that maximize survival and growth. The benefits from managing weeds during establishment of a stand have been demonstrated, but the value of managing understory in older stands is unclear. It is not possible to have water stress (through competition) without some degree of nutrient stress, but the opposite may not be the case in some environments. Managing competition effectively requires a clearer understanding of the dynamics of water × nutrient interactions, as well as the dynamics of the interactions between trees and associated vegetation and how this is modified by silviculture. Experiments where variables are well controlled and supported by simultaneous and regular measurements of both water and nutrients are required.
The mechanical strength of sandy soils under radiata pine plantations was measured with a penetrometer. Resistance to penetration was largely independent of water content in the range sampled, and was directly related to the bulk density of the soil. Soil strength at constant bulk density increased with depth owing to increase in overburden pressure and to a decrease in soil organic matter. Soil under native scrub was consistently less compact than that from adjacent radiata pine plantations on the same soil type. Soil from pasture was usually more compact in the surface 20 cm of soil than that from pine plantations, but was less compact at depth. Soil from second rotation plantations was more compact than soil on some first rotation sites, but on other sites no differences could be established. Radiata pine roots preferentially penetrated areas of lower soil strength. Root penetration was severely restricted above a critical penetration resistance of about 3000 kPa. Saturated soils were highly compacted even by light loads in a laboratory consolidometer compared to unsaturated soil. In the unsaturated condition compaction was greatest under heavy loads on soils at about 1% organic matter. Causes of the observed compaction in the field are discussed and remedial measures are suggested. Soil compaction reduced porosity but had little effect on water storage capacity. Increased organic matter at constant bulk density also reduced porosity, but greatly increased water storage capacity and unsaturated hydraulic conductivity. The importance of organic matter in maintaining a favourable structure in sandy soils and its relation to maintenance of site productivity is discussed.
The interactions of the 4 basic soil physical properties—volumetric water content, matric potential, soil strength, and air-filled porosity—were investigated over a range of contrasting textures and for 3 compaction levels of 4 forest soils in New Zealand, using linear and non-linear regression methods. Relationships among these properties depended on texture and bulk density. Soil compaction increased volumetric water contents at field capacity, at wilting point, and at the water contents associated with restraining soil strength values, but decreased the water content when air-filled porosity was limiting. The integrated effect of matric potential, air-filled porosity, and soil strength on plant growth was described by the single parameter, least limiting water range (LLWR). LLWR defines a range in soil water content within which plant growth is least likely to be limited by the availability of water and air in soil and the soil strength. Soil compaction narrowed or decreased LLWR in most cases. In coarse sandy soil, initial compaction increased LLWR, but further compaction decreased LLWR. LLWR is sensitive to variations in forest management practices and is a potential indicator of soil physical condition for sustainable forest management.
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