Cumulative management impacts on soil physical properties and early growth of Pinus radiata

Abstract

Effects of Pinus radiata D. Don plantation management on soil physical properties and second rotation (2R) tree growth were quantified for a site considered by managers as resistant to compaction. Measurements commenced prior to clearfall and were repeated following harvest and again at 2 and 4 years after second rotation establishment. Effects of earlier thinning on soil physical properties detected prior to clearfall were found to influence the degree of soil change caused during clearfall. Bulk density and soil strength measurements showed that harvesting caused light to moderate compaction of almost 80% of the study area. Moderate compaction over 25% of the area was associated with visible rutting (RUTS class) whilst a lower level of compaction occurred over 54% of the area where visual evidence was minimal (LOW class). Subsoiling of planting lines for second rotation establishment caused a major reduction in penetration soil strength to 0.45 m depth. This effect was still pronounced at 4 years of age. Outside subsoiled areas it was still possible to detect statistically significant compaction in the RUTS class at 2 years of age but not in the LOW class. By 4 years of age, compaction in the RUTS class had declined relative to the LOW class and was no longer detectable in comparison with pre-harvest values. Tree growth at age 2 years was not significantly different between the two dominant disturbance classes although there was slight evidence of a compaction effect. By age 4 years there were no differences in height or volume index between the two disturbance classes. A slash residue-piled-and-burnt site preparation treatment resulted in significantly greater tree volume index at age 2 years. This response was still apparent, but at a diminished level, by age 4 years. The response was assumed to be due to nutrient effects as soil physical properties were not significantly different between the two treatments. Although moderate compaction had occurred across the site, there was no effect on the growth of young P. radiata that would be of immediate concern for site productivity.