Effects of soil compaction on the growth and mortality of planted dipterocarp seedlings in a logged-over tropical rainforest in Sarawak, Malaysia

Abstract

Soil compaction is major determinant of plant growth and/or mortality. Tropical rainforests in Southeast Asia have been degraded by commercial logging, and the use of heavy equipment in these operations has increased soil compaction. We conducted enrichment planting 20years after logging in a tropical forest in Borneo to evaluate the effects of soil compaction on the growth and mortality of planted dipterocarp seedlings over seven years. We planted the seedlings in sites that had been impacted to varying degrees, including a skid trail and undisturbed areas. We measured soil nutrients, moisture content, bulk density, penetration resistance, and light conditions. Seedling mortality, height, and shoot diameter were also determined 0, 12, 24, and 81months after planting. Elongation rates of tap and lateral roots were measured 24months after planting. We also excavated seedlings 81months after planting, and compared heights, shoot diameters, tap root length, and lateral root length between compacted and undisturbed soils. Bulk density varied in the range of 0.98–1.61gcm−3 and correlated with soil penetration resistance. Surface soil (0–20cm depth) in the compacted area had two to three times more resistance to penetration than did undisturbed soils. Surface soil penetration resistance significantly increased seedling mortality during the early planting period (0–12months), but mortality in the later periods (12–24 and 24–81months) did not relate to the soil penetration resistance. The lateral root growth rate in the early planting period (0–24months) was also significantly inhibited by penetration resistance, but tap root growth was not. Penetration resistance did not reduce the seedling growth rate in the periods 0–12 and 12–24months after planting, and higher soil moisture promoted seedling shoot diameter growth in the same time periods. Lateral root elongation did not differ between compacted and undisturbed forest soils 81months after planting, but tap root elongation was inhibited in the compacted area at that time. Our results suggest that soil compaction negatively affect root elongation and initial seedling survival of planted dipterocarp trees even in 20years after logging operation in Bornean tropical rainforest.