Nutrient fluxes in rainfall, throughfall and stemflow in Eucalyptus stands on the Zululand coastal plain, South Africa

Abstract

Atmospheric deposition was assessed at two sites over a four-year period in post-canopy-closure (mature) Eucalyptus stands in the northern Dukuduku and southern KwaMbonambi commercial plantation forestry areas of Zululand, South Africa. The aim of this study was to determine the magnitude and relevance of nutrient addition with rainfall, throughfall and stemflow to commercial forestry in this region. Canopy cation exchange was used with rainfall and canopy drainage to derive wet, dry and total atmospheric deposition. Nutrient concentrations measured in the rainfall, throughfall and stemflow varied widely throughout the study period, and between sources and sites. Rainfall was slightly acidic at both sides, but became less acidic upon passing through the tree canopies. Canopy exchange and collection of dry deposition resulted in increased cation concentrations under the tree canopy, while the canopy generally absorbed nitrogen (N), from the rainfall, reducing the below canopy concentrations. Atmospheric deposition was shown to be responsible for large quantities of nutrients added to the eucalypt stands at each site. Annualised deposition averaged across all years at each site amounted to N, calcium, magnesium, and potassium (K) fluxes of 11.0, 6.0, 2.7 and 10.2 kg ha⁻¹ y⁻¹ at Dukuduku and 10, 10.6, 7.5 and 18.8 kg ha⁻¹ y⁻¹ at KwaMbonambi, respectively. Organic N fluxes contributed a further 8.1 kg ha⁻¹ y⁻¹ at Dukuduku and 7.1 kg ha⁻¹ y⁻¹ at KwaMbonambi to the total N deposition. Although K deposition values were high, additions of all other nutrients, although also high, were within the ranges reported in local and international research. Over the course of a full rotation, the atmospheric deposition levels recorded at these sites may have potential to supply a large proportion of the nutrients that are lost during stem-wood harvesting. This study adds value to understanding of nutritional sustainability of fast-growing plantation forests, demonstrating the importance of atmospheric deposition as a nutrient addition source to plantation-grown eucalypts along the Zululand coastal plain.

Keywords: atmospheric deposition, canopy exchange, clonal Eucalyptus, dry deposition, wet deposition

Southern Forests 2011, 73(3&4): 193–206