The importance of harvest residue and fertiliser on productivity of Pinus patula across various sites in their first, second and third rotations, at Usutu Swaziland

Abstract

Background: Concern is growing about the future of forestry productivity due to intensive nutrient removal, as a result of different harvesting operations. This study aimed to determine the effects on forest productivity when using different slash-retention scenarios with the recommended amounts of mineral fertiliser in Usutu forest. Usutu is a plantation forest that grows mostly softwood where the predominant species is Pinus patula Schiede ex Schltdl. & Cham. Methods: The first trial series (F) comprised of one trial located in each of five forest blocks. It was established in 1971 and compared the effects of various site preparation scenarios (slash removal, slash retention and cultivation) on the early growth of Pinus patula for adjacent first (F1R) and second rotation (F2R) sites (i.e. grassveld and clearfelled first-rotation sites). The second (S) trial series was re-established in April 1991 on exactly the same position as the first trial series, and involved second (S2R) and third rotation (S3R) sites. Three main treatments, standard pitting through harvest residue (control); manual pitting after removal of harvest residue and forest floor (cleared); and manual pitting and broadcast application of dolomitic lime (2 t ha− 1) over the slash (lime), were undertaken in factorial combination with the application of phosphate and potassium fertiliser. The data reported here are for tree volume productivity across the five sites of the first trial series up to age 10 years and three of the five sites of the second trial series up to age 9 years. Results: Slash removal decreased volume productivity by 9 and 13% in the F1R and F2R at 10 years of age and further by 21 and 33% in S2R and S3R, respectively at 9 years of age. However, fertiliser application increased volume productivity by 14 and 15% in the F1R and F2R at 10 years age and further by 18 and 10% in the S2R and S3R, respectively at 9 years of age. Conclusions: In order to sustain or increase productivity, it is recommended that harvest residue slash should be conserved and fertiliser containing phosphate and potassium be applied at planting at Usutu.