Factors influencing shelter in the Woodhill protection strip. (2018)
AuthorsCurry, Mathewshow all
Woodhill Forest, situated on the west coast of the Auckland region, is directly exposed to strong westerly winds coming from the Tasman Sea. To protect the 41km long Pinus radiata forest, a protective strip of trees has remained unharvested between the coast and the remaining forest. The protection strip plays a valuable role in sheltering the forest against strong, salt-laden winds. The aim of this research was to investigate the shelter provided to the production forest by the protection strip.
The protection strip was mapped using aerial imagery. A site was selected where measurement of the protection strip and the production stand immediately adjacent could be undertaken. Protection strip height, basal area, width, health, and crown length were measured at three plots along 24 transects. Production stand volume was measured at three plots at even intervals along the same transects. Regression analysis was used to assess the relationship between protection strip variables and production stand volume at 20m, 90m, and 170m from the protection strip.
The analysis showed that height (adjusted for the elevation difference between the protection strip and the stand) (r=0.56), and the health (r=0.41) of the inland edge of the protection strip were significantly correlated with volume within the first 20m of the stand. The two variables were multiplied together to create a shelter variable which had a significant correlation with stand volume at 20m (r=0.65), and at 90m (r=0.42) away from the strip. When confounding effects of site influence were controlled, the shelter was still deemed to have a significant relationship with stand volume, providing confidence that the relationships detected were not only the result of underlying site conditions. The shelter variable was significantly correlated with distance to the coast (r=0.50).
Despite the relationship between the shelter variable and distance to the coast, the protection strip width did not show a significant relationship with stand volume. It can be concluded that the protection strip can provide sufficient shelter when as narrow as 280m wide, the minimum width tested during this investigation.
These results provide an indication as to how to assess the quality of the protection strip and the key factors to consider when the protection strip is maintained or replaced in the future. They also provide some indication on how to improve the protection strip to achieve greater forest productivity.