Spatial comparisons of productivity and carbon sequestration for Cupressus lusitanica and macrocarpa within New Zealand

Abstract

Cupressus lusitanica and C. macrocarpa are two fast growing conifer species with valuable timber that have been widely established throughout New Zealand. Despite the importance of these two species within New Zealand and in many other countries, little research has investigated how volume, height, and carbon productivity are influenced by environment. Volume growth of these two cypress spp. has been recently described using the 300 Index which quantifies the mean annual volume increment at age 30 yrs for a stand density of 300 stems ha−1. Using New Zealand growth data obtained from 290 plots, the objectives of this study were to (i) develop models and fine resolution national maps of 300 Index and site index, (ii) use these two productivity metrics and allometric equations to develop models of carbon stocks for the two species, and (iii) spatially predict carbon across New Zealand for C. lusitanica and macrocarpa grown under a factorial range of stand densities (400, 650 and 900 stems ha−1) and ages (30, 40 and 50 yrs).Plot measurements showed wide variation in 300 Index which reached 26.9 and 29.0 m3 ha−1 yr−1, respectively, for C. lusitanica and C. macrocarpa. Site index (age 30 yrs) reached values of 31.9 and 34.1 m for C. lusitanica and C. macrocarpa, respectively. Models developed for C. lusitanica had high accuracy for both 300 Index (RMSE = 2.48 m3 ha−1 yr−1; R2 = 0.75) and site index (RMSE = 1.78 m; R2 = 0.84) while models developed for C. macrocarpa had similar, but slightly lower, accuracy for 300 Index (RMSE = 2.86 m3 ha−1 yr−1; R2 = 0.72) and site index (RMSE = 1.86 m; R2 = 0.81).Spatial predictions of the two productivity metrics showed distinct differences between species with C. lusitanica being more sensitive to cold, dry, and wet conditions than C. macrocarpa. Under the mid-range scenario (age 40, 650 stems ha−1) mean carbon sequestration in above and below ground biomass, dead wood, and litter pools for C. macrocarpa was higher than C. lusitanica in both the North Island (321 vs. 296 t C ha−1) and South Island (222 vs. 120 t C ha−1). Carbon was higher for C. macrocarpa than C. lusitanica within, respectively, 62 and 94% of the North and South Islands under the mid-range scenario. Carbon markedly increased with stand density and age and gains were greatest between ages 30 and 40 and from the low to moderate stand density for both species.