Kauri - response to thinning of second-growth stands

Authors: Greg Steward
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2019, Issue N.Z.J.For. 64(4) 2020, pp 27-34, Jan 2020
Publisher: New Zealand Institute of Forestry

Abstract: Six thinning trials established in natural secondgrowth kauri (Agathis australis) forest were analysed to compare the effects of thinning with un-thinned controls, and to develop models of growth and productivity for this forest type. Thinning resulted in statistically significantly greater diameter increment compared to the un-thinned controls within each site. Diameter increments of 0.25 to 0.37 cm/yr were recorded in thinned treatments, while basal area increased by almost 90%. Thinning also reduced the variance in diameter mean annual increment (MAI) by almost 30% and made substantial changes in species composition in some stands. Treatments that combined thinning and fertilising (one site) had the greatest improvement in diameter growth. It also indicated the effects of both persistent high stand density and poor site qualities that are common growth limitations for these forests. In unthinned controls, basal area increased by on average 50% with diameter increments of 0.11 to 0.24 cm/yr. The basal area removed at thinning had not been replaced in the up to 50 years that the trials were monitored. Stand-level models predicted slow volume development in both thinned and un-thinned treatments with MAI exceeding 5.0 m3/ha/yr only by age 200 years. Thinning had little impact on annual height growth with kauri predicted to take 25 years to reach 1.4 m in height. A comparison of models of productivity between second-growth stands and planted kauri has shown that natural stands have significantly less volume per hectare at all ages.