Effect of silvicultural regimes on carbon sequestration in Pinus radiata forest in Canterbury.

Abstract

This study explored effects of silvicultural treatments on carbon sequestration in juvenile Pinus radiata forest in Canterbury. Financial modelling was then employed to determine what regimes would be most profitable given current carbon prices. Deciding how best to manage stands for carbon sequestration is important given the thriving carbon market and the current climate crisis requiring us to reduce net GHG emissions.

Relevant literature noted that biomass growth is highly influenced by local environments and site factors. Analysing biomass assays with a tree-level mixed-effect model to predict biomass through time takes into account variance caused by local factors and was found to be an effective way to predict carbon stocks. A comparison against existing national models found that these could generally be applied to a Canterbury site, however a localised model is superior, with less variance in predictions compared to the existing national models.

A comparison of carbon estimates between the localised model based on biomass assays and the default yield table for Canterbury under the Emission Trading Scheme (ETS) showed that the default yield table markedly underestimated carbon stocks during the initial 15 years. This was most prevalent at the highest stockings where table carbon estimates were 67% less than localised model estimates based on biomass assessments.

The effect of initial stocking was found to significantly ( p<0.05) influence carbon sequestration, and low (625 stems/ha), medium (1250 stems/ha) and high (2500 stems/ha) stockings were all statistically different from each other. The presence of weed control also had a statistically significant effect (p<0.05) on carbon sequestration however fertiliser did not. An interaction effect between stocking and weed control was only present early at age 5 and was likely due to different rates of canopy closure based on the varying stockings. These findings were similar to previous studies on biomass production where treatments that encouraged rapid above-ground growth such as high initial stockings were shown to sequester significantly higher amounts of carbon.

Financial analyses showed that a stand of 2500 stems/ha, no thinning, and a long rotation age of 44 years, returned the highest Land Expectation Value (LEV). Permanent carbon forestry also returned a high LEV and given a longer rotation age (>60 years) may be more profitable than rotational forestry. Forest managers aiming for higher biomass will need to sacrifice large piece size. Growing higher stocked stands with no thinning will require accepting a higher level of risk and ensuring there is a large enough market for lower grade timber.