Biomass and nutrient distribution in radiata pine in relation to previous land use II Nutrient accumulation, distribution and removal

Summary

Above-ground nutrient accumulation and potential nutrient removal through thinning, clearfelling and slash burning were estimated for three areas of 21 year-old radiata pine plantation differing in previous land use prior to plantation establishment. One area carried native eucalypt forest, another had been cleared and grazed (pasture), while the third had been cleared, grazed, and later used for crop production (cultivated pasture).

Stands on the the previously cultivated pasture accumulated the largest amounts of nutrients (and above-ground biomass); an average of 62 kg phosphorus ha⁻¹ in trees and 19 kg phosphorus ha⁻¹ in the forest floor compared with 26 kg phosphorus h⁻¹ and 6 kg phosphorus ha⁻¹ in the trees and forest floor of the ex-native forest site, respectively, and 30 kg phosphorus ha⁻¹ in and 8 kg phosphorus ha⁻¹ in the trees and forest floor of the ex-pasture site, respectively. Total tree nitrogen contents were 325. 418 and 564 kg ha⁻¹ in the ex-native forest, ex-pasture and ex-cultivated pasture sites, respectively.

Thinning at 21 years of age removed between 93.7 and 136.0 t ha⁻¹ of biomass as logs containing 68 to 112 kg nitrogen ha⁻¹ and 6 to 12 kg phosphorus ha⁻¹, with the largest removals from the ex-cultivated pasture site. At clearcutting, 147 (ex-native forest), 173 (ex-pasture) and 200 kg nitrogen ha⁻¹ (ex-cultivated pasture) and 12, 13 and 22 kg phosphorus ha⁻¹, respectively, could potentially be removed in logs. Total residue plus forest floor was estimated to contain 280 (ex-native forest). 381 (ex-pasture) and 536 kg N ha⁻¹ (ex-cultivated), which would be available for the second rotation if retained and re-incorporated through chopper rolling. Clearfelling followed by a moderate slash burn could remove 4.1 to 6.7 % of the total nitrogen capital (to 38 cm depth), and 13.5 % of the exchangeable K, with greater losses (absolute and proportional) in the previously cultivated pasture. Plantations on previously improved pastures, or sites receiving intensive nutrient amendments, appear more vulnerable to nutrient depletion than plantations on ex-native forest sites.

Keywords:

• nutrient content
• nitrogen
• phosphorus
• site preparation
• stem-only harvest