Growth, biomass, and productivity of a bracken (Pteridium esculentum) infested pasture in Marlborough Sounds, New Zealand

Abstract

With cessation of grazing, bracken (Pteridium esculentum) quickly dominated a pasture in Marlborough Sounds sampled at 0, 1, 4, 7, and 17 years. Bracken frond density increased from 1/m² at year 0 to 32/m² at year 7 and declined to 27/m² at year 17 with living fronds peaking at year 4 and dead fronds at year 7. Frond height peaked at 1.83 m at year 7 and weight per frond reached 81 g at year 4. Pasture herb and tree seedling density and height strongly decreased to year 7 as frond density and height increased and then slowly increased to year 17 as the bracken canopy opened.

Biomass peaks of 6800 g/m² above-ground in year 7, of 7100 g/m² below-ground in year 17, and of 12 500 g/m² total in year 17 were higher than other pioneer New Zealand vegetation except Pinus radiata. Biomass accumulation increased to 1480 g/m²/yr in year 4, declined to 250 g/m²/yr in year 17 and was predicted to be nil in year 20. Above-ground net primary productivity (NPP) reached 1500 g/m²/yr in year 4 while maximum below-ground NPP was 1600g/m²/yr in year 17. Minimum total NPP of 2200 g/m²/yr and estimated mean total NPP of 3100 g/m²/yr was 1.6 to 2.6 times higher than earlier global measurements of vegetation with comparable biomass. Bracken stores substantial CO₂ and this study contributes to the current upward revision of the terrestrial carbon pool.

Keywords:

• bracken fern
• Pteridium esculentum
• biomass accumulation
• net primary productivity
• global carbon balance
• plant density
• height growth
• post-grazing regeneration
• Marlborough Sounds