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

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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.