Nutrient losses from commercial picking and cockatoo removal of Banksia hookeriana blooms at the organ, plant and site levels.
Abstract
Losses of dry mass and mineral nutrients (N, P, K, Ca, Mg and Na), occurring through the commercial picking of blooms and their removal by cockatoos, were quantified at the organ, bloom, plant and population-site levels for 3 picked and 3 unpicked populations of B. hookeriana in the scrub-heath of Western Australia. Above-ground dry mass and N and P contents of plants were reduced by 20, 29 and 30%, respectively, from a mean commercial harvest of 29% of blooms over a 9-year period. Cockatoos removed only 1-2% of these during the same period. Removal of cations was in the same order of magnitude. At the site level, losses of N and P from commercial picking were 3103 g N/ha and 152 g P/ha over the last 9 years. Removal of 240 g N/ha and 14 g P/ha by cockatoos was not a loss from the ecosystem as the flower heads were dropped near the plants. Surface soil nutrients, particularly NH4, NO3, Ca and Mg, and organic C were lower in the picked areas, which may indicate nutrient depletion at the ecosystem level. Bloom concentrations of N were higher by 15, 32 and 65% for flower heads, leaves and stems, respectively, in unpicked plants, while P tended to be higher for leaves and stems in picked plants. Total N content of blooms tended to be higher in unpicked plants. Seed production was a very large drain on plant nutrients, with half of above-ground P and a quarter of N allocated to seeds. Equivalent picking of cones for their seeds would cause a loss of 35% of P and 24% of N. Bloom picking not only removed potential nutrient sinks (developing fruits) but also some of the most recent and metabolically active leaves. This may have adversely affected the nutrient status and/or carbon balance of the plant, reducing subsequent growth, nutrient uptake, and bloom production.