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Individual study: The effect of commercial flower picking on a wild restricted-range shrub, Banksia hookeriana, in the Eneabba Plain, Western Australia

Published source details

Witkowski E.T.F., Lamont B.B. & Obbens F.J. (1994) Commercial picking of Banksia hookeriana in the wild reduces subsequent shoot, flower and seed production. Journal of Applied Ecology, 31, 508-520

Summary

In Australian heathlands, commercial wildflower picking has grown in recent years resulting in concerns for the maintenance of populations of some species. Many plants favoured by pickers have restricted distributions and may face other threats such as clearance for agriculture and mining, and overgrazing. One such species is Banksia hookeriana, a shrub with pine cone-like flower heads. It is restricted to an area of sandy plains of only about 33 km x 74 km in extent in Western Australia. The plants are periodically destroyed by fire and are dependant upon canopy-stored seed for regeneration. Fire heat breaks seed dormancy, and seedling recruitment is generally restricted to the first winter/spring after fire following seed release. Owing to concerns for the survival of B.hookeriana, the impact of flower-picking, and also damage to flower heads by cockatoos Calyptorhynchus funereus, on flower and cone production, seed bank dynamics and plant structure, was investigated.

Study site: The study was undertaken in three picked and three unpicked 13-year-old populations of B.hookeriana, growing in scrub-heath on the Eneabba Plain, 285 km north of Perth, Western Australia. The three picked sites were located within Nature Reserve 39744, where traditional picking was still allowed after reserve designation. One of the unpicked sites was situated in an inaccessible part of the reserve, the two others on Crown and private land 9 and 12 km NNE of the reserve respectively.

Plant densities and canopy sizes: Three, 20 x 10 m plots were placed at random in each of the six sites. Banksia numbers and those of other shrubs greater than 50 cm tall were recorded. Ten Banksia shrubs were randomly selected to investigate the effect of bloom picking on canopy size and volume, subsequent bloom production and seed banks, and an additional five were selected to examine plant structure.

Vegetative and reproductive growth: Ten terminal shoots and flower head buds were were tagged on 10 plants in a related phenology study 27 km south of the nature reserve. The distance between the apex of each new season shoot and that of the previous season, and flower head elongation was measured monthly from March 1984 to March 1985. Anthesis (time of expansion of the flower) and fruit development etc. were also measured.

Bloom picking, cone numbers and fertility: Data on cones removed, seed bank dynamics and plant structure were collected in late summer/autumn (February-May) 1991. Numbers of blooms picked was assessed by the clean cut made by secateurs as opposed to the rough cut of those removed by cockatoos, as well as the numbers of cones remaining, were counted each year on the 10 plants at each site. Cones were classified as fertile (follicles present) or infertile.

The results of the study are summarised in Table 1 (attached). B.hookeriana plants take four years to reach reproductive maturity. Bloom picking over the subsequent nine years reduced plant canopy area and volume by 37% and 44% respectively, with a 56% reduction in the number of 1-year-old stems, compared with the unpicked plants. An overall picking level of 29% of blooms resulted in a reduction of cone production of at least 35% in control plants and a reduction in 1-year-old stem apices of 56%.

A total of 13,255 blooms/ha were picked over the nine years, accounting for 29% of total production. Cockatoos removed 2,477 blooms/ha (5% of the total), but removed over 3-times as many blooms on the unpicked plants (7,562/ha, i.e. 7% of the total). However, re-sprouting was much more likely from cockatoo-damaged stems than from picked stems.

Picked plants produced 35% fewer blooms than unpicked plants but percentage cone fertility and the number of follicles per fertile cone did not differ significantly.

Seed production and storage per individual plant were 50% and 57% lower respectively in the picked plants.

There was a slight increase in total insect-eaten seeds (4%), and a slight decrease in viable seeds (9%) in picked plants, but there were no significant differences in the fractions of seeds released, aborted or non-viable.

Conclusions: The study shows that commercial bloom-picking has a detrimental effect on Banksia seed production and the reduced seed store following picking could adversely affect post-fire regeneration, particularly if fires were at short intervals followed by an unusually severe summer drought. Bloom removal did not stimulate compensatory stem and flower production and evidence from this study shows that in fact the reverse happened. It is recommended that no picking be allowed for the first eight years after fire to allow plants to produce a small seed bank and that picking be reduced to one-fifth of the blooms produced each year to ensure that the seed bank remains above 50% of that of unpicked populations.


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