Lupinus arboreus (tree lupin) regeneration from the seed bank following herbicide control, Kaitorete Spit, New Zealand

Background/Question/Methods

New Zealand’s largest tree lupin (Lupinus arboreus Sims) control operation commenced at Kaitorete Spit in October 2007, when an area of 39 ha of lupin was treated with herbicide. Subsequent monitoring documented significant regeneration of lupin from the seed bank after annual applications of herbicide. The current paper examines the impact that adult tree lupin control has had on the lupin seed bank, four years after the commencement of operations. We determined (i) the depth and number of seeds remaining in the treated and adjacent untreated areas; (ii) the viability of this seed; and (iii) the extent to which vegetation and landform development controls seed bank dynamics. We excavated and processed 248 pits, split between non-sprayed and sprayed areas, across three landform strata (foredune, deflation and backdune). Pits measuring 0.5x0.5m were excavated to depths of 0.1m or 0.3m (in 0.1m sections). Efficient seed recovery was achieved by passing bulk samples through a modified rotating drum lined with a 2.3mm stainless steel mesh. Seed viability (20 seeds per sample) and the ability of the hypocotyl to emerge from various depths were also assessed.

Results/Conclusions

Most seed was found in the duff layer or in the top 10cm of soil in the pits excavated to 30cm depth – 34% and 58% in the untreated area and <1% and 82% in the sprayed area, respectively. Less than 1% of seed occurs below 20cm depth in both treated and untreated areas. Seed on, or close to, the surface has fuelled regeneration in the period 2007 to 2011. The tree lupin seed bank is depleted in the treatment area (mean = 37 seeds/0.025m³), compared to the non-sprayed area (mean = 52 seeds/0.025m³). The remaining seed has a high viability (53%), however, much of the seed bank is unlikely to contribute to regeneration in the short term because it is buried too deep. Hypocotyls of lupin seed are unlikely to emerge from depths in excess of 3cm. Vegetation cover may also inhibit germination. An area of dense bracken fern (Pteridium esculentum G. Forst) contains relatively large numbers of seed within the treated zone. The seed bank is not distributed uniformly or at uniform depth across landform strata – the foredune accreted 1m in the period 1991-2011, prior to the commencement of operations, ensuring some seed is now deeply buried. Consequently, we anticipate that the tree lupin control operations will need to extend over many years.