Herbivory, growth, seed production, and resprouting of an exotic invasive shrub Cytisus scoparius

Published source details Bossard C.C. & Rejmanek M. (1994) Herbivory, growth, seed production, and resprouting of an exotic invasive shrub Cytisus scoparius. Biological Conservation, 67, 193-200.
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Summary

In northern California (southwest USA), Scotch broom Cytisus scoparius is a non-native invasive shrub occupying over 250,000 ha in north-west coastal and Sierra Nevada foothill regions where it displaces native plants. Leucoptera spartifoliella (a twig-boring moth) and Apion fuscirostre (a seed weevil) were introduced from Europe in 1960 and 1964 respectively in an attempt to control C.scoparius. In this paper the impact of these insects and vertebrate herbivory on two broom populations were assessed.

The study was undertaken at two sites: an 8 ha foothill area in Eldorado National Forest (ENF) dominated by broom interspersed with Ponderosa pine Pinus ponderosa; and a 2 ha prairie grassland (50% invaded by broom) at Redwood National Park (RNP). Here, broom was sprayed with 2% Roundup in August 1985, then cut and burned in January 1986.

Effects of herbivory at ENF: In April 1987, 75 (50 x 50 cm) plots were placed 60 cm apart along four 40 m transects in as area densely covered with broom seedlings. Five treatments were assigned (15 plots/treatment):

1) control (untreated);

2) plot caged (60 x 60 x 90 cm cage; 1.3 cm diameter chicken wire) to exclude vertebrates;

3) caged and sprayed (hand-held mist sprayer) every 5 weeks (April-October) with 0.03% malathion (insecticide) solution to prevent vertebrate and invertebrate herbivory;

4) caged and mist-sprayed every 5 weeks with water (April-October) to control for any effects of water in the malathion;

5) plots with chicken wire on three sides and half the top (open to herbivores one side) to determine any effects of cage shading.

From May 1987 to November 1990, within the central 25 x 25 cm of each plot, brooms were counted, and height and stem diameter of dominants periodically measured. Ten brooms (outside plots) in each of several height categories were cut, dried and weighed, and volume calculated. Seed productionand the number of seeds damaged by weevils were recorded each year.

Resprouting: 15, 1-2 year old (<l m) and 15, 2-3 yr old (>1 m) brooms at ENF were cut at 0, 5, l0 cm above the soil surface in November 1987, and January, March, May and August 1988. Regeneration was recorded.

A comparison of vertebrate foraging on broom at ENF and RNP was also made.

At RNP, by 1987, 30% of the 2 ha area sprayed with glyphosate became reinfested with broom via the seedbank.

At ENF, the biocontrol agents had no significant effect on any broom life stage; vertebrate herbivory likewise. However, vertebrate herbivory significantly decreased broom biomass at RNP; deer, elk Cervus elaphus, and Botta’s pocket gopher Thomomys bottae were common. Mountain quail Oreortyx pictus, blue grouse Dendragapus obscurus, and Western harvest mouse Reithrodontomys megalotis were also common and significantly reduced the amount of broomseed.

Cutting in August (at the end of the dry season) significantly decreased resprouting with<10 % of shrubs cut resprouting regardless of size or height of cut; those cut at 0 and 5 cm did not resprout. Cutting in January and March was ineffective (over 90% resprouting). Cutting in November and May produced intermediate results.

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