There are several internal and external factors contributing to invasive weed infestations, which threaten the diversity and existence of the native plants. The external factors are the surrounding urban development and disturbance, which introduce and spread exotic plants. The internal factors are traffic impacts (trail and off-trail) that destroy the native plants and disturb and compact the soil. These traffic impacts allow an opportunity for the invasive and exotic plants to colonize the edges of these exposed trail corridors, as well as provide a method of seed dispersal for these plants via shoes, hooves, or tires. The majority of exotic and garden plants coexist with native species and are not ecologically harmful. However a small number of exotic plants are ecologically devastating. These exotic plants are highly invasive and their presence can have numerous negative consequences and effects, such as the following (Pickart and Eicher 2000):

• Invasive plants displace native plants, alter habitat (for flora and fauna) and soils, and frequently form monocultures.



• Invasive plants are the second most important reason for loss of biological diversity after habitat destruction.



• Invasive plants in agricultural and natural areas cost our country 13 billion dollars per year.



• The Bureau of Land Management, the nation's largest public landowner, estimates that 2,300 acres per day of its land are being lost to invasive plants.

The following 13 invasive plants were noted during the field survey in or nearby the redwood forest:
1. Acacia sp. acacia (outside redwood forest)
2. Carduus pycnocephalus Italian thistle
3. Cirsium vulgare bull thistle
4. Conium maculatum poison hemlock
5. Cortaderia jubata weedy pampas grass
6. Cotoneaster pannosa cotoneaster
7. Cytisus scoparius scotch broom
8. Eucalyptus globulus Tasmanian blue gum (outside redwood forest)
9. Genista monspessulana French broom
10. Hedera helix English ivy
11. Ilex aquifolium English holly
12. Senecio mikanioides German-ivy
13. Vinca major greater periwinkle

The majority of these invasive plants are associated with roadsides, trailsides, and openings in the redwood forest, except Tasmanian blue gum and acacia which form...