Reduces light and available space in forest understory. Also changes nutrients in the soil, accelerating the decay of leaves and nutrients (Anderson 2012, Rodgers 2008). Forms dense infestations in invaded range (Anderson 2012) including Southern California where native plants are less dense (see CalFlora pictures).

Anderson, 2012. Stinson et al. 2007. Rodgers et al. 2008a. CalFlora.

Garlic mustard in the eastern US is known to affect pollinators. Several butterfly species have been known to oviposit on garlic mustard however their young do not successfully complete their life cycle on garlic mustard (Renwick et al. 2001, Rodgers et al. 2008). It is likely that as populations of garlic mustard increase host plants will become unavailable to native pollinators in the forest understory where garlic mustard invades (Keeler et al. 2006).

Keeler et al., 2006., Renwick et al., 2001. Rodgers et al., 2008.

Garlic mustard spreads easily in disturbed and undisturbed environments with or without anthropogenic factors (Anderson 2012). In Southern California it is spreading in undisturbed riparian forest habitat. There are reports of garlic mustard invading horticultural landscapes as well as roadsides (Landis and Evans)

Anderson, 2012.

Garlic mustard was first discovered in Southern California in 2020 in Hook Creek. It is unknown when it first arrived there. The infestation is dense in places, with numerous small patches growing in suitable habitat along a 2.5 mile stretch of the creek. In areas where the creek is flat, the patches are large, extend 20-30 feet from the stream's edge, and contain thousands and thousands of plants. Published accounts from other areas suggest garlic mustard spreads rapidly (Rodgers et al. 2008b), and in Hook Creek plants are vigorous with copious amounts of seed. There is no evidence to suggest it does not spread rapidly in Southern California; all current evidence suggests it does spread rapidly. More recently, a separate population has been found along Rush Creek in Trinity County, likely originating on private land and extending into National Forest property.

Rodgers et al. 2008b

People and anthropogenic disturbances seem to be one of the main pathways for garlic mustard to disperse both over medium distances and long distances (Anderson 2012, Rodgers et al. 2008b). Main sources of long-distance dispersal include accidental transport on contaminated tools, clothes and footwear. The seeds in Southern California are readily moving downstream to new habitats, as evidenced by the establishment of numerous small garlic mustard patches (C. McDonald pers. obv., Calflora 2022). The only two known naturalized populations of garlic mustard are hundred of miles apart and themselves isolated by hundreds of miles to the next wild out-of-state populations in Oregon and Utah (iNaturalist). Most seeds fall close to the parent plant (Anderson 2012). Since the fruits are 3-5 feet high the seeds are easily picked up on equipment, the clothes of hikers, and, possibly, wildlife.

Anderson, 2012. Rodgers et al., 2008b. C. McDonald pers. obv. Calflora, 2022.

It grows in the forest understory in the eastern US and Canada as well as roadsides, disturbed areas and trails in mesic and riparian areas (Rodgers et al 2008, Anderson 2012, Landis and Evans).

Note: I had trouble scoring this question. The directions were specifically for similar communities in other states that also occur in California. My knowledge of forest types in the eastern US and pacific northwest is lacking.

Currently growing in riparian forest in Southern California. Other recent iNaturalist sightings suggest it can grow in several habitat types in California and in other western US states (but not AZ, NV or NM as indicated in directions).

Note: another difficult question for me to score, given my lack of knowledge in other forests.