Garlic mustard is not alone: Comparisons among co-occurring invasives and natives in the forest herb layer

Background/Question/Methods

Non-native garlic mustard (Alliaria petiolata; ALPE) can establish dense stands in forest herb layers, nearly excluding other plants, but when colonizing as seedlings must contend with a host of other species in the herb layer, including natives and other invasives. Theory and data lead us to expect invasive plants to be “better” than native plants in many characteristics, but what happens when co-occurring species are other invasives? For example, do they all escape from enemies equally, and more than natives? Are they equally competitive, and more so than natives? We conducted an experiment replicated in three forests, in which we planted seedlings of ALPE and three other species that also colonize the herb layer: the native sugar maple (Acer saccharum; ACSA) and two non-native invasives, Japanese stilt-grass (Microstegium vimineum;  MIVI) and Norway maple (Acer platanoides; ACPL). Single focal plants of each were planted in eight treatments (replicated five times in each forest): alone or with two close neighbors of one of the other three species, and in mammal exclusion cages or open to mammals. For each focal plant we measured foliar symptoms of natural enemy damage in mid-summer, and mortality near the end of the growing season.
Results/Conclusions ALPE sustained the most insect damage overall [ANOVA, P=0.004; mean(SE) percent leaf area damaged:  ALPE, 19.6(2.54); MIVI, 10.3(1.51); ACPL, 15.3(1.82); ACSA, 12.9(1.63)]. Neighboring plants did not affect insect damage on focal ALPE or ACPL, but ALPE neighbors resulted in higher insect damage on ACSA [ANOVA, P=0.004; with ALPE, 22.5(4,82)], and on MIVI [ANOVA, P=0.0001; with ALPE, 20.9(3.77)]. ALPE had highest overall mortality (63%), compared to MIVI (20%), ACPL (29%), ACSA (33%). Caging to exclude deer etc. was only slightly protective for ALPE and MIVI (mortality decreased by 6%), but more protective for ACPL and ACSA (mortality decreased by 19% and 16%, respectively). ALPE mortality was highest when grown with MIVI (87%); MIVI mortality was highest when grown with ALPE (43%); as was ACSA mortality grown with ALPE (52%). The higher insect damage and mortality of garlic mustard, compared to the native and other invasives, illustrate challenges for its invasion in the presence of these other species, particularly MIVI. It has some advantage over both maples, with less of a negative effect from mammalian herbivores (but equal to MIVI), and it appears to increase some competitors’ vulnerability to insect damage and mortality (but itself increases mortality when in competition with MIVI).