Inter-population variation in garlic mustard response to light

Background/Question/Methods

Garlic mustard (Alliaria petiolata) is an invader of Eastern deciduous forests that produces several allelochemicals, including glucosinolates.  Glucosinolates in leaf and root tissue may protect garlic mustard from herbivores and pathogens, and likely gives garlic mustard an advantage over native competitors through negative effects on fungal mutualists.  Garlic mustard invades a range of light environments, from high-light canopy gaps and forest edges to deep-shade understories, making it important to consider the role of light context in mediating garlic mustard’s impact on natives. 

We sought to determine the impact of light availability on growth and glucosinolate production of garlic mustard. Seed collected from eight populations across Indiana, USA was sown into pots outdoors in Bloomington, IN, during the fall of 2011.  Upon germination, plants were subjected to four light treatments: 10%, 30%, 60%, and 95% shade.  Each population x light combination was replicated seven times, for 224 total experimental units.  Of seven replicate blocks, four were harvested as rosettes during the fall of 2012, while the remaining three were left to grow to maturity.  At harvest, leaf samples from each plant were collected and analyzed for glucosinolate content, and above- and belowground biomass were measured.

Results/Conclusions

Across populations, garlic mustard aboveground biomass increased with decreasing light availability (p=0.023), while belowground biomass did not change with response to light (p=0.67).  Interestingly, garlic mustard from different source populations responded differently to light treatments (light x population interaction marginally significant at p=0.076), with some populations showing dramatically increased biomass as light decreased, and others showing weaker or flat responses to light treatment.  Leaf glucosinolate concentration differed amongst populations (p=0.035) but not amongst light treatments (p=0.92).  However, there was a marginally significant interaction between light treatment and source population in determining leaf glucosinolate concentration (p=0.11), with some populations increasing glucosinolate production as light increased, and others decreasing production.

Our results indicate that garlic mustard tends to grow better in deeply shaded conditions than in high-light conditions.  However, there appears to be intraspecific variation amongst populations in response to light availability.  Further work using populations sourced from unique light environments will assist in identifying local adaptation to light environment in garlic mustard populations.  Understanding the role of light context in mediating invasion impact through effects on growth and allelopathic potential may allow land managers to target the highest-impact garlic mustard populations, resulting in more efficient management of this invasive species.