Physiological mechanisms underlying reduced carbon acquisition in a native forest herb during garlic mustard invasion

Background/Question/Methods

Understanding how biotic and abiotic contexts modify the strength of species interactions is a key goal in ecology.  Mutualism effectiveness is highly context-dependent, and invasive species are hypothesized to be one biotic factor that can drive mutualism disruption between native species and their partners.  Garlic mustard (Alliaria petiolata) is a widespread forest invader that releases AMF-toxic allelochemicals into the soil.  Because over 70% of native forest herbs depend on AMF-derived soil resources for physiological function, the loss of AMF is hypothesized to dramatically affect native plant fitness.  Our previous work has shown that exposure to garlic mustard reduces early season carbon acquisition in a native forest herb, Maianthemum racemosum.  Here we assess the extent of mutualism disruption and explore the mechanisms underlying these physiological declines.  In a greenhouse experiment, M. racemosum were treated with either a fungicide – to mimic garlic mustard’s allelochemical effect – or leaves from garlic mustard or Hesperis matronalis, a non-allelochemical producing, yet related, mustard.  Physiological measurements were taken weekly throughout the early growing season, allowing us to determine leaf carboxylation capacity.  Plants were also harvested periodically from June-September and we quantified tissue carbohydrate/nutrient content and soil fungal hyphal length at each time point.

Results/Conclusions

Carboxylation capacity in garlic mustard and fungicide treated M. racemosum was significantly reduced relative to H. matronalis controls (P = 0.0095, P < 0.0001, respectively), indicating that CO₂ in the leaf was not efficiently utilized for photosynthesis.  Because foliar nitrogen and phosphorus concentrations did not differ across treatments, the declines in CO₂ use efficiency were not driven by the loss of AMF-derived nutrients.  Rather, our carbohydrate data indicate that AMF disruption altered source-sink dynamics within the plant.  Garlic mustard treated plants had 17% less stored carbohydrate in their rhizomes, but a significantly greater availability of sucrose – the precursor to AMF sugars (P = 0.0003). Finally, we detected only a weak effect of treatment on soil fungal hyphal length (P = 0.12), but the response was in the predicted direction.  Reductions in fungal hyphal length were nearly identical in the garlic mustard and fungicide treatments and both were lower than the H. matronalis controls.  If the physiological impacts we have observed scale up over time, then mutualism disruption in sites highly invaded by garlic mustard could ultimately diminish native plant population stability.  Allelopathic mutualism disruption may be a generally widespread mechanism underlying the success of exotic species.