Many perennial plant species can lie dormant for multiple growing seasons.  Plants may become dormant in response to stressors such as poor environmental conditions or negative species interactions.  Dormancy could be a non-adaptive reaction to stress, or an adaptive response that creates time for a plant to its reset its physiological functions.  We investigated the causes of dormancy using a 6 yr dataset (2003-2008) of 686 tagged individuals of a long-lived forest perennial herb, Maianthemum racemosum (Liliaceae).  This species is highly palatable to white-tailed deer and depends on mutualistic associations with mycorrhizal fungi (AMF) in its roots to deliver soil resources.  

We conducted this study in a hardwood forest in western Pennsylvania with a high density of deer and was invaded in the late 1990s by an allelopathic invader, Alliaria petiolata (garlic mustard = GM), that kills AMF.  In replicated plots we experimentally manipulated ambient levels of both deer and the presence of GM, creating 4 treatments: 1) deer/GM; 2) deer/GM-; 3) deer-/GM; 4) deer-/GM-.  

We propose that exposure to deer herbivory or GM’s disruption of hyphal networks reduces an individual’s resource status, creating stress.  Thus we predict that the presence of either deer or GM will increase the probability of dormancy, with the highest probabilities occurring in plots where both stressors are present.   Mark-recapture models were used to estimate the annual probability of dormancy in each treatment.  Models were run in MARK using plant length as an individual covariate.
Results/Conclusions

Over all treatments 20% of study plants were dormant for at least 1 year, while 2% were dormant for either 2 consecutive years or had two separate bouts of dormancy.  As expected, the highest dormancy probabilities occurred where both deer and GM were present (0.148±0.066 to 0.264±0.093; ±95% confidence intervals).  Removal of GM when deer are present resulted in significantly lower probabilities of dormancy for all years (0.095±0.047 to 0.178±0.075) relative to the deer/GM plots.  The exclusion of deer significantly decreased dormancy both for plots with GM (0.071±0.035 to 0.136±0.057) and without GM (0.070±0.035 to 0.135±0.058), relative to the deer/GM treatment.  Our results implicate deer and the noxious invader garlic mustard in significantly altering the dormancy dynamics of M. racemosum.  Future work will investigate whether dormancy is an adaption that increases survival and assess its demographic consequences.