Alliaria petiolata, a strict biennial, can have annual alternating high abundance of rosettes and flowering plants. We monitored changes in abundance of rosettes and flowering plants in permanents plots (2004-2014). The alternating yearly cycle was not observed (2007-2008, 2008-2009, and 2013-2014) and years with high rosette abundance were not followed by a year of high flowering plant abundance. To determine if ECEs caused disruption of alternating high abundance of rosettes and flowering plants, we obtained long-term data from a local NOAA station including daily precipitation, days with ≥ 0.13 cm, and maximum, minimum, and mean temperatures. A. petiolata was first observed on our study site in 1988, and was well established by the early 1990’s. To determine if September 2007 significantly differed from other Septembers 1994 to 2014, we used five variables related to drought for our study of 21 years. These variables included (1) total precipitation, (2) mean monthly temperature, (3) number of rainy days, excluding days with trace amount of precipitation, (4) maximum number of contiguous days without precipitation, and (5) days with maximum temperatures > 30 °C. We analyzed these data using PCA and ranked years from (1) high to (21) low plant stress for each variable.
Results/Conclusions
September 2007 met ECEs criteria for mean monthly temperature, total precipitation, and number of days with precipitation > 0.13 cm, ranked 60/64, 6/71, 60/64, respectively, and occurred in 6, 8, and 94 percentiles, respectively. Z-tests for all variables were significant (P < 0.05). September 2007 had the lowest Summed Rank Score (1) indicating that drought conditions were more severe than in the other twenty years. PCA Axes 1 and 2 accumulated 56.2% and 22.4%, respectively, and collectively accounted for 78.6% of the total variance. All five variables had significant correlations with Axis 1. The two variables that reduced moisture stress (total precipitation and number of days with precipitation) had positive correlations with Axis 1, whereas the three remaining variables that increase moisture stress were negatively correlated with Axis 1. Distance from September 2007 for each year in the PCA biplot correlated significantly with Summed Rank Scores. Record-breaking minimum temperatures (ECEs) for January (2009) and February-March (2014) reduced rosette abundance and disrupted reestablishment of the alternating high abundance of rosettes and flowering plants. Our data indicates a single ECE variable, minimum temperature, for 2009 and 2014 and multiple variables related to drought had negative effects on the invasive A. petiolata.