What we know for sure is that the winter of 2011/2012 was a really warm one and March, May and June have been uncharacteristically warm, as well. There has also been less rainfall than usual.
Now, what I'm guessing at is that, since vines require less resources to maintain themselves and grow compared to trees, it seems possible to me that vines have been able to quickly take advantage of the warm spring and start their growth while also being able to continue growing during these more parched months. However, this is just a guess on my part.
What I know is that there are a lot of trees with vines growing on them; the vines themselves are very dense, and there's a lot of poison ivy, too. This radically warm winter, spring, and summer seem to correspond with the basic message of a 2006 paper by Mohan et al. Their abstract reads, in part:
Contact with poison ivy (Toxicodendron radicans) is one of the most widely reported ailments at poison centers in the United States, and this plant has been introduced throughout the world, where it occurs with other allergenic members of the cashew family (Anacardiaceae). ... Rising CO2 is potentially responsible for the increased vine abundance that is inhibiting forest regeneration and increasing tree mortality around the world. In this 6-year study at the Duke University Free-Air CO2 Enrichment experiment, we show that elevated atmospheric CO2 in an intact forest ecosystem increases photosynthesis, water use efficiency, growth, and population biomass of poison ivy. The CO2 growth stimulation exceeds that of most other woody species.We heard news a few months back that the atmospheric concentration of CO2 has increased past 400ppm, which doesn't correspond to the elevated CO2 chambers of Motan et al's study, but as the Motan paper discusses previous studies on increased CO2 and vine growth:
So, poison ivy is expected to be more abundant and a greater lover of the high CO2 future than trees. And this is likely also a cause for the increase vine abundance in general.
With an increase in CO2 concentration and a corresponding increase in photosynthesis, vines can allocate more photosynthate to additional photosynthetic tissue, because of a low allocation to support tissue relative to other woody growth forms (13, 14, 18, 19). Increasing abundance of woody vines is causing increased tree mortality and reduced tree regeneration in forests around the globe (18, 20–23), potentially resulting in shifts in community composition that may impact carbon cycling and biodiversity (23). Although it is unclear how elevated CO2 will affect the growth of vines in forest environments, the contemporary increase in woody vine abundance may be the result of rising atmospheric CO2 concentrations (19, 23).The Motan et al paper doesn't discuss the impacts of warmer winters, springs, and summers on poison ivy growth (nor on the growth of other vine species). However, I am going to guess that a large part of the vine growth this year is likely due to the ever increasing CO2 levels and have been exacerbated this year by the unnaturally warm seasons.