BURLINGTON — Across Northeastern forests, natural causes now account for more tree deaths than harvesting, including in Vermont, according to a recent study from the University of Vermont.  But these natural causes are still cause for concern. Insects, disease and extreme weather are now to blame for the majority of tree deaths in the region, a significant shift from just 15 years ago, the study found. This shift comes at a time when Vermont’s warming climate is already posing dangers to the health of the forest, including worsening the threat of introducing an invasive species. Now, forest health experts are determining what strategies they can use to preserve Vermont’s woods. “Our forests are really resilient,” said Savannah Ferreira, a forest health specialist with the Vermont Department of Forests, Parks & Recreation. “We’re doing a lot of great forest management that really helps increase the health and vigor of our trees.”