One sign of the Christmas season is the tree lots that start popping up. Each person has ideas about what makes the perfect pine or a festive fir—a conical shape, a sharp woodsy smell, or an apartment-friendly size, perhaps. Plant pathologist Gary Chastagner is studying one trait in particular: how well trees hold onto their needles. Conifers, for example, shed their needles for various environmental reasons. Chastagner shares what trees are better at retaining their foliage, along with tips for caring for your Christmas tree.