With temperatures back below zero this week, we are reminded of how important winter survival strategies are for all organisms. We most often associate winter survival strategies with migrating birds and hibernating mammals, but the varied strategies trees use to survive the winter are not so obvious.

One of the greatest challenges facing trees in the winter is a lack of access to liquid water. Deciduous trees address this is- sue by dropping their leaves in the fall. Most of the water trees lose is through the exposed surface of its leaves, so these trees will simply get rid of them in the fall. Conifers, such as pines

and spruces, keep their needles in the winter because they have much less surface area and will lose less water from them. This allows conifers to continue photosynthesizing and growing in the winter, whereas deciduous trees must expend energy growing new leaves each spring. This strategy comes with another risk; if a conifer cannot replace the water it loses in the winter, its cells will begin to dry out and die.

The sheer cold of the winter is also an issue for trees. If ice forms in any individual cell, it is fatal for that cell. However, trees can survive ice that forms in the space between their cells. For this reason, deciduous trees will pull water from the cells to the space between them and the vascular

tissue, or plumbing, of the tree, where it will freeze. The tree will have to repair its damaged vascular tissues in the spring, but its others cells are kept safe. Conifers continue to photosynthesize and transport water in the winter, so they cannot let their vascular tissue be damaged. To address the problem of freezing, they have special valves in their vascular tissues that allow them to start and stop the ow of water if it warms up temporarily. Co- nifer cell walls are also stronger than in those deciduous trees, allowing them to withstand the pressure of expanding ice.