Recently, on December 21st, the earth made another of its remarkable transitions. Winter began as the sun reached its lowest and most southerly position of the year, and for the next six months the days here will become longer and the sun will move higher in the noon sky. This event is called"winter solstice", and for numerous cultures throughout the ages, it has marked the beginning of rebirth of the life that sustains us, and is celebrated with ritual and great joy.

In nature, it can be a time of great calm. In colder climates, all but the hardiest plants and animals are in some state of dormancy. Even in our relatively warm climate, many plants and animals participate in this cycle of dormancy. Trees in particular may loose their leaves and seem to become brittle statues, stark against the winter sky. It is worth learning more about what is happening.

To understand the processes of nature, it can be helpful to look at the world as if we were a particular organism, in this case a tree, moving through the seasons.

As a tree, it is vital to have enough water. Trees have leaves that are essential for growth. These leaves are very efficient at converting sunlight to energy for growth, and very inefficient at holding onto water. The loss of water, or "transpiration" is so significant that plants have to compensate in order to keep from loosing so much water that they become stressed or die.

In colder climates, the winter ground is frozen and water simply is not available to plants. Trees must turn off their water supply systems or they will dry up. Deciduous trees, those that lose their leaves, accomplish this by shedding their leaves and shutting down almost completely. Conifers, trees with needles, have a different strategy. They close off their pores, or "stomata"--organs on all leaves that allows air into the leaf to exchange gases and release water. By closing these stomata, they retain much more water. They can also then produce food through photosynthesis on warmer winter days. If, however, their stomata open too much on sunny days when the ground is still frozen, it is possible for the tree to loose water and to dry out and die. This can be a problem at higher elevations and for interior climates.

Winter cold is still a problem for plants here, though more from frost damage to leaves than from lack of water. Winter is a time of ample water. In summer when the most sunlight is available, there is little water. For this reason, certain tree species like the live and scrub oaks have adapted by evolving a thick waxy outer covering on their leaves that minimizes water loss in summer and provides frost protection in winter. They also can regulate their stomata, like most conifers. They do not loose their leaves each year as do their oak relatives, the valley, black and Oregon oaks. These oak species have access to more water in summer because of deep root systems, and so they do not need to be so frugal with it. As often happens in nature, this is a trade. Their leaves, while very efficient for producing energy, are more fragile, and they must lose them in cold weather. Like their cold climate eastern relatives, they then shut down.

All over the forest, plants have evolved different roles in response to these days of low light, cool or cold nights and ample water. Grasses make use of any warmer sunny day to sprout, grow and mature. Certain annual plants have nearly completed their life cycles before some of the deciduous trees wake up in March. Many native trees like the toyon and the madrone not only retain their leaves but produce their seed or berries in this season--a major boon for the wildlife that lives here or migrates to take advantage of this unusual winter bounty.

It seems then, that more going on in our winter forests than many of us would have expected. At once dormant and productive, this diversity in winter is one of the many reasons that each season in our valley, including winter, is quite special.