I was once told by one of my arborist colleague that the valley oak next door to my home is the largest within our urban area. It is at the northern edge of a very impressive grove of very large valley oaks and coastal live oaks that dominates the neighborhood to the south. The homes and street have been constructed around them so that they could be preserved.

The grove was once even more impressive; but nearly half of the trees have been removed as they have succumbed to root damage caused by development of the neighborhood and subsequent landscape irrigation. Unfortunately, not much was known about their tolerance to such damage at that time.

Ever since I watched one fall seemingly onto my neighbors home, miraculously causing only minor damage as it actually fell across the neighboring front garden, I get rather anxious during windy storms. In fact, the most recent storm broke two cables in the tree next door.

Winter weather is less than pleasurable for the arborists who work out in it, but coincides with the dormancy of most species. Arboricultural procedures that may be performed any time may be less stressful to some sensitive trees now. They emerge from dormancy in spring with no idea of what was done to them. This somehow reminds me of waking up without my wisdom teeth.

Trees that are predisposed to sun scald on formerly shaded stems that become exposed by pruning are less sensitive to it during winter when the sun is lower in the sky. By the time solar intensity is sufficient to cause scald, spring growth provides sufficient protection. Silver maple and English walnut are examples of trees that can be damaged by sun scald in summer, but not during winter when they are completely defoliated anyway.

Winter pruning is also preferred by trees that bleed if pruned while active, such as fruitless mulberry and fig or trees that produce tender new growth prior to frost if pruned at the wrong time, such as jacaranda. It seems that many trees expect to be ‘pruned’ in winter as they would be in a natural ecosystem by severe weather, and become confused if their cycle is violated.

The necessity of arboriculture becomes more obvious during stormy winter weather. Most structural failure caused by wind occurs early in winter when some of the deciduous species are not yet completely defoliated. Such failure later in the season is usually limited to evergreen species that maintain their wind resistance and actually become more unstable as soil becomes saturated.

Various arboricultural procedures may be employed to minimize wind resistance, correct some structural instability and modify weight distribution. In many situations, these procedures are necessary to help trees to adapt to landscape conditions (or prior abuses) that are unnatural for them. Instability is often the result of response to such stimuli. For example, Aleppo pines that are naturally sparse with deep roots are very stable, but are more densely foliated with shallow roots in dense soil that is well irrigated.

Incidentally, not all structural failure is caused by stormy weather, but may occur when the weather is exactly the opposite of stormy. During summer, warm and humid air without breeze can damage trees in a more passive manner. Increased foliar activity stimulated by warmth accelerates translocation of moisture that can not be evapotranspirated (evaporated) into the already saturated air. Consequently, some limbs gain more weight that can be supported. This is occasionally a problem among but not limited to sweetgum, coast live oak and most species of eucalyptus.