fallen coral tree (Erythrina sp.), a frequent occurrence when this tree is planted in a lawn

The Santa Anas of November have stalked the Valley once again, breaking limbs off – and even knocking over – a host of predictable victims: top-heavy trees, hillside trees and overwatered trees with brittle branches and shallow roots. Mother Nature has put us on notice that we have entered the season of turbulent weather, and we would be wise to properly prepare our trees for the storms to come.
Among the trees most susceptible to limb breakage in the Valley are the Shamel ash (Fraxinus uhdei), the silver maple (Acer saccharinum), coral tree (Erythrina sp.) and the many commonly planted species of eucalyptus. A mature Shamel ash will lose many large branches in the course of a typical winter unless it is properly pruned in the fall. The same is true, to a somewhat lesser extent, of the silver maple and the eucalyptus. The susceptibility of these trees to breakage is explained by their rapid growth during the spring and summer. Enormous numbers of shoots and leaves are produced at the ends of branches, creating more weight than a single limb can support, especially when that limb is being hammered by the force of a Santa Ana wind.
The strength of the wood in both ash and eucalyptus trees is further compromised by pests. When the limb of an ash is unevenly broken off at the trunk, the wound created in the trunk does not heal properly. This leads to the invasion of fungi, which weaken the tree.
Ironically, the lerp psyllid insect that has lately stricken the foliage of many Valley eucalyptuses may have decreased the chance of limb breakage in those trees. Since there is less foliage, the weight of eucalyptus branches is less than normal and easier for limbs to support. Also, the fact that the canopy of a eucalyptus is less dense with foliage than usual – due to defoliation caused by the psyllid – will allow potentially destructive winds to blow straight through it.
One tree that seems to make the news every winter when one of its larger specimens falls over – blocking a road or crushing a parked car – is the Italian stone pine (Pinus pinea). This majestic tree, which may reach a height of 80 feet, is recognizable by the flat top of its canopy and the attractive mottling of its bark. Unless it receives light equally from all sides, it has a pronounced tendency to lean in the direction of greatest light exposure. An Italian stone pine that leans unmistakably to one side, and whose lean is not corrected through pruning, will be felled by a storm sooner or later.
On hillsides, trees that have a strong vertical growth habit or lean decidedly away from the slope are also prime candidates to be uprooted in storms.
Too much watering will be detrimental to the stability of trees, whether they are growing on level ground or on slopes. The most reliable trees for Valley slopes are varieties native to California and the Southwest that do not require summer irrigation, such as California incense cedar (Calocedrus decurrens) and Arizona cypress (Cupressus arizonica). The coast live oak (Quercus agrifolia) is also excellent for planting on slopes, as the hundreds of coast live oak specimens on the hillside between the Getty Center and the San Diego Freeway (405) have proven.