Thursday, July 20, 2017

Live Oak Prunning

Because oaks are part of the urban environment and are often at the interface of wild or natural lands and urban landscapes, there are pruning needs. Since oaks attain great size and the wood is heavy, they can cause extreme damage should they fail. For large urban oaks, hazard reduction pruning is often a necessity. Removal of large dead branches is also necessary. Pruning for vehicular and pedestrian clearance is required for oaks used as street trees. Sometimes pruning is specifically for aesthetic purposes; to provide a new view, to expose the branch architecture of the tree or redirect growth of the tree. For young oaks trees, pruning is used to train them and to impose a specific architecture that will ensure sound limb attachments to maintain the tree’s structural integrity over its lifetime.

Pruning is the removal of branches, shoots and buds to achieve specific goals. This usually means removal of oak wood. A few things need to be mentioned about wood and its importance to trees.  Wood is a structural tissue. It provides the mechanical strength necessary for the support of large branches within a tree’s architecture.

Strength of a branch or trunk is proportional to the amount of sound wood (Harris et al., 2004). Wood is also a storage tissue. Trees store sugar as carbohydrate or starch in the wood of twigs and braches.  When disease causing fungi are introduced into the wood system of a tree, these stored carbohydrates are depleted (Fig. 1). The concept of carbohydrate storage is also important when transplanting large trees.

To understand how to prune oaks, two basic principles need to be reviewed. First, pruning retards or slows the growth of whatever is pruned. When leaves are removed, less energy will be captured by the tree and stored. The more you prune a branch or tree, the less its stem will grow, the less it will produce new wood and the less it will store the carbohydrate energy made by leaves. This is why pruning is a growth retarding process.

Sometimes when a tree is heavily pruned, the regrowth is abundant. A pollarded mulberry or sycamore can make over ten feet of growth in a single season. Pruning hardly seems to have slowed the growth of these trees. However, an identical unpruned tree will make more growth on its main stem than a pruned one. I demonstrated this in a study of young oaks with codominant stems. Pairs of very similar size codominant stems were selected and measured. Various pruning treatments (heading, thinning, unpruned) were applied to one stem while the other was left unpruned. In all cases, pruning reduced the stem caliper of the pruned stem relative to its unpruned counterpart.

The other basic pruning principle to remember is that pruning is a bud invigorating process. When branches are removed from trees, buds on the remaining branches are invigorated. For preformed buds such as terminal or axillary buds, this will result in vigorous growth of desired branches. If pruning has been severe (say more than 50% canopy removal), it is possible to invigorate latent buds that will result in the development of epicormic, randomly spaced branches. Epicormic branches or water sprouts are often poorly attached and lead to poorly developed branch architecture in the tree that subjects it to failure at a later date if those branches are not carefully pruned.