Coast Live Oak, Quercus agrifolia, San Diego, California

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• published: 22 Sep 2012
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The coast live oak begins its life as scrubby-looking plant that looks unremarkable and goes unnoticed by many a hiker, but produces the main food source for acorn woodpeckers in riparian zones where it grows. The coast live oak also goes by the names California live oak and Encina, with the scientific name Quercus agrifolia. The foliage is largely unattractive, consisting mostly of leaves that curl backwards, but the acorns are quite unlike the acorns of the oak trees of the Eastern United States. The coast live oak starts out as a shrub in many a riparian zone here in California, but given enough space and light can grow to be a towering giant of up to 30 meters tall and wide with a main trunk diameter of 3-4 meters. The tree can live up to 250 years. Many of the oak species in California hybridize with each other, and the hybrid offspring exchange genetic variation via introgressive hybridization, or the backcrossing between interspecific hybrids with either of the parent species. As you can imagine, this makes field identification of all the oak species tough at times...you never know when you are looking at a hybrid, and what percentages they derive their DNA from different parent species. As it turns out, the leaves start out a paler green and have serrated edges, and are said to have 2-3 layers of photosynthetic cells on the canopy. The leaves underneath the canopy tend to be darker and only have one layer of photosynthetic cells, and have a convex shape with the edges curled back to capture as much residual and reflected sunlight for photosynthesis as possible. It may seem ridiculous that plants would need to have all these adaptations to maximize sunlight absorption in a place like sunny San Diego, but the more sunlight (photons) a tree absorbs, the more sugar and growth it can produce, and hence dominate the landscape and become the tallest plant around. Even though a tree can live about 3 times as long as us, it still is very much locked in competition with the plants around it. Once the coast live oak wins and blocks out almost all of the sunlight from the understory, most of its rivals will be stunted in growth, die, or be unable to germinate, meaning its roots will face less competition for water and less inhibitory toxins from other plants. The tree will then go on to shower the landscape with acorns that acorn woodpeckers eat, and in the process of creating granary trees (see my other videos on acorn woodpeckers) mostly from California sycamores, further eliminates competition from California sycamore trees and provides new land for colonization by coast live oaks.