During a recent visit to Burney Falls State Park (Shasta County CA) the oaks in their fall colors appeared particularly brilliant this year.
There are two native deciduous oaks in our area – the California black oak (Quercus kelloggii) and the Oregon white oak (Quercus garryana). In the spring as the two species begin to bud, the black oak looks reddish because the young leaves are red when they first emerge. The white oak is more green or yellowish green as the leaf buds open. (See “Oaks in Spring” posted on 03-05-2014) Leonard and I wondered if this same color pattern persisted in the autumn. Most guide books state that in the fall white oaks are yellow brown, occasionally red or orange, and black oaks are yellow to yellow orange. Our completely unscientific survey of the oaks at Burney Falls found trees of both species with yellow leaves. The white oaks had yellow and red leaves, while the black oak leaves were yellow. (White and black oaks hybridize so there were a few apparent crosses that did not follow the pattern or that had leaves of a more intermediate orange color.) Generally, the California black oak and the Oregon white oak did not follow the “spring color pattern”.
Leaves have three pigments: 1) green chlorophylls that are continuously produced during the growing season 2) yellow carotinoids which are always present in the leaves and 3) red to blue or purple anthocyanins. Anthocyanins are produced in the fall in response to excess sugars in the leaves. Fall colors depend on the proportion of each pigment present.
The length of the photoperiod begins the chain of events that causes leaves to change from green to their fall colors. Longer nights cause the production of chlorophyll to slow and eventually all the chlorophyll is destroyed. Once the chlorophyll is gone the other pigments can be seen.
Some years the colors appear brighter and more varied. This difference is related to weather conditions, namely temperature and the amount of moisture in the soil, that occur both before and during chlorophyll breakdown. Warm days and cool (not freezing) nights with some rainfall yield the most spectacular colors. Yellow pigments are always present so the amount of yellow does not vary much from year to year. During ideal conditions for red anthocyanin production, sugars produced in the leaves are prevented from moving out of the leaves by a gradual constriction of the leaf veins caused by cooling. Excess sugars in the leaves spur anthocyanin production and thus there are more red leaves. During a year that is optimal for anthocyanins, not all leaves turn shades of red because not all trees produce anthocyanins.
This year conditions must be optimal for anthocyanin production because there seems to be more red among the fall colors.
A previous post (“California Black and Oregon White Oaks” on 08-30-2011) discusses the two oak species.
No matter the mechanisms, we are experiencing a beautiful display of color this fall.