When I rounded the corner on the Lasky Mesa Trail, I could hardly believe my eyes. The Ahmanson Blue Oak was gone. Where there had been a sprawling oak, there was nothing.
Crossing an eroded section of trail and walking over to the edge of the old roadbed, I looked down the slope. Much like this valley oak along Rocky Peak Road, the entire Ahmanson blue oak had fallen from its hillside perch near the bottom of the canyon.
Oaks in the oak-grasslands of Upper Las Virgenes Canyon Open Space Preserve have had a tough time with climate change. The five-year drought from July 2011 to October 2016, increasing temperature, and the 2018 Woolsey Fire have combined to kill a large number of trees.
This blue oak was one of very few found at the southern extent of its range.
Illuminated by a just-risen sun, Goat Buttes reflected sharply on the lake’s surface. Ducks squabbled near some reeds and a bullfrog’s resonant croaking filled the canyon.
Part way through the Bulldog Loop, I’d paused for a moment at Century Lake in Malibu Creek State Park to enjoy the tranquility of the early morning. I snapped a photo and then noticed something very disturbing. The hundred year old coast redwoods across the lake looked brown.
Although coast redwoods have been planted in several areas of Southern California, they do not occur naturally here. The southernmost stand of naturally-occurring coast redwoods is about 200 miles north of Malibu Creek State Park in the Southern Redwood Botanical Area of Las Padres National Forest.
Redwoods have widespread, but shallow, root systems. Drought and warming temperatures are a worst case scenario for these trees, with the upper layer of soil being moisture-starved and baked.
Ironically the redwoods closest to the lake appear to be the most severely affected. This tree away from the lake on the Forest Trail appears to be in better shape, but it too is showing signs of stress.
Malibu Creek State Park isn’t the only locale in Southern California where redwoods are dying. According to this May 2015 San Gabriel Valley Tribune article, 15 redwoods were removed from Verdugo Park in Glendale, and redwoods in other areas of Southern California have also been affected.
As mentioned in an earlier post, the 2004 article “What’s up with the redwoods?” by James Downer, discusses a dramatic decline in coast redwoods planted in Ventura County and describes some of the problems that can affect this tree.
Drought and climate impacts are not limited to redwoods in Southern California. Endemic redwoods, particularly those in the southern extent of their range have also been significantly impacted.
Blue oak (Quercus douglasii) is a characteristic oak of the lower elevations of the Sierra foothills and coastal mountains, its range essentially encircling California’s Central Valley.
Based on its reported range, Upper Las Virgenes Canyon Open Space Preserve (formerly Ahmanson Ranch) contains one of the southernmost populations of blue oak within its range. However, the predominate deciduous oak in the area is valley oak (Quercus lobata), and occurrences of blue oak appear to be rare. Regional climate modeling suggests that over the next century the range of blue oak could shift northward and diminish to nearly 60% of its current range.
Blue oak is reported to hybridize with valley oak, however a 2005 study suggests hybrids of these species may be more rare than generally accepted.