Brett was down for a few days, and as an initial run we did a loop from the Victory Trailhead of Ahmanson Ranch, through part of Las Virgenes Canyon, up onto Lasky Mesa, and then back to the trailhead.
Upper Las Virgenes Creek still flowed. Valley oaks were full with new leaves. The green grasses of the rain season had finally turned, and rare May cumulus clouds were painted on postcard skies.
It was an outstanding run!
According to preliminary NWS reports, Downtown Los Angeles (USC) recorded average monthly temperatures well below normal in November 2022 and January thru March 2023. March was particularly cool, with an average high of about 64 degrees and an average low of about 50 degrees.
Besides high heating bills, Southern California’s unusually cold Winter affected several seasonal processes, one of which was when the valley oaks at Ahmanson budded and grew new leaves.
The leaves of valley oaks at Ahmanson Ranch typically turn in mid-December, and the trees usually begin to leaf out during the latter half of February. Depending on the tree’s microclimate, the emergence of new leaves may vary by a week or more.
The earliest I’ve photographed a valley oak at Ahmanson sprouting new leaves was February 3, 2015. Last year, a valley oak had new leaves on February 10, 2022.
The latest I’ve seen valley oaks sprouting new leaves is this year — from about March 20, 2023, in favored locations to April 4, 2023, in cooler areas such as Las Virgenes Canyon.
Prior to this year, the latest I’ve photographed a valley oak at Ahmanson with new leaves was March 9, 2009.
(Officially named Upper Las Virgenes Canyon Open Space Preserve, most users refer to the open space area as Ahmanson Ranch or simply Ahmanson.)
Some related posts: Revitalized Valley Oak at Ahmanson Ranch, Goldfields Are Blooming and Valley Oaks Greening, Valley Oak Leaf
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.
Related posts: Ahmanson Blue Oak, Ahmanson Valley Oaks Battling Drought, After the Woolsey Fire: Ahmanson Ranch, Fallen Oak, Valley Oak Savanna
Update April 14, 2023. The Ahmanson Blue Oak has died. Please see the post Ahmanson Blue Oak Succumbs to Climate Change.
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.
The photograph of the leaves of a blue oak was taken on a run along East Las Virgenes Canyon to Lasky Mesa on April 24, 2007. A valley oak immediately adjacent to this blue oak was killed in the 2005 Topanga Fire.
Related posts: Ahmanson Blue Oak Succumbs to Climate Change, Valley Oak Savanna, Laskey Mesa Oak
Technical papers:
Modeled regional climate change and California endemic oak ranges.
Kueppers, L.M., M.A. Snyder, L.C. Sloan, E.S. Zavaleta, and Brian Fulfrost. 2005.
Proceedings of the National Academy of Sciences of the United States of America, 102(45): 16281 – 16286.
Limited hybridization between Quercus lobata and Quercus douglasii (Fagaceae) in a mixed stand in central coastal California.
Kathleen J. Craft, Mary V. Ashley and Walter D. Koenig.
American Journal of Botany. 2002;89:1792-1798.