Populus trichocarpa black cottonwood

This native tree, uncommon in San Mateo County, grows in the lower reaches of streams, including Jasper Ridge Biological Preserve, where a close-up look can be afforded near the base of Searsville Dam. Sections of Corte Madera Creek upstream of Searsville Reservoir and San Francisquito Creek below the reservoir also have black cottonwood stands.

The entire San Francisquito Creek watershed below the dam in the Preserve has a notable diversity of native plants, often abundant, in spite of the establishment of french broom and silver wattle along streambanks. Woody plants including white alder, California hazlenut, and brown dogwood are occasional; ninebark is abundant. Locally rare and uncommon perennial understory plants include wild licorice, torrent sedge, durango root, swards of knot grass, water pepper, broadfruit bur-reed, and the vine creek clematis.

Sections of Corte Madera and San Francisquito creeks qualify as black cottonwood vegetative alliance according to the Manual of California Vegetation, even though many overstory cottonwoods have died and fallen in recent years. What is obvious from walking the creekbed is that regeneration is robust and cottonwood thickets are common. New trees are in the making. The same situation is also the case for white alder.

Illustrations: Jasper Ridge photos of P. trichocarpa.