Backcountry Press

Obligate & Facultative Seeding Manzanitas

Elegant crossing of Tule Creek. Photo by Josh Smith.

Last weekend, I hiked in Trinity County along a low-elevation, fire-prone section of the Bigfoot Trail between Highway 3 and Hayfork and was able to witness obligate & facultative seeding in action.

From Field Guide to Manzanitas, Backcountry Press, 2012

One-third of manzanita species are facultative seeders. These are species that regenerate post-fire by both seed and burl resprouting. The remainder are obligate seeders that lose their entire adult population in a fire and depend on a seed bank for regeneration. Obligate seeding is the current model in manzanita evolution.

To understand why, consider the climatic dynamics over thousands, or tens of thousands of years or more. In the case of the resprouting species, particular individuals can live for centuries, resprouting over and over, cloning new individuals as the burls expand with each fire cycle. But in that population, the rate of genetic change is limited, because most individuals live a long time by way of asexual reproduction. This suggests that populations may be unable to respond to rapid climatic changes that might occur in only hundreds of years. The obligate seeders, on the other hand, lose all adults in stand-replacing fires and new post-fire generations have to establish from more genetically diverse seeds. Those populations consequently have greater flexibility to shift and adjust as circumstances require; traits that might have been rare and less important in older generations can emerge through natural selection and become critical in the newer generations within the lifetime of resprouting manzanitas.

Much of this area burned in the summer of 2015. While evidence of the fires were everywhere, there are many signs of the next generation of plants returning to the landscape. This was particularly true on some of the south-facing slopes above Philpot Campground where two species of manzanitas were exploring different reproductive regimes– both obligate & facultative seeding.

Arctostaphylos manzanita is a  facultatative seeder. It can resprout by seed but also from dormant buds. When present, these structures are often prominent and are called burls (also ligno tubers)–seen in the middle of the sprouting leaves.

Arctostaphylos canescens is an obligate seeder and does not have a burl. Instead, genetically unique seedlings resprout from a well-stocked seed bank.

Nature Notes:

A large burned stand of the obligate-seeding hoary manzanita, above Philpot Campground on the Shasta-Trinity National Forest.

One thought on “Obligate & Facultative Seeding Manzanitas

  1. Tom Parker

    This should be a good year to see seed germination and resprouting patterns in manzanitas. Lots of fires!

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