Sara Jo will be giving a talk about seed bank limitation and management at the 21st Annual California Invasive Plant Council symposium, which is being held in October in Sonoma County. She will present results from a study of 131 restoration sites in Orange County, California.
“You don’t just go from artichoke thistle to coastal sage scrub.” -Sara Jo Dickens, talking about undesired restoration trajectories
Dark view from the seed bank
by Joanna Hsu
Prior to assuming the role of graduate student, I had the more glorious (and well-paid) job of being a champion weed fighter at Crystal Cove State Park in southern California. “Environmental services intern” was my real title, but in my mind I was so much more than that. I brought death to the artichoke thistle and stoked fear in the heart of invasive mustards. Exotic annual grasses didn’t stand a chance against me. In my more enthusiastic moments – when my passion for native ecosystems really flared – even non-invasive exotics were not spared. In my arsenal were chainsaws, weed whips, and gallons of Roundup.
On days I wasn’t battling weeds, I was working on a restoration project at an old sewage leach field site. I sowed native seeds by the fistful and put hundreds of sagebrush, buckwheat, orange monkeyflower, and other coastal sage scrub seedlings in the ground. I diligently hand-watered transplants, regularly removed weeds and things aspiring to be weeds, and rubbed the leaves of the few adult native shrubs on the site for good luck.
But as I worked, I wondered – was this enough? Do our weed killing projects and expensive, labor-intensive restoration efforts achieve their goals? What really makes an ecological restoration successful? Would I return to the leach field after my journey to the far-off land of grad school to find a thriving native coastal sage scrub community?
Well, it is now several years later and Sara Jo Dickens, a post-doc in my lab, is working on exactly these questions. She has evaluated outcomes from 131 restoration sites in Orange County, including Crystal Cove State Park. I got a preview of her results at our lab meeting this week. The take-home point was this: restoration needs to account for soil seed banks. Seeds can sit in the ground for months or years before germinating or dying, and this seed bank is important because it is a source of new vegetation at restoration sites. At restoration sites without planting or sowing, it is the main source of new individuals — so what’s in the seed bank matters.
Well, it turns out that Orange County seed banks are rich in exotic species but poor in native ones. By germinating seeds from soil samples at each site, Sara Jo found that on average, less than 1 out of every 100 seeds is from a native local species. Equally troublesome is the fact that less than half of native species are represented in the seed bank. These findings imply that removing invasive vegetation alone is not enough to restore native ecosystems. In the soil, weeds are still present en masse, waiting for the first winter rains to germinate. But this potential for natives is limited at many locations by depleted seedbanks. The lottery is strongly, strongly in favor of the weeds.
We don’t yet know why seed banks in Orange County are so depauperate in natives. Is it a reflection of the long history of exotic annual dominance in these ecosystems? Are native seeds preferentially munched on by animals? Are no seeds arriving from nearby sites with natives? These are all questions that need to be answered. But in the meantime, restoration plans need to account for the fact that our restoration sites are full of seeds that we never want to see grow into adults. Management of the seed bank will be an important strategy going forward. And not all restoration seeding techniques are equal. Sara Jo found that broadcasting and hydro-seeding are effective for herbaceous and woody native species. Native grasses, however, have higher probabilities of establishment with imprint seeding, which involves rolling over seeds with a large machine or tamping of soils post-seeding.
My weed-wacking days at Crystal Cove are over, but the interest in restoration success in Orange County continues. Orange County has lost nearly all native grasslands and a significant portion of the coastal sage scrub community to development and exotic plant invasion. Artichoke thistle alone had covered over 4000 acres of the Nature Reserve of Orange County before management efforts began in the early 1990’s. Sara Jo’s research will directly help land managers in southern California (and future generations of environmental services interns) achieve better restoration outcomes. In addition to looking at seed banks, Sara Jo also studied how other site-specific factors such as soil type, land use history, and management practices influenced restoration success. To learn more, come listen to Sara Jo give a talk at the Cal-IPC Symposium in Sonoma County later this week or read her paper when it comes out.