The conventional wisdom amongst native plant advocates is that native plants will return to the landscape if non-native plants are eradicated. In this post, we will examine this assumption and refute it.
Several different methods are used to eradicate non-native plants, but it doesn’t matter which method is used because the results are the same: native plants do not return when non-native plants are removed.
Spraying herbicides is a popular method of eradicating non-native plants because it is considered the most cost-effective method. In addition to the obvious health risks, the downside of herbicide use is that they are as likely to kill the natives as the non-natives. This problem is illustrated by a USDA study of the effects of a one-time aerial spraying of herbicides on grassland after 16 years. Although the herbicide is assumed to “dissipate” within a few years, the negative effect on the natives persisted 16 years later: “…the invasive leafy spurge may have ultimately increased due to spraying. Conversely, several desirable native herbs were still suffering the effects of the spraying,,,”
Even when native plants are removed, non-native plants occupy the cleared ground. Environmental scientists at UC Berkeley removed native chaparral from experimental plots in Northern California to test fuel reduction techniques using two different methods (prescribed burns and mechanical), in different seasons, over a period of several years. In every test, the result was on average from 23% (for prescribed burns)to 61% (for mechanical methods) non-native plants where they had not previously existed.
Jon E. Keeley (USGS) finds the same tendency for non-natives to replace natives in forests: “Forest fuel reduction programs have the potential for greatly enhancing forest vulnerability to alien invasions.” (1)
A scientist (2) arrived at the same conclusion after attempting to restore an oak-studded grassland on Vancouver Island. He tried several different methods of removing invasive grasses for several years only to find that “…the decline of the native plant species accelerated…”
Those who observe native plant restorations in the San Francisco Bay Area aren’t surprised by these studies. We know that native plant restorations are unsightly failures unless they are aggressively planted, irrigated for several years, and fenced. Examples of successful restorations can be seen at Crissy Field and the summit of Mt. Sutro. The East Bay hills provide examples of the opposite strategy. Where UC Berkeley has clear-cut all non-native trees and vegetation, non-native weeds quickly occupied the barren ground. After a particularly wet winter, the non-native poison hemlock in the East Bay hills is 6 feet tall along the roads.
Why are non-native plants apparently more competitive than native plants? Because the conditions that supported native plants 250 years ago, prior to the arrival of Europeans, have changed. The native plants are no longer well adapted to the current conditions.
Higher levels of CO2 and the associated climate change are promoting the growth of non-native plants. A USDA “weed ecologist” (3) studied the effects of higher temperatures and CO2 on the growth of non-natives (AKA weeds) by growing identical sets of seeds in a rural setting and an urban setting with higher temperatures and CO2 levels. Seeds grown in the urban setting produced substantially larger plants with much more pollen and therefore greater reproductive capability.
Other scientists reach the same conclusions by studying the changing ranges of native plants and insects. An ecologist at UC Berkeley (4) says that “California’s flora face a potential collapse…as the climate changes, many of these plants will have no place to go.” A scientist at the California Academy of Sciences (5) predicts that redwoods will disappear from California by the end of the century.
As the plants move, so do the insects and animals that need them. A study published in Nature magazine in December 2009 found that plants and animals must move as much as 6 miles every year from now to the end of the century to find the habitat they occupy now. An ecologist at UC Davis (6) has been studying native butterflies for over 35 years. He recently reported that native butterflies are moving to higher elevations, where temperatures are lower, but that ultimately, “There is nowhere else to go, except heaven.”
The local environmental organizations and public policy-makers must wake up to this reality and reorder their priorities. Instead of demanding that all non-native plants and trees be eradicated and that native plants be restored where they are no longer sustainable, they must make climate change their highest priority. The easiest and cheapest step to take to address this issue is to quit destroying healthy trees—just because they are non-native–that are sequestering tons of carbon.
(1) “Fire Management Impacts on Invasive Plants,” USGS, Jon E. Keeley, April 2006
(2) Andrew MacDougall, University of Guelph, Ontario, Canada, NY Times Magazine, 6/29/08
(3) Lewis Ziska, USDA, Beltsville, MD
(4) David Ackerly, UC Berkeley, Los Angeles Times, 6/25/08
(5) Healy Hamilton, Cal Academy, Center for Biodiversity Research
(6) Arthur Shapiro, UC Davis, Contra Costa Times, 1/19/10
Conservation Sense and Nonsense 
The Native Plant garden on Mount Sutro’s summit is well-established. It takes a lot of work, though: irrigation in the early years to establish the plantings (and they were planted, not just “allowed to return”); constant monitoring; and ongoing weeding. (They are herbicide-free, which may explain part of the success.) It’s a well-managed garden. This may not be replicable as a wild-land scenario.