The tipping point of invasive plants

Our neglect of invasive plants is locking us in to future consequences that will affect timber, fisheries, agriculture, and tourism industries.

It took 100 years for English ivy to cover 50 percent of Seattle’s urban forests, where invasive trees now outnumber native tree sprouts. Ivy’s spread toward our region’s forests is slow yet relentless, while English holly is increasing almost exponentially, doubling every six years, and having “the potential to become a dominant species in both number of individuals and area covered within a few decades… (transforming) the region’s native forests on a large scale.” (Dr. David Stokes, UW Bothell).

The spread of ivy and holly will soon be unstoppable, if we do nothing. Widespread neglect will result, in another century or two, in the loss of complex diversity needed for pollinator, fish, and wildlife habitat region-wide. This ecological time bomb – horrific for its own sake – is clearly a financial time bomb for our state and economy.

This is a state of emergency, with irreversible consequences. The longer we wait, the more expensive it will be. Without broad cooperation, efforts to restore forests will be constantly compromised by re-introductions of invasive seeds from neighboring land, squandering public and private restoration money. We have no rational choice but to motivate invasive control for all land owners and managers.

Mandates for control will be futile for a public resistant to government regulations. More than 90% of citizens voluntarily restoring their backyard Environmentally Critical Areas avoid the ECA permits and SEPA regulations – regulations that can discourage and delay restoration, crippling our race against time.

An incentive approach could address ECA and SEPA concerns with better outcomes than regulations.

My suggestion is an “eco-rating” program that measures environmental attributes: tree cover, stormwater capacity, and invasive control. King County has a “PBRS” Public Benefit Rating System, but it enrolls at most 200 properties per year. Rather, private certified assessors could calculate public benefits for interested homeowners, obtaining a rating easily applied to property tax assessments.

Property owners could earn individual property tax breaks, plus collective tax breaks where everyone in an eco-district receives a nominal deduction for collectively increasing tree cover, for example (tempering the demand for views), or for eradicating knotweed in a watershed (a plant very harmful to salmon habitat).

An eco-rating would be optional, educational, and motivating to owners wanting to better manage their land.

Property tax breaks are currently not possible given the State’s “Uniformity Clause” constitutionally preventing variable property taxes. Changing this clause is critical to motivating other public goods, like incentives for urban density and emergency preparedness – addressing housing and public safety issues.

Property tax breaks to save our forests would be a good opportunity to shift any revenue losses to other taxes – like carbon taxes to make our taxation more sensible. We need to raise real money for salmon recovery, but a “tax shift” could keep revenue relatively neutral for participating landowners.

If not property tax breaks, then use fees, which can be legally adjusted. Municipalities could partner to bundle stormwater, wastewater, and conservation-district fees to create larger annual, repeating incentives so that restoration investments pay off. Fee breaks could be balanced with an Invasive Species fee, fees on invasive plants that nurseries continue to sell, and compensated by the savings of avoiding new stormwater facilities.

The following are references for the threat level of holly, one of many invasive plants.

Pictures of an Invasion: English Holly (Ilex aquifolium) in a Semi-natural Pacific Northwest Forest. Dr. David L. Stokes, Elliott D. Church, David M. Cronkright, and Santiago Lopez, University of Washington, Bothell, 18115 Campus Way NE, Box 358530, Bothell, WA 98011-8246 USA

“Holly is proliferating and spreading rapidly at two scales: contiguous, primarily vegetative, expansion of tree clumps, and long distance dispersal via seed. Spread by both mechanisms appears to be accelerating, with population and canopy area both increasing approximately exponentially, with doubling times of approximately 6 and 5 years respectively. Projecting past spread patterns forward suggests that holly has the potential to soon become a prominent species both in number and canopy extent, likely at the expense of native plant diversity and forest structure. Based on these results, we offer recommendations for holly management in forested areas in the region.”

“As one of the few invasive plants apparently able to colonize closed-canopy Pacific Northwest forest (Gray 2005), holly may have the potential to transform the region’s native forests on a large scale.”

“Trees > 10 years old appeared to have very low mortality rates and exhibited accelerating rates of size increase and biomass accumulation with age. Native vegetation was greatly reduced under holly canopy.”

“It is a small-to-medium-sized (up to 23 m (70+ feet) height in its native range [Peterken and Lloyd 1967]) dioecious evergreen tree that can reproduce both vegetatively and from seed.”

“The spread of non-native species is emerging as one of the greatest threats to biodiversity (Wilcove et al. 1998, Primack 2010). Invasive non-natives can have diverse and far-reaching effects on native ecosystems, including the elimination of native species and alteration of physical structure and ecological processes (examples in Lockwood et al. 2007). Invasive plants pose a particularly serious threat to natural areas. …While (holly) is widespread, it appears to still be at a level that is amenable to control. Our future projections suggest that delaying control for even a few years will result in a substantial increase in number of holly trees.” Dr. David L. Stokes.