Hitching a ride: Seed accrual rates on different types of vehicles

Abstract

Human activities, from resource extraction to recreation, are increasing global connectivity, especially to less-disturbed and previously inaccessible places. Such activities necessitate road networks and vehicles. Vehicles can transport reproductive plant propagules long distances, thereby increasing the risk of invasive plant species transport and dispersal. Subsequent invasions by less desirable species have significant implications for the future of threatened species and habitats. The goal of this study was to understand vehicle seed accrual by different vehicle types and under different driving conditions, and to evaluate different mitigation strategies. Using studies and experiments at four sites in the western USA we addressed three questions: How many seeds and species accumulate and are transported on vehicles? Does this differ with vehicle type, driving surface, surface conditions, and season? What is our ability to mitigate seed dispersal risk by cleaning vehicles? Our results demonstrated that vehicles accrue plant propagules, and driving surface, surface conditions, and season affect the rate of accrual: on- and off-trail summer seed accrual on all-terrain vehicles was 13 and 3508 seeds km, respectively, and was higher in the fall than in the summer. Early season seed accrual on 4-wheel drive vehicles averaged 7 and 36 seeds km on paved and unpaved roads respectively, under dry conditions. Furthermore, seed accrual on unpaved roads differed by vehicle type, with tracked vehicles accruing more than small and large 4-wheel drives; and small 4-wheel drives more than large. Rates were dramatically increased under wet surface conditions. Vehicles indiscriminately accrue a wide diversity of seeds (different life histories, forms and seed lengths); total richness, richness of annuals, biennials, forbs and shrubs, and seed length didn't differ among vehicle types, or additional seed bank samples. Our evaluation of portable vehicle wash units showed that approximately 80% of soil and seed was removed from dirty vehicles. This suggests that interception programs to reduce vehicular seed transportation risk are feasible and should be developed for areas of high conservation value, or where the spread of invasive species is of special concern.

Running off the road: roadside non-native plants invading mountain vegetation

“…In Australia at least, the positive relationship between non-native species richness on roads and non-native species richness in adjacent habitat suggests that a simple approach to non-native species management will be to reduce non-native species richness on roads. This might be achieved with regular application of herbicide or mechanical removal, and vehicle and visitor hygiene where practicable (Rew et al 2018). Where herbicides are frequently applied, managers should be mindful of secondary invasion; i.e.…”

Mountain roads and non‐native species modify elevational patterns of plant diversity

“…A feature of many mountains is that they have roads that connect lowlands with high elevation sites. Compared to semi‐natural habitats more distant to roads, the habitat directly next to the road is often characterized by high rates of disturbance and reduced competition (Forman & Alexander, ; Spellerberg, ), plus increased propagule pressure and potential for long‐distance dispersal by vehicles (Rew et al, ; Taylor, Brummer, Taper, Wing, & Rew, ; von der Lippe & Kowarik, ). Such habitats also have more homogenous abiotic conditions as a result of similar construction techniques and management practices (e.g.…”

Synergy between roads and disturbance favourBromus tectorumL. invasion

“…Previous studies also show that invasive species progressively spread from initial introduction areas as roadsides (Tyser & Worley, 1992; Gelbard & Belnap, 2003). Roads may be acting in Patagonia as conduits for the spread of B. tectorum seeds as in the USA (Gelbard & Belnap, 2003), where road maintenance and vehicles contribute to human aided dispersal of invasive species seeds at short and intermediate distances (Rauschert, Mortensen & Bloser, 2017; Rew et al, 2018). However, their importance depends on disturbance level.…”

“…As a consequence, road verges may sometimes harbor short lived, fast growing species which allocate a large proportion of their photosynthesis products to seed output (Frenkel, 1977), many of which are introduced species (Gelbard & Belnap, 2003; Trombulak & Frissell, 2000). The linear arrangement of roads, their maintenance works and ordinary traffic, increase plant species dispersal along roadsides, particularly of introduced species (Forman, 2003; Lembrechts et al, 2016; Dainese et al, 2017; Rew et al, 2018). …”