Intact forests provide only weak resistance to a shade‐tolerant invasive Norway maple (Acer platanoides L.)

Abstract

Summary1 Intact, closed canopied forests appear highly resistant to exotic plant invasions, but there are few experimental studies of this observation. To test this issue and explore the conditions that foster resistance, we experimentally added Norway maple (Acer platanoides L.) seeds to intact forests for 3 years and monitored emergence, survivorship and height growth for 5 years. 2 Seed additions (250 seeds −1 m 2 ) were replicated in situ in combinations of light (deep shade vs. small gaps), soil fertility (NO 3 and pH), and variation in soil moisture as influenced by topography (hill slopes vs. flat areas at slope bases) in five eastern deciduous forests in central New York and southern Connecticut. We then parameterized a model with this data to project long-term rates of sapling recruitment. 3 Seedlings had high survival and low growth rates in the deep shade, suggesting that this species readily forms a seedling bank in intact forests. By age 5, annual survivorship reached 93% in deep shade and 98% in small gaps. Median seedling heights were ≤ 10 cm in all treatments after 5 years, though the largest seedlings were markedly taller in gaps. Mean year 5 densities of all experimental seedlings (ages 3-5 years) were 14 seedlings −1 m 2 (± 2.4 SE) in the shade and 19 seedlings −1 m 2 (± 3.5 SE) in small gaps. 4 While seedlings colonized in all conditions, resistance to invasion varied markedly; resistance was highest in deeply shaded, acidic conditions (pH < 4.5). Small gaps initially promoted invasion, but by age 4 shade survivorship rates were equivalent to gaps. Higher soil pH (especially > 6) also significantly increased survivorship, particularly in deep shade. Shading's main effect was to suppress height growth. Overall, the model predicted impressive sapling recruitment; even in deeply shaded, acidic conditions, approximately 1% of seeds emerge and survive to become saplings. 5 Intact forests only weakly resisted A. platanoides colonization, but strongly suppressed its rate of invasion. As such, the frequency of disturbance, though ultimately unnecessary for A. platanoides invasions, will strongly influence its near-term invasiveness. Dispersal limitation and slow stand dynamics appear to be the primary constraints on its current distribution.

Shedding light on shade: ecological perspectives of understorey plant life

“…Thus, intact, closed-canopy forests were believed to be resistant to exotic plant invasions. But intact forests only weakly resisted invasion by Acer platanoides in North America (Martin and Marks 2006). Once established, Acer platanoides exerted a profound change in understorey light quantity, becoming an important driver of suppression of native species and conspecific success in invaded riparian communities (Reinhart et al 2006).…”

Prunus serotina unleashed: invader dominance after 70 years of forest development

“…Shade-tolerant woody species are a particular challenge because they typically have long generation times as well as multiple lag phases during the invasion process (Martin and Marks 2006;Wangen and Webster 2006). This makes the early detection of the invasiveness of shade-tolerant species difficult, and may allow a species to consolidate its position (and its impact on the ecosystem) long before the need for control measures becomes obvious.…”

A Comparison of Invasive Acer platanoides and Native A. saccharum First-Year Seedlings: Growth, Biomass Distribution and the Influence of Ecological Factors in a Forest Understory

“…However, we found no difference in height between A. platanoides and A. saccharum. It is possible that differences in height growth rates (RGR-H) could appear later in their development or after an episode of release, with A. platanoides being a more efficient colonizer of gap openings [19,42]. Indeed, as saplings, A. platanoides have higher height growth rates, in addition to higher high-light radial growths, than A. saccharum [21,35,41].…”

“…Thus, A. saccharum is ecologically very similar to A. platanoides. Because A. platanoides can not only dominate the understory, but also reaches the canopy at least a decade before A. saccharum [14], there are concerns that the invader may be displacing its native congener [14,17,19,23].…”

“…It has been widely planted, in part, to replace Ulmus americana L. (American elm) decimated by Dutch elm disease [10]. While perturbations, such as the creation of edges, roads or disturbed soils, may favour A. platanoides establishment [11][12][13][14][15][16], its shade tolerance, abundant production of wind-dispersed seeds, and ability to establish very dense seedling banks allow this species to invade even relatively undisturbed, closed-canopy forests [15,[17][18][19][20][21].…”