Invasion ecology has traditionally focused on exotic plant species with early successional life‐history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade‐tolerant, late‐successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review reveals that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.

2016-12-24T18:13:07

Aim We examined relationships between climate-disturbance gradients and patterns of vegetation zonation and ecotones on a subtropical mountain range.Location The study was conducted on the windward slopes of the Cordillera Central, Dominican Republic, where cloud forest appears to shift in a narrow ecotone to monodominant forest of Pinus occidentalis.Methods Climate, disturbance and vegetation data were collected over the elevation range 1100-3100 m and in 50 paired plots along the ecotone. Aerial photographs were georeferenced to a high-resolution digital elevation model in order to enable the analysis of landscape-scale patterns of the ecotone.Results A Shipley-Keddy test detected discrete compositional ecotones at 2200 and 2500 m; the distributions of tree species at lower elevations were continuous. The elevation of the ecotone determined with aerial photographs was fairly consistent, namely ± 164 m (SD) over its 124-km length, but it exhibited significant landscape variation, occurring at a lower elevation in a partially leeward, western zone. The ecotone also occurred significantly lower on ridges than it did in drainage gullies. Ecotone forest structure and composition differed markedly between paired plots. In pine paired plots, the canopy height was 1.7 times higher and the basal area of non-pine species was 6 times lower than in the cloud forest directly below. Fire evidence was ubiquitous in the pine forest but rare in the abutting cloud forest. Mesoclimate changed discontinuously around the elevation of the ecotone: humidity and cloud formation decreased markedly, and frost frequency increased exponentially.Main conclusions The discreteness of the ecotone was produced primarily by fire. The elevational consistency of the ecotone, however, resulted from the overarching influence of mesoclimate on the elevational patterns of fire occurrence. Declining temperature and precipitation combine with the tradewind inversion to create a narrow zone where high-elevation fires extinguish, enabling fire-sensitive and fire-tolerant taxa to abut. Once established, mesotopography and contrasting vegetation physiognomy probably reinforce this boundary through feedbacks on microenvironment and fire likelihood. The prominence of the pine in this study -and of temperate and fire-tolerant taxa in subtropical montane forests in general -highlights the importance of climatedisturbance-biogeography interactions in ecotone formation, particularly where fire mediates a dynamic between climate and vegetation.

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.

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Summary Climate is widely assumed to influence physiological and demographic processes in trees, and hence forest composition, biomass and range limits. Growth in trees is an important barometer of climate change impacts on forests as growth is highly correlated with other demographic processes including tree mortality and fecundity. We investigated the main drivers of diameter growth for five common tree species occurring in the Rocky Mountains of the western United States using nonlinear regression methods. We quantified growth at the individual tree level from tree core samples collected across broad environmental gradients. We estimated the effects of both climate variation and biotic interactions on growth processes and tested for evidence that disjunct populations of a species respond differentially to climate. Relationships between tree growth and climate varied by species and location. Growth in all species responded positively to increases in annual moisture up to a threshold level. Modest linear responses to temperature, both positive and negative, were observed at many sites. However, model results also revealed evidence for differentiated responses to local site conditions in all species. In severe environments in particular, growth responses varied nonlinearly with temperature. For example, in northerly cold locations pronounced positive growth responses to increasing temperatures were observed. In warmer southerly climates, growth responses were unimodal, declining markedly above a threshold temperature level. Net effects from biotic interactions on diameter growth were negative for all study species. Evidence for facilitative effects was not detected. For some species, competitive effects more strongly influenced growth performance than climate. Competitive interactions also modified growth responses to climate to some degree. Synthesis. These analyses suggest that climate change will have complex, species‐specific effects on tree growth in the Rocky Mountains due to nonlinear responses to climate, differentiated growth processes that vary by location and complex species interactions that impact growth and potentially modify responses to climate. Thus, robust model simulations of future growth responses to climate trends may need to integrate realistic scenarios of neighbourhood effects as well as variability in tree performance attributed to differentiated populations.

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.

1. The study of invasiveness typically emphasizes early successional life-history traits in exotic plants, which enable the capture of high resources in disturbed environments and rapid growth.A key issue in invasion dynamics is whether such behaviours come at the expense of traits such as low-light survivorship, which allow species to become more dominant later in succession. 2. We used maximum-likelihood analysis to compare the growth and survivorship of two exotic trees, Ailanthus altissima and Acer platanoides, with nine dominant native tree species in closed-canopy forests in Connecticut, USA. Growth was modelled as a function of light and survivorship as a function of recent growth; combining models yielded estimates of light-dependent mortality. 3. The exotic species had strikingly high growth rates, exceeding all native species at light levels ‡ 10% full sun, and growing 2.6 times faster than the fastest-growing native species at 80% full sun. At low-light levels (< 3% full sun), however, growth rates of five native species exceeded both exotics. Exotic species survivorship (as a function of light-driven growth) was strongly dependent on the degree of shading: at 1% full sun, the annual mortality rate of A. platanoides was 10% and A. altissima was 17%; only two native species had higher mortalities. However, at 5% full sun, A. platanoides' mortality was < 1%, superior to all but three native species. Mortality of all species dropped to < 1% by 10% full sun, except A. altissima whose mortality remained high at c. 10%. 4. A life-history trade-off analysis (based on radial growth, height allometry and low-light survivorship) shows a nearly linear trade-off for most species. The native species and Ailanthus follow the common life-history trade-off of low-light survivorship vs. high-light growth. However, A. platanoides diverges from this trade-off pattern by combining very high growth rates with moderately high shade tolerance. 5. Simulations with SORTIE-ND (a forest dynamics model) indicate that poor survivorship of A. altissima will limit it to disturbed sites, whereas A. platanoides' unusual combination of traits makes it invasive in both disturbed and undisturbed forests. Overall, native shade-tolerant trees and slow stand dynamics make undisturbed forests highly resistant to invasion by exotic trees that are intolerant of shade. 6. Synthesis. This study showcases the importance of rapid growth in invasive plants, holding even for exotic tree species known to invade established forests. For A. altissima, high growth rates were accompanied by poor low-light survivorship. A. platanoides departs from the general trade-off pattern that exists among native species and A. altissima, and consequently it can be highly invasive in closed-canopy forests.