Phenological niches and the future of invaded ecosystems with climate change

Abstract

In recent years, research in invasion biology has focused increasing attention on understanding the role of phenology in shaping plant invasions. Multiple studies have found non-native species that tend to flower distinctly early or late in the growing season, advance more with warming or have shifted earlier with climate change compared with native species. This growing body of literature has focused on patterns of phenological differences, but there is a need now for mechanistic studies of how phenology contributes to invasions. To do this, however, requires understanding how phenology fits within complex functional trait relationships. Towards this goal, we review recent literature linking phenology with other functional traits, and discuss the role of phenology in mediating how plants experience disturbance and stress-via climate, herbivory and competition-across the growing season. Because climate change may alter the timing and severity of stress and disturbance in many systems, it could provide novel opportunities for invasion-depending upon the dominant climate controller of the system, the projected climate change, and the traits of native and non-native species. Based on our current understanding of plant phenological and growth strategies-especially rapid growing, early-flowering species versus later-flowering species that make slower-return investments in growth-we project optimal periods for invasions across three distinct systems under current climate change scenarios. Research on plant invasions and phenology within this predictive framework would provide a more rigorous test of what drives invader success, while at the same time testing basic plant ecological theory. Additionally, extensions could provide the basis to model how ecosystem processes may shift in the future with continued climate change.

Keywords: Alien or exotic species; climate change; invasions; non-native; phenology; plasticity; temperate systems..

Figure 1.

Basic invasion theory, built on limiting similarity theory, suggests that species should invade during times when most other species are inactive (vacant phenological niche; see Wolkovich and Cleland 2011). Here we show idealized niche diagrams for four non-native species (purple, dashed-line distributions) and seven native species (grey distributions) in a hypothesized simple mesic temperate system where temperature limits viable periods for plant growth. Across the growing season, variation in stress, disturbance and competition may dictate the optimal phenological strategy, with early-active and late-active species experiencing lower competition but also more variable temperatures, in the mid-season community flowering peaks (see Fig. 2), and thus we expect that mid-season active species may be strong competitors for many resources. With climate change extending viable periods for plant growth (dark blue lines), non-native species with highly plastic phenologies may have an increased opportunity for invasion at the start and end of the growing seasons in temperate mesic systems. As reviewed in Wolkovich and Cleland (2011), there are several major ways in which species may exploit such vacant phenological niches. Species that can track the start of the season closely may exploit even very small vacant niches in the early season via priority effects. Additionally, climate change—by extending growing seasons in many systems—may increase vacant niche space at the start and end of the growing season, possibly allowing for invasions early and late in the season. Non-native species with early phenology and rapid growth strategies may succeed either early or late in the season, while species with greater phenological niche breadth (e.g. longer flowering period) may succeed late in the season.

Figure 2.

Flowering of non-native and native species within a community varies across habitats. This variation in flowering patterns may be strongly driven by climate differences between systems, which impact the various flavours of stress, disturbance and competition that plants experience. Mesic temperate systems such as Chinnor (UK, A) are often defined by temperature (darker blue shading), while other systems such as the tallgrass prairie of Konza (Kansas, USA, B) may have variable drivers across the growing season. In both systems, temperature sets the beginning and end of the season and, as such, early-season species show strong sensitivity to temperature (Cook et al. 2012; Craine et al. 2012b). In Konza, a consistent mid-season drought, however, coincides with a decrease in the number of species flowering at that time (Craine et al. 2012b). We assume that temperature <5 °C limits development, as this is the temperature at which many cell processes slow down dramatically or stop (Larcher 2003) and, further, is the suggested lower threshold temperature for tissue growth and development globally for alpine trees (Korner 1998). Standard deviation (SD) of temperature and the coefficient of variation (CV) of precipitation are given as pentad (5-day) averages. Flowering data are species averages from NECTAR (Wolkovich et al. 2012b), climate data for Chinnor were taken from GHCN UK000056225 and cover 1853–2001, while climate data for Konza were taken from GHCN USC00144972 and cover 1893–2010.

Figure 3.

Predictions for how climate change may promote invasions vary across the growing season, across systems with differing dominant climate regimes, and by how climate shifts (red arrows refer to temperature increases, while blue arrows refer to precipitation increases or decreases). Here we consider three major systems and how dominant climate drivers are projected to shift with climate change, based on recent models (Knutti and Sedlacek 2013). Because models of precipitation shifts are often divergent (Knutti and Sedlacek 2013), for systems with precipitation control we consider increases or decreases in precipitation. In all systems an increase in the dominant climate factor that controls the start of the season may increase invasions by species that can track this shift closely (invader plasticity). Because we suggest that successful invasions are rare in times of very high resource competition and extremely high climatic stress and disturbance, we do not predict invasions during periods when competition is already high (mid-season of many systems) or when climate change increases drought stress (declines in precipitation in semi-arid systems or during mid-season drought in grasslands). When climate change pushes systems far beyond their historic climate regimes, however, native species may be pushed well away from their optimal climate, and we may see an increase in invasions. See the main text for further details, including background assumptions leading to predictions.