Vegetation ecology with anthropic drivers and consequences

1 Pollution

The impacts of anthropogenic pollution on vegetation vary by pollutant (Stevens et al., 2020). Although the most obvious impacts of pollution are the wholesale changes in vegetation structure caused by heavy metal pollution near smelters (e.g. Louback et al., 2016), the most widespread anthropogenic pollution results from nitrogen deposition (e.g. Battye et al., 2017; Oke and Hagar, 2020) and acid rain (e.g. Zhu et al., 2020). Here, we will focus on nitrogen flux.

Despite early work on vegetation response to nitrogen loading (Mikk and Mander, 1995), much of the research on nitrogen flux is within ecosystem and physiological ecology rather than in vegetation ecology (Stevens et al., 2015). However, change in any important plant resource is likely to change the processes underlying plant community composition and be reflected in plant abundance, diversity, and geographic pattern. Further, the biogeochemical processes are controlled by microorganisms, and microorganism diversity directly relates to vegetation diversity (Wagg et al., 2019). Nitrogen flux is a global-scale phenomenon, but its local effects on vegetation are the usual foci of vegetation ecologists. The deposition of atmospheric nitrogen in ecosystems is one target of such globally connected research (Li et al., 2020); however, local sources and sinks also matter (e.g. Liberati et al., 2019). Diekmann et al. (2019) illustrated a vegetation-centered approach to diversity in managed grasslands and found nitrogen deposition was negatively associated with diversity, but only in combination with direct fertilization and management such as mowing and grazing. Also directly linked to human drivers, Wieder et al. (2019) identified changes in the balance of species in boreal fens and bogs in response to experimental nitrogen additions that depicted critical loads associated with deposition from oil sands development in Canada. As an example of the global problem of atmospheric deposition, Zhu et al. (2020) assessed its impact on degraded grassland communities in China and showed that nitrogen deposition decreased stability, among other responses.

Vegetation ecology can also contribute to responses to these problems. Establishing standards for critical loads is an important frontier for communicating vegetation ecology to decision makers (Symstad et al., 2019) and to extend efforts to mitigate impacts (Barwise and Kumar, 2020; Clark et al., 2019). Vegetation ecology can also inform efforts at phytoremediation of excess nitrogen in terms of ecosystem services (Zalesny et al., 2019).

2 Invasive species

Invasive species can change the relative abundance or diversity of plant communities and are a topic of vegetation ecology when not focused on single or paired species. Invasive species and invasiveness are a human dimension of ecology because humans intentionally or unintentionally assist in the movement of species and/or alter land-use conditions in ways that allow expansion (Essl et al., 2020; Richardson and Pyšek, 2006). Vegetation ecologists often focus on the invasibility of plant communities (e.g. Catford et al., 2019), interactions with extant vegetation (Proença et al., 2019), the consequences of invasion in terms of change to community structure and diversity (Vetter et al., 2020; Yu et al., 2016), and the ways in which invasive species are managed (e.g. Mikulyuk et al., 2020). Interaction with other drivers highlighted in this report, such as fire, are also a focus because of the potential for invasives to modify disturbance regimes (e.g. Barker et al., 2019). Additional effort is needed to understand invasions in the context of climate change (Shabani et al., 2020) and the probability of invasions based on functional traits (Mathakutha et al., 2019).

The vegetation ecology of invasive species extends beyond invasive plants when other invasive organisms affect vegetation. It is well-known that invasive pathogens lead to cascades from host species across entire plant communities. Recent sudden oak death and white pine blister rust outbreaks illustrate processes wherein characteristics of the vegetation affect disease spread (i.e. because of alternate hosts) and the mortality of keystone species affects plant community structure and diversity (Dillon and Meentemeyer, 2019; Tomback et al., 2016). Responses to pests and weeds, especially in agriculture, can be informed by vegetation ecology (e.g. more diverse planting strategies) to make them more efficient and environmentally friendly (Stenberg, 2017).

Vegetation per se is also considered in efforts to detect, monitor, and respond to invasions. For the first two, research evaluates the effectiveness of remote sensing methods for these applications, and ranges from spatial and radiometric coarse-scaled satellite imagery to finer scales even including drones, and their combination (Campbell et al., 2020; Kattenborn et al., 2020; Kopeć et al., 2020; Rivas-Torres et al., 2018). Responses also include the development and use of integrated pest management plans that incorporate a diversity of tactics to remove invasive species, minimize herbicide resistance (Owen et al., 2014), and reduce potential impacts to property, people, and the environment (US Environmental Protection Agency, 2019).

3 Climate change

Vegetation change in response to climate change is a burgeoning area of research and is a core dimension of the Intergovernmental Panel on Climate Change assessments of the impacts of climate change (Arneth, 2015). As noted above for nitrogen, change in important plant resources will affect the abundance, diversity, and geographic patterns of vegetation. Water and energy are direct resources for plants, and their availability and usefulness are tied to the primary climatic variables of precipitation and temperature, including soil temperature and length of the growing season (Myers-Smith et al., 2019), and even to secondary ones such as cloudiness and disturbance by storms. Vegetation ecology has addressed ongoing climate change for decades, arising from the work of paleoecologists. At a 1988 symposium, Webb (1994) summarized the lessons derived from research on past vegetation changes and Woodward (1994) did the same for recent changes. Other papers at the symposium also addressed specific types of vegetation (Peters and Lovejoy, 1994). Vegetation ecology can change the human processes affecting the climate through an indirect policy pathway. Research that shows northward and upslope change in species ranges (Anderson et al., 2020; Osland et al., 2017) has recently appeared in popular media to demonstrate the effects of climate change (e.g. https://www.cnn.com/2020/01/10/asia/plants-everest-climate-intl-scli-scn/index.html).

While carbon capture by vegetation is primarily a topic of ecosystem rather than vegetation ecology, the latter is also relevant to changing this driver through human activity. The restoration of forest cover and/or afforestation to help mitigate the impacts of climate change (Bastin et al., 2019) is one area in which knowing the plant community structure and resilience of different forests will be important (Staal et al., 2020). Forest restoration and/or afforestation are likely to have an increasing role in vegetation ecology, but terrestrial carbon capture is not a complete solution to ongoing climate change (Heck et al., 2018).

4 Land-use change and direct exploitation

We combine these categories of global change because the direct exploitation of vegetation is almost always a matter of land use. The primary effect of land-use change on vegetation is through habitat destruction and degradation (Alroy, 2017; Correa Ayram et al., 2016; Gonçalves-Souza et al., 2020; Montibeller et al., 2020), and much of the research in vegetation ecology is on its response (Bähner et al., 2020; Rocha-Santos et al., 2016). Ecologists have studied vegetation responding to many types of human disturbances: agricultural (e.g. De la Pena et al., 2016; Nagy et al., 2015; Owen et al., 2020), forestry (Bolton and D’Amato, 2019; Haapalehto et al., 2017; Hu et al., 2016; Pastur et al., 2020), grazing (Kirkpatrick et al., 2016; Stein et al., 2016; Huang et al., 2017), and multiple other disturbances (Bowd et al., 2018; Ellis et al., 2020; Ton and Krawchuk, 2016) – not all negative (Gedan and Fernández-Pascual, 2019).

Much of the research in vegetation ecology related to land-use change has focused on remnants, which are areas of pre-existing vegetation that were not heavily disturbed by the change. Even remnants, however, are often degraded by their creation, isolation, and the nature of the surroundings (Kupfer et al., 2006; Reider et al., 2018). However, land uses have not always degraded vegetation, and long-term but limited-intensity human activities have the potential to increase biodiversity at local to regional scales (Oberndorfer et al., 2020; Odonne et al., 2019).

New frontiers in this research area aim toward integrating both social and ecological approaches toward practical applications in land-use planning and managing vegetation for a range of social and ecological benefits (Meerow and Newell, 2017; Monteiro et al., 2020). Although calls for more engagement across perspectives has a long history connected to vegetation ecology (Malanson, 2014; Potschin and Haines-Young, 2011), additional research is needed to connect these frames.

1 Fire and vegetation ecology

Wildland fire affects species abundance by causing direct mortality, modifying environmental variables that affect demographic processes, and altering intra- and inter-specific interactions by changing densities and community composition (Burkle et al., 2015; Marshall and Falk, 2020). Fires often increase species diversity locally because of variation in recovery trajectories and return intervals, and regionally because of the mosaic of patches wildfire creates (He et al., 2019). Wildland fires also create and maintain community boundaries by initiating and maintaining state changes such as converting forest to a different forest type or non-forest types (Coop et al., 2020), thereby resetting successional processes and preventing the incursion of trees and shrubs (Transeau, 1935).

Wildland fire is one of the most important natural hazards addressed by vegetation ecologists because vegetation is fuel (Keeley and Syphard, 2019) in an increasing wildland urban interface (Ager et al., 2019). Humans have a long history of directly and indirectly modifying fire regimes (e.g. their frequency, severity, size, and seasonality; Krebs et al., 2010) by increasing ignitions, modifying fuel landscapes (including invasive species), and altering climatic conditions associated with ignition and spread (Abatzoglou et al., 2019; Balch et al., 2017; Fill et al., 2019; Fusco et al., 2019). Wildland fire is a land-use tool (e.g. Marlon et al., 2008); promotion or suppression of fire activity has long facilitated settlement, agriculture, hunting, and foraging (Ellis et al., 2013). Climate change is now altering fuels and fire regimes (Abatzoglou and Williams, 2016; Westerling et al., 2006). The resulting changes in vegetation and climate/vegetation feedbacks affect the probability of future fire occurrence, spread, and severity (Harvey et al., 2016; Taylor et al., 2016). The most notable example of these interacting effects is in Amazonia, where anthropic burning modifies local and regional climate and fuels build-up, which can increase the severity and size of future fires, with already catastrophic consequences (Barlow et al., 2020). The relative contribution of climate and the legacy of land-use policies (e.g. fire suppression) to changing regimes is still under debate (Parks et al., 2016). The future effects of climate change on fire regimes will be mediated by continued land-use change, with directions dependent upon current regimes and actions (Fonseca et al., 2019).

Malanson et al. (2020) noted that Native Americans, First Nations, and other indigenous communities integrate traditional ecological knowledge and vegetation ecology to improve habitat restoration, and wildfire and climate change management. This type of management advances understanding of fire and fuels ecology (e.g. Wynecoop et al., 2019) and promotes diverse and healthy ecosystems that minimize loss of human life and property (Lake et al., 2017). There are numerous collaborations between Native American tribes and government agencies and non-governmental organizations that aim to integrate traditional burning into fire management to improve ecological, cultural, and conservation outcomes (Burr, 2013). Intercultural collaboration and the use of traditional ecological knowledge in regions across the globe help to acknowledge multiple perspectives and build a collective adaptive learning environment for evaluating fire management (Mistry et al., 2019).

2 Urban vegetation

Urban vegetation ecology is the study of plant communities and their interactions with the environment within cities. Various plant communities can be found in urban green spaces such as residential yards, parks, golf courses, community gardens, green roofs and walls, bioswales, green streets, urban forests, and natural remnants (Nielsen et al., 2014). Urban green spaces are often under the pressures of compact space, pollution, and high demand from growing populations and development (Huang et al., 2021). As the population size of many cities grows and land-use activities intensify, the many social and ecological benefits of vegetation within cities are of critical importance (Benedict and McMahon, 2006; Nielsen et al., 2014).

An increasing number of studies address long-standing questions in vegetation ecology with urban data. These include community assembly (Johnson et al., 2018), environmental (e.g. urbanization) gradients (Kuglerová et al., 2019), invasions (Kühn et al., 2017), and relationships of habitat and diversity (Talal and Santelmann, 2019). While intense urbanization is oftentimes correlated with decreased animal and plant species richness, most plant studies show increased plant species richness with moderate levels of urbanization (McKinney, 2008). In addition, when environmental managers preserve large, remnant trees, facilitate habitat connectivity between vegetation patches, and increase native plant species richness, these management actions may also lead to greater animal biodiversity (Barth et al., 2015; Threlfall et al., 2016).

There are many potential ecological and socio-cultural benefits of urban vegetation and green spaces, but these may not be shared equitably across diverse communities (Nesbitt et al., 2019). Urban residents with lower socioeconomic and underrepresented racial-ethnic backgrounds oftentimes have access to fewer acres of urban parks, and the ones that are accessible tend to have less safety, quality, and maintenance (Rigolon, 2016). These residents also tend to live in neighborhoods with greater amounts of heat stress, which is correlated with sparser vegetation and shade, a lack of open space, and high settlement density (Harlan et al., 2006). Women, elderly, and urban residents with mental or physical disabilities also may face limitations in accessing the benefits of urban green space due to inadequate facilities, maintenance, safety, and/or programming (Lynch et al., 2019; Sonti et al., 2020; Zhai et al., 2020).

To help mitigate the legacy of environmental injustices to historically marginalized populations in urban areas, both local and regional urban planning strategies based on community needs are required (Rigolon, 2016). Alternative methodological approaches such as mixed methods (Talal and Santelmann, 2020) and community-based participatory research (Floyd, 2014) may help to promote community engagement and change. There is great potential for urban vegetation management to effectively balance a range of social and ecological goals within cities.