Introduction
Mountains are biodiversity hotspots and provide a wealth of ecosystem functions and benefits to people (Körner & Spehn, 2002; Martín-López et al., 2019; Mengist et al., 2020). At the same time, mountain ecosystems are particularly susceptible to global change. For instance, temperatures are increasing faster at high elevation than at low elevation (Nogués-Bravo et al., 2007; Pepin et al., 2015). In the alpine zone of the European Alps, temperatures have increased approximately twice as much as the northern hemisphere average over the past 100 years (Gobiet et al., 2014). Importantly, amplified warming has enabled many plant species to move to higher elevation (Lenoir et al., 2008; Pauli et al., 2012; Steinbauer et al., 2018). For instance, between 1971 and 1993 native plant species from the forest understorey in the French mountains shifted their elevational range uphill at an average rate of 38 m per decade (Lenoir et al. 2008). Another prominent example is the observed upward shift of most vascular taxa at Chimborazo in Ecuador since Alexander von Humboldt’s visit more than two centuries ago (Morueta-Holme et al., 2015). An expected consequence of such uphill migrations of more competitive lowland species is that less competitive alpine species might locally extinct on mountain summits (Dullinger et al., 2012; Alexander et al., 2018; Guisan et al., 2019; Rumpf et al., 2019). Such local extinctions were recently documented for birds (e.g. Freeman et al., 2018).
In addition to temperature increase, human activities in mountain areas have changed markedly over the last decades (e.g. Peters et al., 2019; Wang et al., 2019; for an overview see Payne et al., 2020). Mountain land use has intensified in many places across the globe (Spehn et al., 2006), driven by booming tourism industries (Pickering & Barros, 2012; Debarbieux et al., 2014), overexploitation of natural resources and ever-increasing demands for agricultural land (e.g. Gillet et al., 2016; Ross et al., 2017). The abandonment of traditional cutting and grazing practices has also occurred in some mountain regions (e.g. MacDonald et al., 2000). Both land use intensification and abandonment can alter plant species distributions and diversity alone (Pellissier et al., 2013; Alexander et al., 2016) and by interacting with climate change (Guo et al., 2018; Elsen et al., 2020).
Further, previously remote areas are becoming increasingly accessible due to construction of new roads and trails, which not only cause a direct disturbance, but also act as corridors for plant species movements (Ansong & Pickering, 2013; Lembrechts et al., 2017; Rew et al., 2018). The role of roads as dispersal corridors is amplified due to increased vehicle traffic, often as a result of recreation and tourism (e.g. Müllerová et al., 2011). Roadside habitats also provide ideal spaces for non-native plants, which generally benefit from reduced competition, increased soil nutrients, more favourable microclimatic and hydrological conditions and intermediate disturbance (Müllerová et al., 2011; Averett et al., 2016). Thus, both native and non-native plant species are known to disperse along mountain roads, from low to high elevation and vice versa (Dainese et al., 2017; Lembrechts et al., 2017; Guo et al., 2018). Indeed, many high elevation areas once free of lowland and non-native species but connected to lowlands by road networks are now harbouring lowland and non-native plant species. Examples for this are the volcanoes of the Hawaiian archipelago (Jakobs et al., 2010), the high Andes (Barros et al., 2020) and the Teide National Park on Tenerife (Dickson et al., 1987). Roadside habitats are also conduits for non-native plants to spread into natural vegetation once established along roadsides (Alexander et al., 2011; Seipel et al., 2012).
The elevational redistribution of plant species, especially non-native species (Dainese et al., 2017), has already significantly impacted mountain ecosystems (Guo et al., 2018) and will continue to do so in the future (Petitpierre et al., 2016). For example, non-native plants can cause biotic homogenization (Haider et al., 2018), reduce the diversity of local native species (Daehler, 2005) and affect important ecosystem functions and services (McDougall et al., 2011b; Pecl et al., 2017). In the mountains of Iceland, non-native Lupinus nootkatensiscompetes strongly with native plant species and modifies soil properties through nitrogen fixation (Wasowicz, 2016). In the alpine zone of the central Chilean Andes, non-native Taraxacum officinale shares pollinators with several native Asteraceae species (Muñoz & Cavieres, 2019), reducing pollinator-visitation rates and seed-set where T. officinale is at high abundances (Muñoz & Cavieres, 2008). Finally, uphill migration of non-native trees and shrubs can increase fire risk at high elevation (Cóbar-Carranza et al., 2014), and transform plant communities through competition (Zong et al., 2016; Nuñez et al., 2017).
While human-driven vegetation change can happen relatively quickly in mountains, it often only becomes apparent at temporal scales beyond the few years covered by most ecological experiments (Mirtl et al., 2018). Thus, data from long-term time series in mountains are essential to identify and follow changes in plant communities (Pauli et al., 2012). There are currently two main types of initiatives which monitor high-elevation vegetation change. At the local or regional scale, some well-established long-term monitoring sites follow a holistic approach and document not only floristic changes, but also modifications for example of soil, hydrology or atmospheric conditions. Examples are Niwot Ridge in the Colorado Rocky Mountains (www.nwt.lternet.edu) or the Sierra Nevada Global Change Observatory in Spain (https://obsnev.es/en/). At the global scale, the Global Observation Research Initiative in Alpine Environments (GLORIA, www.gloria.ac.at; Pauli et al., 2015) is a network monitoring floristic change on mountain summits with a standardized approach. What would complement these highly valuable approaches, is a global long-term monitoring network that covers the full vertical extents of different mountain regions and that allows the detection of species responses to both climate and other human activities.
Here, we present a standardized protocol for monitoring changes in the elevational distribution, abundance and composition of plant biodiversity in mountains as a result of the interaction between climate and human pressures. Importantly, the protocol focuses on large elevation gradients (>1700 m on average; ranging from c. 700 m to >4000 m), allowing vegetation change to be monitored across a broad range of climates and plant community types. It explicitly contrasts anthropogenically disturbed and (semi-)natural vegetation within sampling sites, thus increasing detection of rapid community changes and providing greater insight into the drivers of change. The protocol has been developed by the Mountain Invasion Research Network (MIREN,www.mountaininvasions.org) (Kueffer et al., 2014), a network initially founded in 2005 to study patterns and processes of non-native plant invasions in mountains and recently expanded to more widely understand the effects of global change on mountain plant biodiversity and the distribution of species. The protocol provides a conceptually intuitive yet comprehensive and standardized way to record and monitor native and non-native species along elevation gradients. The survey has been running in some mountain regions of the world since 2007 (Alexander et al., 2011; Seipel et al., 2012) and continues to be implemented in new regions. In this paper, we summarize the most important findings gained over the time of using this protocol, discuss its strengths and weaknesses and outline opportunities and challenges for future work. To achieve broad reach and long-term maintenance of sites, monitoring protocols must be simple, efficient, and inexpensive. Our intention is to promote the use of the MIREN road survey protocol to monitor biodiversity change in mountains, and to generate global, regional and local insights into how plant species and communities are responding to rapid global change in mountains.
Materials and Methods