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