Limitations of the protocol
The focus of the protocol on mountain roads provides excellent
opportunities to disentangle the effects of climate and road
construction on plant species and community redistributions. However,
the protocol also has three important limitations (Figure 5).
First, the protocol excludes the most pristine environments that exist
far from roads and at elevations above where roads reach, so does not
monitor mountain biodiversity as a whole. As such, the protocol is a
complement to the GLORIA protocol, which focuses on long-term climate
change-related vegetation shifts on undisturbed mountain summits (Pauli
et al., 2015). Nevertheless, one regional study has shown that, at least
in northern Scandinavia, the effect of roads on mountain plant diversity
disappears beyond 25 m from the roadside (Lembrechts et al., 2014),
suggesting that the vegetation surveyed in the MIREN survey plot
furthest from the roadside (Figure 3b; 50-100 m distance) may indeed be
representative of regional biodiversity. Yet, using these data beyond
the 100 m reach of the sample site could bring issues for some
applications, such as spatial modelling, where extrapolations for
locations away from the road will suffer from increased uncertainty
(Kadmon et al., 2004). Coupled to this, the restriction of the protocol
to mountain roads means that, depending on the heterogeneity of the
landscape, not all habitat types are necessarily covered relative to
their distribution in the ecosystem. Plot locations may be biased
towards valleys and less steep terrain if road construction favours such
areas. Additionally, while roads represent the most prominent dispersal
pathway present in mountains, they are not the only one (e.g. rivers,
mountain trails, powerline cuttings, cable cars; Foxcroft et al., 2019).
However, the protocol could be easily adapted for other pathways (as
done for trails (Liedtke et al., 2020) and rivers (Vorstenbosch et al.,
2020)), and we suggest that this would be of particular interest in
regions with sparse roads and/or where most of the common non-native
species are wind or water dispersed.
Second, MIREN adopts a discrete temporal and spatial sampling approach.
Specifically, since the protocol focuses on community dynamics and
large-scale patterns it lacks the spatio-temporal resolution to monitor
individual species and populations over time. The relatively low spatial
sampling intensity (i.e. few plots for each elevational belt) and
sometimes large distances between elevational increments (e.g. on
average c. 75 m steps across current MIREN regions) can limit
understanding of local processes, while also biasing sampling against
rarer plant species or habitats. Furthermore, while repeated surveys
facilitate investigation of species range dynamics under global change,
the complete design does not explicitly consider dispersal dynamics
(e.g. through seed rain or seed bank sampling, or seed tracking),
instead assessing such dynamics indirectly through repeated snapshots of
plant community composition.
Third, the standard protocol emphasises simplicity to be as inclusive as
possible and to keep resource use to a minimum. The approach thus
focuses chiefly on plant community composition and coarse estimates of
species abundance (see Supporting Information S1). Other important
variables such as biomass, functional traits, community 3D-structure,
species interactions and other abiotic and biotic variables thus require
additional sampling effort. For the same reason, the protocol is limited
to vascular plants, excluding bryophytes and other taxonomic groups of
potential interest.
Finally, the assumption that elevation can serve as a proxy for climate
is of particular relevance here. Testing how the elevation gradient
correlates with fine-grained climatic gradients requires validation
using high-resolution climate data produced either using in-situmeasurements or downscaling of climate models (Lembrechts et al., 2019).
We therefore recommend participants to include at least one add-on study
that deploys temperature data loggers to allow linking of vegetation
patterns with microclimatic gradients (Lembrechts et al., 2019) –
although this would already add cost.