populations differs across the elevational distribution in Sierra Nevada, where
low-elevation populations of P. sylvestris and J. communis are susceptible to
decline through reduced growth, seed production, and population regeneration
(García et al. 2000; Matías and Jump 2015). Both species presented a significantly
reduced proportion of young individuals at the lowermost populations, and a clear
dominance of older age classes (Matías and Jump 2015; Rabasa et al. 2013;
Benavides et al. 2013; 2015). This contrasts sharply with the pattern found at the
treeline, where a higher proportion of saplings appeared and an even distribution
was found across age classes. These results indicate an ongoing elevational displacement for both species at the southernmost edge of the geographical distribution area (Fig. 16.8a). Still, evidence for lowland contractions in these woody
species is scarce due to generally great individual longevity and relatively slow
decline until survival thresholds are exceeded, especially for slow-growing species
such as J. communis (García and Zamora 2003).
However, factors other than climate affect species growth and reproduction at the
same time such as land-use change and herbivory pressure (Zamora et al. 2001;
Zamora and Matías 2014; Herrero et al. 2012, 2016), and the result of their
interactions are strongly heterogeneous across areas and species. Therefore,
research across altitudinal gradients taking into account other factors in addition to
climate, such as biotic interactions (mostly herbivory), land use, or local adaptations, are strongly recommended for an accurate forecasting of climate-change
consequences on long-lived woody plant species.
Summit vegetation also showed signals of change, with an expansion of some
species, and greater vegetation cover. Many alpine plants have become rarer in the
period 2001–2008 on the high mountains of the European continent, whereas plants
from low elevations become more common (Gottfried et al. 2012). In Sierra
Nevada, over the last 11 years, 13 alpine species have disappeared from monitoring
plots and, at the same time, five new taxa have appeared (Gottfried et al. 2012;
Sánchez-Rojas and Molero Mesa 2016).
Overall, these results for plant and animal species show that a high mountain,
such as Sierra Nevada, plays a vital role in maintaining biodiversity in the context of
global change. Species populations can adapt to climatic changes elevationally by
moving according to their ecological necessities, although plants and animals do not
necessarily need to climb several hundred metres in elevation to find suitable new
habitats in case of warming but may find conditions matching their “thermal niche”
over very short distances, taking advantage of the mosaics of microhabitats characterizing mountains slopes (Scherrer and Koerner 2011). These results indicate that
montane ecological communities that are considered rather stable are, in fact,
undergoing strong spatial and temporal dynamics because of climate warming (and
more so after land-use change). As a result, there is a convergence in biotic responses
to these two major drivers of global change (land-use and climatic changes),
threatening high-mountain species while at the same time opening opportunities for
species from lower elevations. All of this seems to be restricting the distribution
range or leading to the extinction of rare, endemic, and/or specialist species, parallel
to expansions in the distribution range of generalist species. This may be increasing
16 Monitoring Global Change in High Mountains
403
low-elevation populations of P. sylvestris and J. communis are susceptible to
decline through reduced growth, seed production, and population regeneration
(García et al. 2000; Matías and Jump 2015). Both species presented a significantly
reduced proportion of young individuals at the lowermost populations, and a clear
dominance of older age classes (Matías and Jump 2015; Rabasa et al. 2013;
Benavides et al. 2013; 2015). This contrasts sharply with the pattern found at the
treeline, where a higher proportion of saplings appeared and an even distribution
was found across age classes. These results indicate an ongoing elevational displacement for both species at the southernmost edge of the geographical distribution area (Fig. 16.8a). Still, evidence for lowland contractions in these woody
species is scarce due to generally great individual longevity and relatively slow
decline until survival thresholds are exceeded, especially for slow-growing species
such as J. communis (García and Zamora 2003).
However, factors other than climate affect species growth and reproduction at the
same time such as land-use change and herbivory pressure (Zamora et al. 2001;
Zamora and Matías 2014; Herrero et al. 2012, 2016), and the result of their
interactions are strongly heterogeneous across areas and species. Therefore,
research across altitudinal gradients taking into account other factors in addition to
climate, such as biotic interactions (mostly herbivory), land use, or local adaptations, are strongly recommended for an accurate forecasting of climate-change
consequences on long-lived woody plant species.
Summit vegetation also showed signals of change, with an expansion of some
species, and greater vegetation cover. Many alpine plants have become rarer in the
period 2001–2008 on the high mountains of the European continent, whereas plants
from low elevations become more common (Gottfried et al. 2012). In Sierra
Nevada, over the last 11 years, 13 alpine species have disappeared from monitoring
plots and, at the same time, five new taxa have appeared (Gottfried et al. 2012;
Sánchez-Rojas and Molero Mesa 2016).
Overall, these results for plant and animal species show that a high mountain,
such as Sierra Nevada, plays a vital role in maintaining biodiversity in the context of
global change. Species populations can adapt to climatic changes elevationally by
moving according to their ecological necessities, although plants and animals do not
necessarily need to climb several hundred metres in elevation to find suitable new
habitats in case of warming but may find conditions matching their “thermal niche”
over very short distances, taking advantage of the mosaics of microhabitats characterizing mountains slopes (Scherrer and Koerner 2011). These results indicate that
montane ecological communities that are considered rather stable are, in fact,
undergoing strong spatial and temporal dynamics because of climate warming (and
more so after land-use change). As a result, there is a convergence in biotic responses
to these two major drivers of global change (land-use and climatic changes),
threatening high-mountain species while at the same time opening opportunities for
species from lower elevations. All of this seems to be restricting the distribution
range or leading to the extinction of rare, endemic, and/or specialist species, parallel
to expansions in the distribution range of generalist species. This may be increasing
16 Monitoring Global Change in High Mountains
403