Timberline Patterns and Dynamics Depend on Forest Type, Regional Climate, and Topography in the Cantabrian Mountains (Spain)

Questions

Forest timberlines are globally influenced by climatic and anthropogenic factors, with regional differences in species composition, spatial patterns, and temporal dynamics. We studied mountain forests in a climatically heterogeneous region to analyze (I) the distribution and structure of the timberline across local climatic and topographic gradients, and (II) the temporal dynamics of regional timberlines over the last 70 years and their drivers.

Location

Cantabrian Mountains, Northwestern Spain.

Methods

We used local vegetation maps describing habitat distributions with high spatial and thematic resolution to characterize montane forest types forming natural timberlines. We assessed the elevation distribution of the timberline for each forest type and fitted generalized linear models to analyze their main environmental and anthropogenic drivers at the regional scale. We also examined current and historical aerial imagery to explore timberline changes since the 1950s and fitted generalized linear mixed models to evaluate their driving factors.

Results

The elevation of regional timberlines varied from 1700 to 2000 m, with the highest elevations found in Betula celtiberica and Quercus orocantabrica forests. Regional variation of timberline was primarily influenced by mean annual temperature and solar radiation, and secondarily by the distance to roads. We detected upward shifts of timberlines dominated by Betula celtiberica (10.9 ± 11.6 m), Quercus petraea (7.7 ± 8.5 m), and Fagus sylvatica on acid (6.4 ± 9.2 m) and non-acid (3.0 ± 3.9 m) soils, while Quercus orocantabrica forests largely remained unchanged. Beyond the forest type, elevation shifts were mainly explained by northness, eastness, and slope.

Conclusions

Our study indicates that within-regional variation in spatial patterns and dynamics of timberlines is primarily determined by the forest type and its relationships with climate and topography. We also provide evidence of the continentality–oceanity gradient in determining the elevation of the natural timberline in middle latitudes. Although higher timberline elevations are reached under continental conditions, stronger upward shifts of regional timberlines under oceanic conditions may be explained by the dispersal abilities of Betula species.