Abstract
• Key messageThe shrub species richness in Spanish forests is mainly linked to climatic variables and the importance of the groups of variables scarcely differs among forest types. Forest surrounding the Mediterranean Basin exhibit the highest levels of shrub richness.• ContextShrub species account for a high proportion of the plant diversity in Spanish forests and are a determinant factor in forest dynamics and ecosystem functionality.• AimsTo investigate the relative importance of climatic, forest stand features, soil and topographic variables in explaining shrub richness in Spanish forests and if the relative importance of these four groups of variables reflects variations among forest types.• MethodsWe used the Spanish National Forest Inventory and a boosted regression trees approach to identify which climatic, soil, stand and topographic variables (N = 19 variables) are related to the richness of shrub species in Spanish woodlands.• ResultsThe shrub species richness is mainly related to climatic variables followed by soil variables whereas stand and topographic variables play a minor role. The importance of the groups of variables scarcely differs among forest types although forests located around the Mediterranean Sea display the highest levels of shrub richness.• ConclusionShrub richness in Spain is primarily driven by climatic and soil variables, both at country and forest-type scales. Forests surrounding the Mediterranean Basin account for the highest richness of shrub species but are also those most threatened by global change. Therefore, special attention must be paid to the monitoring and assessment of these forest ecosystems.
Highlights
The Mediterranean basin is considered a hotspot of diversity as it harbors many endemic species (Myers et al 2000; Hewitt 2011; Molina-Venegas et al 2017; Di Pasquale et al 2020)
The main aim of this study is to analyse the spatial patterns of shrub richness and to identify the climatic, forest stand features, soil and topographic variables related to shrub species richness in Spanish woodlands
Similar values were found when analyzing the data by biogeographical region, except for the Macaronesian region, where the percentage of plots with shrub species dropped to 75 %
Summary
The Mediterranean basin is considered a hotspot of diversity as it harbors many endemic species (Myers et al 2000; Hewitt 2011; Molina-Venegas et al 2017; Di Pasquale et al 2020). Shrubs have an important role in the natural forest dynamics, especially during the first stages of tree development With this regard, several studies have highlighted the importance of shrub presence on the establishment and development of new seedlings in areas with intense, severe drought (Smit et al 2008; Moreno-Fernández et al 2019), and under milder climatic conditions where water stress intensifies from the lack of soil structure (Torroba-Balmori et al 2015; Alday et al 2016), the relationship between shrubs and trees seems to be speciesspecific (Gómez-Aparicio et al 2004; Madrigal-González et al 2014; Cruz-Alonso et al 2020). Under the current context of global warming, the shrub stratum gains importance as shrubs depict more tolerance to drought and aridity than tree species (Liu et al 2011; Bateman et al 2018) and can be one of the last barriers against desertification and soil erosion in water-limited environments (Maestre et al 2009)
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