By Carlos Lara-Romero, Paloma Ruiz-Benito & Pilar Castro-Díez
Non-native tree species (NNT) have been planted worldwide to provide different types of benefits, from resources, such as wood, tannins, or fiber, to ornamental assets. Yet, some non-native species may cause harm to the recipient ecosystems, or even induce socio-economic losses in the region (e.g. when they increase fire frequency or deplete soil nutrients and water). These drawbacks are exacerbated when the NNT show a strong potential to expand and displace the native vegetation.
In peninsular Spain, several NNT have been introduced and expanded mostly during the ‘50s and ‘60s of the last century to produce wood or pulp, prevent soil erosion, or produce other benefits or amenities. We focused on the 17 NNT that occurred more often in the study area. Our aim was to assess whether these species have been expanding over the last three decades and to identify drivers explaining contrasting trends across species. We used data from the Spanish Forest Inventory, which does systematic sampling of forest plots all over the country every ten years. This information allowed us to calculate changes in the species occupancy (i.e. area of Spain occupied by each species), tree density per plot, and size of individual trees over time. We selected several species traits (i.e. species properties which affect their strategy to acquire resources), propagule pressure (i.e. the amount of pre-existing individuals able to produce propagules), and the people’s interest in the species (assessed by means of the Google Trends tool), as potential predictors of NNT performance over time.
Results showed that 13 out of the 17 explored NNT expanded their occupancy over the last three decades. The most expansive species were two wattles (Acacia melanoxylon and A. dealbata), while two eucalypts (Eucalyptus globulus, E. camaldulensis), Norway spruce (Picea abies) and larch (Larix spp.) showed a small reduction in the occupancy. All species increased their density, basal area (area of the plot occupied by stems), and tree growth through time. The species with larger increments were those with greater tolerance to water stress (e.g. Lawson cypress, Chamaecyparis lawsoniana), and those with high propagule pressure (e.g. Monterrey pine, Pinus radiata). Taller species also increased basal area more than shorter species. Overall, we found that aridity is the main factor preventing the expansion and growth of non-native trees in peninsular Spain, and that species with higher tolerance to this factor have greater success. The overall increase in occupancy suggests that there is room for expansion of NNT in Spain. This knowledge will help to predict the dynamics of NNT already present in Spain and identify risks for forest biodiversity.
This is a plain language summary of the paper of Lara-Romero et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13131). The post was prepared by Pilar Castro-Díez.