35 years of vegetation changes in a suburban national park
Protected areas are essential to preserve species against extinction drivers. Concerns have recently been raised about the capacity of national parks to achieve their conservation objectives over the long term. Few studies have been conducted in parks to understand changes in plant communities over several decades, which could provide information on their efficiency to protect native species. In the few known examples, parks gained exotic species and lost native species, resulting in a decline of beta diversity. However, these parks were essentially small in size and located in urban settings; to what extent conclusions could be applied to other parks in suburban areas or in the countryside remains to be substantiated. This study aimed to determine whether, over a period of 35 years, a small suburban park (8.9 km2) preserved its flora despite anthropogenic and natural disturbances.
We resurveyed in 2013, with the same methodology, 133 vegetation-plots that were first sampled in 1977 at Mont-Saint-Bruno National Park in Quebec. We analysed changes in native species richness, exotic species richness and species composition. Distance-based redundancy analysis was used to analyse changes in beta diversity over time. We also gathered information on species traits and tested for functional homogenization and indicator traits.
The number of native species in each vegetation-plot tripled over the last 35 years, while exotic species increased by an average of only one species per plot. The among-year variation in species composition was mainly due to species enrichment rather than species replacement (turnover). Taxonomic homogenization occurred (decline in beta diversity in 2013), but did not result into functional homogenization since functional diversity components (richness, dispersion, evenness) were significantly higher in 2013 than in 1977. Indicator trait analysis revealed that in 2013, vegetation-plots were more likely to contain grass, sedge or fern species, and plants with a storage organ. In contrast, in 1977, sampling-plots were more likely to be dominated by highly palatable species.
Our results suggest that white-tailed deer – recently overabundant in the park – is the dominant driver of plant community changes as deer-resistant plants increased in frequency. The net result was an increase in the richness of native plants and a homogenization of forest communities. This homogenization was apparently not associated with the spread of invasive exotic plant species (as frequently reported for other parks), but with an invasive mammal which can have major effects on plant assemblages, even in protected areas.