Predicting decline of threatened species, invasiveness of alien species, and invasibility of seminatural habitats: A case study from threatened coastal heathlands and semi-natural grasslands in western Norway
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Semi-natural habitats are characterised by a large biodiversity, but also associated with a large susceptibility to invasion by non-native species. Coastal heathlands and semi-natural grasslands in Norway, are under pressure from many different drivers, like invasion of non-native species, fragmentation, intensification and currently climate change, leading to concern about the cumulative effects from multiple global change drivers. Success of a non-native species in a new environment and a native species in a changing environment would be dependent on the tolerances to any changes in abiotic and biotic conditions. Thus, to make predictions about potentially invasion or extinction risk of a plant species, information about the whole life cycle of the species, the environmental drivers, and how this translates into any changes in vital rates and population growth rate need to be understood. This thesis aims to investigate the main drivers behind changes in species abundances over time and space, exemplified by (i) invasion of the non-native conifer Picea sitchensis (Sitka spruce, (Bong.) Carr.) and the native species Pinus sylvestris (Scots pine, L) in coastal heathlands (Paper I, III) and (ii) the decline of the threatened peripheral species Arnica montana (L) in semi-natural grasslands (Paper II). In addition, we zoom out to investigate (iii) the impact of the non-native conifer species Pinus mugo coll. (Dwarf mountain pine) on species community and landscape scales (Paper IV). Methods variated from demographic data analysed by means of matrix projection models or integral projections models, field experiments, to data on species composition. Spread of Sitka spruce was mainly driven by the seed rain from plantations and consequently the naturalisation of Sitka spruce was most abundant near plantations, but with scattered trees at longer distances from plantations. Sitka spruce had a high population growth rate. Both Sitka spruce and Scots pine invade every successional stage of heathland vegetation, but with decreasing recruitment success in later successional stages. Although Sitka spruce seems to invade all vegetation types in adjacent heathlands, our results suggest larger abundance on well-developed and moist soil conditions. The probability of establishment further seems to be constrained by competition from Juniperus communis, another native species which invade the coastal heathlands after land-use abandonment. Decline of A. montana was mainly driven by increasing precipitation. Life-table response experiments revealed that the temporal variability in population growth rate was driven by survival and clonality, whereas the spatial variation was driven by clonality. Our results suggest that A. montana has a threshold response to increasing precipitation, likely due to adaptions to local climatic conditions. Surprisingly, the results suggest no effects from habitat quality and population size. The stochastic growth rate was negatively influenced by climate change, indicating an increased extinction risk for marginal populations, possibly leading to range contraction of A. montana as climate change proceeds. Islands with the introduced species P. mugo Coll. had more vascular plants than islands with the native species P. sylvestris. The latter had forest floor vegetation dominated by bryophytes, reflecting a low light regime and thus lower species richness than the more open P. mugo habitats. P. mugo islands harboured more species associated with semi-natural habitats compared to the P. sylvestris islands which had a more closed late-successional canopy. Thus, habitat and species richness were higher and increased with area on P. mugo islands but not on P. sylvestris islands. In conclusion, this thesis highlights the need to include information about the whole life cycle of the species, the main environmental drivers behind the population growth rate and the underlying vital rate to make assessment about the invasion potential of non-native species and threatened species. In addition, it shows that impact of a non-native species on resident vegetation is highly dependent on the specific traits of the species and the resident species community.
Består avPaper I: Vikane J. H., Rydgren, K., Jongejans, E. & Vetaas, O. R. (2019). Spread of Picea sitchensis from plantation into adjacent coastal heathland. The article is not available in BORA.
Paper II: Vikane, J. H., Rydgren, K., Jongejans, E. & Vandvik, V. (2019). Rainfall and temperature change drive Arnica montana population dynamics at the Northern distribution edge. Oecologia. In press. The article is not available in BORA.
Paper III: Vikane, J. H., Vandvik, V., & Vetaas, O. R. (2013). Invasion of Calluna heath by native and non-native conifers: the role of succession, disturbance and allelopathy. Plant ecology, 214 (7), 975-985. The article is not available in BORA due to publisher restrictions. The published version is available at: https://doi.org/10.1007/s11258-013-0223-9
Paper IV: Vetaas, O. R., Vikane, J. H., Saure, H. I., & Vandvik, V. (2014). North Atlantic Islands with native and alien trees: are there differences in diversity and species‐area relationships? Journal of Vegetation Science, 25 (1), 213-225. The article is available in the main thesis. The article is also available at: https://doi.org/10.1111/jvs.12045