Population re-establishment and spatial dynamics of crowberry (Empetrum nigrum ssp. hermaphroditum), a foundation species in restored alpine ecosystems

Many ecosystems are defined and shaped by one or a few common, foundation species. Even though such species hold a key role in the restoration of these ecosystems, the demographic processes involved in their re-establishment have rarely been studied. Foundation species' population dynamics, re-establishment history, and the abiotic and biotic factors that affect individual establishment at restored sites can be studied by addressing population spatial patterns and age structure. Such an approach to studying population dynamics is particularly relevant for long-lived species with low mortality, such as shrubs in alpine areas.

We studied a population of the foundation species Empetrum nigrum ssp. hermaphroditum at an alpine spoil heap site and found evidence of population re-establishment starting within a decade after construction. High Empetrum densities close to the spoil heap edges indicated that short distances to seed sources in the surroundings had a strong positive effect on establishment of individuals. Empetrum individuals were significantly clustered, which indicated intraspecific facilitation. As revealed by spatial analyses of recruits and older, established individuals, clustering developed gradually over time, which indicated a shift from no interaction to increased facilitation. We conclude that intraspecific facilitation promotes Empetrum reestablishment at the studied alpine spoil heap.

We show that population spatial patterns and age structure can be successfully used to unveil the re-establishment history of a foundation species in a restoration context. Efficient seed dispersal and intraspecific facilitation seem to be important factors behind Empetrum's successful re-establishment at alpine spoil heaps. Identification of abiotic and biotic factors determining foundation species' establishment success at restored sites can support planning and improve success of restoration.

Methods
This dataset contains positional and age data of a population sample of Empetrum nigrum ssp. hermaphroditum (approx. 2300 individuals) collected in autumn 2015-spring 2016 at Fossane spoil heap, Aurland municipality, Norway (approx. 60°47'22.0"N 7°23'46.6"E)

Positional data of Empetrum individuals collected in the field by survey-grade GNSS equipment from contiguous transects, processed in ArcGIS and R.

Empiric and modelled age data of Empetrum individuals: empiric data obtained from counting of growth rings on root collar cross-sections from sampled Empetrum individuals, modelled data based on individuals' crown size (principal crown diameter).

For details, see article in Ecology and Evolution (open access).