DOI: 10.5061/dryad.fd20c
title: Data from: A portfolio effect of shrub canopy height on species richness in both stressful and competitive environments

description: Facilitating effects of benefactor plants on plant species richness have
 been commonly tested in stressful habitats because competitive effects are
 assumed to predominate in more productive habitats. Here, we examine this
 assumption by testing whether benefactor plants can nonetheless be
 facilitating in competitive environments. We provide a conceptual
 framework describing how a trait of benefactor plants, canopy height of
 shrubs, can have a portfolio of facilitative effects on species richness
 in more competitive environments, and we provide an empirical assessment
 of this portfolio effect in tundra plant communities. Across tundra plant
 communities representing an extensive gradient in aboveground live biomass
 ranging from 11 to above 800 grams per m2, we found that species richness
 exhibited a humped-back relationship. Increasing canopy height of shrubs
 to a maximum height of what defines the dwarf shrub tundra, that is 40 cm,
 consistently and significantly increased species richness along the entire
 biomass gradient tested. The positive effect of shrub canopy height was
 not confounded with herbivore intensity, competitive interference or
 abiotic factors such as bedrock-weathered mineral availability, moisture
 availability or temperature. However, we cannot rule out that the general
 presence of large mammalian herbivory may have been central to the
 positive effect of shrub canopy height in reducing herbivore impacts on
 species richness. In this study, conceptual and empirical evidence support
 that increasing canopy height of shrubs facilitates species richness
 regardless of relative abiotic stress levels within tundra ecosystems. We
 propose that positive interactions can play an important ecological role
 in systems where competitive effects are observed or assumed. For tundra
 plant communities where climate change is currently causing encroachment
 of shrub species, the effects of increasing canopy height may have
 unprecedented effects on plant species richness.
description: Data from: A portfolio
 effect of shrub canopy height on species richness in both stressful and
 competitive environmentsData file with the
 following variables: District pair - number from 1-10 where each pair
 number represents two neighbouring reindeer districts where one has
 relatively higher reindeer density and the other relatively lower reindeer
 density; Relative reindeer density - Indicating whether the reindeer
 district belongs to the high or low reindeer density category; District -
 actual district number given by the authorities of reindeer herderers
 (Reindriftsforvaltningen); Landscape area - ID of the 2 km x2 km areas;
 Sampling square - ID of the 200 m x 200 m squares; Julian date - date of
 sampling; Bedrock nutrient category - category explanation provided in the
 paper; Curvature index - category explanation provided in the paper; E
 coord UTM 33; N coord UTM 33; Temperature - in ºC; Shrub canopy height -
 in cm; Total biomass - in grams; Total species richness - the count of
 species; Grass biomass - in grams; Emeptrum nigrum presence - 0 for
 absence and 1 for presence; Shrub biomass - in grams; Species richness
 excluding shrub species - the count of species; Cover of lichens and
 bryophytes - point estimatesBrathen and Lortie portfolio
 effect.xlsx