DOI: 10.5061/dryad.gtht76hpm
title: Data: Interactive effects of drought and edge exposure on old-growth forest understory species

description: Context: Both climatic extremes and land-use change constitute severe
 threats to biodiversity, but their interactive effects remain poorly
 understood. In forest ecosystems, the effects of climatic extremes can be
 exacerbated at forest edges. Objectives: We explore the hypothesis that an
 extreme summer drought reduced the richness and coverage of old-growth
 forest species, particularly in forest patches with high edge exposure.
 Methods: Using a high-resolution spatially explicit precipitation dataset,
 we could detect variability in drought intensity during the summer drought
 of 2018. We selected 60 old-growth boreal forest patches in central Sweden
 that differed in their level of drought intensity and amount of edge
 exposure. The year after the drought, we surveyed red-listed and
 old-growth forest indicator species of vascular plants, lichens and
 bryophytes. We assessed if species richness, composition, and coverage
 were related to drought intensity, edge exposure, and their interaction.
 Results:  Species richness was negatively related to drought
 intensity in forest patches with a high edge exposure, but not in patches
 with less edge exposure. Patterns differed among organism groups and were
 strongest for cyanolichens, epiphytes associated with high-pH bark, and
 species occurring on convex substrates such as trees and logs.
 Conclusions:  Our results show that the effects of an extreme
 climatic event on forest species can vary strongly across a landscape.
 Edge exposed old-growth forest patches are more at risk under extreme
 climatic events than those in continuous forests. This suggest that
 maintaining buffer zones around forest patches with high conservation
 values should be an important conservation measure.
description: The data consists of
 60 inventoried
 old-growth boreal forest patches in central Sweden that
 differed in their level of drought intensity and amount of edge exposure.
 In these sites we conducted surveys of
 red-listed and old-growth forest indicator species of bryophytes, lichens
 and vascular plants (a subset of the species list from the monitoring
 program of woodland key habitats, see more in the main article method
 section). Inventories were conducted between the end of June and
 mid-August 2019. We subdivided each site into 20 × 20 m subplots. For each
 present species, we estimated their total cover in each subplot
 (dm<sup>2</sup>) and in the case of the
 orchid <em>Goodyera repens</em> (L) R.Br., we also estimated
 the percentage of flowering individuals for the first encounter in each
 subplot. We categorized all surveyed species based on organism groups and
 substrate association. Organism groups were: lichens (divided into
 cyanolichens and chlorolichens), bryophytes and vascular plants. For
 substrate association we separated species as epiphytic (divided into
 high-pH bark and low-pH bark), epixylic, epilithic and
 epigeic. Drought intensity was calculated based on a
 high-resolution precipitation dataset called HIPRAD (HIgh-resolution
 Precipitation from gauge-adjusted weather RADar, Berg et al. 2016). We
 calculated three drought measures for each site: i) total precipitation
 during the whole summer of 2018 (1<sup>st</sup> May - August
 31<sup>st</sup>); ii) precipitation during the extreme drought
 period during the summer of 2018 (22<sup>nd</sup> June -
 27<sup>th</sup> July); iii) the summed precipitation before
 and after this extreme drought period in ii
 (1<sup>st</sup> May - 21<sup>st</sup> June and
 28<sup>th</sup> July -
 31<sup>st</sup> August).
 We calculated two background
 climatic variables: the average summer precipitation from
 1<sup>st</sup> May to 31<sup>st</sup> August over
 the years 2010 till 2017, and growing degree days (GDD, measured as days
 with mean temperatures over 5°C (based on the dataset from
 Meineri &
 Hylander, 2017). Data on the size and vegetation type in the 60
 sites (woodland key habitats) from the Swedish Forest Agency. The amount
 of laying and standing dead wood was counted (trees &gt; 30 DBH), and
 the canopy cover was determined based on five hemispherical photos,
 processed in ImageJ. Edge effects and edge exposure were determined
 by categorizing the strength of microclimatic edge effects for each
 subplot in each site as either strong, weak or absent, based on previous
 measures of how microclimatic edge effects penetrate edges with different
 edge orientation at forest/clear-cut interfaces. To get a measure of edge
 exposure for each site, we calculated as the proportion of subplot-area
 that were exposed to edge effects (including both weak and strong edge
 effects). More details can be found in the
 method section 2.1 - 2.4 in the paper. 
description: The following datasets are included: 1)
 Sitelevel data for the 60 woodland key habitats, including: woodland key
 habitat ID (Swedish Forestry Board, Betecknings_ID), coordinates, Site
 size, standing and laying dead wood, canopy cover, presense of high pH
 bark, edge exposure, the drought indices, and the background climatic
 variables.  2) A species coverage by site
 matrix 3) Species richness of the different groups at
 site level 4) Species richness at subplot
 level 5) The coverage of the four common species at
 site level 6) The coverage of the four common species
 at subplot level (four files, one for each species).  A
 list of the species organism group and substrate affility can be found in
 Online appendix, Table S2.