DOI: 10.5061/dryad.q573n5tjk
title: Changes in forest structure drive temperature preferences of boreal understory plant communities

description: 1. The local climate in forest understories can deviate substantially from
 ambient conditions. Moreover, forest microclimates are often characterized
 by cyclic changes driven by management activities such as clear-cutting
 and subsequent planting. To understand how and why understory plant
 communities change, both ambient climate change and temporal variation in
 forest structure has to be considered. 2. We used inventories from 11436
 productive forest sites in Sweden repeated every 10th year 1993 - 2017 to
 examine how variation in forest structure influences changes in the
 average value of minimum and maximum temperature preferences of all
 species in a community, i.e. community temperature indices (CTI). We then
 evaluated to what extent these changes were driven by local extinctions
 and colonizations, respectively, and to what extent the difference in CTI
 value between two inventories were related to changes in forest density
 and in macroclimate. Lastly, we tested whether effects on CTI change by
 these two drivers were modified by topography, soil moisture and tree
 species composition. 3. CTI values of the understory plant communities
 increased after clear-cutting, and decreased during periods when the
 forest grew denser. During the period immediately after clear-cutting,
 changes were predominately driven by colonizations of species with a
 preference for higher temperatures. During the forest regeneration phase,
 both colonization by species preferring lower temperatures and local
 extinctions of species preferring higher temperatures increased. The
 change in understory CTI over 10-year periods was explained more by
 changes in forest density, than by changes in macroclimate. Soil moisture,
 topography and forest tree species composition modified to some extent the
 effects of changes in forest density and in macroclimate on understory CTI
 values. 4. Synthesis. Via stand manipulation, forest management impacts
 the effects of regional climate on understory plant communities. This
 implies that forest management by creating denser stands locally even can
 counterbalance the effects of regional changes in climate by creating
 denser stands locally. Consequently, interpretations of changes in the
 mean temperature preference of species in forest understory communities
 should take forest management regimes into account. 
description: The data consist of 11436 plots inventoried for the National
 Inventory of Forests in Sweden between 1993-2017. The dataset has plot ID,
 year of inventory, basal area, basal area difference between inventories,
 proportion of spruce and pine, region, elevation, soil moisture index,
 relative elevation, northerness of slope, macroclimate temperature change
 (10 years), Community Temperature Indices of maximum and minimum
 temperatures of the understory communities (CTImax and CTImin).
 CTI values were calculated from presence/absence data of
 understory plants that were inventoried in a 100 m2 circular plot. Forest
 structures such as basal area and forest tree composition were measured
 from a larger circular plot of 300 m2 with the same centre. CTI were
 calculated as an average value of species-specific temperature preferences
 for all species present in the inventory for maximum and minimum
 temperatures, respectively. Elevation, Slope and Aspect
 were retrieved from a 50 meter resolution Digital Elevation Model of
 Sweden (the Swedish mapping, cadastral and land registration authority,
 www.lantmateriet.se). Relative Elevation was calculated
 from the Digital Elevation Model. We calculated relative elevation as the
 difference between the elevation of the plot and the minimum elevation in
 a 500 meter radius. We log-transformed relative elevation, as cold air
 pooling has a negative logarithmic relationship with relative
 elevation. Soil moisture index was extracted a 10 meter
 resolution map from the Swedish Environmental Protection Agency
 (http://www.naturvardsverket.se/Sa-mar-miljon/Kartor/Nationella-Marktackedata-NMD/). Macroclimate temperatures were extracted from TerraClimate (Abatzoglou, J. T., Dobrowski, S. Z., Parks, S. A., & Hegewisch, K. C. (2018). TerraClimate, a high-resolution global dataset of monthly climate and climatic water balance from 1958-2015. <i>Scientific Data</i>, <i>5</i>, 1–12. https://doi.org/10.1038/sdata.2017.191) References for NFI data collection: Fridman, J. (2016). <i>The Swedish National Forest Inventory</i>. Swedish University of Agricultural Sciences. https://www.slu.se/en/Collaborative-Centres-and-Projects/the-swedish-national-forest-inventory/about-us/how-we-work/ Fridman, J., Holm, S., Nilsson, M., Nilsson, P., Ringvall, A. H., & Ståhl, G. (2014). Adapting National Forest Inventories to changing requirements - The case of the Swedish National Forest Inventory at the turn of the 20th century. <i>Silva Fennica</i>, <i>48</i>(3), 1–29. https://doi.org/10.14214/sf.1095