DOI: 10.5061/dryad.jf2n3
title: Data from: Positive shrub-tree interactions facilitate woody encroachment in boreal peatlands

description: Boreal ecosystems are warming roughly twice as fast as the global average,
 resulting in woody expansion that could further speed up the climate
 warming. Boreal peatbogs are waterlogged systems that store more than 30%
 of the global soil carbon. Facilitative effects of shrubs and trees on the
 establishment of new individuals could increase tree cover with profound
 consequences for the structure and functioning of boreal peatbogs, carbon
 sequestration and climate. We conducted two field experiments in boreal
 peatbogs to assess the mechanisms that explain tree seedling recruitment
 and to estimate the strength of positive feedbacks between shrubs and
 trees. We planted seeds and seedlings of Pinus sylvestris in microsites
 with contrasting water-tables and woody cover and manipulated both shrub
 canopy and root competition. We monitored seedling emergence, growth and
 survival for up to four growing seasons and assessed how seedling
 responses related to abiotic and biotic conditions. We found that tree
 recruitment is more successful in drier topographical microsites with
 deeper water-tables. On these hummocks, shrubs have both positive and
 negative effects on tree seedling establishment. Shrub cover improved tree
 seedling condition, growth and survival during the warmest growing season.
 In turn, higher tree basal area correlates positively with soil nutrient
 availability, shrub biomass and abundance of tree juveniles. Synthesis.
 Our results suggest that shrubs facilitate tree colonization of peatbogs
 which further increases shrub growth. These facilitative effects seem to
 be stronger under warmer conditions suggesting that a higher frequency of
 warmer and dry summers may lead to stronger positive interactions between
 shrubs and trees that could eventually facilitate a shift from moss to
 tree-dominated systems.
description: Figure 1data_Exp
 2Figure 1 data: Condition
 of experimental seedlings in hummocks with contrasting shrub density and
 tree canopy in Experiment 2: No Trees - Low Shrub biomass (NTLS), No Trees
 - High Shrub biomass (NTHS), Present Trees - Low Shrub biomass (PTLS) and
 Present Trees - High shrub biomass (PTHS) during the warmest growing
 season (2011) and at the end of the experiment (2013). Seedling condition
 was defined as: healthy (&lt; 50% of the needles turned yellow or brown)
 or unhealthy (&gt; 50% of the needles turned yellow or brown). Seedlings
 were 1 month old at plantation time in the July 2010.Table 1_environmental
 conditions_Exp 1Table 1 data:
 Environmental conditions and vegetation characteristics in hummocks
 (circular and bands) and lawns for Experiment 1. Water table depth below
 surface is an average for the four growing seasons
 (2010-2013)Table 2_ photosynthesis
 data_Exp 1Table 2 photosynthesis
 data: Photosynthesis rates for experimental pine seedlings in hummocks
 (circular and bands) versus adjacent lawns for Experiment
 1.Table 2_seedling
 responses_Exp 1Table 2 data: Responses
 of experimental pine seedlings in hummocks (circular and bands) versus
 adjacent lawns for Experiment 1 after 4 growing seasons. ST: Seeds
 inserted on top of moss; SB: Seeds inserted below moss; Small seedling (1
 month old at plantation time); Large seedling (2 months old at plantation
 time). Emergence = % of planted seeds emerged after 1 year. Condition = %
 healthy seedlings. Stem growth corresponds to vertical stem growth for
 germinating (ST and SB) seedlings and new stem growth for older (small and
 large) seedlings.Table 3_regression
 seedling-environment_Exp 1Table 3 data for
 generalized linear models assessing the responses of experimental pine
 seedlings in hummocks (circular and bands) and adjacent lawns for
 Experiment 1 during the whole experimental period (2010-2013). ST:
 Seedlings from seeds inserted on top of moss; SB: Seedlings from seeds
 inserted below moss; Small seedling (1 month old at plantation time);
 Large seedling (2 months old at plantation time). Condition = % healthy
 seedlings. Growth = stem growth.Table 4_Environmental
 data_Exp 2Table 4: Environmental
 conditions in hummocks with contrasting shrub density and tree canopy in
 Experiment 2: No Trees - Low Shrub biomass (NTLS), No Trees - High Shrub
 biomass (NTHS), Present Trees - Low Shrub biomass (PTLS) and Present Trees
 - High shrub biomass (PTHS).Table 4 and Table
 S5a_seedling performance_Exp 2Table 4: Seedling
 performance in hummocks with contrasting shrub density and tree canopy in
 Experiment 2: No Trees - Low Shrub biomass (NTLS), No Trees - High Shrub
 biomass (NTHS), Present Trees - Low Shrub biomass (PTLS) and Present Trees
 - High shrub biomass (PTHS). Seedling emergence, condition and survival
 from seeds inserted below the moss (SB), and from small planted
 seedlings.Table S3_cox regression
 (survival analysis)_Exp 1Table S3: Data for Cox
 survival analysis for experimental pine seedlings in hummocks (circular
 and bands) versus adjacent lawns during 2010-2013. ST: Seedlings from
 seeds inserted on top of moss; SB: Seedlings from seeds inserted below
 moss; Small seedling (1 month old, 10 cm tall at plantation time); Large
 seedling (2 months old, 30 cm tall at plantation time).Table S4_ regression
 seedling-environment 2011_Exp 1Table S4: Data for
 generalized linear models assessing the responses of experimental pine
 seedlings in hummocks (circular and bands) and adjacent lawns for
 Experiment 1 in 2011. Small seedling (1 month old, 10 cm tall at
 plantation time); Large seedling (2 months old, 30 cm tall at plantation
 time). Condition = % healthy seedlings. Growth = stem
 growth.