germination

To assess germination, we introduced seeds to Experiment 1 in the third week, corresponding to the time when pot water contents had stabilized. Three seeds were placed around the seedling of the same species, on the capitulum of a moss individual, using 3 x 140 = 420 seeds in total. Germination was checked twice a week until harvest, 5 weeks later. We considered a seed germinated when the integument had broken and a ‘shoot’ of at least 1 mm had emerged from the seed

Seedlingtraitdataexperiment2

Experiment 2 Morphological traits were assessed independently of Experiment 1. We grew tree seedlings under optimal conditions by planting pre-grown 4 weeks old tree seedlings ( see plant material) into the center of a (10 cm wide) pot, using a density of one seedling per pot. The pots were filled with sterilized organic soil, watered daily and kept under the same glasshouse light and humidity conditions as Experiment 1. Pots were arranged in five replicated blocks. Both the blocks and the pots within a block were randomly moved once a week. For more information on columnheadings see Table 1 in the associated M

Traitsandsurvival

Traits assessed in Experiment 2 were used to relate to seedling survival in Experiment 1. This file contains trait data from experiment 2 and seedling survival of seven conifer species in experiment 1 kept under contrasting moisture conditions (Dry, Wet

Mossgrowth and seedling performance

The file contains growth and survival of seedlings grown on moss in experiment 1 as well as the moss growth itsel

Post-thaw variability in litter decomposition best explained by microtopography at an ice-rich permafrost peatland

<p>Litter decomposition, a key process by which recently fixed carbon is lost from ecosystems, is a function of environmental conditions and plant community characteristics. In ice-rich peatlands, permafrost thaw introduces high variability in both abiotic and biotic factors, both of which may affect litter decomposition rates in different ways. Can the existing conceptual frameworks of litter decomposition and its controls be applied across a structurally heterogeneous thaw gradient? We investigated the variability in litter decomposition and its predictors at the Stordalen subarctic peatland in northern Sweden. We measured <i>in situ</i> decomposition of representative litter and environments using litter bags throughout two years. We found highly variable litter decomposition rates with turnover times ranging from five months to four years. Surface elevation was a strong correlate of litter decomposition across the landscape, likely as it integrates multiple environmental and plant community changes brought about by thaw. There was faster decomposition but also more mass remaining after two years in thawed areas relative to permafrost areas, suggesting faster initial loss of carbon but more storage into the slow-decomposing carbon pool. Our results highlight mechanisms and predictors of carbon cycle changes in ice-rich peatlands following permafrost thaw.</p

Water balance components.

<p>Water balance components (means, SE, n = 5) were calculated over the summers of 2008–2010 (period V, calendar weeks 19–32). Water balance components (P, I, R and ΔW are in mm summer⁻¹, S is in mm mm⁻¹ and ET in mm day⁻¹. Positive values of water storage refer to changes in water volume as a result of a net rise in the water table between the first and last date of period V, whereas negative values refer to changes in water volume as a result of a net decrease in the water table between the first and last date of period V. Different letters denote statistically significant differences between tree density treatments for the same year based on 2-way ANOVAs with treatment as factor and block as random factor. Ns = P<0.10, (*) = 0.10≥P<0.05, * = P≤0.05, ** = 0.05>P≤0.01, *** = P<0.01.</p

Relationship between plot-LAI and mesocosm evapotranspiration (ET) for the summers of 2008, 2009 and 2010.

<p>ET of the mesocosms with trees (LT and HT) were averaged over the summer (period V) for each year separately and standardized by dividing by the ET from mesocosms without trees (NT mesocosms). Symbols above the dashed line indicate a higher evapotranspiration than NT mesocosms, whereas symbols below this line indicate lower evapotranspiration than NT mesocosms. The solid line indicates a weak, but significant, (<i>P</i><0.05), linear relationship (linear regression, R² = 0.25, y = 0.1x+1.1).</p

Seasonal changes in tree density effects on mesocosm water table.

<p>Bars represent mean water tables ±1 SE (n = 5) in cm relative to a fixed point per week, from week 38 in 2007 (38) until week 37 in 2008 (37). Positive values indicate a water table closer to the surface. Water table level was measured relative to a fixed point (the overflow outlet), which was 10–15 cm below the moss surface. Birch density treatments are identified by differently shaded bullets. NT = control without trees, LT = low tree density, HT = high tree density. Arrows indicate onsets of leaf senescence in 2007 and leaf emergence in 2008. * = week during which storage coefficient has been determined, ** week in which demineralized water has been added to each mesocosm. ETp periods indicates periods (I-V) differing in solar radiation and potential evapotranspiration, over which evapotranspiration has been averaged for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g001" target="_blank">Figures 1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g004" target="_blank">4</a>. Note: mesocosm trees were planted in December 2007.</p

Vegetation composition understory.

<p>Abundances (means, SE, n = 5) of vascular plants, litter and moss in the mesocosms over the experiment (2007–2010). Data on 2008 are missing, due to a malfunctioning voice recorder. 100% of frame = species hit in all 150 points of the point-quadrat frame (see methods). For plants with horizontal (planar) leaf orientation 100% frame roughly corresponds to LAI = 1. Values over 100 indicate multiple hits per point. Statistics give results of repeated measures ANOVAs with treatment as within subject factor and year as between subject factor. Ns = P<0.10, (*) = 0.10≥P<0.05, ** = 0.05>P≤0.01, *** = P<0.01.</p

Seasonal changes in tree density effects on mesocosm evapotranspiration.

<p>Bars represent means +1 SE (n = 5) per birch density treatment averaged over periods (I–V) in order of increasing atmospheric demand for water. Periods I and II cover late autumn -early spring, whereas periods III–V represent mid spring-mid autumn (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-t002" target="_blank">Table 2</a>). Measurements spanned 1 year from week 38 in 2007 until week 37 in 2008, the only year for which we had water table data for all seasons. NT = control without trees, LT = low tree density, HT = high tree density. Different letters above the bars denote statistically significant (<i>P</i><0.05) differences between tree density treatments within a period based on five separate 2-way ANOVAs with treatment as factor and block as random factor, one ANOVA for each period.</p