DOI: 10.5061/dryad.wstqjq310
title: Long-term low-level nutrient additions significantly impact a low arctic mesic tundra plant community, but species responses differ from high-level fertilization: Implications for predicting climate warming impacts

description: Arctic climate warming is expected to enhance the soil’s supply of
 growth-limiting nutrients, resulting in changes in plant community
 composition. Much of our understanding of nutrient influences on tundra
 plants is derived from experiments with very high levels of fertilization.
 Here, aboveground biomass of all species in a mesic birch hummock
 community, and shoot extension of Betula glandulosa, were measured in
 response to 11-19 years of factorial annual low-level nitrogen (N) and
 phosphorus (P) additions that simulate climatically realistic anticipated
 increases in soil fertility. Only the low N addition treatment had
 significant effects, enhancing the mosses and some infrequent vascular
 species’ biomass, reducing others, while the dominant shrubs were
 unaffected. Hence, overall community composition was altered, but in
 markedly different ways than classic high-level fertilization responses.
 By contrast, when measured in much larger sampling areas (9 m2), birch
 shrub new shoot extension was clearly stimulated by the separate low-level
 N and P additions, and even more so by their combination, indicating that
 its growth was NP co-limited. Overall, our results demonstrate
 that this tundra plant community is sensitive to low-level
 (climatically-realistic) increases in nutrient availability, and suggest
 that changes will be slow (multiple decades), and likely to favour species
 whose growth is primarily N-limited.
description: # Long-term low-level nutrient additions significantly impact a low arctic
 mesic tundra plant community, but species responses differ from high-level
 fertilization: Implications for predicting climate warming impacts Dataset
 DOI: [10.5061/dryad.wstqjq310](https://doi.org/10.5061/dryad.wstqjq310) ##
 Description of the data and file structure **Birch shoot length
 measurements** At the end of the 2022 growing season, we measured total
 shoot length and current year’s new shoot extension of all individual
 birch stems in centrally located 3 m x 3 m sampling areas within 3
 replicates of each of the low N and low P factorial experimental plots at
 Daring Lake, NWT. For each birch individual ramet within the sampling
 area, the lengths of all stems arising from the soil surface and their
 associated branches were measured. Concurrently, any new shoot apical
 extension (i.e. current year’s primary growth) was identified and measured
 for each stem. Since a very considerable time was required to identify all
 *Betula glandulosa* individual ramets within each of the 3 m x 3 m plots,
 and then to measure all total and new shoot growth of each individual
 shoot and its branches, we were logistically restricted to measuring only
 three randomly selected plots from the five experimental replicates in
 each treatment. **Biomass sample collection** Aboveground plant biomass
 was harvested in 1 m x 1 m sampling areas of all five replicate plots of
 each treatment in the factorial low N x P experiment in late summer of
 2023. For sorting, each live shoot of all vascular plants was followed
 down to the first adventitious root or where the shoot met the rhizome -
 whichever occurred first, at which point the shoot was clipped and sorted
 by species. Lichen pieces &gt; 1 cm diameter were plucked from the top
 soil/moss surface and all lichen species were grouped together for
 biomass. For mosses, the green-brown boundary was used to differentiate
 above- from belowground growth, and all green moss tissue for all species
 was clipped and grouped together. Standing dead biomass of vascular
 species, leaf litter, and dead moss were discarded (i.e. not included in
 the sorted sample components). For birch, live leaves were separated from
 stems to separately quantify each component of aboveground biomass.
 Finally, all sorted biomass components were dried at 65 °C in a
 fan-assisted oven for ~7 days and weighed immediately afterwards. **Soil
 measurements** We returned to the site to measure multiple soil variables
 in the treatment plots a year later (August 25th, 2024) at points directly
 adjacent to each plot’s harvested area that were covered by comparable
 intact vegetation to the harvested community. The active layer (i.e. late
 August soil thaw depth) was measured 15 cm away from each of the four
 corners of the harvest area and averaged. Soil bulk density and late
 August gravimetric soil moisture content were determined by using a
 serrated breadknife to cut out a 20 cm x 20 cm block of soil that was
 carefully extracted to prevent disturbance of soil structure/volume. The
 density and soil moisture properties were then determined on carefully
 measured subsamples (~5 x ~5 x ~10 cm) cut from the outermost side of the
 main block (furthest from the harvested sample area) by measuring first
 the fresh weight, and then the dry weight after oven-drying samples at 65
 °C for 5 days. Finally, the transition from organic to mineral soil was
 identified in the 20 cm x 20 cm excavated hole, and its depth from the
 soil surface (i.e. the green-brown transition in the moss layer) was
 measured to quantify the organic layer depth. **Files and variables:**
 **File: BirchMeasurements_DaringLake_LNLP_ArcticScience2025.csv** Plot -
 unique numeric/letter plot identifier also denoting treatment Treatment -
 C = Control, LN = low nitrogen addition, LP = low phosphorus addition,
 LNLP = low nitrogen and phosphorus addition Birch Individual - code
 ascribed to birch individuals in a given plot Shoot Length - Measurements
 of birch individuals’ shoot length and of all associated branches in cm
 New Shoot Extension - Measurements of a birch individuals’ current year’s
 growth in cm   **File: Biomass_DaringLake_LNLP_ArcticScience205** Plot -
 unique numeric/letter plot identifier also denoting treatment Treatment -
 C = Control, LN = low nitrogen addition, LP = low phosphorus addition,
 LNLP = low nitrogen and phosphorus addition Species - Species name or name
 of growth form Biomass - Total dry weight of biomass from a given species
 in a given plot (g) 
 
 **File: ActiveLayerDepths_DaringLake_LNLP_ArcticScience2025.csv** Plot -
 unique numeric/letter plot identifier also denoting treatment Treatment -
 C = Control, LN = low nitrogen addition, LP = low phosphorus addition,
 LNLP = low nitrogen and phosphorus addition Active Layer Depth -
 Measurements of active layer depth using a metal probe (cm) 
 
 **File: SoilMeasures_DaringLake_LNLP_ArcticScience2025.csv** Plot -
 unique numeric/letter plot identifier also denoting treatment Treatment -
 C = Control, LN = low nitrogen addition, LP = low phosphorus addition,
 LNLP = low nitrogen and phosphorus addition active_layer_avg - the mean
 active layer depth across four measurements for a given plot (cm)
 org_layer - soil depth at the transition from organic to mineral soil (cm)
 soil_fresh_weight - weight of soil subsamples (5 x 5 x 10 cm) directly
 after harvest (g) soil_dry_weight - weight of soil subsamples following
 oven drying (g) bulk_density - mass of dry soil per unit volume (g/cm^3)
 soil_moisture - gravimetric water content obtained by (fresh weight - dry
 weight) / dry weight x 100% 
 
 Data Files:  * ActiveLayerDepths_DaringLake_LNLP_ArcticScience2025.xlsx *
 Biomass_DaringLake_LNLP_ArcticScience2025.xlsx *
 BirchMeasurements_DaringLake_LNLP_ArcticScience2025.xlsx *
 README_(Arctic_Science_2025).docx * Description: a word file copy of this
 README. * SoilMeasures_DaringLake_LNLP_ArcticScience2025.xlsx