Data from: Canopy cover and soil moisture influence forest understory plant responses to experimental summer drought
Data files
Jul 16, 2024 version files 186.56 MB
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All_species_growth_sc.xlsx
135.67 KB
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All_species_vit_sc.xlsx
44.86 KB
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Hemisperical_canopy_cover_photos.zip
186.07 MB
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README.md
9.47 KB
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Reproduction.xlsx
28.32 KB
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sitelevel_data.xlsx
12.14 KB
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SLA.xlsx
27.80 KB
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Species_responses_raw.xlsx
212.65 KB
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TMS4_soil_moisture_raw.zip
15.51 KB
Abstract
Extreme droughts are globally increasing in frequency and severity. Most research on drought in forests focuses on the response of trees, while less is known about the impacts of drought on forest understory species and how these effects are moderated by the local environment.
We assessed the impacts of a 45-day experimental summer drought on the performance of six boreal forest understory plants, using a transplant experiment with rainout shelters replicated across 25 sites. We recorded growth, vitality and reproduction immediately, two months, and one year after the simulated drought, and examined how differences in ambient soil moisture and canopy cover among sites influenced the effects of drought on the performance of each species.
Drought negatively affected the growth and/or vitality of all species, but the effects were stronger and more persistent in the bryophytes than in the vascular plants. The two species associated with older forests, the moss Hylocomiastrum umbratum and the orchid Goodyera repens, suffered larger effects than the more generalist species included in the experiment. The drought reduced reproductive output in the moss Hylocomium splendens in the next growing season, but increased reproduction in the graminoid Luzula pilosa. Higher ambient soil moisture reduced some negative effects of drought on vascular plants. Both denser canopy cover and higher soil moisture alleviated drought effects on bryophytes, likely through alleviating cellular damage.
Our experiment shows that boreal understory species can be adversely affected by drought and that effects might be stronger for bryophytes and species associated with older forests. Our results indicate that the effects of drought can vary over small spatial scales and that forest landscapes can be actively managed to alleviate drought effects on boreal forest biodiversity. For example, by managing the tree canopy and protecting hydrological networks.
Dataset 1: sitelevel data
Provides site-level data for the 25 sites
Site_ID: Unique ID for each site
Latitude: Latitudinal coordinate
Longitude: Longitudinal cordinate
gap_fraction: The % gap in the canopy (inverse of canopy cover).
Canopy_cover: The % canopy cover
TMS_may: Proportional soil moisture in May before the drought treatment
Dataset 2: All_species_growth_sc
Provides data on the scaled growth of the species over the different time intervals
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Species: Abbreviations of the six study species: Gr = Goodyera repens, Lp =Luzula pilosa, Oa = Oxalis acetosella, Hs = Hylocomium splendens, Hu = Hylcomiastrum umbratum, Bl = Barbilophozia lycopodioides
Group: Two level factor: vascular plants (vasc) or bryophytes (bryo)
Growth_July_sc: Scaled growth between the pre-drought and post-drought period. n/a values are due to missing data, for example destroyed transplants or missing markings for the moss shoots (see dataset 6).
Growth_Sept_sc: Scaled growth between post-drought and two months after the drought. n/a values are due to missing data.
Growth_2022_sc: Scaled growth between two months after the drought and one year after the drought. n/a values are due to missing data.
Growth_Tot_sc: Scaled growth between pre-drought and one year after the drought. n/a values are due to missing data.
Growth_22seg_sc: Scaled growth of the next seasons segment for the mosses H. umbratum and H. splendens. Only applicable to the mosses, n/a for the other species.
Dataset 3: All_species_vit_sc
Provides data on the scaled vitality of the species over the different time intervals
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Species: Abbreviations of the six study species: Gr = Goodyera repens, Lp =Luzula pilosa, Oa = Oxalis acetosella, Hs = Hylocomium splendens, Hu = Hylcomiastrum umbratum, Bl = Barbilophozia lycopodioides
Group: Two level factor: vascular plants (vasc) or bryophytes (bryo)
Vit_Before_sc: Scaled pre-drought vitality
Vit_July_sc: Scaled vitality post-drought. n/a values are due to missing data, for example completely dead plants or destroyed transplants so that vitality could not be scored.
Vit_Sept_sc: Scaled vitality two months after the drought. n/a values are due to missing data.
Vit_22_sc: Scaled vitality one year after the drought. n/a values are due to missing data.
Dataset 4: SLA
Provides data on the Specific Leaf Area for the vascular plants
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Species: Abbreviations of the three vascular plants: Gr = Goodyera repens, Lp =Luzula pilosa, Oa = Oxalis acetosella
Leaf area (cm2): Leaf area in cm2, traced with the software ImageJ. n/a values are due to missing data, for example when there the plants were dead and no leaves could be collected.
Dry weight (mg): Dry weight in mg. n/a values are due to missing data, for example when there the plants were dead and no leaves could be collected.
Dry weight (g): Dry weight in g. n/a values are due to missing data, for example when there the plants were dead and no leaves could be collected.
SLA (cm2.g-1): Specific leaf area in cm2.g-1 calculated by dividing the leaf area by weight in gram. n/a values are due to missing data, for example when there the plants were dead and no leaves could be collected.
Dataset 5: Reproduction - Sheet HS
Provides data on the reproductive output of Hylocomium splendens
Site_ID: Unique ID for each site
Treatment: Two level factor: T= drought treatment and C= ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Sporophytes_produced: The number of sporophytes for each moss patch
Spore_count: Number of spores counted on a sedgewick chamber. n/a values are due to missing data, for example there were no sporophytes produced.
Aborted: the % of aborted spores. n/a values are due to missing data, for example there were no sporophytes produced.
Dataset 5: Reproduction - Sheet LP
Provides data on the reproductive output of Luzula pilosa
Site_ID: Unique ID for each site
Treatment: Two level factor: T= drought treatment and C= ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Nr_leaves_22: Number of leaves at the time of reproductive effort scoring
Weight: Average seed weight. n/a values are due to missing data, for example there were no fruits produced.
Vital_seed_fruit: Average number of vital seeds per fruit (based on 10 fruits). n/a values are due to missing data, for example there were no fruits produced.
Nr_Infl: Number of inflorescenses.
Avr_fruits_infl: Average number of fruits per inflorescence.
Dataset 6: Species responses_raw - sheet Vasc plants
Provides the raw data on the species vitality and size for each site, plot and transplant
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Species: Abbreviations of the study species: Gr = Goodyera repens, Lp =Luzula pilosa, Oa = Oxalis acetosella
Nr_leaves_before: Number of leaves before the drought treatment (either April or June 2021)
Vitality_before: Vitality (1-7 scale) before the drought treatment (either April or June 2021)
Vitality_july: Vitality (1-7 scale) in July 2021.
Nr_leaves_July: Number of leaves in July 2021
Vitality_sept: Vitality (1-7 scale) in September 2021. n/a values can occur when the plant was completely gone.
Nr_leaves_sept: Number of leaves in September 2021
Vitality_22: Vitality (1-7 scale) in July 2022. n/a values can occur when the plant was completely gone.
Nr_leaves_22: Number of leaves in July 2022
Dataset 6: Species responses_raw - sheet Mosses
Provides the raw data on the species vitality and size for each site, plot and transplant
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Species: Abbreviations of the six study species: Hs = Hylocomium splendens, Hu = Hylcomiastrum umbratum
Size_June: Segment size (mm) in June 2021 (before the drought treatment)
Vitality_June: Vitality (1-7 scale) in June for each transplant patch (hence the vitality is written at the first row of each site-plot-transplant, rest is n/a)
Size_July: Segment size (mm) in July 2022 (after the drought treatment)
Growth_July: Growth from June 2021 to July 2021 in mm. When rows are n/a it is because we lost the initially marked shoot, and marked a new one with the same color.
Vitality_July: Vitality (1-7 scale) in July for each transplant patch (hence the vitality is written at the first row of each site-plot-transplant, rest is n/a)
Size_September: Segment size (mm) in September 2022
Growth_September: Growth from July 2021 to September 2021 in mm. When rows are n/a it is because we lost the initially marked shoot, and marked a new one with the same color.
Vitality_September: Vitality (1-7 scale) in September for each transplant patch (hence the vitality is written at the first row of each site-plot-transplant, rest is n/a)
Size_2021: Size (mm) of the entire segment produced in 2021, measured in July 2022
Growth_sep_july22: Growth from September 2021 to July 2022 in mm. When rows are n/a it is because we lost the initially marked shoot, and marked a new one with the same color.
Size_july_2022: Size (mm) of the segment produced in 2022 (growing from the 2021 segment).
Vitality_2022: Vitality (1-7 scale) in July 2022 for each transplant patch (hence the vitality is written at the first row of each site-plot-transplant, rest is n/a)
Dataset 6: Species responses_raw - sheet Bl
Provides the raw data on the species vitality and size for each site, plot and transplant for the liverwort Barbilophozia lycopodioides
n/a values can occur when the liverwort patch was gone (cannot scale vitality) or if we had a missing datapoint such as a loss of a photograph.
Site_ID: Unique ID for each site
Treatment: Two level factor: drought treatment and ambient control
Transplant: Transplant ID within each plot (3 transplant patches for each species)
Vitality_june: Vitality (1-7 scale) in June 2021
Patch_size_june: Path size (mm2) in June 2021
Vitality_july: Vitality (1-7 scale) in July 2021
Patch_size_july: Path size (mm2) in July 2021
Vitality_sept: Vitality (1-7 scale) in September 2021
Patch_size_sept: Path size (mm2) in September 2021
Patch_size_2022: Path size (mm2) in July 2022
Vitality_2022: Vitality (1-7 scale) in July 2022
Dataset 7: Hemispherical canopy cover photos (zip file)
Hemispherical photos at each site and each plot from which we calculated canopy cover
Dataset 8: TMS soil moisture raw (zip file)
Excel files for each site and plot with raw (uncalibrated) soil moisture values, as well as temperature, at a temporal resolution of 10 minutes
Experimental set-up and site-level variables
We experimentally induced a summer drought at 25 sites, representing a range of ambient canopy cover and soil moisture levels, in a managed forest landscape in central Sweden (Ekopark Färna, Västmanland, Sweden). We experimentally induced a 45-day drought during the summer 2021 (2th of June - 16th of July), by installing rainout shelters that completely intercepted all rainfall during this period over one of the plots at each site. The rainout shelters were 2 × 2 m, to allow for a 0.5 m buffer zone on each side of the drought-treated plots. The control plots received ambient rainfall, which added up to 87 mm.
In all treatment and control plots, we tracked soil moisture levels using TMS-4 loggers (TOMST). These loggers recorded soil moisture of the topsoil in the middle of each plot every 15 minutes. Raw soil moisture measures, which are based on conductivity pulses, were calibrated to percentage soil moisture volume. We calculated site-level measures of soil moisture and canopy cover, in order to test how among-site variation in these factors interacted with the drought. For soil moisture, we used the average value of moisture in May 2021, the month before the drought from both control and treatment plots (drought plots were at that time still untreated). To calculate canopy cover for each site, we used hemispherical photographs taken with a Lumix Panasonic camera (DMC-G80M) and a circular fisheye lens (MFT 4mm F2.8 210°). We used the Hemispherical 2.0 package in the software ImageJ (version 1.53 a) to process the photos into binary images.
Plant responses
We transplanted three vascular plant species (Goodyera repens, Luzula pilosa, Oxalis acetosella) and three bryophytes (Hylocomiastrum umbratum, Barbilophozia lycopodioides, Hylocomium splendens) under rainout shelters and in control plots.
We recorded plant drought responses in terms of vitality, growth and reproductive output. We assessed the immediate post-drought effects, as well as the effects two months and one year after the drought. For the vascular plants, we calculated growth by taking the log-ratio of the difference in number of leaves (L) at t+1 and t, using the formula log(Lt+1) – log(Lt). We also calculated the specific leaf area (SLA) for a few leaves (if possible, 3 for each transplant, 9 per plot for each species) that were produced during the drought period, by dividing the leaf area (cm2) by its dry-mass (g). For the mosses H. splendens and H. umbratum, we marked five shoots per transplant (15 per plot, 750 shoots in total for the experiment). These mosses produce a distinct growth segment each year, on top of the previous years’ segment. We calculated growth by measuring the difference in length of the marked segment (from the start of the segment until the apical tip) over sampling intervals (t and t+1). We also measured the segment that was produced during the growing season one year after the drought. In addition, we recorded number of new segments produced in 2022 on the marked shoot as a measure of branching. For the liverwort B. lycopodioides, we measured growth as the relative increase in patch size (St+1 / St). We photographed patches together with a scale bar and measured patch size by manually tracing the patches in ImageJ. Vitality was assessed for vascular plants and bryophytes using a scale from 1 to 7 developed by Dynesius et al. (2008) to describe vitality after exposure to drought: 1) dead; 2) some leaves green; 3) some shoots green; 4) half of the shoots alive; 5) alive but affected; 6) most of the shoots vigorous; and 7) the entire plant fresh and growing. We assessed reproductive output only for L. pilosa and H. splendens. For L. pilosa, we measured the number of inflorescences per transplant, mean number of fruits per inflorescence (based on fruit count of two inflorescences), the number of developed seeds per fruit, and the average weight of developed seeds. Developed seeds were distinguished from non-developed based on appearance. For H. splendens, we counted the number of sporophytes per transplant, the average number of spores per capsule, and the proportion of spores that were aborted.