Data from: Canopy cover and soil moisture influence forest understory plant responses to experimental summer drought

Description

Notes

Methods

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 (2^th of June - 16^th 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(L_t+1) – log(L_t). 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 (S_t+1 / S_t). 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.