DOI: 10.5061/dryad.qfttdz0kg
title: Functional traits and their plasticity shift from tolerant to avoidant under extreme drought

description: Under climate change, extreme droughts will limit water availability for
 plants. However, the species-specific responses make it difficult to draw
 general conclusions. We hypothesized that changes in species’ abundance in
 response to extreme drought can be best explained by a set of water
 economic traits under ambient conditions in combination with the ability
 to adjust these traits towards higher drought resistance. We conducted a
 four-year field experiment in temperate grasslands using rainout shelters
 with 30% and 50% rainfall reduction. We quantified the response as the
 change in species abundance between ambient conditions and the rainfall
 reduction. Abundance response to extreme drought was best explained by a
 combination of traits in ambient conditions and their functional
 adjustment, most likely reflecting plasticity. Smaller leaved species
 decreased less in abundance under drought. With increasing drought
 intensity, we observed a shift from drought tolerance, i.e. an increase
 leaf dry matter content, to avoidance, i.e. a less negative turgor loss
 point (TLP) in ambient conditions and a constancy in TLP under drought. We
 stress the importance of using a multidimensional approach of variation in
 multiple traits and the importance of considering a range of drought
 intensities to improve predictions of species’ response to climate change.
description: Study
 system The experiment was set up in four sites on the
 Swabian Alb, southern Germany (Appendix S1: Table S3). Mean annual
 precipitation in the region is 784 mm (1861-2016; missing data for 1862
 and 1866). During the timeframe of the experiment, mean annual
 precipitation varied between 546 mm and 849 mm, mean annual temperature
 varied between 9.4 °C and 10.4 °C, with average monthly temperatures in
 January at 1 °C and in July at 19 °C (DWD 2020). The plant communities are
 temperate mesic grasslands of the Arrhenatherum elatioris (classified
 after Wilmanns 1993). They contain on average 22 species of vascular
 plants per square meter. In most of Central and Western Europe, open
 grasslands are conditioned by mowing and/or grazing management, which is
 centuries old and results in grasslands with stabilized species
 composition. To sustain the original ecosystem and mimic the traditional
 management, experimental grasslands were mown once a year in early July
 (Site KB) or twice a year in June and September (sites GW, FP, SC). The
 mowing dates during the experiment were set up to be synchronous with the
 common management practice. Experimental
 design In March and April 2017, rainout shelters were
 set up and after four years of experimental manipulation (in 2020),
 abundance and trait data were collected . Rainfall manipulation aimed to
 represent two different extreme droughts. A reduction in precipitation of
 30% corresponded to a 25-year interval. In 2020, this resulted in the
 reduction in annual precipitation down to 512 mm. A reduction in
 precipitation of 50% corresponded to a 555-year interval. In 2020, this
 resulted in reduction in annual precipitation down to 366 mm. Each
 treatment (ambient conditions, 30% rainfall reduction and 50% rainfall
 reduction) was replicated twice per study site, resulting into a total of
 24 plots (4 sites × 3 treatments × 2 plots). Treatments were randomly
 assigned to the plots which were placed 8-10 m apart. The rainout shelters
 covered a surface of 6 × 6 m. To minimize the edge effects, only the inner
 4 × 4 m were considered for data sampling and this area was further
 divided into 16 subplots of 1 × 1 m. Shelter design minimized unintended
 effects on microclimate (see Appendix S1: Section S1, Appendix S1: Figure
 S1, Appendix S1: Table S1 and Appendix S1: Table S2 for more information
 on rainout shelters and minimized artifacts). Species
 selection The target species were selected per
 treatment and site based on the following criteria: (1) minimum abundance
 of each species of 2% to enable enough plant material for trait
 collection, and (2) cumulative abundance of at least 80% of total plant
 cover to ensure that these species represent well the studied communities
 (Májeková et al. 2016). These criteria yielded 14 common plant species
 comprising grasses and forbs in a balanced ratio characteristic for the
 mesic temperate grasslands (Appendix S1: Table S3). All plants were
 perennials with a C3 photosynthetic pathway. Species
 abundance Vegetation survey was conducted at the peak
 of the vegetation season in July 2020, approximately one month after the
 mowing was applied at each site. Nested within each plot, four subplots
 were semi-randomly assigned to the vegetation survey (Latin Square
 design). Species abundance was measured as a visual estimate [% of cover].
 Percentages were recorded for total abundance, with values approximated in
 10% increments when coverage exceeded 10%. Total coverage could exceed
 100% and was later back calculated to relative abundance.
 Leaf functional traits  Four key plant
 functional traits were selected: leaf area (LA; cm2), specific leaf area
 (SLA; m2 kg-1), leaf-dry-matter content (LDMC; mg g-1), and turgor loss
 point (TLP; MPa). Trait measurements were performed on plant individuals
 with fully developed and undamaged leaves following the standardized
 protocols of Pérez-Harguindeguy et al. (2013) for LA, SLA, and LDMC, and
 of Májeková et al. (2019) for TLP (see Appendix S1: Section S2 for
 detailed information on trait sampling and measurements). For further
 analyses, absolute values of TLP were used, where higher values indicate
 higher osmotic concentration in leaf cells, delayed stomatal closure and
 higher drought tolerance. Mean abundance was calculated
 over 8 subplots (4 subplots × 2 plots per site and treatment) and mean
 traits values were averaged over 10 (five for TLP) individuals collected
 in per site and treatment.