DOI: 10.5061/dryad.905qfttmc
title: Intraspecific trait variation and species turnover mediate grazing impacts on above- and below-ground functional trait composition of plant communities

description: Although grazing has significant impacts on plant functional traits and
 community composition in grasslands, few studies have simultaneously
 explored how plant above- and below-ground traits and community functional
 composition respond to grazing. Using a grazing manipulation experiment
 with seven levels of grazing intensity (0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9
 sheep ha-1) in the Inner Mongolia grassland, we partitioned the roles of
 intraspecific trait variation (ITV) and species turnover underlying the
 grazing induced changes in above- and below-ground functional trait
 composition of plant communities.  Six aboveground traits (i.e.
 plant height, plant aboveground biomass, plant density, leaf area,
 specific leaf area (SLA) and leaf density) and three root traits (average
 root diameter, ARD; specific root length, SRL; and root tissue density,
 RTD) of the first-, second- and third-order roots (1st-, 2nd-, and 3rd-)
 were measured at the plant individual level.  At the community
 level, plant above-ground traits shifted towards grazing avoidance
 strategy (e.g. plant height and leaf area decreased), and below-ground
 traits shifted towards conservative strategy (i.e. 1st-SRL and 2nd-SRL
 decreased, 1st-RTD and 2nd-ARD increased) with increasing grazing
 intensity.  Functional tradeoffs were found between plant
 individual biomass and plant density, and between leaf area and leaf
 density under grazing.  However, community-weighted mean SRL
 (SRLCWM) and ARD (ARDCWM) of different root orders exhibited functional
 coordination under grazing pressure.  SLACWM and SRLCWM also
 showed synergistic responses to grazing.  The ITV plays a
 predominant for the changes in above- and below-ground functional trait
 composition at the community level.  However, changes in mean
 trait values among plant species with different resource use strategies
 were mainly triggered by species turnover.  For species with
 different resource use strategies, grazing exhibited a coordinated effects
 on ITV but an offset effect on species turnover. Synthesis. Our results
 demonstrate that both the above- and below-ground trait composition of
 plant communities shifted toward conservative strategy under long-term
 grazing.  This study highlights the effects of ITV and species
 turnover that govern the grazing-induced changes in functional trait
 composition of plant communities, and has important implications for
 grazing management in semiarid grasslands.
description: <b>Study
 site and experimental design</b>
 This study was
 conducted at the Inner Mongolia Grassland Ecosystem Research Station
 (IMGERS, 43º38'N, 116º42'E) of the Chinese Academy of Sciences,
 located in the Xilin River Basin of Inner Mongolia, China.  The altitude
 is about 1200 m above the sea level (Bai et al. 2004).  The study area has
 a semi-arid continental climate, with the mean annual temperature of 0.5
 ºC and mean annual precipitation of 334.6 mm (1970-2010). About 60-80% of
 the precipitation occurs in the growing season (May to August). Loamy sand
 is the main zonal soil types in the experimental area (Calcic Chernozem
 according to ISSS Working Group RB, 1998).  In this area, the dominant
 species are the perennial bunchgrass <i>Stipa grandis</i> and
 the perennial rhizomatous grass <i>Leymus
 chinensis</i>. The experimental
 plots were located at the Sino-German grazing experimental site and were
 established in June 2005 (Schönbach et al. 2011). The gazing manipulation
 experiment has seven levels of grazing intensity (GI): 0, 1.5, 3.0, 4.5,
 6.0, 7.5 and 9.0 sheep ha<sup>-1</sup>. Each grazing intensity
 level included one standard plot, given the high intrinsic spatial
 heterogeneity in soil and plant community composition between the plots
 and high management cost (Schönbach et al. 2011). Size of each standard
 plot was 2 ha. To achieve a minimum of six sheep per plot, the plot size
 was 4 ha in GI=1.5. To prevent sheep migration between plots, we isolated
 each plot via using fences at the beginning of the experiment.  During the
 period of vegetation growth from June to September, sheep were kept in
 each grazing plot to graze continuously.    
 <b>Field
 sampling</b> Plant functional
 traits of co-occurring species were measured by soil core method (Fig. S1)
 in August 2013 when most species were mature (Bai et al. 2008). In each
 grazing plot, three enclosed cages
 (4 m × 4 m) were randomly set up to ensure that the plants in the cages
 are not disturbed by grazing in 2013. In each cage, three cores
 (with a diameter of 20 cm and a depth of 50 cm) were set up, thus, a total
 of 9 soil cores were excavated for each grazing plot. The soil cores were
 kept 40 cm underground and 10 cm above ground to obtain most roots, given
 that most species are shallow root system in the Inner Mongolia grassland
 (Cheng et al. 2015). We measured the plant height (PH, for vegetation
 branch) and plant density (PD) of each species in soil cores before they
 were taken. After the soil cores were fully irrigated, plants were taken
 out completely from the plots and transported to the laboratory to obtain
 the leaf and root functional traits.   
 <b>Traits
 measurement </b> The plant
 functional traits of all plant species were measured at individual-level
 in a given soil core. We separated and recorded all the vegetative leaves
 for each plant individual, then measured the projected area of all these
 leaves by using Li-3100 (Li-COR, Lincoln, NE, USA). LA was the total leaf
 area divided by leaf number, and SLA was the ratio of the total leaf area
 to oven-dried mass (oven-dried at 70 ºC for 48 h to a constant mass). The
 soil column was immersed in water for 24 h and then carefully washed to
 obtain the roots of co-occurring species. On the basis of root color,
 texture and the connection with its shoots, we identified and separated
 the plant individual-specific root systems for each species.  Following
 the steps described in Pregitzer (2002), the whole root systems were
 completely classified into three orders (the most distal roots represent
 the first-order roots) for all species occurring in grazing plots (Cheng
 et al. 2015). After classification, first-, second- and third-order roots
 of plant individual were scanned separately by using the Epson Perfection
 V750 Pro (SEIKO EPSON, Nagano, Japan) with the resolution of 300 dpi. The
 roots were dried and weighted separately after scanning, and the root
 images were analysed by WinRHIZO Pro 2019 software (Regent Instruments,
 Quebec, Canada) to obtain root traits including root length, ARD, and
 volume for each root order. SRL was calculated as the total root length
 divided by the root dry mass, and RTD was calculated as the root dry mass
 divided by the root volume. Finally, the shoot and other portion of
 underground (rhizome, and the connection between root and shoot) of
 individual plant was oven-dried to obtain the biomass of different plant
 organs, including plant aboveground part, leaf, and roots of different
 orders.  In this study, each soil core can be considered as a
 meta-community. Each soil core contained two to eight plant species, and
 16 plant species were measured in this study.