DOI: 10.5061/dryad.547d7wmds
title: Belowground traits and biomass data from California grassland in 2015

description: Understanding precipitation controls on functional diversity is important
 in predicting how change in rainfall patterns will alter plant
 productivity in the future. Trait-based approaches can provide predictive
 knowledge about how certain species will behave and interact with the
 community. However, how functional diversity relates to above- and
 belowground biomass production in variable rainfall conditions remains
 unclear. Here, we tested the role of mass ratio and niche complementarity
 hypotheses in shaping above- and belowground biomass-functional diversity
 relationships in seasonal drought. We implemented a fully crossed
 experiment that manipulated drought timing (fall dry, spring dry,
 consistent dry, and ambient rainfall) and community composition
 (grass-dominated, forb-dominated, and mixed grass-forb) in a California
 annual grassland. Plant communities with mixed functional groups showed
 higher above- and belowground biomass than either the grass- or
 forb-dominant communities. We found divergent functional diversity-biomass
 relationships for above- and belowground biomass. Aboveground biomass
 decreased with community-weighted means (CWMs) of SLA and height,
 supporting the mass ratio hypothesis, which posits that dominant species
 with specific traits drive biomass production of the community.
 Belowground biomass showed no evidence of either mass ratio hypothesis or
 niche complementarity. While biomass was largely unaffected by the timing
 of drought in one season, we found community-wide functional trait shifts
 in response to rainfall treatments. Aboveground traits shifted to higher
 SLA in consistent dry compared to ambient. Belowground traits shifted to
 longer, finer and denser roots in fall and consistently dry, and shorter
 and coarser roots in spring dry. Functional diversity buffered biomass
 production by enabling shifts in above- and belowground functional traits
 across variable rainfall conditions.
description: This study was conducted at the
 University of California Sierra Foothills Research and Extension Center
 (SFREC), which is located north of Sacramento in Browns Valley, California
 (39º15’ N, 121º17’ W). Experimental Design
 In October 2014, we set up an
 experiment that manipulated the quantity and timing of rain and the plant
 community composition (Hallett et al., 2019; Shaw et al., 2022). We did
 not need permits for fieldwork. Water year 2015 (October 2014 to May 2015)
 was the final year of a 6-year drought, among the worst on state record
 since record-keeping began in 1895 in California (California Department of
 Water Resources 2017). In a random-block design, plant community
 composition treatments were nested within rainfall treatment plots in 4
 blocks (Fig. S1). Rainfall treatments consisted of control (ambient
 rainfall), consistent dry (50% of rain blocked from October–May), fall dry
 (50% of rain blocked from October–January), and spring dry (50% of rain
 blocked from February–May). A 50% rain reduction represents roughly a
 one-in-ten-year drought. Rainfall treatments were effective in their
 respective windows (e.g., fall dry and consistently dry lowered volumetric
 soil moisture in the fall), although duration of the drought effect in
 fall was shorter compared to that in spring due to a late start to the
 season (Fig. S2). Within rainfall treatments, three 1 x 2 m community
 composition subplots were established for a total of 48 subplots: 4
 rainfall plots x 3 composition subplots x 4 blocks. Community composition
 treatments were two single functional group treatments (only annual
 grasses or only forbs) and a mixture of both functional groups. Prior to
 seeding the composition treatments, we removed litter and applied
 post-emergence herbicide during a sunny period when seedlings were around
 1 inch tall. We used Poast herbicide (BASF Ag Products) to remove grass
 seedlings in the only forb plots, and 2,4D herbicide (Dow Chemical) to
 remove forb seedlings in the only annual grass plots. No herbicide was
 applied in the mixed plots. We followed the herbicide application with
 hand weeding of legumes in all three composition treatments. We seeded 4
 g/m<sup>2</sup> of <em>Erodium</em>
 <em>botrys</em> in the forb-only plots, 4
 g/m<sup>2</sup> of <em>Bromus</em>
 <em>hordeaceus</em>, <em>Lolium</em>
 <em>multiflorum</em>, and <em>Avena</em>
 <em>barbata</em> in the grass-only plots, and nothing in the
 mixed plots. We seeded these species because they are the most dominant
 forb and grass species at the field site. We did not think the difference in number of species sown
 would disproportionately increase functional trait diversity in the grass
 plots, because forbs from the seedbank emerged after seeding, and
 functional diversity in this system is largely influenced by forb
 abundance (Hallett et al. 2017). Because <em>E. botrys</em> densities
 were variable across blocks, we transplanted individuals into the plots to
 reach a density of at least 10 individuals/m<sup>2</sup> in
 the forb and mixture plots. Apart from <em>E. botrys</em>
 transplants, the mixed functional group treatment was simply what emerged
 from the existing seed bank. Species composition and
 biomass Following one growing season, peak species
 composition and biomass were collected in May 2015. A 1
 m<sup>2</sup> quadrat was laid out within each subplot, and
 all plants present in the quadrat were identified to species and visual
 estimates of their percent cover were recorded. Additionally, a visual
 estimate of the percent cover of grass, forb, bare ground, and litter
 cover was also recorded. Aboveground net primary productivity (ANPP) was
 harvested by clipping plant biomass down to the soil surface from a 0.25 m
 x 0.25 m quadrat. Fresh biomass was placed in a drying oven at 60°C for 48
 h. Samples were weighed after drying. Belowground net primary productivity
 (BNPP) was harvested by separating roots from a 5 cm diameter x 30 cm deep
 soil core in the same location as the ANPP clipping. Briefly, the core was
 divided into three 10 cm segments and roots were picked out of each
 segment with forceps in 10-minute intervals, for a total of 40 minutes per
 segment (Metcalfe et al., 2007). Roots were gently washed with tap water
 over a 2 mm sieve to remove any soil particles (Fisher Scientific No. 10),
 dried in a 60 °C oven for 48 h, and then weighed. ANPP and BNPP data are
 presented as grams of dry biomass per
 m<sup>2</sup>. Plant traits
 For 16 out of 37 species present
 at our site, we used a trait database available from Butterfield &
 Suding (2013). We replicated their methods to collect traits on the
 remaining 21 species, with 5 species not included because they were rare
 members of the community or did not germinate (Table S1). Specifically, we
 collected plant traits from individuals grown in a greenhouse for one
 season (6 weeks after germination). We used the mean trait value of six
 individuals as the trait value for each species. The following aboveground
 traits were measured: plant height, specific leaf area (SLA), and leaf dry
 matter content (LDMC). Height was measured from the tip of the newest
 tiller to the bottom of the oldest tiller using a ruler. One leaf (second
 newest, mature leaf) per individual was cut, scanned, and weighed fresh
 for fresh leaf area and weight. Then, these leaves were dried in a 60 °C
 oven for 48 h and weighed to obtain dry leaf weight. Resource-acquisitive
 species are generally taller and have larger and fleshier leaves (i.e.,
 high SLA; low LDMC) than resource-conservative species. These traits are
 consistent predictors of aboveground biomass (e.g., Butterfield &
 Suding, 2013; Cheng et al., 2021; Finegan et al., 2015).
 The
 following belowground traits were measured: root tissue density, specific
 root length of coarse (&gt; 2 mm diameter) and fine roots (≤ 2 mm
 diameter) separately, coarse root diameter, and proportion of fine roots.
 Roots were washed with tap water over 2 mm sieve, stored in 50% ethanol in
 a 4°C refrigerator, then scanned and analyzed using WinRhizo (Regent
 Instruments, Siante-Foy, Quebec, Canada) to measure belowground traits.
 Resource-acquisitive species have finer roots with low root tissue density
 and high specific root length compared to resource-conservative species
 (Reich, 2014; Tjoelker et al., 2005; Weemstra et al., 2016). Specific root
 length is a trait that has been related to the root’s efficiency to water
 and nutrient acquisition, since it indicates the amount of root length
 achieved per unit root mass invested (Lambers et al., 2006; Ostonen et
 al., 2007). Root tissue density has been linked to drought tolerance in
 arid environments (Butterfield et al., 2017).