Belowground traits and biomass data from California grassland in 2015
Data files
Jun 16, 2023 version files 41.77 KB
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BNPP_MayHarvest_2015.csv
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ClimVar_2015_species-cover_wide.csv
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ClimVar_ANPP-peak.csv
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GHtraits.csv
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README.md
Abstract
- 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.
Methods
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/m2 of Erodium botrys in the forb-only plots, 4 g/m2 of Bromus hordeaceus, Lolium multiflorum, and Avena barbata 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 E. botrys densities were variable across blocks, we transplanted individuals into the plots to reach a density of at least 10 individuals/m2 in the forb and mixture plots. Apart from E. botrys 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 m2 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 m2.
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 (> 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).