Plant Soil https://doi.org/10.1007/s11104-019-04107-9 REGULAR ARTICLE Comparing water-related plant functional traits among dominant grasses of the Colorado Plateau: Implications for drought resistance David L. Hoover & Kelly Koriakin & Johanne Albrigtsen & Troy Ocheltree Received: 7 September 2018 /Accepted: 24 April 2019 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2019 Abstract above- and belowground biomass, and morphology, Background and aims Water is the primary limiting then assessed how these traits varied by species, and factor for plants in drylands, which are projected to photosynthetic pathway. become even drier with climate change. Plant functional Results Individual water-related traits varied widely, but traits related to water influences individual performance, did not consistently vary by photosynthetic pathway. community composition, and can provide insight into We identified three unique functional trait syndromes which species will be most vulnerable to drought. that could be classified as either conservative or non- Methods Here, we used a trait-based approach to exam- conservative with regard to water use. ine key water-related traits of five perennial grasses of Conclusions Variation in water-related traits may be the Colorado Plateau, with the goals of identifying func- key to the coexistence of species in drylands, but there tional trait syndromes, and assessing vulnerability to is uncertainty as which traits or functional trait syn- drought. We examined 14 traits including hydraulic, dromes will be most vulnerable to changes in climate. Based on the traits examined here, and forecast changes Responsible Editor: Ian Dodd. in climate for the region, we predict that the cool-season, C3 grasses will be most vulnerable in this drying, more Electronic supplementary material The online version of this drought-prone region. article (https://doi.org/10.1007/s11104-019-04107-9)contains supplementary material, which is available to authorized users. * Keywords Drylands Hydraulic Morphology Roots D. L. Hoover ( ) . Rangeland Resources & Systems Research Unit, U.S. Department Aboveground Belowground of Agriculture, Agricultural Research Service, Fort Collins, CO, USA e-mail: [email protected] Introduction K. Koriakin Department of Wildlife Ecology and Conservation, University of Water availability is the one of the primary limiting Florida, Gainesville, FL, USA factors to growth and survival of terrestrial plants, and J. Albrigtsen in no environment is water more limiting than in dry- Department of Ecology & Evolutionary Biology, University of lands (Safriel and Adeel 2005; Aschehoug et al. 2016). Colorado, Boulder, CO, USA Global climate models predict an increase in tempera- T. Ocheltree ture, aridity, and the frequency and magnitude of Department of Forest & Rangeland Stewardship, Colorado State droughts, all of which will exacerbate water limitations University, Fort Collins, CO, USA (IPCC 2013). Such changes in water availability in Plant Soil drylands will likely alter plant community composition shrub, tree), and life history (annual, perennial). For and the critical ecosystem functions they provide example, differences in photosynthetic pathways sug- (Maestre et al. 2016). Thus, understanding which spe- gest that C4 plants should have higher drought tolerance cies are most vulnerable to drought (e.g. low drought than C3 plants, due to higher water use efficiencies and resistance), is key to predicting ecological impacts of differences in growth and allocation patterns (Pearcy climate change in drylands. and Ehleringer 1984;Sage2004; Atkinson et al. Functional traits related to water in plants reflect evo- 2012). Much like traits or functional trait syndromes, lutionary history, and influence individual performance, classifying species into PFTs makes predicting re- community composition, ecosystem function, and pro- sponses to environmental change simpler. However, vide insight into which plants will most likely be affected PFTs are often broad classifications, and include multi- by changes in water availability (Reich 2014). In many ple functional trait syndromes (or plant strategies), dryland regions, stable coexistence of species within a which may be insufficient to capture functional differ- plant community is due to variation in the functional ences (Verheijen et al. 2016). Therefore it is important to traits, which can lead to spatial and temporal niche use a trait-based approach to identify potential vulnera- partitioning (Schwinning and Ehleringer 2001; bilities of plant species to drought in drylands. Silvertown et al. 2015; Butterfield et al. 2017). Variation The Colorado Plateau of the southwestern US, is in water-related functional traits may include: 1) occupy- predicted to become hotter and drier with climate ing unique soil moisture regimes, 2) varied recruitment change (Seager et al. 2007;USGCRP2017). The unique patterns, and 3) different acquisition strategies (e.g. ecohydrology of this semi-arid dryland allows for mixed rooting depth, phenology; Silvertown et al. 2015). There- communities of shrubs, forbs, and perennial grasses, fore, assessing variation in such traits may provide key which may be differentially impacted by increased wa- insights into evolutionary strategies and help identify ter limitations (Munson et al. 2011; Gremer et al. 2015; which species are most vulnerable to drought. Hoover et al. 2015, 2017; Witwicki et al. 2016; Thoma Plant species occupying a given hydrological niche et al. 2018). Perennial grasses with C3 and C4 photo- are predicted to coordinate physiological, hydraulic, and synthesis co-occur in this region and even overlap phe- morphological traits (Reich 2014; Silvertown et al. nologically for much of the year. Although cool, wet 2015). However, tradeoffs exist, leading to functional springs are ideal growing times for C3 grasses, C4 trait syndromes, such as fast growing species with grasses often initiate growth and can complete all phe- resource-acquisitive traits, or slow growing species with nological stages during that period (Comstock and resource-conservative traits (Fort et al. 2013; Reich Ehleringer 1992). Conversely, if monsoons deliver suf- 2014). The Bsafety vs efficiency^ tradeoff suggests that ficient moisture during the hot summer, both grass types plants can either have high rates of hydraulic conduc- can be active, despite hotter conditions often favoring tance, which supports high growth rates but are prone to C4 grasses (Comstock and Ehleringer 1992;Schwinning hydraulic failure (cavitation), or low rates of hydraulic et al. 2008). Climate projects for this region suggest that conductance, with low growth rates but high resistance water availability will decrease due to increases in tem- to cavitation (Manzoni et al. 2013). Thus species with perature and changes in precipitation (Seager et al. resource-acquisitive trait syndromes may be more vul- 2007;IPCC2013;USGCRP2017). Increased droughts nerable to water stress and mortality when conditions due to hotter summers and reduced cool-season precip- are dry than those with resource-conservative trait syn- itation may have large impacts on grasses in this region. dromes. It is hypothesized that these syndromes arise Indeed, recent observations and experiments suggest from selection along trait tradeoffs, resulting in coordi- that both C3 and C4 grasses may decline with continued nation among above- and belowground traits (Reich warming and more frequent drought (Munson et al. 2014). 2011; Hoover et al. 2015; Witwicki et al. 2016). How- Plant functional types (PFTs) aggregate plant species ever, uncertainty remains if such classifications based on into different classes based on particular traits that dif- photosynthetic pathway alone is sufficient to predict ferentiate in terms of functional responses to the envi- responses to drought. ronment (Verheijen et al. 2016). PFTs are defined using Here, using a trait-based approach, we examined key many classes and combinations of classes including: hydraulic, biomass, and morphological traits of five photosynthetic pathway (C3 vs C4), growth form (grass, dominant perennial grass species of the Colorado Plant Soil Plateau to identify water-related functional trait syn- 50 ml tubes of deionized (DI) water and kept cool in the dromes and identify species vulnerable to drought. This shade until they were transported back to the lab. In the research addressed three questions: 1) How do the lab, stems were cut under DI water and placed in vials water-related traits vary by species and photosynthetic overnight in the dark to fully rehydrate. The next day, pathway? 2) Is there evidence for unique trait syn- each sample was freshly cut under water, the leaf surface dromes? and 3) What are the implications for drought was then dried, the sample was weighed immediately, vulnerability? To accomplish this, we focused on two and water potential was measured with a Scholander- general types of traits. First, we examined hydraulic type pressure chamber (PMS Instruments, Albany, OR, traits, because they are mechanistically linked to water USA). The leaf surface was then placed on lab bench to transport and provide insights into plant responses to dry in order to generate a range of leaf water potential changes in water availability (Griffin-Nolan et al. 2018). values. Measurements of leaf water potential and leaf Second, we examined above- and belowground biomass weight were