Intraspecific variation in precipitation responses of a widespread C_(4) grass depends on site water limitation

Aims Variation in precipitation strongly influences plant growth,species distributions and genetic diversity.intraspecific variation in pheno-typic plasticity,the ability of a genotype to alter its growth,morphol-ogy or physiology in response to the environment,could influence species responses to changing precipitation and climate change.Despite this,the patterns and mechanisms of intraspecific varia-tion in plasticity to variable precipitation,and the degree to which genotype responses to precipitation are influenced by variation in edaphic conditions,remain poorly understood.Thus,we determined whether genotypes of a widespread C_(4) grass(Panicum virgatum L.,switchgrass)varied in aboveground productivity in response to changes in precipitation,and if site edaphic conditions modified genotype aboveground productivity responses to precipitation.We also determined if genotype productivity responses to precipitation are related to plasticity in underlying growth and phenological traits.Methods Nine P.virgatum genotypes originating from an aridity gradient were grown under four treatments spanning the 10th to the 90th percentiles of annual precipitation at two sites in central Texas:one site with deep,fine-textured soils and another site with shallow,coarse-textured soils.We measured volumetric soil water content(VWC),aboveground net primary productivity(ANPP),tiller pro-duction(tiller number),average tiller mass,canopy height,leaf area index(LAi)and flowering time on all plants at both sites and examined genotype responses to changes in precipitation.Important Findings Across precipitation treatments,VWC was 39%lower and more variable at the site with shallow,coarse-textured soils compared to the site with deep,fine-textured soils.ANPP averaged across genotypes and precipita-tion treatments was also 103%higher at the site with deep,fine-textured soils relative to the site with shallow,coarse-textured soils,indicating substantial differences in site water limitation.Where site water limi-tation was higher,ANPP of most genotypes increased with increasing precipitation.Where site water limitation was less,genotypes expressed variable plasticity in response to precipitation,from no change to almost a 5-fold increase in ANPP with increasing precipitation.Genotype ANPP increased with greater tiller mass,LAi and later flowering time at both sites,but not with tiller number at either site.Genotype ANPP plasticity increased with genotype tiller mass and LAi plasticity at the site with deep,fine-textured soils,and only with genotype tiller mass plasticity at the site with shallow,coarse-textured soils.Thus,variation in genotype ANPP plasticity was explained primarily by variation in tiller and leaf growth.Genotype ANPP plasticity was not associated with temperature or aridity at the genotype’s origin.Edaphic factors such as soil depth and texture may alter genotype ANPP responses to precipitation,and the underlying growth traits contributing to the ANPP response.Thus,edaphic factors may contribute to spatial variation in genotype performance and success under altered precipitation.

Productivity of well-watered Panicum virgatum does not increase with CO_(2) enrichment

Aims Rising atmospheric CO_(2) has been shown to increase aboveground net primary productivity(ANPP)in water-limited perennial grasslands,in part by reducing stomatal conductance and transpiration,thereby reducing depletion of soil moisture.However,the benefits of CO_(2) enrichment for ANPP will vary with soil type and may be reduced if water limitation is low.Little is known about CO_(2) effects on ANPP of Panicum virgatum,a perennial C4 tallgrass and potential bioenergy crop.We hypothesized that if water limitation is minimized,(i)CO_(2) enrichment would not increase P.virgatum ANPP because photosynthetic rates of this C4 grass would not increase and because decreased transpiration at elevated CO_(2) would provide little additional benefit in increased soil moisture and(ii)soil type will have little effect on P.virgatum CO_(2) responses because of high overall soil moisture.Methods Growth and leaf physiology of P.virgatum cv.‘Alamo’were studied as plants established for 4 years on silty clay and clay soils along a 250 to 500 ll l-1 gradient in atmospheric CO_(2) located in central Texas,USA.Plants were watered to replace evapotranspiration,fertilized with NO_(3)NH_(4) and P_(2)O_(5) and clipped to standard height during mid-season.Important Findings ANPP increased through the third year of growth.Soil moisture(0–20 cm),ANPP,tiller numbers and leaf area index were 8–18%higher on the clay than on the silty clay soil.ANPP did not increase with CO_(2) except in the planting year.However,biomass removed with clipping strongly increased with CO_(2) in years 2 and 3,suggesting that CO_(2) enrichment increased the early-to mid-season growth of establishing P.virgatum but not later regrowth or that of fully established plants.Furthermore,CO_(2) enrichment differentially affected two components of ANPP in years 2 and 3,increasing tiller mass and reducing tiller numbers.This reallocation of resources in clipped P.virgatum suggested increased meristem limitation of productivity with CO_(2) enrichment.CO_(2) enrichment had little effect on photosynthesis but increasingly reduced stomatal conductance and transpiration as the plants established.As a result,water use efficiency became increasingly coupled to CO_(2) as leaf area increased during establishment.These results suggest that for well-watered and clipped P.virgatum,ANPP differed between soil types,was not affected by CO_(2) enrichment when fully established but interacted with clipping to alter allocation patterns during establishment.Soil type effects on ANPP-CO_(2) responses will likely become more apparent when water is more limiting.