Plant–plant interactions and soil moisture might be important in determining ozone impacts on grasslands

Abstract

Ozone effects on plant species mixtures could depend on the characteristics of the species involved, their mixing ratio, or on environmental conditions. Predicting long-term effects on the dynamics of plant communities requires an understanding of the interactions involved. The present experiment was designed to determine the effects of ozone on grassland species in relation to mixing ratio and soil water content (irrigation) using binary mixtures. The grass Trisetum flavescens was grown in potted replacement-series mixtures with Centaurea jacea (Experiment A) or Trifolium pratense (Experiment B). The plants were exposed to three concentrations of ozone in open-top chambers in two irrigation treatments. Total above-ground dry weight over three growth periods was measured. The competitive ability of T. flavescens was expressed as the competitive ratio (CR T ). In Experiment B, total above-ground dry weight was reduced by elevated ozone and by reduced soil moisture, and significant interactions were found for ozoneiirrigation and ozoneiratio. In Experiment A these effects were not significant. Under well watered conditions, CR T tended to be reduced by elevated ozone in Experiment A, but increased significantly in Experiment B, indicating the importance of the competing species in modifying the ozone effect on T. flavescens. In both experiments reduced irrigation decreased the magnitude of ozone effects on biomass production, which could be related to observed reductions in specific leaf conductance. The results suggest that under well watered conditions the effect of elevated ozone on the competitive balance between species depends on the species mixture, but that the mixing ratio is less important.

Effects of combined ozone and nitrogen deposition on the in situ properties of eleven key plant species of a subalpine pasture

“…But species respond diVerently to elevated O 3 depending on speciWc O 3 uptake, mesophyll exposure, and detoxiWcation capacity (Fuhrer and Booker 2003). Thus, in plant communities, eVects on plant-plant interactions can lead to shifts in species composition (Nussbaum et al 2000).…”

Plant-Mediated Ecosystem Effects of Tropospheric Ozone

“…Reduced proportions of injured leaves on O 3 exposed grasses due to the presence of a competitor have been described. For example, Trisetum flavescens responded more strongly in mixture with the low stature species Centaurea jacea than in mixture with Trifolium pratense (Nussbaum et al 2000). Similarly, Leontodon hispidus exhibited a larger increase in O 3 -induced senescence observed in the more open canopy of A. odoratum compared to the denser canopy of D. glomerata (Hayes et al 2011).…”

“…If the total biomass of the species mixture was largely influenced by the more sensitive component, a decline in total yield of the mixture has been recorded under O 3 exposure (e.g. for Trifolium repens/Lolium perenne, Hayes et al 2009; for Trifolium pratense/Trisetum flavescens, Nussbaum et al 2000). On the other hand, a range of O 3 exposure experiments resulted in unchanged total yield quantities (e.g.…”

“…This result derives from the fact that an O 3 -induced decline in the relative yield of clover entailed an increase in the relative yield of the grass component. In addition, it has been argued that altered root/shoot ratios (Haldemann and Fuhrer 2005) and reduced remobilisation of reserves after grazing (Nussbaum et al 2000) due to the O 3 impact could facilitate the less sensitive species. Thus, O 3 may interact with cutting or grazing by reducing the capacity for regrowth from energy reserves .…”

Effects of ozone on species composition in an upland grassland

“…Studies of the effects of ozone on both individual grassland species and constructed communities in chamber studies have identified that a range of grassland species are sensitive to ozone and that ozone can change species composition (e.g. Ashmore and Ainsworth 1995;Nussbaum et al 2000;Rämö et al 2006, Thwaites et al 2006. Based on the sensitivity of individual species, Mills et al (2007) suggested that calcicolous and mesotrophic grasslands are among the most ozone-sensitive ecosystems in Europe.…”