The use of an unenclosed field fumigation system to determine the effects of elevated ozone on a grass–clover mixture

Abstract

SUMMARYA mixture of Trifolium repens L. (var. Grasslands Huia) and Loliiim pereniie L. (var. Melle) was established in an area with low ambient ozone concentrations and exposed to episodes of ozone using a simple, unenclosed field fumigation system. Control plots were exposed to ambient air. There were two cutting frequencies to simulate grazing and silage production respectively. This paper reports on the performance of the system and the effects on yield and species composition in the first year of fumigation.The system operated satisfactorily when the wind speed was between 1 and 5 m s"'. When the wind dropped suddenly below 1 m s"' there were short periods with high concentrations but these events were rare and did not last more than a few minutes. Above 5 m s"' there was lateral downwind spillage of ozone and at higher wind speeds it was not possible to maintain the target concentration of 50-70 nl T'. The plots were fumigated only when the weather was suitable for ozone formation and when the wind speed was stable. Because the weather was poor, fumigation was restricted to 22 d between July and September. The mean 4-7 h concentrations were 50-70 nl 1^Ŝ even-hour mean ambient concentrations over the same period were 10-28 nl 1"'. There were significant effects of ozone on yield and herbage composition. The effects varied between harvests, probably because of differences in the timing of cutting relative to the episodes, and to the diflerences in phenology between the species. Most of the loss of yield was due to effects on Trifolium, which was much more sensitive than Lolium. There was a persistent eflect of ozone on the stolon density of Trifolium which lasted into the following year. It is suggested that this arose because the clover replaced damaged leaves very quickly, maintaining the canopy at the expense of the stolons. Persistent effects on stolons might lead to poorer winter survival and lower nitrogen fixation. Gaps in the sward as a result of loss of Trifolium might allow rapid weed invasion. There was no interaction between cutting frequency and ozone, which may have been due to the fact that the poor summer restricted the number of cuts and therefore decreased the difl'erence between the two cutting frequencies.

Effects of ozone on wild plants

“…competition modifies the physiological and ecological amplitude of species so the ecological effects of ozone have to be investigated in this context. The most comprehensive information available is for simple grass\clover mixtures (Bennett & Runeckles, 1977 ;Kochhar et al, 1980 ;Montes, Blum & Heagle, 1982 ;Blum et al, 1983 ;Kohut et al, 1988 ;Rebbeck et al, 1988 ;Becker et al, 1989 ;Heagle et al, 1989Heagle et al, , 1991Fuhrer et al, 1994 ;Nebel & Fuhrer, 1994 ;Nussbaum et al, 1995 ;Wilbourn et al, 1995 ;' Pleijel et al, 1996). The subject has been reviewed recently by Fuhrer (1997).…”

“…In simple grass-clover mixtures the relationship between clover and its companion grasses is unstable, with the clover tending to be at a competitive disadvantage, particularly in early spring (Evans, Williams & Mason, 1990 ;Wilbourn et al, 1995). Management keeps the balance by appropriate defoliation and fertilizer practices.…”

“…There are several reasons why visible symptoms might be under-recorded in herbaceous vegetation. In white clover, damaged leaves die and are replaced very quickly, so observation has to be made within a few days of an episode (Wilbourn, Davison & Ollerenshaw, 1995). In addition, symptoms induced by ozone can be non-specific, appearing as chlorosis, bronzing, diffuse pigmentation or senescence.…”

“…Nevertheless, the experiment shows the potential effects introduced by defoliation. In order to try to simulate defoliation as realistically as possible, Wilbourn et al (1995) used two cutting regimes to simulate grazing (frequent cut) and silage production (infrequent cut) in an experiment in which a rye grass-clover mixture was fumigated in an open-field system. Cutting was not at defined intervals but was based on an agronomic criterion, the height of the sward.…”

Consistent ozone-induced decreases in pasture forage quality across several grassland types and consequences for UK lamb production

“…Most such studies have focussed on reductions in the legume fraction as this could affect productivity of ruminant herbivores that feed on the pasture either directly via grazing or through silage production due to the higher nitrogen content of legumes. Reductions in the legume fraction, but not the grass fraction, of harvested biomass as a result of ozone exposure have been demonstrated using both intact managed pasture (Fuhrer, 1994, Trifolium repens -Lolium perenne mixtures established in field conditions from seed (Wilbourn et al, 1995), and T. repens -L. perenne established in mesocosms (Hayes et al, 2009). Similarly, reductions in the clover:grass ratio due to ozone have been reported by Nussbaum et al (1995) and Heagle et al (1989) using T. repens and L. perenne and T. repens and Festuca arundinacea respectively, which can also occur if the above-ground biomass of the grass component increases due to reduced competition from the legume content.…”