© 2001 Heron Publishing—Victoria, Canada
Blue wild-rye grass competition increases the effect of ozone on ponderosa pine seedlings
Christian P. Anderson (1), William E. Hogsett (1), Milton Plocher (2), Kent Rodecap (2) and E. Henry Lee (1)
1. US EPA, Western Ecology Division, National Health and Environmental Effects Research Laboratory, 200 SW 35th Street, Corvallis,
OR 97333, USA / 2. Dynamac Corporation, 200 SW 35th Street, Corvallis, OR 97333, USA / Received December 17, 1999
Summary
Individual ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were grown in mesocosms with three densities of blue wild-rye grass (Elymus glaucus Buckl.) (equivalent to 0, 32 or 88 plants m–2) to determine if the presence of a natural competitor alters the response of ponderosa pine seedlings to ozone. After 3 years
of ozone exposure, grass presence reduced total ponderosa pine dry mass by nearly 50%, whereas ozone alone had no significant
effect on ponderosa pine growth. The combination of ozone and grass further reduced needle, stem and branch dry mass significantly
below that induced by grass competition alone. Root:shoot ratios increased in response to the combined grass and ozone treatments.
Grass competition significantly reduced soluble sugar concentrations in all ponderosa pine tissue components examined. Starch
concentrations were highly variable but did not differ significantly between treatments. Ozone significantly reduced soluble
sugar concentrations in fine roots and stems. In the absence of grass, ozone-treated seedlings tended to have higher tissue
N concentrations than controls. In the presence of grass, ozone-treated seedlings had lower N concentrations than controls,
resulting in a significant interaction between these two stresses in 1- and 2-year-old needles. Needle C:N ratios decreased
in response to grass competition, as a result of increased N concentration and no change in C concentration. The opposite
response was observed in ozone-treated seedlings as a result of decreased N concentrations, indicating that ozonetreated seedlings
were unable to take up or retain as much nitrogen when grown in the presence of grass. We conclude that ponderosa pine seedlings
are more susceptible to ozone when grown in competition with blue wild-rye grass.
Keywords:
C:N ratio, Elymus glaucus, Pinus ponderosa, root:shoot ratio.
