Bj. Wilsey, UREA ADDITIONS AND DEFOLIATION AFFECT PLANT-RESPONSES TO ELEVATED CO2IN A C-3 GRASS FROM YELLOWSTONE-NATIONAL-PARK, Oecologia, 108(2), 1996, pp. 321-327
A common grass from Yellowstone National Park, Stipa occidentalis, was
grown in a factorial experiment to determine if its response to the d
irect effects of elevated CO2 would be affected by defoliation, and ur
ea additions simulating the N in a urine hit. Plants were grown in tal
l pots (to mimic rooting depth in the field) in growth chambers under
elevated (700 ppm) and ambient (370 ppm) CO2, were defoliated or left
undefoliated, and given N-supply rates based on field mineralization r
ates (untreated) or with an additional 40 g N/m(2). Growth increases i
n response to elevated CO2 were largest when plants remained unclipped
and received urea additions, and were found primarily in crowns and r
oots (storage organs). Aboveground biomass, which is the part of the p
lant consumed by grazing mammals, was not affected by elevated CO2. Th
e elevated CO2 treatment caused a reduction in leaf percent N. However
, there was a significant interaction between the CO2 and urea treatme
nts, resulting in a larger difference in leaf percent N between urea-t
reated and control plants under elevated than under ambient CO2. Hence
, elevations in atmospheric CO2 may cause an increase in the amount of
urine-hit-induced spatial variability in temperate grasslands. Since
food quantity remained largely unchanged in response to elevated CO2,
and forage N content went down, grazing mammals may be negatively affe
cted by increases in atmospheric CO2.