Vol. 51—1997 - NorthEastern Weed Science Society

74
GSNOW ECOSYSTEM PROJECT:
WHITE SPRUCE GAS EXCHANGE
R n I I, J.A. Simpson", A.M. Gordon", F.W. Bell3, R.A. Lautenschlager',
D.A. Buckley! and D.A. Gresch'
1 C nadian Forest Service, 1219 Queen St. East, Sault Ste. Marie, ON. P6A 5M7
2 Univ rsity of Guelph, Department of Environmental Biology, Guelph, ON. NIG 2Wl
3 Ontari Forest Research Institute, 1235 Queen St. East, Sault Ste. Marie, ON. P6A 5N5
Stomatal onductance (gs), transpiration (E), and photosynthesis (NA) were measured using a
LiCor U- 200 Portable Photosynthesis Unit for 225 white spruce seedlings in late July 1994 on
3 blocks nd 5 treatments/block (harvested control, brushsaw, Silvana Selective, Vision" raj.
glyphosat ], and Release" raj. triclopyrl), 1 year post-treatment. Five seedlings were tagged at
each of 3 soil temperature/moisture cell locations/treatment. Photosynthetic radiation (PAR)
reaching e upper crowns of these seedlings was quantified during gas exchange measurements,
along wi air and leaf temperatures and leaf vapour pressure deficits (VPD). Concurrently,
midday a d pre-dawn plant water potentials of these seedlings were measured using a pressure
bomb. He ght and stem density of shrub, deciduous, and coniferous species surrrounding (1 m 2 )
each seed ing were measured. A competition index (CI = ht x density) was calculated. All data
were anal zed using ANOVA procedures.
Condition for gas exchange were similar on each of three measurement days (1 day/block). Air
temperat re averaged 28 0 C and relative humidity 85% during measurements (1200 to 1800 h
EST). S il moisture averaged 38% over all treatments. Vapour pressure deficits were well
maintain during measurements, averaging 22.0 kPa, and did not differ among treatments. PAR
averaged 1200 umol S-I m- 2 for all conifer release treatments and was reduced by half for the
control tr atment. Because of frequent rains in 1994, seedlings were not stressed for water and
demons ed considerable capability to recover from minimal to mild (- 0.7 to 1.2 MPa) midday
water str ss. Midday water potentials did not differ significantly between treatments. However,
the highe t midday water potentials were observed for the 2 herbicide treatments.
Transpir tion rates did not differ among treatments, but were highest for Release, Silvana
Selective and Vision treatments and lowest for control and brushsaw treatments. Stomatal
conducta ce differed among treatments, following a similar trend to that observed for
transpirat n. Photosynthesis increased 1.4 x over controls following the herbicide treatments, and
increases were slightly less (1.3 x) following the cutting treatments. Greater photosynthesis for
the 2 he bicide treatments was attributed to greater PAR and higher soil moisture for these
treatmen s as compared with the 2 cutting treatments. Although water use efficiency (WUE =
NA/E) v lues averaged 1.2 x higher for all conifer release treatments, they did not differ from
each oth r or from that for the control treatment.