Gas exchange response of western hemlock seedlings from various dormancy-induction treatments to reforestation site environmental conditions

Abstract

Western hemlock ( Tsuga heterophylla (Raf.) Sarg.) seedlings from four dormancy-induction treatments (i.e. long-day dry, long-day wet, short-day dry, short-day wet) had needle conductance ( g wv) and net photosynthesis ( P n) patterns monitored over the first field growing season. The objective was to compare stocktype response with environmental conditions on a coastal reforestation site in British Columbia. Both P n and g wv response to photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) were determined using boundary line analysis. This defined the stocktypes' maximum gas exchange response (i.e. P nMAX. and g wvMAX.) to field site environmental conditions. Short-day wet and long-day dry seedlings had greater P nMAX. with increasing PAR after frost. Short-day seedlings had greater P nMAX. with increasing PAR during summer conditions compared with long-day seedlings. The g wvMAX. response to PAR was suppressed after frost in all seedlings. Short-day seedlings had greater g wvMAX. response to PAR from March through June. All treatments showed a linear decrease in P nMAX. with increasing VPD, though short-day seedlings had the greatest P nMAX. with increasing VPD. One-year-old needles from seedlings in all treatments had very little change in g wvMAX. with increasing VPD. For current-year needles, all treatments had very little change in g wvMAX. as VPD increased, though short-day seedlings had the greatest g wvMAX.. Both P n and g wv decreased at lower shoot water potentials in all treatments, but between −0.5 and −1.0 MPa short-day seedlings had greater P n and g wv. At g wv measurements greater than 1.0 mm s −1, short-day wet seedlings had the least reduction in non-stomatal P n limitations implying a greater water-use efficiency.