Inflence of dormancy induction treatments on the photosynthetic response of field planted western hemlock seedlings

Abstract

Nursery cultural treatments can improve field performance by pre-acclimatizing seedlings to field site conditions. Four dormancy induction treatments (DIT) (i.e. long-day wet, long-day dry, short-day wet, and short-day dry) were examined for their influence on net photosynthesis ( P n) of western hemlock ( Tsuga heterophylla (Raf.) Sarg.) seedlings. Seedlings were planted on a control site with high soil water potential, adequate nitrogen levels and no plant competition. Net photosynthesis was measured in response to a full range of photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) conditions on 1-year-old and current-year needles. The response of P n to both PAR and VPD was defined by response surface models used to forecast field performance over the range of anticipated environmental conditions. All models had greater P n values as PAR increased, but increased VPD resulted in a suppression of P n. For 1-year-old needles, short-day compared with long-day seedlings, had greater or equivalent P n response over the range of measured PAR and VPD conditions. For current-year needles, short-day treated seedlings had greater P n response at high VPD (e.g. 3.8 kPa). Current-year, compared with 1-year-old needles had from 45 to 110% greater P n over a wide range of PAR (0.5–2.2 mmol m −2 s −1) and VPD (0.5–2.4 kPa) conditions. The results were compared with the testing of these DIT on a reforestation site to determine the validity of measuring physiological responses on a control site as a method for improving nursery cultural treatment selections.