Drought conditioning influences conifer seedling height but not survival three growing seasons after planting

Reforestation success in areas impacted by high-severity wildfire and seasonal moisture limitations require nursery-grown seedlings capable of establishing and surviving in harsh environments. Drought conditioning (i.e. restricted irrigation regimes) has the potential to improve planting outcomes, although assessments of effectiveness based on replicated field trials in operational settings are rare. In 2022, we conducted a drought conditioning experiment on second-year stock of ponderosa pine (Pinus ponderosa), western white pine (Pinus monticola) and two seed sources of western larch (Larix occidentalis) using well-watered (control) and water-limited (drought) treatments. Seedlings were outplanted in the spring of 2023 at two sites (a hot/dry, low elevation site and a cool/moist, higher elevation site) within a high-severity burn scar in western Montana, USA. To test treatment effectiveness, we measured seedling survival and height growth over three growing seasons along with pre-dawn water potential in the first growing season. Survival was not affected by drought conditioning (p > 0.05); instead, site and seed source explained seedling survival (p < 0.001). While ponderosa pine had high survival at both sites, the hot/dry site resulted in high mortality for western white pine and western larch. Average seedling height by the third growing season revealed interesting treatment effects: shorter drought conditioned ponderosa pine (5.6–6.2 cm difference), taller droughted western white pine (6.8 cm difference), and while initially shorter, droughted western larch height converged with control seedlings. Our results highlight the importance of following greenhouse experiments with field trials and the value of monitoring beyond the first growing season.