The effect of nursery drought preconditioning on summer plantation performance of Pinus tabuliformis: high winter mortality mediated by trade off between seedling carbohydrate and field growth

In temperate monsoon climate zones, which feature low precipitation in winter and spring, summer planting of Pinus tabuliformis Carr. seedlings may improve site restoration. The application of moderate water stress to improve seedling drought resistance may be an option to harden summer-plant seedlings. The objective of this study was to assess the effect of watering intensity and duration of acclimation treatments on seedling morphology and physiology attributes in the nursery and on the field performance of Pinus tabuliformis. We applied three watering intensities at 45%, 60%, 75% (control) of container capacity (CC) in the nursery for three durations (2, 3 and 4 weeks) during early summer and then correspondingly outplanted in three dates across July (i.e., 11th, 18th, and 27th). We measured seedling morphology, non-structural carbohydrates (TNC) and nitrogen (N) concentrations in the nursery, and in the field. Additionally, we measured field mortality and plant growth for three years. Drought hardening led to greater bud set and TNC reserves, along with lower shoot-to-root mass ratio (S/R). Longer drought hardening duration produced seedlings with larger stem volume, lower S/R and lower N concentration, but with greater root mass and bud set. Field mortality primarily occurred in the winter, with the three-week drought hardening causing increased winter mortality but enhanced field growth. Mortality during the first winter was negatively related to shoot TNC reserves but positively related to seedling field growth measured at the beginning of the fall, cold season. Mortality during the second winter was negatively related to seedling size but positively related to S/R measured at the end of second growing season. In contrast, drought hardening intensity had no effect on outplanting mortality and only a minor effect on field growth. Field growth in subsequent growing seasons was directly related to the size of the plant, N and TNC content at the beginning of the first fall cold season. This study demonstrates that higher TNC reserves were associated with smaller stem volume during the first growing season, suggesting a trade-off between growth and TNC storage. This suggests that under non-stressed conditions the TNC concentration and growth of seedlings before winter strongly affected their mortality during winter. Study results suggest that summer planting of P. tabuliformis container seedlings is feasible without the application of drought hardening acclimation treatments in geographic locations typically having summer rains. Optimizing the outplanting date during the summer and initial seedling size appear to be critical for good mid-term growth and low winter mortality.