The dark side of nursery photoperiod reduction on summer plantation performance of a temperate conifer: High winter mortality mediated by reduced seedling carbohydrate and nitrogen storage

Abstract

Planting during summer is considered as a suitable option for forest plantations in regions with a temperate monsoon climate. However, seedlings planted early in the summer are not dormant and experience active growth, which makes them vulnerable to planting stress. The artificial reduction of the photoperiod in the nursery is used to harden seedlings of boreal and alpine tree species for summer planting. Little, however, is known about the effect of artificial reduction of photoperiod with respect to summer planting in temperate monsoon climate tree species.The objective of this study was to assess the effect of length and timing of photoperiod reduction on seedling morphology and physiology attributes in the nursery and on the field performance of Pinus tabuliformis. We applied four photoperiod treatments (6 h, 8 h, 10 h, control) in the nursery on four dates in the summer (June 23, July 3, July 13, and July 23). The nursery and field site were located near Beijing, Northern China. We measured seedling morphology and non-structural carbohydrates and nitrogen concentrations in the nursery and in the field. Additionally, we measured field mortality and plant growth for three years.The reduction of the photoperiod did not have any effect on bud set, but it reduced seedling growth and non-structural carbohydrate concentration, while increasing plant N concentration. A delay in the application of photoperiod treatments increased plant size and the proportion of seedlings that developed buds, and it decreased plant non-structural carbohydrates and nitrogen concentration. High field mortality was typically observed in the winter, and the early application of photoperiod treatments increased winter mortality but enhanced field growth. Mortality during the first winter was negatively related to plant non-structural carbohydrates and nitrogen concentrations at the beginning of the cold season. In contrast, photoperiod had no effect on outplanting mortality and a minor effect on field growth. Field growth in subsequent growing seasons was directly related to the size of the plant at the beginning of the first cold season. However, field growth during the growing season immediately after planting was negatively related to seedling size, and positively related to non-structural carbohydrates and nitrogen concentrations at planting.This study demonstrates that summer planting of container seedlings is feasible for P. tabuliformis, and there is no need to reduce the photoperiod in the nursery before outplanting. Optimizing the planting date during the summer appears to be critical for good mid-term growth and low winter mortality.