Year-round production of forest seedlings under LED lamps : Biological and energetic implications of indoor cultivation

Abstract: Year-round cultivation of forest seedlings under light emitting diodes (LED) is a technology receiving increased attention from nurseries in the boreal forest regions. In these areas, the climate is characterized by strong seasonal fluctuations, resulting in short vegetation periods with narrow time windows for seedling transplanting and outdoor growth. An indoor pre-cultivation phase under LEDs would offer nurseries the possibility to extend their production throughout the year.LED lamps present several advantages compared to traditional light sources such as higher efficacies, longer lifetimes and advanced controls. In order to test their feasibility for seedling cultivation, three different LED lamps were compared against fluorescent lights. Biological effects of the light quality were studied through pre-cultivating seedlings of Picea abies (L.) Karst. and Pinus sylvestris L. under each spectrum in a growth room with controlled conditions. LEDs showed equal or better results than the control also after a forest field trial.Effects of light quantity were studied for both species using the most promising LED spectrum. Unlike the energy input, which can be considered proportional to the photon flux output, seedling growth exhibited a non-linear behavior with an optimum light intensity depending on the specific requirements of each species. The year-round cultivation concept enables seedlings to be produced outside of the vegetation period. However, these batches need to be cold stored until the transplanting window opens. Thus, protocols to induce cold hardiness in very young seedlings need to be established. Short-day treatments comparing different temperatures and photoperiods using LED lamps were investigated. Lower temperatures had a significant effect on inducing cold hardiness, especially for Pinus sylvestris seedlings.In contrast to some horticultural crops, which can be cultivated entirely under LEDs, forest seedlings need to be transplanted outdoors at a very young age. This poses a risk for photodamage due to high sunlight intensity and exposure to ultraviolet radiation. Light shock mitigation treatments were investigated by exposing seedlings either to higher LED light intensity, ultraviolet radiation indoors or a transient phase outdoors using shading cloths. Despite some initial stress mitigation, none of the treatments resulted in improved adaptation.Regardless of how efficient LED lamps become, indoor cultivation will always require electricity. Integrating photovoltaics throughout the nursery could compensate some of this and applying adaptive lighting controls could reduce unnecessary consumption. With proper optimization, these technologies could enable a year-round cultivation under LEDs even in the boreal forest regions.