Agashashok Treeline Warming x Fertilization White Spruce Seedling Needle Nutrients and Isotopes (2024)

Seedling establishment and recruitment is required to maintain or advance the northern treeline. The prevailing hypothesis states that treeline position is determined by temperature. Warming is thus expected to enhance seedling growth and recruitment. The goal of this experiment was to test for the independent and combined effects of warming and fertilization on height growth of treeline seedlings at a Dryas-lichen treeline in northwest Alaska. Passive warming was achieved by installing an 18 centimeter (cm) tall, 7.6 cm inside diameter open-top clear acrylic cylinder around individual seedlings. Active warming was achieved by installing within the cylinders a 10 watt (W) 12 volt (V) cartridge heater that was controlled by a datalogger to provide heat whenever the temperature difference between an actively warmed seedling and a nearby control seedling fell below 3 degrees celsius (deg C). The experiment was established in 16 blocks with a control, a passively warmed, and an actively warmed seedling in each block. Granular fertilizer was applied in early June of 2023 and again in early June of 2024 to all seedlings in every-other block at a rate of 10 grams of nitrogen (g N), 5 grams of phosphorus (g P), and 10 grams of potassium (g K) per square meter per year. Measurements of needle N concentration and isotopes of C and N were made to gain insights into the effects of our treatments on seedling physiology and nutrient uptake.