PASSIVE NIGHTTIME WARMING FACILITY FOR FOREST ECOSYSTEM RESEARCH

A nighttime warming experiment is proposed. Over the last four decades a significant rise in nighttime mini mum temperature has been determi ned from analysis of meteorological records from a global distribution of locations. The experiment involves nighttime deployment of infrare d (IR) reflecting curtains around four sides of a forest canopy and ac ross the top of the forest to mimic the top-down warming effect of clo ud cover. The curtains are deployed with cable and pulley systems moun ted on a tower and scaffolding structure built around the selected for est site. The trunk space is not enclosed except as an optional manipu lation. The curtains reflect long-wave radiation emitted from the fore st and ground back into the forest warming the trees, litter, and soil . Excellent infrared reflection can be obtained with commercially avai lable fabrics that have aluminum foil bonded to one side. A canopy war ming of 3 to 5 degrees C is expected on cloudless nights, and on cloud y nights, a warming of 1 to 3 degrees C is anticipated relative to a c ontrol plot. The curtains are withdrawn by computer control during the day and also at night during periods with precipitation or excessive wind. Examples of hypothesized ecosystem responses to nighttime warmin g include: (1) increase in tree maintenance respiration (decreasing ca rbon reserves and ultimately tree growth), (2) increase in the length of the growing season (increasing growth), (3) increase in soil respir ation, (4) increase in litter decomposition, (5) increase in mineraliz ation of N and other nutrients from soil organic matter, (6) increase in nutrient uptake (increasing growth), and (7) increase in N immobili zation in litter. Hypothesis 1 has the opposite consequence for tree g rowth to Hypotheses 2 and 6, and thus opposite consequences for the fe edback regulation that vegetation has on net greenhouse gas releases t o the atmosphere. If Hypothesis 1 is dominant, warming could lead to m ore warming from the additional CO2 emissions. Site-specific meteorolo gical, ecophysiological. and phenological measurements are obtained in the warming treatment and in a carefully selected control plot to inv estigate site-specific hypotheses, Measurements made on both plots for a baseline period and during the period of curtain deployment provide data to test the hypotheses statistically by the ''before-after-contr ol-impact'' method applicable to unreplicated experiments. The enclosu re has a modular design that can be adapted and combined with other fo rest-scale manipulation experiments such as free air CO2 enrichment an d throughfall displacement.