SUPPLEMENTAL FIGURE 2: Inter-site variation in vapor pressure deficit. White triangles indicate mean values. Vapor pressure deficit (VPD) is a measurement of the amount of moisture in the air and, given the temperature, its propensity to either condense or evaporate. Higher values of VPD correspond to environments in which moisture is more likely to evaporate, indicating a dry environment. The extremely low VPD values in the persisting cold and wet site and extirpated warm and wet site indicated that the air was nearly saturated and had a high propensity to condense on surfaces.