Canopy wetting and drying has a variety of effects on the function of plant foliage, ranging from increased risk of pathogenic infection to reduced diffusion of gases to enhanced leaf water status in plants capable of foliar water uptake (FWU). Projected shifts in rainfall regimes and increases in summertime vapor pressure deficit will likely change the timing and duration of canopy wetting, yet current patterns of wetting are poorly understood. In this study, we investigated patterns of wetting by source (rain, dew, or frost), at different canopy heights, and at annual, seasonal and diurnal time scales using leaf wetness sensor data collected over a 4-year period in an old growth Douglas-fir tree in a temperate wet forest. We found that canopy layers were wet for roughly half the year with strong seasonal variation, staying wet 83% of the cold winter season but only 1.9% of the dry season. Upper canopy layers experienced higher wetting frequency and shorter wetting duration in all seasons compared to lower canopy layers. Outside of the dry season, wetness was predominantly caused by rain, while in the dry season the predominant source was dewfall. Throughout the year and particularly in the dry season, dewfall was restricted to the upper canopy, occurring on 28.5% of dry season nights. Multiple models which use meteorological variables to predict dewfall timing and length were developed and evaluated. Using in-tree observations, dry season dewfall was best predicted with a logistic model using dewpoint depression as a predictor. Using observations from a nearby weather station in a clearing, dry season dewfall was best predicted with the Penman equation, a biophysical model. The most important determinant of dry season dewfall in our study was sufficient nighttime cooling of the air, suggesting that increasing nighttime temperatures will lead to a decrease in dew formation frequency in the future.

冠层润湿和干燥对植物叶子的功能有多种影响，从增加病原感染的风险到减少气体扩散，再到提高植物叶面吸水能力 (FWU) 的叶水状态。预计降雨状况的变化和夏季蒸汽压不足的增加可能会改变树冠润湿的时间和持续时间，但目前的润湿模式却知之甚少。在这项研究中，我们使用在 4 年期间收集的叶片湿度传感器数据，调查了不同来源（雨、露水或霜）、不同冠层高度以及年度、季节和昼夜时间尺度的润湿模式。温带潮湿森林中的花旗松树。我们发现树冠层大约半年都是潮湿的，季节性变化很大，83% 的寒冷冬季保持湿润，但只有 1.9% 的旱季保持湿润。与下层冠层相比，上层冠层在所有季节都经历了更高的润湿频率和更短的润湿持续时间。在旱季之外，潮湿主要是由降雨引起的，而在旱季，主要来源是露水。全年，特别是在旱季，露水仅限于上层树冠，发生在 28.5% 的旱季夜晚。开发和评估了多个使用气象变量预测露水时间和长度的模型。使用树内观察，使用露点降低作为预测因子的逻辑模型可以最好地预测旱季露水。使用附近空旷气象站的观测数据，使用 Penman 方程可以最好地预测旱季的露水，一个生物物理模型。在我们的研究中，旱季露水的最重要决定因素是夜间空气充分冷却，这表明夜间温度升高将导致未来露水形成频率降低。