Canopy openness is an important forest characteristic related to understory light environment and productivity. Although many methods exist to estimate canopy openness, comparisons of their performance tend to focus on relatively narrow ranges of canopy conditions and forest types. To address this gap, we compared two popular approaches for estimating canopy openness, traditional spherical densiometer and modern smartphone hemispherical photography, across a large range of canopy conditions (from closed canopy to large gaps) and forest types (from low-elevation broadleaf to high-elevation conifer forests) across four states in the northeastern United States. We took 988 field canopy openness measurements (494 per instrument) and compared them across canopy conditions using linear regression and t-tests. The extensive replication allowed us to quantify differences between the methods that may otherwise go unnoticed. Relative to the densiometer, smartphone photography overestimated low canopy openness (<10%) but it underestimated higher canopy openness (>10%), regardless of forest type.

中文翻译：

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使用球形密度计和智能手机半球形摄影表征大型森林景观的树冠开放性

冠层开阔度是与林下光环境和生产力相关的重要森林特征。尽管存在许多估计冠层开放度的方法，但它们的性能比较往往集中在相对狭窄的冠层条件和森林类型范围内。为了解决这个差距，我们比较了两种流行的树冠开放度估算方法，传统球形密度计和现代智能手机半球形摄影，涵盖了大范围的树冠条件（从封闭的树冠到大间隙）和森林类型（从低海拔阔叶树到高海拔-海拔针叶林）横跨美国东北部的四个州。我们进行了 988 次野外冠层开放度测量（每台仪器 494 次），并使用线性回归和 t 检验在冠层条件下对它们进行了比较。广泛的复制使我们能够量化方法之间的差异，否则这些差异可能会被忽视。相对于密度计，智能手机摄影高估了低冠层开放度（<10%），但低估了较高的冠层开放度（>10%），无论森林类型如何。