The land surface models that provide surface fluxes of energy and mass to the atmosphere in weather forecast and climate models typically represent plant canopies as a homogenous single layer of phytomass without vertical structure (commonly referred to as a big leaf). This modeling paradigm harkens back to a 30–40-year-old debate about whether big-leaf models adequately simulate fluxes for vegetated surfaces compared to more complex and computationally costly multilayer canopy models. This article revisits that scientific debate. We review the early literature to place our findings in context and discuss recent advancements in roughness sublayer theory, observations of canopy structure and leaf traits, and computational methods that facilitate the use of multilayer models. Using a model with variable vertical resolution, we compare a multilayer canopy representation with the equivalent one-layer canopy to ask how well the one-layer canopy replicates the multilayer benchmark and to identify why differences occur. Comparisons with flux tower measurements at several forest sites spanning multiple years show that sensible heat flux, latent heat flux, gross primary production, and friction velocity for the one-layer canopy degrade in comparison to the benchmark multilayer canopy. For the forest sites considered, 5–10 canopy layers sufficiently reproduce the observed fluxes. Vertical variation of within-canopy air temperature, specific humidity, and wind speed in the multilayer canopy alters fluxes compared with the one-layer canopy. The vertical profile of leaf water potential, in which the upper canopy is water-stressed on dry soils, also causes differences between the one-layer and multilayer canopies. The differences between one-layer and multilayer canopies suggest that the land surface modeling community should revisit the big-leaf surface flux parameterizations used in models.

在天气预报和气候模型中向地面提供能量和质量的表面通量的陆地表面模型通常将植物冠层表示为单层的植物茎，没有垂直结构（通常称为大叶）。这种建模范式可以追溯到30到40年以前的争论，即与更复杂且计算成本较高的多层冠层模型相比，大叶模型是否可以充分模拟植被表面的通量。本文重新讨论了科学辩论。我们回顾了早期文献，将我们的发现放在背景下，并讨论了粗糙度亚层理论的最新进展，对冠层结构和叶片性状的观察以及促进多层模型使用的计算方法。使用垂直分辨率可变的模型，我们将多层树冠表示与等效的一层树冠进行比较，以询问一层树冠复制多层基准的效果如何，并确定为什么会出现差异。与跨多年森林的流量塔测量结果进行的比较表明，与基准多层冠层相比，单层冠层的显热通量，潜热通量，初级总产值和摩擦速度会降低。对于所考虑的森林地点，5-10个冠层可以充分再现观测到的通量。与一层顶篷相比，多层顶篷的顶篷内空气温度，比湿度和风速在垂直方向上的变化会改变通量。叶片水势的垂直剖面，其中上层冠层在干旱土壤上受水胁迫，也导致单层和多层遮篷之间的差异。一层和多层冠层之间的差异表明，陆地表面建模界应该重新研究模型中使用的大叶表面通量参数化。