Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, USA.
School of Life Sciences, University of Technology Sydney, Ultimo, NSW, Australia.
Glob Chang Biol. 2021 May;27(9):1802-1819. doi: 10.1111/gcb.15555. Epub 2021 Mar 3.
Tropical forests are an important part of global water and energy cycles, but the mechanisms that drive seasonality of their land-atmosphere exchanges have proven challenging to capture in models. Here, we (1) report the seasonality of fluxes of latent heat (LE), sensible heat (H), and outgoing short and longwave radiation at four diverse tropical forest sites across Amazonia-along the equator from the Caxiuanã and Tapajós National Forests in the eastern Amazon to a forest near Manaus, and from the equatorial zone to the southern forest in Reserva Jaru; (2) investigate how vegetation and climate influence these fluxes; and (3) evaluate land surface model performance by comparing simulations to observations. We found that previously identified failure of models to capture observed dry-season increases in evapotranspiration (ET) was associated with model overestimations of (1) magnitude and seasonality of Bowen ratios (relative to aseasonal observations in which sensible was only 20%-30% of the latent heat flux) indicating model exaggerated water limitation, (2) canopy emissivity and reflectance (albedo was only 10%-15% of incoming solar radiation, compared to 0.15%-0.22% simulated), and (3) vegetation temperatures (due to underestimation of dry-season ET and associated cooling). These partially compensating model-observation discrepancies (e.g., higher temperatures expected from excess Bowen ratios were partially ameliorated by brighter leaves and more interception/evaporation) significantly biased seasonal model estimates of net radiation (R ), the key driver of water and energy fluxes (LE ~ 0.6 R and H ~ 0.15 R ), though these biases varied among sites and models. A better representation of energy-related parameters associated with dynamic phenology (e.g., leaf optical properties, canopy interception, and skin temperature) could improve simulations and benchmarking of current vegetation-atmosphere exchange and reduce uncertainty of regional and global biogeochemical models.
热带雨林是全球水热循环的重要组成部分，但驱动其地气交换季节性的机制在模型中难以捕捉。在这里，我们（1）报告了在亚马逊地区四个不同的热带森林站点的潜热（LE）、显热（H）和长、短波外逸辐射通量的季节性变化——从亚马逊东部的卡西亚万纳和塔帕若斯国家森林到马瑙斯附近的森林，再到杰鲁保护区的南部森林，跨越了赤道；（2）调查了植被和气候如何影响这些通量；（3）通过将模拟与观测进行比较，评估陆地表面模型的性能。我们发现，先前模型未能捕捉到观测到的旱季蒸散量（ET）增加的情况，这与模型对（1）比辐射率（Bowen 比）的幅度和季节性的高估有关（与季节性观测相比，Bowen 比中显热仅占潜热通量的 20%-30%，表明模型夸大了水分限制），（2）冠层发射率和反射率（反照率仅为入射太阳辐射的 10%-15%，而模拟值为 0.15%-0.22%），（3）植被温度（由于低估了旱季 ET 以及相关的冷却）。这些部分补偿了模型观测之间的差异（例如，过高的 Bowen 比预计会导致更高的温度，但较亮的叶片和更多的截留/蒸发会部分缓解），显著偏向了季节性模型对净辐射（R）的估计，净辐射是水热通量的关键驱动因素（LE0.6R 和 H0.15R），尽管这些偏差在不同的站点和模型之间有所不同。更好地代表与动态物候学相关的能量相关参数（例如，叶片光学特性、冠层截留和表皮温度）可以改善当前植被-大气交换的模拟和基准测试，并减少区域和全球生物地球化学模型的不确定性。
