Amazonian evergreen forests show distinct canopy phenology and photosynthetic seasonality but the climatic triggers are not well understood. This imposes a challenge for modeling leaf phenology and photosynthesis seasonality in land surface models (LSMs) across Amazonian evergreen forest biome. On continental scale, we tested two climatic triggers suggested by site observations, vapor pressure deficit (VPD), and short-wave incoming radiation (SW) for defining leaf shedding and incorporated VPD- and SW-triggered new canopy phenology modules in the ORCHIDEE LSM (hereafter VPD-AP and SW-AP versions). Our results show that both VPD and SW are plausible precursors of large scale litterfall seasonality across the basin by comparing against in situ data from 14 sites. Specially, both VPD-AP and SW-AP correctly capture the increases in litterfall during the early dry season, followed by a flush of new leaves with increasing photosynthetic rates during the later dry season. The VPD-AP version performs better than the SW-AP version in capturing a dry-season increase of photosynthesis across the wet Amazonia areas where mean annual precipitation exceeds 2,000 mm yr⁻¹, consistent with previous satellite data analysis. Both VPD-AP and SW-AP model versions perform well in northern, central and southern Amazon regions where the SW seasonality is unimodal, but miss the seasonality of satellite GPP proxies in the eastern region off the coast of Guyana shield where SW seasonality is bimodal. Our findings imply that atmospheric dryness and sunlight availability likely explain the seasonality of leaf shedding and leaf flush processes, respectively, and consequently control canopy photosynthesis in Amazonian evergreen forests.

中文翻译：

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蒸气压不足和阳光解释亚马逊常绿阔叶林叶片物候和光合作用的季节性

亚马逊常绿森林显示出明显的树冠物候和光合季节性，但气候触发因素尚不清楚。这对模拟亚马逊常绿森林生物群落的地表模型 (LSM) 中的叶片物候和光合作用季节性提出了挑战。在大陆范围内，我们测试了现场观测建议的两种气候触发因素、蒸汽压差 (VPD) 和短波入射辐射 (SW)，用于定义叶片脱落，并在 ORCHIDEE LSM 中加入了 VPD 和 SW 触发的新冠层物候模块（以下称为 VPD-AP 和 SW-AP 版本）。我们的结果表明，通过与原位比较，VPD 和 SW 都是整个流域大规模凋落物季节性的合理前兆来自 14 个站点的数据。特别是，VPD-AP 和 SW-AP 都正确地捕获了旱季早期凋落物的增加，随后是旱季后期光合速率增加的新叶冲洗。VPD-AP 版本在捕捉平均年降水量超过 2,000 mm yr ^-1的潮湿亚马逊地区的旱季光合作用增加方面比 SW-AP 版本表现更好，与之前的卫星数据分析一致。VPD-AP 和 SW-AP 模型版本在西南季节性为单峰的亚马逊北部、中部和南部地区表现良好，但错过了圭亚那盾海岸东部地区卫星 GPP 代理的季节性，其中 SW 季节性为双峰. 我们的研究结果表明，大气干燥度和阳光可用性可能分别解释了落叶和叶面冲洗过程的季节性，从而控制了亚马逊常绿森林中的冠层光合作用。