We evaluate the influences of elevation and climate on the spatio-temporal distribution of wetland and dryland biomes during the Pennsylvanian and early Permian in tropical Pangea. The longstanding “upland model” places drought-tolerant vegetation in elevated habitats, where slope and drainage created moisture-limited substrates under a humid climate that simultaneously promoted peat accumulation in contemporaneous lowlands. Upland plants were periodically transported to, and buried in, lowlands. Rare preservation of dryland vegetation thus reflects its general absence in basins, and taphonomic vagaries of long-distance transport. The alternative “climate model” proposes that drought-tolerant plants dominated tropical habitats when climate was seasonally dry, with wetland vegetation reduced to scattered refugia. Environmental changes attending glacial-interglacial cycles caused alternating wetter-drier conditions, and the relative abundance of wetland versus dryland biomes in basinal lowlands thus varied with climatic oscillations. The paucity of drought-tolerant plants reflects a preservational megabias against habitats with seasonal moisture deficits.

The environmental signal of “mixed” plant-fossil assemblages, comprising taxa characteristic of both wetland and dryland biomes, may help resolve these debates. We review key Pennsylvanian and lower Permian mixed assemblages from tropical Euramerican Pangea, and interpret their original habitats and climatic contexts based on multidisciplinary lines of evidence, including sedimentology, taphonomy, physiology, and paleoecology. Evaluations also consider patterns of vegetational distribution and taphonomy in modern tropical environments. We suggest that even a cursory view of current tropical plant distribution exposes flaws in the upland model. Where tropical climate is sufficiently humid to support peat swamps, slopes and elevated habitats do not host drought-tolerant vegetation, but are occupied by plants similar to those in lowland settings. This occurs because equable, high precipitation strongly dampens water-table variation across entire landscapes. Furthermore, taphonomic studies indicate that most plant-fossil assemblages record vegetation living near the burial site. Fossil floras thus reflect environmental conditions near their growth site, excluding an upland origin for most occurrences of drought-tolerant taxa. Conversely, the climate model is consistent with modern tropical vegetational distribution and soundly explains late Paleozoic floristic patterns. When Pangean tropical lowlands experienced seasonally dry conditions, plants tolerant of moisture deficits dominated most habitats, whereas wetland vegetation was restricted to wetter sites with greater preservation potential. This occurred because topographic variations are magnified under seasonal precipitation regimes, creating a complex habitat mosaic with wetland patches in a landscape subject to seasonal drought. Accordingly, we propose that a macrofloral assemblage with even rare drought-tolerant taxa indicates seasonality in the broader landscape.

At larger spatio-temporal scales, disagreement also persists about whether tectonic uplift or long-term climatic drying was the primary driver of changes in late Paleozoic floristic patterns and areal extent of tropical peat swamps. We argue that tectonic activity alone cannot explain the drastic reduction in peat swamps or coincident changes in dominance-diversity of wetland vegetation. Rates of plant dispersal and evolution far outpace that of mountain building, and peat-forming wetlands persisted in elevated habitats well into the Late Pennsylvanian. Therefore, progressive late Paleozoic aridification was the most probable driver of changing floral patterns and the distribution of wetland and dryland biomes in tropical Pangea.

我们评估了海拔和气候对热带泛大陆宾夕法尼亚纪和二叠纪早期湿地和旱地生物群落时空分布的影响。长期存在的“高地模式”将耐旱植被置于高架栖息地，在那里斜坡和排水在潮湿气候下创造了水分有限的基质，同时促进了同期低地泥炭的积累。高地植物被定期运送到低地并埋在低地。因此，旱地植被的罕见保护反映了其在盆地中普遍缺失，以及长途运输的变幻无常。另一种“气候模型”提出，当气候季节性干燥时，耐旱植物在热带栖息地占主导地位，湿地植被减少为分散的避难所。伴随冰川-间冰期循环的环境变化导致交替的干湿条件，因此盆地低地湿地与旱地生物群落的相对丰度因此随气候振荡而变化。耐旱植物的缺乏反映了对季节性水分不足的栖息地的保护性大偏见。

“混合”植物化石组合的环境信号，包括湿地和旱地生物群落特征的分类群，可能有助于解决这些争论。我们回顾了来自热带欧美洲 Pangea 的宾夕法尼亚和下二叠纪的主要混合组合，并根据多学科证据（包括沉积学、埋藏学、生理学和古生态学）解释它们的原始栖息地和气候背景。评估还考虑了现代热带环境中的植被分布和埋藏模式。我们建议即使是对当前热带植物分布的粗略观察也会暴露高地模型中的缺陷。在热带气候足够潮湿以支持泥炭沼泽的地方，斜坡和高架栖息地没有耐旱植被，但被类似于低地环境的植物所占据。发生这种情况是因为均匀的高降水强烈抑制了整个景观的地下水位变化。此外，埋藏学研究表明，大多数植物化石组合记录了生活在墓地附近的植被。因此，化石植物群反映了它们生长地点附近的环境条件，不包括大多数耐旱类群的高地起源。相反，气候模型与现代热带植被分布一致，并能很好地解释晚古生代植物区系模式。当盘古热带低地经历季节性干燥条件时，耐受水分不足的植物占据了大多数栖息地，而湿地植被则仅限于具有更大保护潜力的湿润地点。这是因为在季节性降水情况下地形变化被放大了，在受季节性干旱影响的景观中创建带有湿地斑块的复杂栖息地马赛克。因此，我们建议即使是罕见的耐旱类群的大型植物组合也表明更广泛的景观具有季节性。

在更大的时空尺度上，关于构造隆升或长期气候干燥是否是晚古生代植物区系模式和热带泥炭沼泽面积范围变化的主要驱动因素也存在分歧。我们认为，仅靠构造活动并不能解释泥炭沼泽的急剧减少或湿地植被优势多样性的同时变化。植物扩散和进化的速度远远超过了造山的速度，形成泥炭的湿地在高地栖息地一直持续到宾夕法尼亚晚期。因此，渐进的晚古生代干旱化是热带盘古大陆花卉图案变化以及湿地和旱地生物群落分布的最可能驱动因素。