Pittermann Jarmila, Baer Alex, Campany Courtney, Jansen Steven, Holmlund Helen, Schuettpelz Eric, Mehltreter Klaus, Watkins James E
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, 95060, USA.
Department of Biology, Shepherd University, Shepherdstown, WV, 25443, USA.
New Phytol. 2023 Mar;237(5):1745-1758. doi: 10.1111/nph.18667. Epub 2023 Jan 5.
The Cretaceous-Cenozoic expansion of tropical forests created canopy space that was subsequently occupied by diverse epiphytic communities including Eupolypod ferns. Eupolypods proliferated in this more stressful niche, where lower competition enabled the adaptive radiation of thousands of species. Here, we examine whether xylem traits helped shape the Cenozoic radiation of Eupolypod ferns. We characterized the petiole xylem anatomy of 39 species belonging to the Eupolypod I and Eupolypod II clades occupying the epiphytic, hemiepiphytic, and terrestrial niche, and we assessed vulnerability to embolism in a subset of species. The transition to the canopy was associated with reduced xylem content and smaller tracheid diameters, but no differences were found in species vulnerability to embolism and pit membrane thickness. Phylogenetic analyses support selection for traits associated with reduced water transport in Eupolypod 1 species. We posit that in Eupolypod epiphytes, selection favored water retention via thicker leaves and lower stomatal density over higher rates of water transport. Consequently, lower leaf water loss was coupled with smaller quantities of xylem and narrower tracheid diameters. Traits associated with water conservation were evident in terrestrial Eupolypod 1 ferns and may have predisposed this clade toward radiation in the canopy.
热带森林在白垩纪 - 新生代的扩张创造了林冠空间，随后被包括真水龙骨蕨类植物在内的各种附生群落占据。真水龙骨蕨类植物在这个压力更大的生态位中大量繁殖，在那里较低的竞争使得数千个物种得以适应性辐射。在这里，我们研究木质部特征是否有助于塑造真水龙骨蕨类植物的新生代辐射。我们对属于真水龙骨蕨I类和真水龙骨蕨II类分支的39个物种的叶柄木质部解剖结构进行了表征，这些物种占据附生、半附生和陆地生态位，并且我们评估了一部分物种对栓塞的脆弱性。向林冠的转变与木质部含量减少和管胞直径变小有关，但在物种对栓塞的脆弱性和纹孔膜厚度方面未发现差异。系统发育分析支持对真水龙骨蕨I类物种中与减少水分运输相关的特征的选择。我们认为，在真水龙骨蕨类附生植物中，选择更倾向于通过更厚的叶子和更低的气孔密度来保持水分，而不是更高的水分运输速率。因此，较低的叶片水分流失与较少量的木质部和更窄的管胞直径相关。与水分保持相关的特征在陆地真水龙骨蕨I类蕨类植物中很明显，并且可能使这个分支倾向于在林冠中辐射。
