Tree ferns are arborescent plants characterized by an upright, usually unbranched trunk with weak secondary growth, typically classified as pachycaulous in terms of growth form. Unlike leptocaulous woody plants that rely on continuous thickening via the vascular cambium, tree ferns mainly depend on primary tissues for mechanical support and vertical growth. Consequently, clear quantitative constraints on their height-stem diameter relationships are lacking. Furthermore, quantitative studies on the allometric growth of pachycaulous plants remain relatively scarce, leading to considerable uncertainty in reconstructing the height of fossil plants such as tree ferns. This study systematically evaluates the allometric characteristics of tree ferns using 463 individuals of the extant tree fern Cyathea (Cyatheaceae) from the Leshan area of Sichuan Province and 127 fossil tree ferns (Psaroniaceae) from the Taiyuan Formation of the Wuda Coalfield, Inner Mongolia. The fossils include 113 partly permineralized Psaronius stems and 14 impressions of Caulopteris stems. Using reduced major axis regression, quantile regression, and coefficient of variation analysis, we assessed the variability of the plant height–stem diameter relationships of the tree ferns. The results show that the height growth rate of Cyathea is significantly higher than its diameter growth rate—approximately double—reflecting a "rapid height increase, slow diameter increase" growth strategy. The allometric scaling exponent of plant height–stem diameter in Cyathea is significantly higher than that of leptocaulous woody plants, exhibiting a large coefficient of variation in plant height and significant individual differences at identical diameters. Quantile regression further reveals the potential range of height growth in Cyathea. Comparative analysis indicates that the slenderness ratio of Psaroniaceae is significantly higher than that of extant Cyathea. Models based on Cyathea generally underestimate the actual height of Psaroniaceae, suggesting distinct height-diameter relationships between the two groups. Ultimately, models based on leptocaulous plants or modern tree ferns are not suitable for Paleozoic pachycaulous groups. These Paleozoic pachycaulous plants exhibit unique, highly variable allometric growth patterns that require further targeted exploration for accurate height restoration.
YANG et al. (Fri,) studied this question.