Ferns evolved from 400 million years ago show various functional traits and ecological strategies in extant species, and over 80% of them belong to the youngest order Polypodiales. How the functional traits and strategies of ferns have changed during their evolutionary history remains unexplored. Here, we measured functional traits that sensitive to environmental light and water availability of 345 fern species across the fern phylogeny, and reconstructed their evolutionary histories. We found that ferns, mainly Polypodiales, have developed diversified functional traits in response to forest environments. Terrestrial species, especially Thelypteridaceae and Athyriaceae in eupolypods II, showed decreased leaf mass per area (LMA) and area-based leaf nitrogen (N_area) but increased mass-based leaf nitrogen (N_mass) than early-derived polypods since the late Jurassic. Epiphytic species, mainly those in Polypodiaceae, showed reductions in N_mass and individual leaf area (Area) since the late Cretaceous. The adaptation of functional traits of Polypodiales to forest environment may have played a crucial role in fern radiation since the late Jurassic. Integrative analysis of functional traits especially the numerical ones may shed new light on plant evolution.

 In the field, we sampled 950 sporophytes of 345 fern species identified according to the Flora of China (Lin et al., 2013), which covers 87 genera, 28 families and 9 out of 11 orders in the PPG I system (PPG I, 2016) (Table S1). For each species except a few rare ones, at least three individuals were collected as three samples; and for each individual three or more fully expanded and healthy leaves were collected. For each sampled individuals, we measured four numeric leaf functional traits, including LMA, N_area, N_mass and Area (Pérez-Harguindeguy et al., 2013), and recorded three categorical traits, including growth form, leaf venation type and leaf type. Each sample with single leaf petioles or compound leaf rachis removed was scanned using an image scanner (LiDE110, Cannon, Japan). Projected leaf area (cm²) of each sample was measured using the WinFOLIA system (Regent, Canada). Leaves were oven dried for at least 48 h at 60 °C to a constant weight, and the leaf dry mass was weighed to the nearest milligram. For each sample, LMA (g m^-2) was calculated as the leaf dry mass divided by the projected area; N_mass (mg g^-1) was measured using an elemental analyser (Elementar, Germany); N_area (mg m^-2) was calculated as N_mass × LMA. The growth form (terrestrial, lithophyte or epiphytic), leaf type (single or compound) and leaf venation type (open, semi-reticulate or reticulate) were also recorded (Table S1). The traits data at species level were averaged from all the sampled individuals within the species accordingly (Table S2).

Table S1. Functional traits of 950 individuals of 345 fern species.

Table S2. Functional traits of 345 fern species in Fig. 2.

Table S3. Functional traits of ferns and seed plants in Fig. 3.

Data S1. Time-calibrated phylogeny of 345 fern species in Fig. 2.

Figure S1. Variation of four functional traits of ferns explained by three climatic factors and growth form.