2.4.1 Growth of C. purpureus and morphology of S. cataractae
We measured plant vegetative growth as a proxy for fitness. For S. cataractae , we measured the following morphological traits in 10 gametophores from each population and treatment: i) plant length, as the length in mm of the green part of the gametophyte; ii) leaf length, as the average length in mm of three leaves per shoot from the apical most part of the gametophore; and iii) leaf width, as the average width in mm of the same three leaves. These same traits were also measured in 50 gametophores from each field population of S. cataractae .
Because the area of the protonemal mat is considered a suitable proxy for biomass production in mosses (Burtscher, List, Payton, McDaniel, & Carey, 2020), for C. purpureus we calculated total growth by subtracting the area of each protonemal mat (mm2) at the beginning of the experiment to the area at the end of the experiment, and averaged the growth of 5 mats for each replicate.
All morphological and growth measurements were made by processing the photos taken of S. cataractae and C. purpureus with ImageJ v1.51 (Schneider, Rasband, & Eliceiri, 2012).