Size and reproduction of Thelypteris limbosperma and Athyrium distentifolium along environmental gradients in Western Norway

The performance of the ferns Thelypteris limbospgrma and Athyrium distentifolium , as expressed by number of son on pinnules and frond size, has been related to environmental gradients. Data have been collected from homogenous fern-dominated stands from different geographical areas in Western Norway. Number of sori produced, which was assumed to indicate fern fertility, was highly variable. There were significant correlations between frond size and fertility, but only 35–46 % of the variation in fertility could be explained by differences in frond size. The highest number of sori was found on plants of medium size, and for A. distentifolium there was a statistically significant reduction in sori number on fronds taller than 125 cm. For the linear relationships between fertility and size, it is suggested that the small X-intercept and slope defines the high competitive ability of A. distentifolium in sites with a long-lasting snow cover. The ferns may dominate in large stands even though their fertility is low or zero, which indicates that they under certain ecological conditions mainly reproduce vegetatively. Both ferns show decreased fertility both toward their altitudinal and lowland distribution limits, but their fertility optima were different. The performances of the two fern species showed highly different responses to geographical and climatical variables. The fertility of T. limbosperma could be predicted by mean July- and January temperatures, humidity and canopy cover, while the fertility of A. distentifolium could not be predicted by any of the site environmental variables investigated.