Latitudinal patterns and environmental drivers of moss layer cover in extratropical forests

Aim The forest moss layer plays an important role in water retention, nutrient cycling and forest regeneration. We quantified broad-scale latitudinal patterns and environmental drivers of moss layer cover in extratropical upland forests where such information has been missing. Location Time period One thousand, nine hundred and seventy-seven sites outside the tropical belt (i.e., beyond 26 degrees N and S) across longitudes (from 154 degrees W to 140 degrees E) and elevations (from 0 to 3,950 m a.s.l.). Current (1969-2016). Major taxa studied Methods Moss layer bryophytes (mosses, liverworts and hornworts). We compiled three large datasets from: (a) a global literature search, (b) a national forest inventory database in the USA and (c) an extensive survey along the Appalachian Trail (Eastern USA) to contrast the moss layer cover along latitudinal gradients and among regions. We used correlation analysis and simple (meta- and linear-) regression to test whether moss layer cover increased towards the poles across and within forest biomes, and we used multiple (meta- and beta-) regression to test whether and how climate, forest composition and age and topography influence forest moss layer cover at broad spatial scales. Results Main conclusions Forest moss cover increased towards the North along a clear latitudinal gradient by c. 2% per 1 degrees of latitude, and it decreased with increasing maximum temperature by c. 1%-5% per 1 degrees C. Thus, cool northern regions (e.g., northern Europe) can be considered forest moss layer hotspots. Moss layer cover was driven mainly by canopy composition when all potential drivers were considered. Compared with broad-scale systematic surveys, the data from specialized literature overestimated moss cover in broadleaf deciduous forests where moss cover was low and variable. Our findings highlight that systematic surveys complement specialized literature in accurately modelling abundance on broad scales. Given that temperature, rather than precipitation or seasonality, limited the abundance of forest moss layer on broad scales, the moss layer might be a promising tool for monitoring forest ecosystem responses to climate warming.