Moss-associated cyanobacteria (MAC) are important source of new N into high latitude ecosystems. However, biological nitrogen fixation (BNF) by MAC is affected by an important spatiotemporal variability. Our poor understanding of the drivers controlling this variability limits our ability to produce reliable N input estimates and to predict the effect of global climate change on high latitude N input through MAC. In this study, we will evaluate the role of moss color phenotypes (green versus yellow), canopy gaps and moss species in structuring the spatial heterogeneity of BNF. We collected samples of the three dominant moss species (Pleurozium schreberi, Ptilium crista-castrensis and Hylocomnium splendens) in 15 black spruce forests in June 2017 along a 500 km south-north transect in northeastern Canada. Additionally, one site was sampled monthly over the growing season (May to September 2016, n = 9). Following a similar sampling design we collected P. schreberi and P. crista-castrensis, sorted according to their perceived color (green or yellow), from 24 paired or contiguous sites showing contrasted canopy openness (canopy gaps, windfall). N2 fixation activity was assayed in-situ and under controlled conditions (22°C, 24h light) using the acetylene reduction assay. Species coverage, canopy picture and meteorological data were collected.
Results/Conclusions
Pleurozium schreberi and Ptilium crista-castrensis account for 20 to 80% of the total BNF. Shoot color and canopy openness in those species account respectively for 26% and 17% of the BNF variance (p < 0.001). Increase of BNF was linear with increasing canopy openness at the stand level (0.2 < r² <0.8). P. schreberi BNF shows strong response to canopy opening (~ +800%) and was more abundant in open canopy area (65 % vs 51%). P. crista BNF increase with canopy gaps (~ +240 %) was less important and this moss was less abundant in large canopy gaps (30% vs 20%). The role of temperature, precipitation, light, throughfall and N deposition on the canopy effect will be discussed.
Based on these results, we can draw several important conclusions. Firstly, canopy openness can structure the relative abundance of moss species and the rate of BNF at the stand level. As a result, sylvicultural practices (e.g. stand thinning) and natural disturbances (e.g. windfall) that alter canopy openness could positively impact N input via BNF. Finally, this study highlight the importance of considering moss species, colors and spatial variation in canopy openness to obtain more reliable estimates of BNF at the landscape scale.