Autumn water sources for understory vegetation and fungi in a boreal forest : an evaluation using stable isotopes

Abstract: Understory vegetation and fungi are regarded as important ecological drivers of processes like productivity and nutrient cycling in boreal forests. Whilst those processes are linked to soil water content, relatively little is known about the sources of soil water for these forest components. During early autumn in boreal forests, temperature falls and large events of rain are frequent which may influence soil water availability. To better understand the autumn plant-soil-fungi water relationships in this ecosystem, I used stable isotopes techniques in this study to examine the water sources for ericaceous shrubs and fungi in a Scots pine forest following a large, early autumn rain event. I hypothesize that ericaceous shrubs of two functional groups (evergreen vs. deciduous) utilize different soil water sources as a result of differences in their morphology. I also hypothesize sporocarps of saprotrophic and ectomycorrhizal fungi utilize different water sources based on previous studies that have shown a vertical separation of these fungi within the soil profile. My isotopic results showed xylem water δ18O values did not differ between evergreen and deciduous shrubs (means ranged between -9.25 and -9.98 ‰). Using a two source mixing model, it appeared that saprotrophic fungi drew 20-100 % of its water from shallow sources (organic matter -1 cm deep), whereas in general, ectomycorrhizal fungi used deeper water sources (4-75 cm deep). Moreover, rather than using water at different depths, uptake patterns and sources of water for understory vegetation and fungi appeared to be greatly influenced by a large rain event that occurred two weeks prior to sampling. This study clearly shows the importance of autumn large rain events for understory vegetation and highlights the need for further examining if the mechanisms observed are the same year to year. Therefore, more comprehensive studies integrating seasonality, soil water availability and the phenological characteristics of the plants and fungi would provide a more integrated picture of the soil water-plant-fungi continuum in the boreal forests.