Effects of climate warming on the performance of three boreal tree seedling species in interaction with moss cover and fire history

Abstract: Forest regeneration in the form of successful seedling establishment is a key factor for the persistence of a healthy forest ecosystem. Tree seedlings grow in a complex environment shaped by over- and understory and competition for light, nutrients and water. Not much research has been done on how climate change is impacting on seedling performance through direct and indirect warming effects including possible interactions with the growth environment i.e. understory vegetation. To elucidate these, seedlings of B. pubescens, P. sylvestris and P. abies were planted into a full-factorial experiment consisting of randomly assigned plant functional group (moss) removal in combination with passive warming (open-top chambers [OTCs]) along a post-fire chronosequence. Each seedling’s survival and growth had been surveyed over a period of three years (not as part of this thesis). At the end of the experiment, growth assessment in terms of biomass determination was done, by harvesting selected individuals. Growth has been found to be species-specific but without evidence of a direct or interactive warming effect. The survival analysis highlights that mosses promoted a positive warming effect on survival in the young and intermediate successional stages regardless of seedling species. In the old successional stage warming reduced survival regardless of moss presence explicitly for B. pubescens and P. sylvestris. If, as anticipated, climate change induces a shift toward younger forest stands by altering the fire frequency and climate warms, moss cover can therefore become a critical factor for seedling survival in the boreal forest.