Abstract
In the long-term absence of major disturbances ecosystems enter a state of retrogression, which involves declining soil fertility and consequently a reduction in decomposition rates. Recent studies have looked at how plant traits such as specific leaf mass and amounts of secondary compounds respond to declining soil fertility during retrogression, but there are no comparable studies for lichen traits despite increasing recognition of the role that lichens can play in ecosystem processes. We studied a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. We used this system to explore how specific thallus mass (STM) and carbon based secondary compounds (CBSCs) change in three common epiphytic lichen species (Hypogymnia phsyodes, Melanohalea olivacea and Parmelia sulcata) as soil fertility declines during this retrogression. We found that STMs of lichens increased sharply during retrogression, and for all species soil N to P ratio (which increased during retrogression) was a strong predictor of STM. When expressed per unit area, medullary CBSCs in all species and cortical CBSCs in P. sulcata increased during retrogression. Meanwhile, when expressed per unit mass, only cortical CBSCs in H. physodes responded to retrogression, and in the opposite direction. Given that lichen functional traits are likely to be important in driving ecological processes that drive nutrient and carbon cycling in the way that plant functional traits are, the changes that they undergo during retrogression could potentially be significant for the functioning of the ecosystem.
Highlights
As ecosystems develop during succession they undergo an initial build-up phase which is characterized by an accumulation of nutrients, leading to a maximum biomass phase [1,2,3]
Specific thallus mass was positively correlated with bark pH, humus depth, soil N to P ratio and total N for all three species, and light for two of the three species
Cortical carbon based secondary compounds (CBSCs) per unit mass were overall affected by island size, and there was a significant island size6species interaction because CBSCs for H. physodes declined with decreasing island size while those for P. sulcata were unresponsive (Fig. 2, Table 1)
Summary
As ecosystems develop during succession they undergo an initial build-up phase which is characterized by an accumulation of nutrients, leading to a maximum biomass phase [1,2,3]. A growing number of studies have used retrogressive chronosequence for exploring how plant functional traits change as soil fertility declines over time [8,9,10,11,12,13]. STM could potentially have afterlife effects by controlling lichen litter quality and decomposability, as has been shown for comparable measures for vascular plants such as specific leaf mass (SLM, leaf mass per unit area; the reciprocal of specific leaf area) [19,20]. Previous studies have shown SLM to increase as soil fertility declines such as occurs during retrogression [11,21], but whether STM for lichens shows similar responses remains unexplored
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