Can dominant species traits and soil chemistry explain stability in Kalmia heath?

Abstract

In eastern Canada, black spruce (Picea mariana)-Kalmia angustifolia forests exist in one of three states: Kalmia heath, shrub savanna (SS), and black spruce forest. My main objective was to determine whether the strength of a plant-soil feedback (PSF) implicating abiotic soil properties distinguishes these three states. It is thought that traits of ericaceous shrubs (recalcitrant litter, low foliar nutrients, and high production of phenolic compounds) create nutrient-poor conditions in Kalmia heath and that this PSF explains long-term black spruce regeneration failure in heath. To test the importance of this PSF, I measured four above-ground Kalmia traits related to litter quality (leaf dry matter content, specific leaf area, foliar C:N, and foliar total phenolics) and six indicators of soil nutrient availability (soil respiration, pH, total N, inorganic N, organic N to inorganic N ratio, and total phenolics) at eight sites in and around Terra Nova National Park, Newfoundland, Canada. The eight sites consisted of four vegetation types, which were, in order of least to most spruce dominance and post-fire spruce canopy expansion, Kalmia heath, early-stage SS (eSS), mature SS (mSS), and black spruce forest. I found that the four vegetation types differed based on Kalmia traits, and that Kalmia litter quality increased from heath to eSS, to mSS, to forest. I also found that forest had higher soil nutrient availability than heath, eSS, and mSS. This means that the strength of a PSF creating nutrient-poor soil was weaker in the forest than in the other vegetation types. Contrary to expectation, the heath, eSS, and mSS vegetation types were not different based on the soil nutrient availability indicators measured. Therefore, a PSF involving the abiotic soil properties measured here did not explain the improved black spruce colonisation success in SS compared to Kalmia heath.