Lichens and mosses promote alternate stable plant communities in the New Jersey Pinelands

Abstract

Severely burned areas in the pitch pine‐dominated forests of the New Jersey Pinelands may remain open and only vegetated with mats of lichens and mosses and sparse, scattered vascular plants for many decades. We hypothesize that climate‐driven alternation between moss and lichen domination of the cryptogam mats may foster and inhibit, respectively, vascular plant development. We propose that these processes are mediated by the inhibitory effects of lichens on seed germination and seedling establishment versus a facilitating effect of mosses. We tested the hypothesis by 1) detailed surveys of the composition of cryptogam mats and their association with vascular plants, 2) experimental studies of the effects of tissue leachates on seed germination, 3) observations of mycorrhizal infection in field‐collected plants, and 4) experimental tests of seedling emergence from mats. Lichen dominance in the mats was correlated with low densities of vascular plants (graminoids and ericoid shrubs), thin organic horizons on the soil, and high levels of light availability; moss dominance was correlated with higher vascular plant densities, thicker organic horizons, and lower light. Tissue extracts of lichens strongly inhibited seed germination, while moss extracts had no effect. Similarly, mycorrhizal infection by both ecto‐ and endomycorrhizae was lower in plants growing within lichen mats than in moss mats or in bare soil. However, thick mats of both types of cryptogam inhibited seedling emergence. We observed that moss‐dominated patches became overgrown with lichens during a series of very dry, hot summers during the study. These observations all support the hypothesis that fluctuating warm and dry versus cool and moist conditions allow alternative stable states (open cryptogam mats vs succession to pine forest) to develop.