| Vegetation scientists and alpinists alike have long noted the impressive collection of cryptogams in alpine plant communities, yet few have endeavored to uncover what ecological roles they may have. I attempted to address the question of whether and how cryptogams affect the higher plants with which they cohabitate, using several manipulative experiments involving the soil-plant-cryptogam matrix. Studies were conducted in the alpine and subalpine zones of mountains in Washington State and Chile. In most experiments, I measured the performance of several alpine plant species in the midst of either abundant or no cryptogam cover, and I coupled this with measurements of the physical and biogeochemical status of the rhizosphere. The results of these experiments cautiously support the view that some cryptogams influence the growth of neighbor plants. Most notable were the reductions in growth of grass-like species seen in plots where fruticose (large-bodied) cryptogams were removed; Festuca contracta lost approximately 40% of its initial cover when the lichen Usnea trachycarpa was removed from the spaces between plants, and Carex scirpoidea lost approximately 10% of its initial cover when the mixture of Flavocetraria cucullata and Cetraria islandica was removed from its surroundings. These growth reductions coincided with greater leaf temperature swings and some profound changes in thermal and biogeochemical properties of the rhizosphere. In particular, all of the cryptogams with tissues extending above the soil surface (U. trachycarpa, F. cucullata - C. islandica, Polytrichum piliferum) helped buffer soil temperature extremes, and in some cases buffered leaf temperature extremes and retained soil moisture. Finally, all three cryptogam surfaces studied were associated with higher soil N and C, and both removals and additions of F. cucullata- C.islandica were associated with lower NH4-N. While the mechanisms behind the observed shifts in soil resources are unknown, microclimate data provide sufficient evidence to suspect indirect effects of temperature and moisture on mineralization processes. In summary, I found moderate evidence that cryptogams affect alpine plants, and that this influence might occur through a number of physical and biochemical pathways. |
