Interactions between epiphytes during canopy soil formation: an experiment in a lower montane cloud forest of southeast Mexico.

In several montane forests around the world, epiphytes coexist in mats, sharing the rhizosphere and forming histosol-type soils rich in nutrients. The role of these epiphytes in the formation of canopy soil and the fitness costs that epiphytes face when cohabiting in these mats are unknown. In a lower montane cloud forest in central Veracruz, Mexico, a 2-year factorial experiment was carried out with the presence/absence of ramets of Phlebodium areolatum (Polypodiaceae), Tillandsia kirchhoffiana, T. multicaulis and T. punctulata (Bromeliaceae). We examined (i) which epiphyte species contribute to the formation of canopy soil, (ii) the role of epiphyte composition in the soil nutrient composition, and (iii) the fitness costs faced by epiphytes when cohabiting. Canopy soil formation highest when P. areolatum is present. Soil nutrient content does not change with epiphyte composition, is influenced by the microbiota, and P content decreases with the presence of epiphytes. The fitness costs show that the species compete, decreasing their survival and growth, but the competitive capacity differs between the species. We conclude that P. areolatum is an ecosystem engineer that promotes the creation of canopy soil but is a poor competitor. The results coincide with the model of succession by facilitation. Canopy soil is a slow-created component whose nutrient content does not depend on the epiphytic flora. In epiphyte mats, the dominant interactions are competitive, but there is also facilitation.

Epiphyte associations and canopy soil volume: nutrient capital and factors influencing soil retention in the canopy.

Canopy soil (CS) volume reflect epiphyte community maturity, but little is known about the factors that retain CS or species succession within it. Humus fern species (e.g. Phlebodium areolatum) appear capable of retaining CS. In ten Quercus spp. we sampled 987 epiphyte mats to examine the role of the common epiphyte species and crown traits determining CS volume, in order to infer successional stages and identify pioneer and late successional species. Branch traits (height, diameter and slope), CS volume and cover of the epiphyte species were determined for each mat. Nutrient content was determined in CS random samples of 12 epiphyte associations and sizes (one sample from each size quintile). A total of 60% of the mats lack CS. Cover of P. areolatum was the main variable explaining CS volume, and this species was present in 46.8% of those with CS. Epiphyte composition was highly variable, but pioneer (species appearing in monospecific mats, without CS) and late successional species could be identified. Canopy soil nutrient content was similar among the associations of epiphytes. Magnesium, Ca and pH decreased with CS volume, while P and N increased. Phlebodium areolatum is associated with high CS volumes and could act as a key species in its retention. Monospecific mats of pioneer species lack CS or have low volumes, while CS is much higher in mats with late successional species, but the mechanisms of CS formation and nutrient retention in response to interactions between epiphyte species remain to be tested.