Efectos ecologicos de perturbaciones humanas en bosques de niebla de las islas canarias. Ensayo con comunidades de briofitos
- Autores: Jairo Patiño Llorente
- Directores de la Tesis: Juana María González Mancebo (dir. tes.), Olaf Werner (codir. tes.)
- Lectura: En la Universidad de La Laguna ( España ) en 2010
- Idioma: español
- Tribunal Calificador de la Tesis: Esperanza Beltrán-Tejera (presid.), Ana Losada Lima (secret.), Alain Vanderpoorten (voc.), Rosa María Ros Espín (voc.), Rosalina María De Almeida Gabriel (voc.)
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- Resumen
The aims of this thesis were to study the effects of clearcutting on species composition, species richness, biomass and genetic variability of epiphyte bryophytes, as well as the possible variations of these effects according to the host tree identity and zone within the tree (Chapters 3, 4 and 5). In a previous chapter, it was explored which factor, forest type or host tree identity, is the most important in influencing the composition of the epiphyte bryophyte communities (Chapter 2). In the last part of the present thesis, the aim was to investigate whether the responses of subtropical bryophytes to clearcutting vary depending on the type of freshwater environment and substrate (Chapter 6). This information is crucial for understanding global and local ecological impacts of clearcutting and designing forest management strategies in intensified harvested cloud forest landscapes.
In studies of succession after anthropogenic forest disturbance, the chronosequence approach is commonly used. However, (space for time) substitutions exhibit certain disadvantages, such as possible small differences among stands with regard to site history, environmental conditions or availability of propagules. Despite such limitations, chronosequence (based) studies have been found to be a useful method for studying temporal succession. The investigations based on (space for time) substitutions and epiphyte distribution studies were performed in cloud forests located in the Canary Islands, Spain. The studies related to successional changes after clearcutting of epiphyte bryophytes were carried out on La Palma Island (Chapters 3 and 4). The studies of the disturbance impacts on the genetic variability of a moss species and freshwater bryophyte responses to clearcutting, as well as factors controlling the structuring of epiphyte bryophyte communities in different types of cloud forests were performed on La Gomera Island (Chapters 2, 5 and 6).
In the Chapter 2, the importance of the forest type (ericaceous versus broadleaved laurel forests) and the host tree identity (Erica arborea, Ilex canariensis, Laurus novocanariensis, Myrica faya) on the structuring of epiphyte bryophyte communities was analysed in 51 ancient forest stands on La Gomera Island. The low trunk bryophyte composition was different between two groups of host tree species, E. arborea and M. faya versus Ilex canariensis and Laurus novocanariensis. Such tree groups showed different distribution patterns of their epiphyte communities. The epiphyte composition of the host tree group formed by I. canariensis and L. novocanariensis differed significantly between forest types, while we failed to find variations for the host tree group shaped by E. arborea and M. faya. At the scale of our study system, particular host tree traits, such as funnelling ratios, were among the most important factors for epiphyte community structuring between forest types.
In the Chapter 3 and 4, four different postharvest successional forest stages (8, 15, 25, and 60 years after clearcutting) and three host tree species (Erica arborea, Laurus novocanariensis and Myrica faya) were studied. Species turnover after clearcutting and temporal changes in species richness and biomass of epiphyte bryophytes varied according to the host tree identity. Most of the later successional bryophytes with narrower ecological requirements were only present on L. novocanariensis, but with low abundances. Contrastingly, epiphyte communities on E. arborea and M. faya were dominated by early successional species throughout the chronosequence. Biomass, cover and species richness of bryophytes increased through the chronosequence, both at the level of each plot and overall for L. novocanariensis. Most of the biomass (53%) and richness (81%) was concentrated on L. novocanariensis, in plots for which 60 years had elapsed since the last clearcutting. In contrast, E. arborea and M. faya showed increasing richness during the second period (between 15 and 25 years) followed by a drop in richness during the last period (between 25 and 60 years). Trunks supported greater bryophyte biomass and richness than canopies, even in the oldest plots. Current commercial requirements in the Canary Islands do not realistically allow managers to change the short rotation cycle (harvesting regime of between 7 and 10 years) that is currently in widespread use. Therefore, it is essential to consider retaining mature patches that exhibit as much original host tree diversity as possible across landscapes dominated by harvested secondary forests.
In the Chapter 5, interpopulation and intrapopulation genetic variability of Isothecium myosuroides was assessed in two ancient and four disturbed forest stands, which were classified according to their vegetation, forest age and degree of disturbance. ISSR analysis of 144 colonies with 8 primers resulted in 211 reliable bands. Population structure increased and genetic diversity decreased in populations in disturbed stands compared with the levels observed in ancient forest populations. Local scale environmental conditions and particular life traits seemed to have an impact on the genetic variation of I. myosuroides. Strong differentiation among populations at different spatial scales (insular and local) and differences in molecular diversity indices are probably related to the characteristics of the founders and the availability of suitable (micro) habitats after disturbance.
In the Chapter 6, the response of subtropical freshwater bryophytes to forest cleacutting was investigated in of 56 study sites associated with dripping walls, streams, and waterfalls, and belonging to three successional forest stages following clearcutting (>250 years without harvesting; between 20 and 50 years; and between 5 and 15 after clearcutting). There was a general pattern of decline in species richness and change in species composition after clearcutting, which was stronger for streamside assemblages, compared to assemblages on dripping walls and in waterfalls. The change in species numbers on rocks was larger than that on soils, because a guild of species growing on soil (but not on rocks) were favoured by disturbance and thus increased in the disturbed sites. Most of the sensitive species could be classified as typical laurel forest species. Mosses were generally more tolerant to forest clearcutting than were liverworts. We suggest that streamsides are more sensitive to disturbance than waterfalls and dripping walls because of a larger variation in microclimate before than after clearcutting, and because they are more easily invaded by early successional species (both bryophytes and highly competitive vascular plants). Eventually, special care should be taken along small streams and dripping walls within disturbed watersheds if bryophyte assemblages and threatened species should be protected.
This thesis reveals the great importance of maintaining the natural diversity of host tree species along environmental gradients in order to preserve the greatest biological diversity for epiphytes in mountain cloud forests. Thus, the protection of ancient forest stands containing trees of selected species can contribute to the survival of epiphytic bryophytes in harvested cloud forest landscapes, as well as the maintenance of the genetic variability of focal epiphyte species. Furthermore, there was major variation in the responses of freshwater bryophyte communities to clearcutting, which were related to the freshwater environment and substrate. In conclusion, understanding forest succession in the context of different ecological, spatial and temporal dimensions is one of the key challenging tasks to design appropriate conservation and management strategies for the restoration of freshwater environments dominated by disturbed riparian vegetation and secondary forests.
