Restauración de la biodiversidad y los servicios de los ecosistemas ribereños y otros humedalesmeta-análisis global y evaluación de especies útiles en el trópico húmedo mexicano

Resum

Riparian ecosystems harbor high biodiversity levels and their ecological functions stand for key ecosystem services to societies, such as flood control and water purification. Anthropogenic disturbances frequently exceed the natural resilience of these ecosystems, leading to degradation processes that claim for restoration actions. In the humid tropics of Mexico, riparian forests are deforested for the establishment of livestock or agricultural activities, depending on their location and soil type. This has negative effects on the associated aquatic ecosystems and also on landscape connectivity. In this region, Lacandona rainforest (Chiapas State) is one of the most important remnant forest patches in the country and in North America due to its large extent (600,000 hectares) and biodiversity. Lacandona originally occupied 1.8 million hectares, but its area has been reduced to less than one third in the last three decades. Given this deforestation, seven protected natural areas were decreed in Mexico. The Montes Azules Biosphere Reserve, with 331,200 hectares, is the largest area and holds the highest ecological significance. This Reserve adjoins the Marqués de Comillas municipality, which has been intensively deforested -including the riparian forests- in the last decade. The restoration of riparian forests is a key issue for the recovery of local streams and landscape connectivity. The main goal of this Doctoral Thesis is to evaluate the effects of ecological restoration on the provision of biodiversity and ecosystem services of riparian ecosystem and other wetlands at the global scale and to identify native tree species and some environmental filters that may limit the active restoration of the degraded tropical riparian forests. First, in Chapter 2, we meta-analyzed 70 ecological restoration studies from around the world to determine their effectiveness and factors affecting it. We compared selected ecosystem performance variables between degraded and restored wetlands and between restored and natural wetlands using response ratios and random-effects categorical modeling. Biodiversity showed excellent recovery in most studies, though the precise recovery depended strongly on the type of organisms involved. Restored wetlands showed, on average, 36% higher levels of provisioning, regulating and supporting ecosystem services than did degraded wetlands. In fact, wetlands showed levels of provisioning and cultural services similar to those of natural wetlands; however, their levels of supporting and regulating services were, respectively, 16% and 22% lower than in natural wetlands. Biodiversity and ecosystem services recovery positively correlated, indicating a win-win restoration outcome for ecosystem and society alike. The extent to which restoration increased biodiversity and ecosystem services above their values in degraded wetlands depended primarily on the main agent of degradation, restoration actions, experimental design, and ecosystem type. In contrast, residual differences in biodiversity and ecosystem services between restored and natural wetlands depended primarily on the specific restoration actions used. These results highlight the importance of comprehensive, multifactorial assessment to determine the ecological status of degraded, restored and natural wetlands and thereby evaluate the effectiveness of ecological restorations. Future research on wetland restoration should also seek to identify which restoration actions work best for specific habitats. Secondly, in Chapter 3, we proposed a species selection framework through a “species selection index” (SSI) using five independent criteria related to ecological, social and technical information. SSI targeted species that (1) are important in the reference forest, (2) are less likely to establish following disturbance, (3) are not specific to a particular habitat, (4) are socially accepted, and (5) their propagation requires a reasonable time and financial investment. SSI may range between 0 and 50, with higher values meaning higher potential for restoration purposes. The high number of tree species found in the reference ecosystem suggests that the species pool for ecological restoration is large. Out of a local pool of 97 species, we identified 30 target tree species that together represented >60% of total Importance Value Index in the reference riparian forests. SSI averaged 28.3±1.0 over the studied species, suggesting that species with high values are not frequent. For twenty species reintroduction by means of active forest restoration was deemed necessary. Species that established through natural regeneration, following secondary regrowth, had lower social value among local farmers. Nearly half of the identified species showed technical constraints for easy propagation and seeding. The proposed procedure is useful for selecting species to initiate forest restoration projects and of other woody ecosystems that harbor high biodiversity, and is suitable for several stakeholders interested in restoration. Third, in Chapter 4, we distinguished species that can be passively restored by natural regeneration from those requiring active restoration. We quantified tree species dominance (measured by an Importance Value Index, IVI) and used abundance-size correlations to select those species suitable for passive and/or active restoration. We sampled riparian vegetation in a 50m × 10m transect in each of six reference and five disturbed riparian ecosystems. Those species representing more than 50% of total IVI in each ecosystem were selected and Spearman rank correlation between abundance and diameter classes was calculated. For eight species it was determined that passive restoration could be sufficient for their establishment. Another eight species should be introduced (active restoration). Five species regenerated well in only one ecosystem type, suggesting that both restoration strategies could be used depending on the degradation degree. Finally, two species were determined as not suitable for restoration (based on the above selection criteria) and were not selected during this initial stage of our restoration project. Fourth, in Chapter 5, we investigate whether soil compaction and grass competition limit the establishment of tree species in riparian abandoned pastures to assess vegetation restoration projects. We run a two-year experiment to assess the effects of grass clearing and soil tilling on the performance of seven native tree species planted into pastures resulting from the conversion of tropical riparian forest in Mexico. We also analyzed the effects of seasonal variation of vertical distance to water level, which is related to both drought and flooding events, and changes in microclimate, vegetation biomass and tree dbh after planting. Overall, seedling survival was low (19±3%), ranging between 3% (Brosimum alicastrum, Moraceae) and 38% (Pachira aquatica, Bombacaceae). Clearing but not tilling enhanced seedling survival. Survival was negatively correlated to vertical distance to water level, highlighting the importance of the short but severe dry season occurring in the study region, which may reduce survival by >60%. Flooding events also produced high seedling mortality (e.g. 80% after the first two events). Clearing significantly increased seedling growth of some species, suggesting competition release. Tilling did not have any consistent effect on growth, but it counteracted the positive effects of clearing. Although survival of planted trees was relatively low, both pre-existing and planted trees ameliorated microclimate to produce better conditions for establishment of new trees following natural regeneration. We conclude that, overall, clearing enhanced seedling establishment in an abandoned tropical pasture, but other revegetation treatments intended to reduce soil drying and rooting out by flooding during the first year should be used. Their cost-benefits should be also evaluated to achieve successful riparian forest revegetation. Finally, in Chapter 6, we developed a methodological proposal to implement restoration actions at the municipality level, since restoration of riparian ecosystems can increase landscape connectivity, using a geographic information system. We estimate the economic costs of restoring riparian forest in Marqués de Comillas, and also analyzed the legal aspects that should be considered to implement and promote restoration actions in Mexico. The reintroduction of native trees is one of most frequently techniques used for the restoration of riparian ecosystems, but it must be combined with other ones such as direct seeding, site preparation, passive restoration, and mulching. The major challenge is not demonstrating the ecological and practical benefits of restoration, but identifying the correct strategies to reduce economic costs, plan actions at the landscape scale, and address the socio-political issues that limit restoration. These restoration efforts will not be successful if they are not accompanied by an effective and efficient legal framework. The incorporation of riparian ecosystems as a specific component of the landscape in land management programs and public policies is crucial for their conservation, management and restoration.