Flora and vegetation of the Guayana highlands : past dynamics, global warming and conservation guidelines = Flora i vegetació de les Terres de Guayana : dinàmica del passat, escalfament global i directrius de conservació

Aquesta tesi té com a objectiu l'estudi de la flora i la vegetació de les Terres Altes de Guayana (TAG), un conjunt de -50 muntanyes tabulars (anomenades tepuis) situades al nord d'Amèrica del Sud, amb especial èmfasi en la potencial migració altitudinal de les plantes vasculars com a resposta a l'escalfament global. Els cims dels tepuis contenen una diversitat i endemisme de plantes vasculars excepcional. Aquest estudi suggereix que el 30-50% de les espècies endèmiques de les TAG podrien desaparèixer per pèrdua total d'hàbitat cap a l'any 2100 segons els escenaris d'escalfament B1 (més optimista) i A2 (més pessimista), respectivament. S'ha calculat un Valor d'Impacte Ambiental per a cada espècie amenaçada per a la posterior classificació d'aquestes espècies en categories de prioritat (10 pel B1 i 13 per l'A2), les quals haurien de ser utilitzades en una seqüència cronològica per orientar les accions de conservació i recerca. Posteriorment, s'ha establert una línia base de diversitat florística, endemisme i vegetació al cim del Roraima-tepui per a la verificació del fenomen migratori a les TAG. L'inventari florístic d'aquest cim tepuià conté 227 espècies, incloent 44 nous registres, una espècie nova per a la ciència i 13 espècies exòtiques. S'han identificat cinc tipus de comunitats vegetals principals, amb les seves corresponents espècies diagnòstiques. També s'ha dut a terme una caracterització ambiental d'aquestes comunitats. Finalment, l'estudi paleoecològic de la seqüència sedimentària del Uei-tepui ha permès documentar la resposta de les espècies als canvis ambientals dels últims 2000 anys. Els resultats suggereixen que el foc ha estat el principal factor que ha modelat la vegetació del Uei durant aquest període. Els esdeveniments de focs locals molt probablement van ser causats per activitats humanes i van assolir el cim des de la Gran Sabana afavorits per les condicions climàtiques. La preservació de la flora de les TAG enfront de l'escalfament global s'hauria d'adreçar, com a mínim parcialment, mitjançant tècniques de conservació ex situ, a causa de la naturalesa de l'amenaça (pèrdua d'hàbitat). No obstant, la documentació en aquest estudi d'altres impactes humans (trepig de la vegetació, espècies exòtiques, canvis en les propietats del sòl, etc.) i l'aparició del foc com a potencial agent pertorbador del bioma tepuià posen de manifest que la conservació in situ mitjançant àrees protegides pot ser també molt important.This thesis is aimed at the study of the flora and vegetation of the Guayana Highlands (GH), a set of -50 table mountains (called tepuis) located in the Guayana Shield of northern South America. Special emphasis is put on the potential upward migration of vascular plants as a response to global warming and the suggestion of conservation strategies. The tepui summits contain exceptional diversity and endemism of vascular plants, with more than 2400 known species, of which 30% would be endemics. In this study, the results of the Altitudinal Range Displacement analysis suggest that 30-50% of the GH endemic species could disappear due to total habitat loss by 2100 according to the most optimistic (B1) and pessimistic (A2) warming scenarios, respectively. An Environmental Impact Value has been calculated for each threatened species for a subsequent classification of these species into priority categories (10 for B1 and 13 for A2), which should be used in a chronological sequence to guide conservation and research actions. Subsequently, flora and vegetation approaches have been applied for the verification of the migratory phenomenon in the GH. Since this verification is based on the comparison of inventories conducted at different times, the first necessary step is to establish a baseline. The Roraima-tepui summit (2810 m elevation) has been selected for that purpose, as it is the best well-known tepui from the botanical point of view. The floristic survey of this tepuian summit has yielded 227 species, including 44 novelties since the publication of the Flora of the Venezuelan Guayana, one species new for science (Epidendrum sp. nov.), and 13 exotics, among which Poa annua and Polypogon elongatus (Poaceae) show the highest invasive potential. Five main plant communities have been identified: three herbaceous, one shrubland, and one forest formation, with their corresponding diagnostic species. An environmental characterization of these communities has been also performed. Finally, palaeoecological records may provide valuable information of the species' responses to past environmental changes, which is not available from short-term ecological observations. In this thesis, a palynological analysis of a sedimentary sequence from Uei-tepui (2150 m elevation) has been conducted to record the vegetation changes occurred in the last 2000 years in this tepui and to investigate the potential drivers involved. At -810 cal yr BP (AD 1140), a peak of fire incidence, coeval with a decrease in regional precipitation, reduced the woody elements of meadows (mainly Biophytum) and the Bonnetia cloud forests. Forest recovery to its previous abundance occurred slowly and elapsed several centuries to be completed after fire disturbance. In the mid-18th century, forests and meadows were replaced by Cyrilla racemiflora shrublands, coinciding with intense fires, synchronous with the arrival of Europeans to the Gran Sabana (GS) uplands. These results suggest that fire has been the main factor in the shaping of Uei¬tepui vegetation during the last two millennia. The documented local fire events were most likely caused by human activities and reached the summit from the GS, probably favoured by climatic conditions. The last local fire leading to forest clearing and the establishment of present-day vegetation most likely took place in the AD 1920s. The preservation of the GH flora in front of global warming should be addressed, at least partially, through ex situ conservation techniques, due to the nature of the threat (habitat loss). However, the documentation in this study of other human impacts (trampling on vegetation, exotic species, changes in soil properties, etc.) and the emergence of fire as a potential disturbing agent of the GH biome highlight that in situ conservation through protected areas may also be very important. A management plan with the involvement of local indigenous communities should be implemented in the GH region (i.e., lowlands, uplands and highlands), in order to reduce the risks posed by global warming, human impacts, and the potential synergy among them