PhD Thesis

Linking in-situ and remote sensing data for a better understanding of ecological changes in forest ecosystems of La Palma, Spain

Frank Weiser (05/2017-05/2023)

Support: Carl Beierkuhnlein, Manuel Steinbauer, Cyrus Samimi

Biodiversity is shaped and influenced by a complex multitude of processes and environmental gradients on different spatial and temporal scales. One influence, the disturbance regime, acts as a filter during the evolution of species, but also causes short term changes of the species composition of ecosystems. Another, the arrival of novel species, can increase diversity but also threaten existing diversity. In recent decades, both the disturbance regime and invasion processes have been rapidly altered due to climate change and anthropogenic influence. It is therefore even more important to keep track of and understand the effect of these processes on local biodiversity. The recent rise in availability of remote sensing data offers many opportunities for combined analytic approaches using in situ data jointly with satellite images to significantly increase the frequency of observations, to aid in better understanding existing patterns and face challenges of a changing world. Islands are hotspots of biodiversity and harbour a disproportionate number of endemic species while the overall number of species richness is generally lower compared to the mainland. Island biogeography investigates patterns in the distribution of these species and how this extraordinary richness evolved over millions of years. Islands are seen as perfect “natural laboratories” as they allow studying a multitude of environmental gradients and their effects on both speciation and community assembly, such as isolation, topography, island age or height. Island biogeography therefore allows insights into processes that are otherwise overlayed by other factors. In addition, in recent decades, the biodiversity of islands has come under threat due to climate change, invasion or anthropogenic influence such as land use change. One factor often not paid due attention in island biogeography is the role of disturbances and the disturbance regime, which can range from large events effectively sterilizing islands (e.g. hurricanes or volcanic eruptions) to small, recurring, local events. These events have been linked to the evolution of endemic species and their traits, e.g. insular woodiness or fire adaptations. Additionally, disturbances affect current species communities and change their composition. The manuscripts in this thesis investigate the effects of these events on the forests of La Palma. The study subject of most manuscripts in this thesis, the Canary Pine forests, has evolved under the influence of volcanic eruptions and fires. The name-giving species, Pinus canariensis, is well adapted to survive even severe fires and can recover from significant damage caused by volcanic eruptions. I investigate the effects of one of the frequently occurring wildfires on α- and β-diversity patterns of the generally species poor plant communities of the understory of one of these forests. Two other manuscripts investigate how P. canariensis reacted to the 2021 Tajogaite volcanic eruption, which poisoned the surrounding forests with large quantities of sulphur gases and covered them in a thick tephra layer. One manuscript detects spatial patterns of damage the canopy suffered while another manuscript explores needle sulphur, nitrogen and carbon content and how they link to the observed canopy damage. The second focus of this thesis is to explore synergies between remote sensing and field data. The usage of remote sensing has strongly increased in the last decade, mainly due to the increasing wealth of openly available datasets and new technological advancements, such as improved, cheaper unmanned aerial vehicles or freely available hyperspectral or lidar data. Alas, remote sensing is still underused in island biogeography, in parts because high elevation islands pose similar difficulties for remote sensing analysis as mountain ranges (e.g. steep slopes make more pre-processing necessary). Nonetheless, especially the remoteness of many islands and the extreme isolation of many habitats makes remote sensing highly useful for island biogeography. Two of the manuscripts presented here combine remote sensing data with data sampled on the ground. The final study on this thesis uses ground truth data combined with remote sensing data to map the current extent of an invasive species threatening the laurel forest on La Palma and then further combines the results with modelling to map the future niche of this invasive species. The aim of this thesis is therefore twofold: I aim to contribute to a better understanding of the effects of disturbances on the Canary Pine forests of La Palma. In addition, I explore benefits of combining remote sensing approaches with more traditional fieldwork methods to offer new possibilities for island biogeography.