Woody plant distributions along an Amazon-to-Andes elevation gradient: evolutionary trends and tendencies
Griffiths, Andrew R.
With high taxonomic turnover and exceptional levels of endemism, tropical montane forests are one of the most biodiverse ecosystems on the planet. The taxa within these forests frequently occupy narrow elevational ranges, and display upslope migration rates insufficient to track predicted temperature increases. Consequently, tropical montane forests and the diversity within them are expected to be susceptible to declines in abundance and potential extinction under ongoing environmental change. Substantial changes to biodiversity patterns across elevation may have significant consequences for the carbon and nutrient cycles as well as the regulation of hydrological processes provided by these forests. In this thesis I examine the diversity and distribution trends of woody plants across a tropical montane forest elevation gradient on the Amazonian flank of the Peruvian Andes, stretching from 425 to 3625 m asl. I consider the influence of the major environmental changes which occur over elevation, such as decreasing temperature. A particular focus is on the transitions which occur at the cloud-base ecotone, above which forests are defined by frequent and prolonged cloud immersion. I apply an evolutionary perspective, using phylogenetic approaches throughout this research. Closely related evolutionary lineages are frequently similar in terms of their ecology and functional characteristics, yet the influence of this tendency on the elevational distribution pattern of woody plants within tropical montane forest has received limited attention Using census data from a network of one-hectare plots, in combination with phylogenetic information, covering a breadth of vascular plant diversity, from angiosperms and gymnosperms to pteridophyte tree ferns, I investigate the influence of evolutionary heritage on elevational distribution trends at the genus level. I further employ these plot data to reflect on the differences between taxonomic and phylogenetic richness patterns across elevation. A deeper consideration of the way variation in the evolutionary age structure of communities can influence diversity trends is also undertaken. In order to test the consistency of elevational patterns between genus and species scales, I conducted independent and detailed sampling within the tribe Miconieae (Melastomataceae), along the same gradient. Within Miconieae, I also measured functional leaf-traits associated with species resource-investment strategies, allowing investigation of the potential mechanistic processes underlying distribution trends across elevation. I reveal a tendency for closely related woody plant lineages to occupy similar mean elevations and display phylogenetic clustering both above and below the cloud-base ecotone. A few exceptional lineages are able to occupy broad elevations, yet they are not each other’s closest evolutionary relatives. I further show that, across both taxonomic and phylogenetic measures, the diversity of mid-elevation tropical montane forests may rival, or even exceed that found in the tropical lowlands, especially when the full evolutionary history of lineages is represented. Deviation among diversity measures is driven by variation in the evolutionary age structure of communities across elevation. Older evolutionary lineages are more numerous at middle to high elevations, while many evolutionarily younger lineages are restricted to distributions at lower elevation. Utilising Miconieae as a test lineage, I show that elevational diversity and distribution patterns at the species-level largely echo genus-level trends. Lastly, I reveal that closely related species tend to have similar functional leaf trait values, with certain traits displaying elevational trends. Amongst measured traits, only specific leaf area (SLA) appears to share an evolutionary correlation with species’ elevational distributions. It may be that unmeasured traits or resource investment strategies unrelated to SLA, are more significant drivers of species constrained elevational distributions. Overall, the work presented in this thesis adds to the body of evidence demonstrating that evolutionary factors, such as niche conservatism, form an important lens through which to understand the spatial organisation of biodiversity. I demonstrate that the broad relevance of evolutionary processes applies to woody plant distributions within unique and vulnerable tropical montane forests and may prove important to our understanding of the ecological response of this system to ongoing global environmental change.