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Palm hydraulics linking biodiversity and functioning of tropical forests under climate change

Periodic Reporting for period 1 - PalmHydraulics (Palm hydraulics linking biodiversity and functioning of tropical forests under climate change)

Reporting period: 2016-08-01 to 2018-07-31

Global climate is largely dependent on tropical forest functioning as the carbon that they store is equivalent to more than half of total atmospheric carbon. In the largest remnant of tropical forest, the Amazon, 50% of carbon is stored by only 227 hyper-dominant species (<2% of the total tree species richness). Six of the top ten hyper-dominant species are palms but the role of palms as ecosystem engineers has been neglected until now. Plant species responses and effects to climate are mediated by plant hydraulics. Palm hydraulics are virtually unknown as palms are understudied relative to typical trees. This is especially problematic because palms, as monocots, show significant differences from other trees in their structure with important consequences for biomass allocation, forest structure, and demography. Palms also deliver critical provisioning services to humankind (for example, coconuts, dates, and açais are palms) and an understanding of their functional constraints may help to secure palm resources for future generations. The overall aim of PalmHydraulics was to understand how palm unique hydraulic structure is related with their colonization of warmer and drier biomes, abundance variation and hydraulic functioning.
To better understand palm hydraulic which traits are needed for palms to colonize warmer and drier biomes, we asked if there is a particular phenotype (as a combination of traits) that could be associated to each biome. For this, we took advantage of the unrivaled palm collection from the Royal Botanic Gardens Kew to sample palms from disparate biomes while controlling for phylogenetic relationships. For over 50 palm species in five continents, we collected data on at least eight functional traits (plant height and diameter, leaf area and leaf mass per area, stomata size and density, parallel and cross venation density). In addition to that, compiled and cleaned occurrence data for over 55K palms to calculate biome probability for over a thousand of palms species (~ 30% of all palm species). The first important result from PalmHydraulics project (Emilio et al, in prep) was to show that species that are able to colonize non-tropical rainforest biomes are equally numerous in the two main palm sub-families (Arecoideae and Coryphoideae). This was an unexpected result meaning that cold and dry affiliation on the palms family has evolved multiple times. Biome affiliation on palm species can be predicted from the traits palms possess. This is a very important result as by combining these models with species phenotypic plasticity is possible to identify the thresholds responses of palm species to climate and land cover changes.
To document the global patterns of palm abundance variation, we strength our collaboration with researchers from University of Aarhus Ecoinformatics and Biodiversity group (palm ecology and distribution experts) and University of Leeds Ecology and Global Change group (host of the largest tropical forest plot inventory repository). From the data collected by over a hundred field ecologists and botanists, we were able to assemble the most comprehensive (2,544 plots, 661,194 individual palms) dataset of arborescent palm abundance variation at the global scale. The second important result from PalmHydraulics project (Muscarella & Emilio et al. in preparation) was to show that palms are not abundant elsewhere as they are in the Neotropics. At least not large arborescent palms. Small understorey and climbing palms are largely unsampled in forests inventories and expect to increase the contribution of palms to biomass in over of 80% of the locations outside of the Neotropics. We also found that although it is not unexpected that estimates of aboveground biomass are more affected in locations were palm abundance is higher, the direction and strength of this effect are unpredictable, varying from +5% to -16%.
To gain a more mechanistic view on palm functioning we started a collaboration with a world-leading research group on plant hydraulics from INRA/University of Bordeaux (France). Together we performed in vivo visualization of embolism formation in palms for the first time. The main experiments we conducted at Soleil Synchrotron in the Spring of 2017, followed by additional experiments at BIOGECO in 2017 and 2018. The third important result (Emilio et al., under review) was to show that palms are not as vulnerable to drought-induced embolism (the main cause of plant mortality under drought) as we previously thought from their hydraulic structure. For the first time, we discover that palms show the entire range of embolism resistance of angiosperms in general. Besides that, we demonstrate with high-resolution micro-CT images that hydraulic functioning of palm leaves is supported by a large amount of water stored in their parenchyma.
PalmHydraulics contributed to the advance of our knowledge on the hydraulics of palms, tropical forests biodiversity, and functioning. We expect that our pioneering studies on the hydraulics of arborescent monocots using a range of imaging techniques will be instrumental for the next steps on tropical plant hydraulics. Our documentation of palms abundance at the global scale will set the baseline for evaluating where and when is necessary to include palms or a different approach on estimating palm biomass to reduce errors in aboveground biomass estimates. Finally, by identifying the traits that predict biome affiliation while controlling for phylogenetic relationships we expect to offer the first step on parametrizing trait-based biodiversity-ecosystem dynamic models. The results of the project were presented to a range of audiences in the 68th Brazilian Congress of Botany, 2018 European Conference of Tropical Ecology (by Robert Muscarella), XVIII Annual meeting of the European Network of Palm Specialists and 6th International Conference on Comparative Biology of Monocotyledons and finally at Royal Botanic Gardens Kew. The results of the project were also disseminated during the supervision of five students (sandwich internship, final year project, MSc internship and thesis) and in the two major training events (IV Plant functional trait field course in Brazil and BES Novel Methods in Ecology Skills Workshop in UK) were the Fellow provided training to over 40 young plant scientists. As follow-up of the PalmHydraulics project, the Fellow will continue the studies on functional ecology and biogeography of palms with focus on hydraulics to understand plant abundance variation across an aridity gradient from the semi-arid Caatinga until Amazon rainforests in Northern South America.