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Patterns of species diversity have been associated with changes in climate across latitude and elevation. However, the ecological and evolutionary mechanisms underlying these relationships are still actively debated. Here, we present a complementary view of the well-known tropical niche conservatism (TNC) hypothesis, termed the multiple zones of origin (MZO) hypothesis, to explore mechanisms underlying latitudinal and elevational gradients of phylogenetic diversity in tree communities. The TNC hypothesis posits that most lineages originate in warmer, wetter, and less seasonal environments in the tropics and rarely colonize colder, drier, and more seasonal environments outside of the tropical lowlands, leading to higher phylogenetic diversity at lower latitudes and elevations. In contrast, the MZO hypothesis posits that lineages also originate in temperate environments and readily colonize similar environments in the tropical highlands, leading to lower phylogenetic diversity at lower latitudes and elevations. We tested these phylogenetic predictions using a combination of computer simulations and empirical analyses of tree communities in 245 forest plots located in six countries across the tropical and subtropical Andes. We estimated the phylogenetic diversity for each plot and regressed it against elevation and latitude. Our simulated and empirical results provide strong support for the MZO hypothesis. Phylogenetic diversity among co-occurring tree species increased with both latitude and elevation, suggesting an important influence on the historical dispersal of lineages with temperate origins into the tropical highlands. The mixing of different floras was likely favored by the formation of climatically suitable corridors for plant migration due to the Andean uplift. Accounting for the evolutionary history of plant communities helps to advance our knowledge of the drivers of tree community assembly along complex climatic gradients, and thus their likely responses to modern anthropogenic climate change. 

Abstract We introduce the FunAndes database, a compilation of functional trait data for the Andean flora spanning six countries. FunAndes contains data on 24 traits across 2,694 taxa, for a total of 105,466 entries. The database features plant-morphological attributes including growth form, and leaf, stem, and wood traits measured at the species or individual level, together with geographic metadata (i.e., coordinates and elevation). FunAndes follows the field names, trait descriptions and units of measurement of the TRY database. It is currently available in open access in the FIGSHARE data repository, and will be part of TRY’s next release. Open access trait data from Andean plants will contribute to ecological research in the region, the most species rich terrestrial biodiversity hotspot. 

Abstract It is largely unknown how South America’s Andean forests affect the global carbon cycle, and thus regulate climate change. Here, we measure aboveground carbon dynamics over the past two decades in 119 monitoring plots spanning a range of >3000 m elevation across the subtropical and tropical Andes. Our results show that Andean forests act as strong sinks for aboveground carbon (0.67 ± 0.08 Mg C ha −1 y −1 ) and have a high potential to serve as future carbon refuges. Aboveground carbon dynamics of Andean forests are driven by abiotic and biotic factors, such as climate and size-dependent mortality of trees. The increasing aboveground carbon stocks offset the estimated C emissions due to deforestation between 2003 and 2014, resulting in a net total uptake of 0.027 Pg C y −1 . Reducing deforestation will increase Andean aboveground carbon stocks, facilitate upward species migrations, and allow for recovery of biomass losses due to climate change. 

Abstract AimEcological niches shape species commonness and rarity, yet, the relative importance of different niche mechanisms within and across ecosystems remains unresolved. We tested the influence of niche breadth (range of environmental conditions where species occur) and niche position (marginality of a species’ environmental distribution relative to the mean environmental conditions of a region) on tree‐species abundance and occupancy across three biogeographic regions. LocationArgentinian Andes; Bolivian Amazon; Missouri Ozarks. Time period2002–2010. Major taxa studiedTrees. MethodsWe calculated abiotic‐niche breadths and abiotic‐niche positions using 16 climate, soil and topographic variables. For each region, we used model selection to test the relative influence of niche breadth and niche position on local abundance and occupancy in regional‐scale networks of 0.1‐ha forest plots. To account for species–environment associations caused by other mechanisms (e.g., dispersal), we used null models that randomized associations between species occurrences and environmental variables. ResultsWe found strong support for the niche‐position hypothesis. In all regions, species with higher local abundance and occupancy occurred in non‐marginal environments. Observed relationships between occupancy and niche position also differed from random species–environment associations in all regions. Surprisingly, we found little support for the niche‐breadth hypothesis. Observed relationships between both local abundance and niche breadth, and occupancy and niche breadth, did not differ from random species–environment associations. Main conclusionNiche position was more important than niche breadth in shaping species commonness and rarity across temperate, sub‐tropical and tropical forests. In all forests, tree species with widespread geographic distributions were associated with environmental conditions commonly found throughout the region, suggesting that niche position has similar effects on species occupancy across contrasting biogeographic regions. Our findings imply that conservation efforts aimed at protecting populations of common and rare tree species should prioritize conservation of both common and rare habitats.