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Wood density (WD) is a key functional trait related to ecological strategies and ecosystem carbon dynamics. Despite its importance, there is a considerable lack of information on WD in tropical Andean forests, particularly regarding its relationship with forest succession and ecosystem carbon cycling. Here, we quantified WD in 86 upper Andean tree and shrub species in central Colombia, with the aim of determining how WD changes with forest succession and how it is related to productivity. We hypothesized that WD will increase with succession because early successional forests will be colonized by acquisitive species, which typically have low WD, while the shaded understory of older forests should favor higher WD. We measured WD in 481 individuals from 27 shrub and 59 tree species, and quantified aboveground biomass (AGB), canopy height, net primary production (NPP) and species composition and abundance in 14, 400-m², permanent plots. Mean WD was 0.513 ± 0.114 (g/cm³), with a range between 0.068 and 0.718 (g/cm³). Shrubs had, on average, higher WD (0.552 ± 0.095 g/cm³) than trees (0.488 ± 0.104 g/cm³). Community weighted mean WD (CWMwd) decreased with succession (measured as mean canopy height, AGB, and basal area); CWMwd also decreased with aboveground NPP and stem growth. In contrast, the percentage of NPP attributed to litter and the percent of shrubs in plots increased with CWMwd. Thus, our hypothesis was not supported because early successional forests had higher CWMwd than late successional forests. This was related to a high proportion of shrubs (with high WD) early in succession, which could be a consequence of: 1) a low seed availability of trees due to intense land use in the landscape and/or 2) harsh abiotic conditions early in succession that filter out trees. Forest with high CWMwd had a high %NPP attributed to litter because they were dominated by shrubs, which gain little biomass in their trunks. Our findings highlight the links between WD, succession and carbon cycling (biomass and productivity) in this biodiversity hotspot. Thus, WD is an important trait that can be used to understand upper Andean forest recovery and improve forest restoration and management practices. [ABSTRACT FROM AUTHOR]

Along with habitat transformation, climate change has profound impacts on biodiversity and may alter ecosystem services on which human welfare depends. Many studies of the carbon cycle have focused on lowland tropical forests; however, upland forests have been less explored despite their pivotal role in carbon sequestration. Here, I synthesized the state of knowledge on the allocation of carbon in its different stocks (aboveground, belowground, and soil) as well as in its main fluxes (plant decomposition, respiration, and litterfall) in tropical upland ecosystems of the planet. In November 2020, a systematic review was carried out to identify references published from 2000 to 2020 through a combination of key terms in Google Scholar and Scopus databases, thus analysing bibliographic, geographical, methodological, and carbon cycling information of the global upland tropics (between 23.5 ∘ N–23.5 ∘ S). After analysing a total of 1967 references according to inclusion–exclusion criteria, 135 references published in the last 20 years were selected. Most of the studies were conducted in the tropical and subtropical moist broadleaf forest of South America. The main factors studied were elevation and forest type. Forest structure and soil variables were largely associated when studying carbon cycling in these ecosystems. Estimations of carbon stocks comprised three-fourths of the total studies, while the remaining fraction focused on carbon fluxes. Aboveground biomass and carbon in soils were highly investigated, while plant decomposition and respiration were the components that received the least attention. Even though in the last 20 years there was a slight increase in the number of studies on carbon cycle in tropical upland forests, I found bias associated with the biomes and ecoregions studied (especially in the Andes). Elevation was the main factor examined but other essential aspects such as the successional gradient, landscape management, diversity–productivity relationship, faunal and microbial effect, trophic cascades, and Gadgil effect require more attention. The inclusion of different litter species and origins (i.e. roots and stems) and theoretical frameworks including home-field advantage, substrate–matrix interaction, and phenology–substrate match may provide explanatory mechanisms to better understand litter decomposition over these forests. Despite respiration being a paramount link that is closely tied to above- and belowground compartment, this flux constitutes one of the important gaps to fulfil in future research. For a comprehensive understanding of the carbon cycle in upland forests, it is necessary to obtain information on its main fluxes and integrate them into climate change mitigation plans. [ABSTRACT FROM AUTHOR]