Government and international agencies have highlighted the need to focus global change research efforts on tropical ecosystems. However, no recent comprehensive review exists synthesizing humid tropical forest responses across global change factors, including warming, decreased precipitation, carbon dioxide fertilization, nitrogen deposition, and land use/land cover changes. This paper assesses research across spatial and temporal scales for the tropics, including modeling, field, and controlled laboratory studies. The review aims to (1) provide a broad understanding of how a suite of global change factors are altering humid tropical forest ecosystem properties and biogeochemical processes; (2) assess spatial variability in responses to global change factors among humid tropical regions; (3) synthesize results from across humid tropical regions to identify emergent trends in ecosystem responses; (4) identify research and management priorities for the humid tropics in the context of global change. Ecosystem responses covered here include plant growth, carbon storage, nutrient cycling, biodiversity, and disturbance regime shifts. The review demonstrates overall negative effects of global change on all ecosystem properties, with the greatest uncertainty and variability in nutrient cycling responses. Generally, all global change factors reviewed, except for carbon dioxide fertilization, demonstrate great potential to trigger positive feedbacks to global warming via greenhouse gas emissions and biogeophysical changes that cause regional warming. This assessment demonstrates that effects of decreased rainfall and deforestation on tropical forests are relatively well understood, whereas the potential effects of warming, carbon dioxide fertilization, nitrogen deposition, and plant species invasions require more cross-site, mechanistic research to predict tropical forest responses at regional and global scales.

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Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, we know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions, and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, we identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. We also identify interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing us to better predict tropical forest-climate feedbacks.

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Abstract. Native bark beetles and wildfire are among the most prominent natural disturbances in the conifer forests of western North America. Using multiple approaches and scales, researchers have sought to understand how each disturbance may, or may not, influence the subsequent likelihood or severity of the other disturbance. Our objectives were to complement existing research on the linkage of mountain pine beetle and wildfire disturbances by combining large-scale spatial data at variable temporal resolutions to characterize the discrepancy between observed and expected overlap areas of fire and beetle disturbances. Multiple time lags, differing by 1-yr intervals, spanning the 30-yr period 1984-2014 in the U.S. states of Washington and Oregon were examined for wildfire following bark beetle epidemics and for beetle epidemics following wildfire. We used existing spatial data sets of wildfires and insect pest disturbances to perform overlay analyses to characterize the two relationships. We also compared years of peak vs. nonpeak fires, as well as the influence of several individual, large disturbances. We found a clear association of increased bark beetle epidemic shortly after wildfire. The evidence of an association between increased fire extent and severity following bark beetle epidemics was less straightforward, although it suggested that the nature of the observed patterns varied with fire severity. In some cases, individual disturbance seasons or events were highly influential on the overall trends. Variability in the results highlights the challenges of quantifying landscape-level relationships among disturbance types and supports the existing calls for further analyses with additional ecological variables.