Understanding the effects of climate change on natural systems has been referred to as "a grand challenge in ecology" (Thuiller 2007). Nowhere is this challenge more poignant, and perhaps more daunting, than in tropical forests. Despite covering a tiny fraction of the Earth's surface, tropical forests harbor the majority of known and unknown species (Raven 1988, Dirzo and Raven 2003, Joppa et al. 2011a), sequester vast amounts of carbon (Saatchi et al. 2011, Baccini et al. 2012), and support many millions of humans directly through the production of food and other resources (Fa et al. 2002, Milner-Gulland and Bennett 2003), or indirectly through a diverse range of ecosystem services (Costanza et al. 1997, Cincotta et al. 2000, Naidoo et al. 2008). In this review, we discuss the potential effects of ongoing and future climate change on tropical forests and specifically tropical forest tree species, with an acknowledged bias towards Amazonian and Andean species. There are only four possible responses of any species, including tropical trees, to global climate change: (1) individuals can acclimate to changes in climate through phenotypic plasticity Abstract. In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (i.e., "migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth's species and are facing especially rapid rates of climate change relative to current spatial or temporal variability. Considering the rapid changes in climate predicted for the tropics, coupled with the apparently low capacities of tropical tree species to either acclimate or adapt to sustained changes in environmental conditions, it is believed that the greatest hope for avoiding the loss of biodiversity in tropical forest is species migrations. This is supported by the fact that topical forests responded to historic changes in climate (e.g., post glacial warming) through distributional shifts. However, a great deal of uncertainty remains about whether tropical forest plant species can migrate, and if so, whether they can migrate at the rates required to keep pace with accelerating changes in multiple climatic factors in conjunction with ongoing deforestation and other anthropogenic disturbances. In order to resolve this uncertainty, as will be required to predict, and eventually mitigate, the impacts of global climate change on tropical and global biodiversity, more basic data are required on the distributions and ecologies of tens of thousands of plant species, in combination with more directed studies and large-scale experimental manipulations.

Understanding the effects of climate change on natural systems has been referred to as "a grand challenge in ecology" (Thuiller 2007). Nowhere is this challenge more poignant, and perhaps more daunting, than in tropical forests. Despite covering a tiny fraction of the Earth's surface, tropical forests harbor the majority of known and unknown species (Raven 1988, Dirzo and Raven 2003, Joppa et al. 2011a), sequester vast amounts of carbon (Saatchi et al. 2011, Baccini et al. 2012), and support many millions of humans directly through the production of food and other resources (Fa et al. 2002, Milner-Gulland and Bennett 2003), or indirectly through a diverse range of ecosystem services (Costanza et al. 1997, Cincotta et al. 2000, Naidoo et al. 2008). In this review, we discuss the potential effects of ongoing and future climate change on tropical forests and specifically tropical forest tree species, with an acknowledged bias towards Amazonian and Andean species. There are only four possible responses of any species, including tropical trees, to global climate change: (1) individuals can acclimate to changes in climate through phenotypic plasticity Abstract. In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (i.e., "migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth's species and are facing especially rapid rates of climate change relative to current spatial or temporal variability. Considering the rapid changes in climate predicted for the tropics, coupled with the apparently low capacities of tropical tree species to either acclimate or adapt to sustained changes in environmental conditions, it is believed that the greatest hope for avoiding the loss of biodiversity in tropical forest is species migrations. This is supported by the fact that topical forests responded to historic changes in climate (e.g., post glacial warming) through distributional shifts. However, a great deal of uncertainty remains about whether tropical forest plant species can migrate, and if so, whether they can migrate at the rates required to keep pace with accelerating changes in multiple climatic factors in conjunction with ongoing deforestation and other anthropogenic disturbances. In order to resolve this uncertainty, as will be required to predict, and eventually mitigate, the impacts of global climate change on tropical and global biodiversity, more basic data are required on the distributions and ecologies of tens of thousands of plant species, in combination with more directed studies and large-scale experimental manipulations.

The tropics have long been a focal point of interest in ecology and evolutionary biology-but where actually are the tropics? Classically, the tropics have been defined as all areas lying between 23.4° North and South as these zones receive direct overhead solar radiation at some point during the year. However, a suite of different environmental and climatic characteristics have also been employed to classify regions as tropical or not. The aims of this paper are to 1) briefly review some of the different criteria commonly used to define the tropics, 2) map the extent and distribution of tropical land areas according to these different criteria, and 3) asses the concordance between these criteria and a sample of recent "tropical" studies. More specifically, we review eight criteria that are frequently used (implicitly or explicitly) to define the tropics. We then map the location and extent of land areas that are "definitely tropical" (i.e., the core tropics) and areas that are "tropical by most definitions". Finally, we examine how the different classifications apply to tropical research through an analysis of the study locations of >200 recent tropical biology articles. Depending on the definition, the extent of the terrestrial tropics ranges from 23 million to 66 million km 2-a nearly threefold difference. Likewise, the classification of many areas as being tropical vs. non-tropical depends on the specific criterion employed. Of the tropical studies reviewed here, only 44% were based on data collected from the core tropics, and 12% of tropical studies were based on data collected from sites outside of the geographic tropics but with tropical climates. Many different criteria are used to classify areas as tropical vs. non-tropical, leading to inconsistencies when estimating the extent of tropical areas and variation in the classification of ecosystems and species as being tropical vs. non-tropical.

The consequences of rising temperatures for trees will vary between species based on their abilities to acclimate their leaf thermoregulatory traits and photosynthetic thermal tolerances. We tested the hypotheses that adult trees in warmer growing conditions (1) acclimate their thermoregulatory traits to regulate leaf temperatures and (2) acclimate their thermal tolerances such that tolerances are positively correlated with leaf temperature, and that (3) species with broader thermal niche breadths have greater acclimatory abilities. To test these hypotheses, we measured leaf traits and thermal tolerances of seven focal tree species across steep thermal gradients in Miami’s urban heat island. We found that some functional traits varied significantly across air temperatures within species. For example, leaf thickness increased with maximum air temperature in three species, and leaf mass per area and leaf reflectance both increased with air temperature in one species. Only one species was marginally more homeothermic than expected by chance due to acclimation of its thermoregulatory traits, but this acclimation was insufficient to offset elevated air temperatures. Thermal tolerances acclimated to higher maximum air temperatures in two species. As a result of limited acclimation, leaf Thermal Safety Margins (TSMs) were narrower for trees in hotter areas. We found some support for our hypothesis that species with broader thermal niches are better at acclimating in order to maintain more-stable TSMs across the temperature gradients. These findings suggest that trees have limited abilities to acclimate to high temperatures and that thermal niche specialists may be at a heightened risk of thermal stress as global temperatures continue to rise.