1 INTRODUCTION

Tropical forests represent the world's most biodiverse ecosystems, as well as providing its largest stores of living carbon and contributing more to biomass productivity than any other biome on the planet. Changes here can therefore have global consequences, potentially nowhere more so than in Amazonia where between 6,000 and 16,000 tree species exist (Cardoso et al., 2017; Ter Steege et al., 2013) and as much as 100 Pg of carbon is stored in biomass (Feldpausch et al., 2012). While the physical, chemical, and biological environment have all been changing over recent decades, it is the changes in climate—both documented and projected—which are widely expected to cause some of the most profound changes in forest communities and ecosystem processes (Esquivel‐Muelbert, Baker, et al., 2017; Thomas et al., 2004). For example, higher temperatures and intensifying drought may threaten larger trees due to hydraulic failure (McDowell & Allen, 2015; Rowland et al., 2015), which could eventually compromise forest biomass and productivity. In Amazonia, because forest diversity is concentrated in the wetter, least seasonal forests (Francis & Currie, 2003; Gentry, 1988), a persistent lengthening of the dry season might threaten a large portion of tropical biodiversity. Here we aim to better understand how Amazonian trees have responded to the last 30 years of environmental change, by analysing floristic records from long‐term tree monitoring in the Neotropics to assess the potential compositional changes to date.

Changes in biodiversity attributed to climate change have already been documented in a wide variety of ecosystems (e.g. Bowler et al., 2017; Chen et al., 2009), including in some tropical locations, but so far there is remarkably little evidence of widespread impacts of climate change on the composition of tropical ecosystems which harbour much of the planet's diversity (Duque, Stevenson, & Feeley, 2015; Fauset et al., 2012; Van Der Sande et al., 2016). In contrast, there is evidence for widespread changes in the structure (i.e. aboveground biomass) and dynamics (e.g. mortality and productivity) of old‐growth tropical forests. In many forests, apparently undisturbed by humans, both aboveground biomass and the rate of ecological processes such as growth and recruitment have increased (e.g. Brienen et al., 2015; Lewis, Lopez‐Gonzalez, et al., 2009; Phillips & Gentry, 1994; Qie et al., 2017), while in Amazonia increases in mortality have caused a recent weakening of the biomass carbon sink (Brienen et al., 2015). However, it remains unclear whether these structural and dynamic changes are also associated with concerted changes in the species richness and composition of Amazonian forests.

In Amazonia, as elsewhere, climate change is a potential leading driver of changes to the ecosystem. During the last few decades, the climate of Amazonia has become more extreme—the length of the dry season and its intensity have increased, while precipitation has become more intense during the wet season (Gloor et al., 2015; Hilker et al., 2014). Extreme climate events in recent years include the three strong droughts within a decade (Erfanian, Wang, & Fomenko, 2017; Jiménez‐Muñoz et al., 2016; Lewis, Brando, Phillips, Van Der Heijden, & Nepstad, 2011; Marengo, Tomasella, Alves, Soares, & Rodriguez, 2011; Marengo et al., 2008) and several large‐scale episodes of extreme rainfall (Espinoza et al., 2014; Marengo & Espinoza, 2016). In addition to the repeated drought events, precipitation has declined in the south and south east of the basin (25% reduction in rainfall between 2000 and 2012) (Hilker et al., 2014) and higher temperatures are likely to have intensified seasonal evaporative stress across the basin (Jiménez‐Muñoz, Sobrino, Mattar, & Malhi, 2013). These changes are consistent with model‐based predictions (Duffy, Brando, Asner, & Field, 2015), implying that the Amazon may already have entered a new regime of a hotter, more variable climate. The forest has clearly responded to these recent fluctuations in climate—for example, tree mortality rates increased markedly during and after drought events causing at least temporary losses of standing biomass (Brienen et al., 2015; Feldpausch et al., 2016; Phillips et al., 2009; Zuleta, Duque, Cardenas, Muller‐Landau, & Davies, 2017). The 2010 drought also impacted on the basin‐wide exchange of carbon dioxide between the vegetation and the atmosphere, with the vegetation becoming a net source of CO 2 during 2010 (Gatti et al., 2014).

In addition to the changes in climate, atmospheric CO 2 concentrations have increased globally from 320 ppm to over 400 ppm over the past 50 years (Conway & Tans, 2016). Carbon dioxide is a fundamental resource for photosynthesis, and higher concentrations are expected to stimulate plant growth (Lloyd & Farquhar, 1996). Indeed, the increase in atmospheric concentrations of CO 2 is a potential driver of the observed increase in aboveground biomass and turnover rates in tropical forests (Brienen et al., 2015; Lewis, Lloyd, Sitch, Mitchard, & Laurance, 2009; Pan et al., 2011; Zhu et al., 2016). Additionally, under higher CO 2 concentrations plants may increase their water‐use efficiency with less water being required per unit of carbon fixed. Thus, by allowing plants to use less water, higher CO 2 concentrations could alleviate the effect of increasing aridity (Lloyd & Farquhar, 2008; Van Der Sleen et al., 2015).

Interpreting potential shifts in tropical floristic composition and linking them with possible drivers is a considerable challenge due to the very high diversity of tropical forests and their large spatial extent. However, if sufficient high‐quality, long‐term floristic monitoring is available, then the approach of analysing shifts in a suite of functional traits to describe how communities change over time can be used to link floristic changes to their drivers (McGill, Enquist, Weiher, & Westoby, 2006; Violle, Reich, Pacala, Enquist, & Kattge, 2014). For tropical tree species, two largely independent trait axes may have value in addressing these questions. One axis, the life‐history trade‐off between growth and mortality, can be represented by wood density as it is negatively correlated to growth and mortality rates. This is a reflection of slow‐growing trees tending to invest more in wood structure (Chave et al., 2009; Muller‐Landau, 2004; Nascimento et al., 2005; Poorter et al., 2010). The other axis is related to the potential size that taxa can attain, representing the capacity of taxa to compete for light (Falster & Westoby, 2005; Poorter, Bongers, Sterck, & Wöll, 2005).

Environmental changes could have different impacts along each of these ecological axes (Chave et al., 2008; Lewis, Lloyd, et al., 2009). Most notably, with more extended or intense periods of soil water deficit, large trees and those with low wood density may be at greatest risk of hydraulic failure due to cavitation (McDowell & Allen, 2015; Rowland et al., 2015). Large trees have been shown to be particularly affected by artificially‐imposed drought (McDowell & Allen, 2015; Nepstad, Tohver, Ray, Moutinho, & Cardinot, 2007; Rowland et al., 2015) and drought events (Bennett, Mcdowell, Allen, & Anderson‐Teixeira, 2015; Phillips et al., 2010). On the other hand, several observations from tropical forests show a decline of small understory taxa associated with increases in soil water deficit (e.g. Condit, Hubbell, & Foster, 1996; Enquist & Enquist, 2011; Fauset et al., 2012; Feeley, Davies, Perez, Hubbell, & Foster, 2011). To explain these observations, it has been hypothesized that small taxa have shallower roots and are therefore more vulnerable compared to large, deep rooted trees to long‐term drying trends (Condit et al., 1996; Fauset et al., 2012). Although the link between rooting depth and tree size is still unclear (Stahl et al., 2013), this hypothesis is consistent with wetter areas tending to have more densely populated understoreys (Malhi et al., 2002; Pitman et al., 2002) and taller forests being less sensitive to precipitation variability (Giardina et al., 2018). The ongoing increase in atmospheric carbon dioxide is also expected to cause changes in species composition as it is predicted to favour those trees that have greater competitive capacity to access light (Coomes, Lines, & Allen, 2011; Laurance, 2004), consequently increasing the mean potential tree size within the community (e.g. Laurance et al., 2004) and to favour fast‐growing trees, potentially leading to communities with lower wood density.

Given the uncertainty about how tree size relates to responses to a drying climate, the geographic distributions of species over precipitation gradients offer an alternative source of information with which to predict and to infer the effects of drought on floristic composition. The spatial distribution of tree taxa over precipitation gradients has been shown to provide a valuable proxy for drought tolerance in observational studies and experiments (Engelbrecht et al., 2007; Esquivel‐Muelbert, Galbraith, et al., 2017). If drought is increasingly affecting Amazonian forests, we might therefore expect concerted shifts in tree communities towards more dry‐affiliated components. A compositional shift towards more dry‐tolerant taxa as a consequence of an increase in moisture stress has been reported locally for sites in southern Ghana (Fauset et al., 2012), Central America (Enquist & Enquist, 2011; Feeley, Davies, et al., 2011), and parts of the Amazon (Butt et al., 2014).

Here, we aim to quantify the shifts in the floristic composition of Amazonian tree communities, and test the hypothesis that recent climatic drying is already impacting the composition of Amazonian forests. The data set derives from 106 long‐term, tree inventory plots distributed across intact closed‐canopy moist forests in lowland Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru and Venezuela (Supporting Information Appendix S1). We analyse monitoring records from as early as 1985 onwards to as recently as 2015, deliberately excluding any possible influence of the 2015–2016 El Niño drought from our analysis. We investigate trends within the overall community composition, as well as among recruits, trees that died, and in the growth rate that occurred within each census interval (Figure 1). We analyse compositional shifts along these three trait axes, which we demonstrate to be independent: life‐history (using wood density as a proxy), potential size, and bioclimatic distribution (Supporting Information S2). Based on predictions from plant physiology supported by experimental studies, we expect increases in dry season duration or intensity to shift floristic composition towards dry‐affiliated and smaller‐statured genera with high wood density (McDowell & Allen, 2015; Rowland et al., 2015). Additionally, we examine trends in abundance for individual genera, which allows us to understand which taxa dominate the changes in functional composition.