Old‐growth Neotropical forests are shifting in species and trait composition

Abstract

Tropical forests have long been thought to be in stable state, but recent insights indicate that global change is leading to shifts in forest dynamics and species composition. These shifts may be driven by environmental changes such as increased resource availability, increased drought stress, and/or recovery from past disturbances. The relative importance of these drivers can be inferred from analyzing changes in trait values of tree communities. Here, we evaluate a decade of change in species and trait composition across five old‐growth Neotropical forests in Bolivia, Brazil, Guyana, and Costa Rica that cover large gradients in rainfall and soil fertility. To identify the drivers of compositional change, we used data from 29 permanent sample plots and measurements of 15 leaf, stem, and whole‐plant traits that are important for plant performance and should respond to global change drivers. We found that forests differ strongly in their community‐mean trait values, resulting from differences in soil fertility and annual rainfall seasonality. The abundance of deciduous species with high specific leaf area increases from wet to dry forests. The community‐mean wood density is high in the driest forests to protect xylem vessels against drought cavitation, and is high in nutrient‐poor forests to increase wood longevity and enhance nutrient residence time in the plant. Interestingly, the species composition changed over time in three of the forests, and the community‐mean wood density increased and the specific leaf area decreased in all forests, indicating that these forests are changing toward later successional stages dominated by slow‐growing, shade‐tolerant species. We did not see changes in other traits that could reflect responses to increased drought stress, such as increased drought deciduousness or decreased maximum adult size, or that could reflect increased resource availability (CO2, rainfall, or nitrogen). Changes in species and trait composition in these forests are therefore most likely caused by recovery from past disturbances. These compositional changes may also lead to shifts in ecosystem processes, such as a lower carbon sequestration and “slower” forest dynamics.

Compositional response of Amazon forests to climate change

“…Amazonian forests have been hypothesized to be recovering from previous disturbances that are either natural (Chambers et al, 2013;Van Der Sande et al, 2016) or human-driven, particularly in pre-Colombian times (McMichael, Matthews-Bird, Farfan-Rios, & Feeley, 2017). Conceivably one or more of these could have pervasive impacts on mature forests across the basin to rival or exceed any climate impacts.…”

“…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). 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).…”

Shifting species and functional diversity due to abrupt changes in water availability in tropical dry forests

“…We evaluated four functional traits that are associated with drought avoidance and/or resistance and can potentially affect demographical processes, and hence should indicate species responses to changes in water availability (van der Sande et al, , Chave et al, ): a stem trait (wood density, WD) and three leaf traits (specific leaf area, SLA; deciduousness, and compound/simple leaves). Wood density is positively related with plant carbon storage and drought resistance (Pérez‐Harguindeguy et al, ; Santiago et al, ).…”

“…Community‐weighted mean (CWM) trait values and functional diversity indices (FD) were calculated per plot for the overall community (at T 0 and T 10 ) and the three demographical groups. We weighted by species abundance to equally weight recruiting and dying trees on changes in CWM values (van der Sande et al, ). CWM values were calculated for WD and SLA.…”

“…Climate change and other human impacts on abiotic conditions are causing global shifts in ecosystem dynamics, even in old‐growth tropical forests, thought to be in a stable state (Meir & Ian Woodward, , Brienen et al, , van der Sande et al, ). Abiotic conditions can shift in multiple ways (e.g., increased drought, rain or temperature), each of which can result in distinct changes in the vegetation.…”