Source and sink carbon dynamics and carbon allocation in the Amazon basin

Abstract

Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field-based data. Here we show seasonal cycles of the complete carbon cycle for 14, 1 ha intensive carbon cycling plots which we separate into three regions: humid lowland, highlands, and dry lowlands. Our data highlight three trends: (1) there is differing seasonality of total net primary productivity (NPP) with the highlands and dry lowlands peaking in the dry season and the humid lowland sites peaking in the wet season, (2) seasonal reductions in wood NPP are not driven by reductions in total NPP but by carbon during the dry season being preferentially allocated toward either roots or canopy NPP, and (3) there is a temporal decoupling between total photosynthesis and total carbon usage (plant carbon expenditure). This decoupling indicates the presence of nonstructural carbohydrates which may allow growth and carbon to be allocated when it is most ecologically beneficial rather than when it is most environmentally available.

The variation of productivity and its allocation along a tropical elevation gradient: a whole carbon budget perspective

“…In order to explore variation in forest carbon production and allocation, we ask: what parameters explain the variation in total NPP, aboveground coarse wood productivity (NPP acw ; hence tree growth rates), and aboveground biomass among sites? To resolve this question, we apply a systematic framework to decompose the relationship between NPP stem and GPP into several terms in a productivity–allocation–turnover chain, that we previously introduced to analyse carbon cycling along wet–dry gradients in lowland Amazonia (Malhi et al ., ) and temporal responses to carbon allocation, seasonality and drought events are explored in (Doughty et al ., ,b): …”

Global change effects on humid tropical forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem scale

“…For example, a 4 year observation study in two 1 ha Bolivian plots found that photosynthesis decreased during a strong drought, but plant NPP remained unchanged, and canopy and fine root growth actually increased during the 6 months following the drought [ Doughty et al ., ]. An analysis of 14 1 ha plots in tropical humid lowlands, humid highlands, and dry lowlands showed a negative effect of dry season on wood growth, but root and leaf growth were sustained during the dry season and total NPP was unchanged [ Doughty et al ., ]. In this study, nonstructural carbohydrates stored in plants were used for growth when dry periods reduced photosynthetic activity.…”

Amazon Forest Ecosystem Responses to Elevated Atmospheric CO2 and Alterations in Nutrient Availability: Filling the Gaps with Model-Experiment Integration

“…Therefore, allocation schemes based on functional relationships among biomass fractions that vary with resource availability perform best in capturing field-based observations . Recent observations indicate that photosynthesis and plant carbon usage are temporally decoupled allowing C to be allocated when it is ecological beneficial, rather than when C is environmentally most available (Doughty et al, 2015b). Hence, seasonal reductions in wood NPP were found associated with carbon preferentially allocated to either root or canopy NPP during the dry season (Doughty et al, 2015b).…”

“…Recent observations indicate that photosynthesis and plant carbon usage are temporally decoupled allowing C to be allocated when it is ecological beneficial, rather than when C is environmentally most available (Doughty et al, 2015b). Hence, seasonal reductions in wood NPP were found associated with carbon preferentially allocated to either root or canopy NPP during the dry season (Doughty et al, 2015b). Due to significant differences in turnover times of plant tissues (i.e., leaves, wood, fine roots) this suggests that projected increases in temperature and dry season length could strongly affect tropical C storage by shifting C allocation away from wood NPP (Hofhansl et al, 2015) and toward canopy and root NPP to alleviate droughtinduced resource limitation (Doughty et al, 2014).…”