Allocation trade‐offs dominate the response of tropical forest growth to seasonal and interannual drought

Abstract

What determines the seasonal and interannual variation of growth rates in trees in a tropical forest? We explore this question with a novel four-year high-temporal-resolution data set of carbon allocation from two forest plots in the Bolivian Amazon. The forests show strong seasonal variation in tree wood growth rates, which are largely explained by shifts in carbon allocation, and not by shifts in total productivity. At the deeper soil plot, there was a clear seasonal trade-off between wood and canopy NPP, while the shallower soils plot showed a contrasting seasonal trade-off between wood and fine roots. Although a strong 2010 drought reduced photosynthesis, NPP remained constant and increased in the six-month period following the drought, which indicates usage of significant nonstructural carbohydrate stores. Following the drought, carbon allocation increased initially towards the canopy, and then in the following year, allocation increased towards fine-root production. Had we only measured woody growth at these sites and inferred total NPP, we would have misinterpreted both the seasonal and interannual responses. In many tropical forest ecosystems, we propose that changing tree growth rates are more likely to reflect shifts in allocation rather than changes in overall productivity. Only a whole NPP allocation perspective can correctly interpret the relationship between changes in growth and changes in productivity.

Sensitivity of tropical forest aboveground productivity to climate anomalies in SW Costa Rica

“…Adding to this, recent studies suggest that C allocation strategies in tropical forests trigger a temporal decoupling between NPP components [Doughty et al, 2014]. During their 4 year high temporal resolution study in two forest plots in the Bolivian Amazon, a strong 2010 drought strongly reduced photosynthesis, whereas NPP remained constant and even increased in the 6 month period following the drought.…”

“…Whereas the deep-soil plot was inhabited by species typical of humid regions, the shallow-soil plot comprised species more typical of a dry deciduous forest with more seasonal leaf and wood production. As a result, the shallow-soil plot showed a more regular seasonal but also substantial interannual variation in NPP, suggesting a greater interannual drought sensitivity of dry deciduous tree species [Doughty et al, 2014]. Therefore, local topoedaphic characteristics associated with water availability have the potential to determine the composition of tree species, which in turn could have important consequences for the tolerance of tropical lowland forests to climate anomalies.…”

“…What determines the seasonal and interannual variations of tropical ecosystem processes has become a recent subject of studies investigating the productivity of tropical forest ecosystems [Rowland et al, 2013;Malhi et al, 2014;Doughty et al, 2014]. So far, it remains unknown whether direct effects of climate on tropical NPP or rather inherent phenological rhythms in growth allocation dominate the growth response of tropical forests ecosystems to seasonal climatic signals.…”

Environmental drivers of drought deciduous phenology in the Community Land Model

“…6 that CLM is dramatically overestimating LAI across the mesic regions of the terrestrial biosphere. This issue, often masked when only mean annual values or zonal means are considered, deserves more attention, and it is likely that recent detailed studies of carbon allocation (e.g., Doughty et al, 2014) could improve this part of the model.…”

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

“…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). As a result, tropical C storage will differ considerably depending on if C is stored in long-lived wood or allocated to plant tissue with reduced lifespan (Körner et al, 2005).…”