Asynchronous forest: the role of rainfall seasonality controlling fine-roots and litterfall productivity in Central Amazonia

Seasonal phenological patterns in the Amazon Forest result from interactions among climate and turnover rates of different plant tissues. Changes in productivity rates and allocation are predicted to occur with climate change, particularly for dynamic tissues such as fine-roots and leaves. Accurate measurements of fine-roots and litterfall dynamics and their interactions with climate are key to understanding the fate of carbon and nutrients in these ecosystems, which will improve climate model predictions.

In this study we quantified fine-root dynamics up to 90 cm soil depth and asked if there were any differential allocation patterns between fine-roots and leaf litterfall productivity in a Central Amazon forest. We hypothesized that rainfall seasonality would affect such trends.

Fine-root (diameter <2mm) were measured monthly for 3 years (November 2016 – November 2019) using minirhizotrons cameras at the AmazonFACE site in a tropical rainforest in Central Amazonia near Manaus, Brazil. We divided root analysis in three soil layers: 0-30 cm (n=9), 30-60 cm (n=7) and 60-90 cm (n=6). Leaf litterfall was collected biweekly at the same site and period using 24 50 x 50 cm litter traps (0.25 m²) installed one meter above the ground.

The total fine-root biomass (0-90 cm) was 11.13 ± 0.2 Mg ha^-1 and decreased with soil depth (0-30 cm: 4.97 ± 0.2; 30-60 cm: 3.43 ± 0.2; 60-90 cm: 2.73 ± 0.2 Mg ha^-1). Fine-roots productivity also decreased with depth, ranging from 4.27 ± 0.31 Mg ha year^-1 in the top 30 cm to 1.15 ± 0.18 Mg ha year^-1 between 60 to 90 cm. As a result, turnover rates were faster in the first layer (1.08 year^-1), and slower in the deeper layers (30-60 cm: 0.63; 60-90 cm: 0.45 year^-1), being 0.78 year^-1 for the whole soil profile.

Mean total fine-roots productivity up to 90 cm depth was 7.22 ± 0.82 Mg ha year^-1 and mean leaf litterfall productivity was 5.94 ± 0.39 Mg ha year^-1, with a marked seasonal trade-off between these two components. In the dry season (June to October) litterfall peaked, reaching 9.06 ± 0.22 Mg ha year^-1 while fine-roots reached its lower values: 4.66 ± 0.54 Mg ha year^-1. The opposite trend occurred in the wet season (November to May), when fine-roots reached 9.03 ± 1.18 Mg ha year^-1 and litterfall dropped to 3.6 ± 0.08 Mg ha year^-1. Rainfall was positively correlated with fine root productivity and negatively correlated with leaf litterfall, explaining 34% and 48% of the variation, respectively.

Our results show that, although commonly neglected, deep fine-roots account for a high proportion of forest productivity in the Amazon, once they are also very dynamic at deeper layers. Moreover, since new leaf production has been found to be temporally synchronized with litterfall production in this forest, such trends point to a possible shift in total plant carbon allocation between above and belowground compartments driven by the seasonal changes in rainfall regime. Deeper understanding of phenological mechanisms in the Amazon forest could therefore improve predictions of its long-term response and resiliency to changing climate.