Trade-Offs Among Aboveground, Belowground, and Soil Organic Carbon Stocks Along Altitudinal Gradients in Andean Tropical Montane Forests.

Lydia De La Cruz-Amo,Alicia Ledo,Carlos I Espinosa,Guillermo Bañares-De-Dios,Luis Cayuela,Norma Salinas,Manuel J Macía,Íñigo Granzow-De La Cerda,Victoria Cala

doi:10.3389/fpls.2020.00106

Abstract

Tropical montane forests (TMFs) play an important role as a carbon reservoir at a global scale. However, there is a lack of a comprehensive understanding on the variation in carbon storage across TMF compartments [namely aboveground biomass (AGB), belowground biomass (BGB), and soil organic matter] along altitudinal and environmental gradients and their potential trade-offs. This study aims to: 1) understand how carbon stocks vary along altitudinal gradients in Andean TMFs, and; 2) determine the influence of climate, particularly precipitation seasonality, on the distribution of carbon stocks across different forest compartments. The study was conducted in sixty 0.1 ha plots along two altitudinal gradients at the Podocarpus National Park (Ecuador) and Río Abiseo National Park (Peru). At each plot, we calculated the amount of carbon in AGB (i.e. aboveground carbon stock, AGC), BGB (i.e. belowground carbon stock, BGC), and soil organic matter (i.e. soil organic carbon stock, SOC). The mean total carbon stock was 244.76 ± 80.38 Mg ha–1 and 211.51 ± 46.95 Mg ha–1 in the Ecuadorian and Peruvian plots, respectively. Although AGC, BGC, and SOC showed different partitioning patterns along the altitudinal gradient both in Ecuador and Peru, total carbon stock did not change with altitude in either site. The combination of annual mean temperature and precipitation seasonality explained differences in the observed patterns of carbon stocks across forest compartments between the two sites. This study suggests that the greater precipitation seasonality of colder, higher altitudes may promote faster turnover rates of organic matter and nutrients and, consequently, less accumulation of SOC but greater AGC and BGC, compared to those sites with lesser precipitation seasonality. Our results demonstrate the capacity of TMFs to store substantial amounts of carbon and suggest the existence of a trade-off in carbon stocks among forest compartments, which could be partly driven by differences in precipitation seasonality, especially under the colder temperatures of high altitudes.

Highlights

Tropical forests are the most important terrestrial carbon sink (Pan et al, 2011)
Our results show that carbon storage at each compartment of the forest (AGC, belowground carbon (BGC), and soil organic carbon (SOC)) follows a distinct pattern along the altitudinal gradient that differs between sites
The mean total carbon stock found in our study sites (229.02 ± 68.06 Mg ha–1) was within the low to middle range (87–754 Mg ha–1) reported in a meta-analysis of Tropical montane forests (TMFs) by Raich et al (2006), and similar to the results recently reported for Colombian TMFs (241.3 ± 37.5 Mg ha–1; Phillips et al, 2019)

Summary

Introduction

Both the increase in temperatures and the loss of tropical forest cover may have diminished their effectiveness in mitigating the effect of climate change (Gibbs et al, 2007; Saatchi et al, 2011; Liu et al, 2017; Mitchard, 2018) In this context, it is paramount to make accurate estimations of the carbon content of tropical forests. 1000 to over 3600 m; Spracklen and Righelato, 2014) and environmental, making them unique for understanding the influence of climate on carbon stocks (Malhi et al, 2010) They are important by providing ecosystem services and as biodiversity hotspots (Bruijnzeel et al, 2011), but in comparison to moist lowland tropical forests, their role as a carbon sink is still poorly understood This fact can be relevant at higher altitudes, where hydromorphic processes (i.e. water saturation in the soil associated under conditions of reduction) can prevail over podsolisation processes (i.e. downward migration of aluminium, iron and organic matter, and their accumulation in lower layers), resulting in lower rates of organic matter decomposition (Schawe et al, 2007) and larger concentration of soil organic carbon (SOC)

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