Quantifying above‐ and belowground biomass carbon loss with forest conversion in tropical lowlands of Sumatra (Indonesia)

Abstract

Natural forests in South-East Asia have been extensively converted into other land-use systems in the past decades and still show high deforestation rates. Historically, lowland forests have been converted into rubber forests, but more recently, the dominant conversion is into oil palm plantations. While it is expected that the large-scale conversion has strong effects on the carbon cycle, detailed studies quantifying carbon pools and total net primary production (NPPtotal ) in above- and belowground tree biomass in land-use systems replacing rainforest (incl. oil palm plantations) are rare so far. We measured above- and belowground carbon pools in tree biomass together with NPPtotal in natural old-growth forests, 'jungle rubber' agroforests under natural tree cover, and rubber and oil palm monocultures in Sumatra. In total, 32 stands (eight plot replicates per land-use system) were studied in two different regions. Total tree biomass in the natural forest (mean: 384 Mg ha(-1) ) was more than two times higher than in jungle rubber stands (147 Mg ha(-1) ) and>four times higher than in monoculture rubber and oil palm plantations (78 and 50 Mg ha(-1) ). NPPtotal was higher in the natural forest (24 Mg ha(-1) yr(-1) ) than in the rubber systems (20 and 15 Mg ha(-1) yr(-1) ), but was highest in the oil palm system (33 Mg ha(-1) yr(-1) ) due to very high fruit production (15-20 Mg ha(-1) yr(-1) ). NPPtotal was dominated in all systems by aboveground production, but belowground productivity was significantly higher in the natural forest and jungle rubber than in plantations. We conclude that conversion of natural lowland forest into different agricultural systems leads to a strong reduction not only in the biomass carbon pool (up to 166 Mg C ha(-1) ) but also in carbon sequestration as carbon residence time (i.e. biomass-C:NPP-C) was 3-10 times higher in the natural forest than in rubber and oil palm plantations.

Carbon and nitrogen stocks in dead wood of tropical lowland forests as dependent on wood decay stages and land-use intensity

“…Pregitzer & Euskirchen (2004) reported that the mean C pool of woody debris across tropical forests amounted to 10% of the total carbon stocks. Dead wood contributed 2.3% of total aboveground C stocks in the natural forest and 1.9% of that in the jungle rubber (Kotowska et al 2015). These values were five times lower compared to 33% of the C stock in Costa Rican natural forests (Clark et al 2002).…”

“…In the natural forest total aboveground tree biomass (AGB) was more than two times higher and aboveground net primary productivity (NP-P wood ) was also higher than in the jungle rubber systems (Kotowska et al 2015). The high AGB and NPP wood in the natural forest could have one reason for the higher dead wood mass, bearing on its capacity to store stocks of C and N. The larger mean diameter and height of living trees in the aboveground structure of the natural forest provided evidence for a higher dead wood biomass (Kotowska et al 2015). As a result, jungle rubber systems apparently lack large dead wood pieces which is more of a structural characteristic of natural forests.…”

“…We assumed that high lignin stocks in the natural forest served as the C stock source are expected to retain a long-term storage in this system supporting the longevity of dead wood on the forest floor. A dense canopy cover and high basal area in the natural forest are clear reasons for lower air temperature and higher air humidity than in the jungle rubber system (Kotowska et al 2015). These microclimate properties in the natural forest plots represent favourable conditions for microorganism degrading dead wood (Zhou et al 2007).…”

“…The jungle rubber system was a rubber agroforest with natural tree cover where forest trees were logged and rubber trees planted in the gaps between 1973 and 2006. Canopy cover and total basal area in the natural forest were 92.1 ± 0.5% and 30.1 ± 0.9 m 2 ha -1 , respectively, whereas in the jungle rubber the numbers were 87.6 ± 0.8% and 18.3 ± 1.1 m 2 ha -1 , respectively (Kotowska et al 2015).…”

“…Wood density (ρ) was measured using Pilodyn 6J wood tester (PROCEQ SA, Zürich, Switzerland). The penetration depth (h) of the pin was converted using a calibration equation derived by Kotowska et al (2015) (Eq. 1), based on 204 trees which wood core samples were analyzed.…”