Deforestation and Reforestation Impacts on Soils in the Tropics
Total Page：16
File Type：pdf, Size：1020Kb
REVIEWS Deforestation and reforestation impacts on soils in the tropics Edzo Veldkamp 1 ✉ , Marcus Schmidt 1, Jennifer S. Powers2,3 and Marife D. Corre1 Abstract | Soils under natural, tropical forests provide essential ecosystem services that have been shaped by long- term soil–vegetation feedbacks. However, deforestation of tropical forest, with a net rate of 5.5 million hectares annually in 2010–2015, profoundly impacts soil properties and functions. Reforestation is also prominent in the tropics, again altering the state and functioning of the underlying soils. In this Review, we discuss the substantial changes in dynamic soil properties following deforestation and during reforestation. Changes associated with deforestation continue for decades after forest clearing eventually extend to deep subsoils and strongly affect soil functions, including nutrient storage and recycling, carbon storage and greenhouse gas emissions, erosion resistance and water storage, drainage and filtration. Reforestation reverses many of the effects of deforestation, mainly in the topsoil, but such restoration can take decades and the resulting soil properties still deviate from those under natural forests. Improved management of soil organic matter in converted land uses can moderate or reduce the ecologically deleterious effects of deforestation on soils. We emphasize the importance of soil knowledge not only in cross-disciplinary research on deforestation and reforestation but also in developing effective incentives and policies to reduce deforestation. 1,2 27 (Fig. 1) Slash- and- burn Soils in the tropics provide essential functions (such as natural forest losses . Substantial area that was 3–5 management nutrient storage and recycling , carbon (C) storage and formerly cleared for agriculture has also been abandoned, Cutting down and burning of greenhouse gas (GHG) emissions6,7, erosion resistance8 commonly because of soil degradation, and now pro- vegetation in an area, often as and water storage, drainage and filtration9–11), and soil vides areas for forest regeneration by secondary succession preparation for agricultural use. variability within and amongst landscapes promotes or replanting28. Indeed, replanted forest area has 12–14 Shifting cultivation biodiversity of tropical forests . The properties of increased rapidly, at approximately 1 million ha per Agricultural system in which soils under natural tropical forests reflect long- term year between 1990 and 2015, a trend that is projected the area is fallowed in between soil- vegetation feedbacks, influenced by the high pro- to continue27. Currently, the total area of primary forests periods of cultivation, allowing ductivity, regular litter input and permanent deep root in tropical countries (541 million ha) is only approxi- natural vegetation to return 15 and soils to recover. systems that are characteristic of these forests, all of mately half of regenerating forests (1,172 million ha), which stimulate activity of soil organisms16,17. Compared illustrating their global significance29,30. with agricultural land uses, tropical forests promote Deforestation and reforestation can lead to pro- 1Faculty of Forest Sciences efficient soil- nutrient recycling18,19 and modulate soil found changes in dynamic soil properties that directly and Forest Ecology, Soil temperature and moisture20. and indirectly affect many soil functional processes31,32. Science of Tropical and Humans have shaped tropical forests and their soils For example, deforestation generally leads to lower Subtropical Ecosystems, 21,22 6,7,33–35 University of Goettingen, for millennia, especially since the onset of agriculture . soil organic carbon (SOC) stocks , higher soil Goettingen, Germany. For centuries, slash- and- burn management during bulk density36 and changes in soil pH (REF.37). Changes 2Department of Ecology, shifting cultivation was a main cause of forest clearing, in dynamic soil properties occur most rapidly in the Evolution, and Behavior, but was offset by natural regrowth during fallow periods23, organic- matter- rich topsoils with the highest biolog- University of Minnesota, thus, preventing significant net deforestation24. Tropical ical activity6,38. Deeper soil horizons (>50 cm) are also St. Paul, MN, USA. deforestation as a global process started during colonial affected by deforestation and reforestation, although 3 Department of Plant and times and intensified in the second half of the twentieth changes there occur more slowly and often become Microbial Biology, University 38 of Minnesota, St. Paul, century as a result of the demand- driven expansion of substantial only after several decades . However, most 25 MN, USA. logging and agricultural land , typically supported by studies do not report long-term changes in dynamic soil 26 ✉e- mail: [email protected] government policies . At 5.5 million hectares (ha) per properties — of those reviewed here, only 48% included https://doi.org/10.1038/ year (based on the 2010–2015 period), tropical deforest- land uses ≥25 years old. Nevertheless, most studies s43017-020-0091-5 ation continues to be the largest contributor of global assumed that a steady state was reached within the period 590 | NOVEMBER 2020 | VOLUME 1 www.nature.com/natrevearthenviron REVIEWS 42 Key points owing to the input of carbonate- containing ashes that buffer soil pH to values between 6.5 and 7.5 (REF.41). • Deforestation leads to profound changes in dynamic soil properties that degrade In the absence of liming during land management, the most soil functions. net nutrient export by harvested products and leaching • The rate and degree of soil degradation following deforestation are a function of the losses43 cause a net release of H+ that can eventually lead inherent soil fertility and land- use intensity. to a drop in soil pH over time41. Indeed, in >25- year- old • Changes in dynamic soil properties continue for decades following deforestation and croplands, soil pH decreases to values below those of the eventually extend to deep subsoils. original forest soils in the top 10 cm (FIG. 3a). Changes • Reforestation reverses some of the undesirable effects of deforestation on dynamic in pH following slash- and- burn management are less soil properties; however, the resulting soil conditions and their functions are pronounced in HAC soils compared with LAC soils substantially different from the previous soils under natural forests. (Supplementary Fig. 1). Conversion of forests to pastureland also rapidly studied or that further changes were insignificant7,35. increases soil pH owing to ash input44 (24% increase In combination with the undersampling of deeper soil in the top 10 cm and 9% increase in 10–50 cm, FIG. 3c), horizons, this assumption has led to the relatively wide- followed by a slow decrease in pH in subsequent years44 spread view that only topsoils are affected by deforest- (Fig. 3c). In contrast to croplands, >25- year- old pastures ation and reforestation, and that within approximately maintain soil pH values above those of the original for- one or two decades following deforestation, most ests (3–9% increase, Fig. 3c), likely because only small dynamic soil properties have reached steady- state or amounts of base cations are exported through harvest equilibrium values7,35. and leaching11. As with crop and pastureland, tree cash Fallow periods In this Review, we examine the impacts of deforest- crop plantations (such as for rubber and oil palm) are Time during which arable land ation and reforestation on soils in the tropics. We often established using slash-and- burn management45,46; is not actively used in crop compile changes in dynamic soil properties following soil pH increases with age in these plantations, likely production. forest clearing and during forest regeneration, explicitly because of regular liming45 (Fig. 4a). Deforestation including results from studies that covered several dec- Most soils are negatively charged and retain posi- Removal of forest and ades and/or measurements from deeper subsoils. These tively charged nutrients (including K+, Ca2+, Mg2+ and conversion of land for other + quantitative results are framed in a broader context by NH4 ) in exchangeable form in the soil, referred to as uses. reviewing evidence of how changes in dynamic soil ECEC when measured at field pH (REF.40) (BOx 1). LAC Forest regeneration properties affect important soil functions. Finally, we soils have low, pH- dependent ECEC, with a value of −1 Re- establishment of forest discuss how soil-management practices in land-use tran- 3.20 ± 0.15 cmolc kg in the natural forest soils included after disturbance. sitions affect soil functionality and identify important here. The ECEC is higher in HAC soils under natu- knowledge gaps. ral forests, with a value of 9.95 ± 1.05 cmol kg−1 in the Secondary succession c Ecological changes during the reviewed studies. Following slash- and- burn manage- regeneration of an ecosystem Impacts on dynamic soil properties ment, the inherently low ECEC of LAC soils quickly on disturbed or damaged land. Dynamic soil properties, which we use here to describe increases in both croplands42 (up to 35% increase com- characteristics that change over years to decades pared with forests, Supplementary Fig. 1a) and pastures44 Primary forests Native forests that lack owing to land- use change and management, include (25–31% increase, Supplementary Fig. 1c). In contrast, substantial signs of human soil pH, effective cation- exchange capacity (ECEC), the inherently