Environmental impacts and benefits of agroforestry in the Galapagos Islands
(2023)
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- Abstract
- The suitability of volcanic tropical islands for sustained agriculture is challenging and often contentious. Islands are particularly vulnerable to global change due to their limited size and remoteness. Notably, small tropical islands in the Pacific Ocean are highly vulnerable to climate change, which might limit sustainable farming activities as air temperatures increase, sea level increases, and storm frequency and intensity are expected to increase by 2050. Sustainable land use practices based on nature-based solutions can help local communities to adapt the agricultural systems. This empirical study took Santa Cruz Island (Galapagos Archipelago) as a case study. About 707 km2 of the continental surface area of Santa Cruz is nowadays protected, and a buffer zone of 152 km2 separates it from the non-protected area. A 112 km2 non-protected area was designated on the island's windward side so that a self-sufficient community could live and develop agricultural activities. This study aims to answer three questions: 1) How dynamic is the land use of tropical volcanic islands, and what are the biophysical and socioeconomic factors that are associated with land use dynamics after land colonization? 2) How can agroforestry management practices safeguard physical and hydraulic soil properties and contribute to adapting to climate change? 3) How can agroforestry management practices contribute to maintaining soil nutrient stocks after the land is colonized for agriculture? A multi-source data integration procedure was followed to reconstruct the land cover change between 1961 and 2018. Existing land cover products were integrated into a homogeneous land cover time series to reduce imprecision and inconsistencies that may result from comparing heterogeneous datasets. In the central part of the island, where agriculture was traditionally concentrated, we monitored sites with three different management practices: (i) conservation of native forest, (ii) traditional agroforestry, and (iii) abandoned farmland in passive restoration. Per agroforestry management practice, two replicates were equipped with environmental sensors to monitor hydrometeorology and soil moisture and temperature, and the sites were characterized for key soil hydro physical and biogeochemical properties, including soil nutrients (carbon, nitrogen, exchangeable cations, total phosphorus). Monitoring started in July 2019 and continues to date, and the data from 30-month period (July 2019-December 2021) were analysed for this thesis. The central part of the island experienced significant land cover changes, with native forests and grasslands being converted to agricultural land. In 1961, ninety-four percent of the non-protected area was still covered by native vegetation, which was reduced to only seven percent by 2018. Most agricultural expansion occurred in the 1960s and 1970s, resulting in an anthropogenic landscape comprising 67% of agricultural land and 26% of the land with dominance of invasive species. At the beginning of agricultural colonization, the native forest was cleared on easily accessible land with fertile and deep soil. While the early settlements were large, isolated farmsteads, the average size of the farms decreased over time as the number of farms doubled. More recent farmsteads are small and occupy less favorable land for agriculture. Over the last two decades, the rapid rise of tourism activities alleviated pressure on natural resources and led to the abandonment of agricultural land. The comparative analyses showed that agroforestry management practices significantly control soil temperature, moisture availability, and nutrient contents. When protected by forest vegetation, the soils are protected from direct solar radiation, and trees keep the soil 12% cooler than soils converted to agricultural land. Soil moisture is, on average, 20% higher under forest than under traditional agroforestry or abandoned farmland, and forest soils have a lower bulk density, lower saturated hydraulic conductivity, and higher water retention capacity. The forest and passive restoration sites stored in the first 1 m more than 377 Mg C. ha-1, about 50% more than under traditional agroforestry. The loss of soil organic carbon in agricultural sites is probably related to soil mixing as a result of tillage which increases soil pore connectivity and saturated hydraulic conductivity and facilitates decomposition of soil organic carbon and leaching of base cations. At the same time, fertilisation with organic manure and/or nitrogen-phosphorus-potassium changed the soil pH and enhanced soil nutrient leaching. Meanwhile passive restoration sites left the farmland to rest and the soil organic carbon levels recovered, and the CEC and base saturation increased. The present work highlights the importance of soil quality, access to markets and the possibilities of alternative incomes in decision making process of land cover change process. Therefore, it is suggested that future land policy designs should consider the diversity in the farm size types and the factors that make agricultural activities feasible. Additionally, this empirical work demonstrated that agroforestry management practices can be used as effective strategies to mitigate increasing soil temperatures, agricultural drought, and decline in soil nutrient content. Preserving forest remnants in an agricultural landscape has measurable effects on the soil environment, with a reduction of soil warming by 12 %), a reduction of soil drying by 20%, and better preservation of soil organic carbon stocks compared to traditional agroforestry. Passive restoration helps to increase carbon content, CEC, and base saturation but has no measurable effects on soil hydrological properties. Further research topics might include the study of the nutrient cycle, including plant cycling (e.g., root uptake and overturn rates in native species and crops), the importance of atmospheric inputs in soil development, or modeling the nutrient, water, and energy cycle in different agroforestry management practices. However, it can also be related to further optimizing agroforestry management practices to mitigate carbon emissions and improve water management.
- Affiliations
UCLouvain SST/ELI/ELIC - Earth & Climate
Citations
Alomia Herrera, I. M. (2023). Environmental impacts and benefits of agroforestry in the Galapagos Islands. https://hdl.handle.net/2078.5/27249