Impacts of forestation on water and soils in the Andes: What do we know?

POLICY BRIEF: This brief summarizes the findings of a systematic review on the impacts of forestation on water and soils in the Andes (detailed in Bonnesoeur et al., 2019). For each finding, we report the confidence level in the evidence found in the literature reviewed. Forestation impacts on water supply, hydrological regulation and soil erosion. The results of the synthesis also helped identify research gaps, which are described at the end

Ecohydrology and ecosystem services of a natural and an artificial bofedal wetland in the central Andes

High-altitude wetlands of the Central Andes, locally known as bofedales, provide important ecosystem services, particularly carbon storage, forage provisioning, and water regulation. Local communities have artificially expanded bofedales by irrigating surrounding grasslands to maximise areas for alpaca grazing. Despite their importance, biophysical processes of both natural and artificial bofedales are still poorly studied, which hinders the development of adequate management and conservation strategies. We analyse and compare the vegetation composition, hydrological variables, groundwater chemistry, and soil characteristics of a natural and an artificial bofedal of at least 10 years old in southern Peru, to understand their interrelations and the consequences for ecosystem service provisioning. We do not find statistically significant differences in the soil, water, and vegetation characteristics. Soil organic carbon (SOC) content, which we use as a proxy for carbon storage, is negatively correlated to dissolved oxygen, pH, and soil water temperature. In addition, Non-Metric Multidimensional Scaling analysis shows a positive relation between plant community composition, SOC content, and water electric conductivity. Our results suggest a three-way interaction between hydrological, soil, and vegetation characteristics in the natural bofedal, which also holds for the artificial bofedal. Vegetation cover of two of the most highly nutritious species for alpaca, Lachemilla diplophylla and Lilaeopsis macloviana with 19-22% of crude protein, are weakly or not correlated to environmental variables, suggesting grazing might be obscuring these potential relationships. Given the high economic importance of alpaca breeding for local communities, expanding bofedales artificially appears an effective strategy to enhance their ecosystem services with minimal impact on the ecohydrological properties of bofedales

A review of the literature on the relationships between trees, land use, and hydrological processes in the Andes

In the Andean region, interest is growing in the use of trees to restore degraded areas through afforestation, reforestation or agroforestry. In addition to the economic benefits that these interventions can generate, particularly in the case of commercial plantations with exotic trees, one of the main arguments used to support these actions has been their purportedly positive effects on the recovery of the water provision and regulation functions of ecosystems. However, the relationship between tree cover and water is complex and may have positive or negative effects on the ecological functions of watersheds, depending on diverse factors. In this study, we compiled and synthetized the literature on the relationships between woody plants, land use and hydrologic processes in the Andes. The results showed that there is very limited and fragmented knowledge, with significant gaps in specific areas. While most studies focus on native forests and on agricultural land uses, there is very little research on paramos (Andean moorlands) or on agroforestry, the latter limited to coffee agroforestry. Furthermore, the results from different studies are often not comparable and are sometimes contradictory because of varying research design and methods. Our qualitative review highlighted the importance of montane cloud forests in water provision, in particular through the interception of horizontal precipitation. In addition, native forests play an important role in water regulation in comparison with crop and pasture lands, as they reduce runoff and store more water. Our results also showed that paramos provide higher water regulation than forested areas with exotic species and other land covers by maintaining base flows. Plantations with exotic taxa, such as pines and eucalypts, provide overall lower water provision than other land uses. However, their effect on water regulation defies simple comparisons with other uses: they have lower base flow and higher peak flow than other uses, and lower infiltration and water storage than forests and paramos, but perform better in these respects than natural pastures. The reviewed studies indicated that water provisioning from coffee agroforestry systems is lower than in coffee systems without shade, but that these levels vary according to the tree species. The tree species, and in particular the management practices, influence the water regulatory role of coffee agroforestry systems with shade as compared to systems without shade: while run-off is higher in systems with exotic taxa (e.g. pine, eucalypts), the level of infiltration varies according to management and leaf characteristics. Even though there remain important knowledge gaps on the role of forests and trees in the hydrology of Andean watersheds, especially regarding the combined impacts of land use and land cover change, our review results can help inform degraded land restoration practices and policies in the Andean region

Impacts of forests and forestation on hydrological services in the Andes: A systematic review

Several Andean countries have planned to restore forest cover in degraded land to enhance the provision of multiple ecosystem services in response to international commitments such as the Bonn Challenge. Hydrological services, e.g. water supply, hydrological regulation and erosion mitigation, are particularly important to sustain the life of more than fifty million Andean people. While rapid and important forest cover changes have occurred during recent decades, critical information on the impact of forestation on hydrological services has not yet been synthesized in the context of Andean ecosystems. We define forestation as the establishment of forest by plantation or natural regeneration on areas that either had forest in the past or not. To help improve decision-making on forestation in the Andes, we reviewed the available literature concerning the impacts of forestation on water supply, hydrological regulation and mitigation of erosion and landslides. We also examined available data on the most relevant hydrological processes such as infiltration, evapotranspiration and runoff in forest stands. Hydrological services from native forests were also included as a reference state for comparing processes and services provided by forestation. Following systematic review protocols, we synthesized 155 studies using different methods, including meta-analyses and meta-regressions. Results show that forestation has had clear impacts on degraded soils, through reducing water erosion of soils and risk of moderate floods, increasing soil infiltration rate by 8 and topsoil organic matter (SOM). We found that 20 years of tree plantation was sufficient to recover infiltration rate and sediment yield close to the levels of native forests whereas SOM, soil water storage and surface runoff of native forests could not be recovered by forestation in the time scales examined. The benefits in terms of hydrological regulation are at the expense of a reduction in total water supply since forest cover was associated with higher water use in most Andean regions. Forestation with native species was underrepresented in the reviewed studies. The impact of forestation on landslides has also been largely overlooked in the Andes. At high elevations, exotic tree plantations on Andean grasslands (e.g. páramo and puna) had the most detrimental consequences since these grasslands showed an excellent capacity for hydrological regulation and erosion mitigation but also a water yield up to 40% higher than tree plantations. People engaged in forest restoration initiative should be aware that hydrological services may take some time for society and the environment to show clear benefits after forestation

Contribution of high-andean ecosystems in providing the water regulation ecosystem service

El servicio ecosistémico de regulación hídrica es uno de los más importantes que ofrecen los ecosistemas altoandinos. Sin embargo, el conocimiento sobre su aporte en términos hídricos es aún escaso y su estimación difícil, debido a los complejos procesos eco-hidrológicos, climáticos y características ambientales de los Andes. Por ello, se estimó la influencia de tres tipos de ecosistemas (pajonal de puna húmeda, bofedal y bosque de Polylepis) sobre flujos de agua, particularmente los flujos que se dirigen hacia el almacenamiento subterráneo, que equivalen al servicio ecosistémico de regulación hídrica. Dicho estudio se desarrolló durante el año hidrológico 2018 - 2019 en la unidad hidrográfica de Rontoccocha, entre los 3 900 a 4 635 msnm, en el Departamento de Apurímac, Perú. Para ello se modeló el balance hídrico por cada tipo de ecosistema con la herramienta eco-hidrológica Hydrobal. Se usaron variables de: a) vegetación, b) parámetros climáticos y c) características del suelo. Los resultados revelan el aporte de la cobertura vegetal en la regulación hídrica. En cada ecosistema, cerca del 15% de toda la lluvia que precipita en la cuenca logra llegar al almacenamiento subterráneo. Dichos datos extrapolados a toda la unidad hidrográfica, muestran que el pajonal de puna húmeda regula el 80%, el bofedal el 17% y el bosque de Polylepis el 3%. A pesar de que la evaluación se realizó por separado para cada ecosistema, para fines de gestión, es necesario abordarlo de manera integral dado que entre ellos se establecen relaciones de interdependencia

Impactos de las zanjas de infiltración en el agua y los suelos: ¿Qué sabemos?

Las intervenciones de manejo de la infraestructura natural para la seguridad hídrica gozan de un interés creciente a nivel nacional e internacional para enfrentar problemas relacionados al agua o al cambio climático. En muchas partes del mundo, se han implementado tecnologías de conservación de agua y suelos, como andenes, cercos vivos, control de cárcavas, zanjas de infiltración, terrazas de formación lenta, reforestación, entre otras. En este contexto, es importante evaluar las experiencias pasadas y actuales para entender mejor cómo las intervenciones en la infraestructura natural pueden contribuir a aumentar la seguridad hídrica y mejorar las condiciones de vida y las actividades productivas de las comunidades