The Andes of South America are one of the most biologically rich and diverse regions in the world. They are also undergoing significant change, not least in terms of land cover. In order to build a better picture of the extent and impact of this, new MRI-supported research published in the journal Global Change Biology evaluates the distribution of woody vegetation in the tropical Andes between 2001 and 2014.

“The tropical Andes are particularly important in terms of biodiversity and ecosystem services,” says study author and MRI SLC member Ricardo Grau, Director of the Institute of Regional Ecology at the National University of Tucumán, Argentina. “Yet, in part due to the difficulties caused by rough topography and high cloud cover, there were no comprehensive assessments of vegetation cover change – so we aimed to fill this research gap.”

Addressing this gap was important for a number of reasons. The interactions between climate and land-use change are dictating the distribution of flora and fauna, and reshuffling biotic community composition around the world. Land‐cover change, particularly the distribution of woody vegetation, has significant implications for biodiversity conservation and the provision of vital ecosystem services such as watershed and soil protection, carbon sequestration, and food production. The tropical Andes are particularly sensitive to this due to relatively high human population density, a long history of agriculture, range-restricted species, and high beta diversity as a result of steep elevational gradients.

“This research is a comprehensive and updated assessment of woody vegetation cover change in one of the most important mountain regions globally, and woody cover is a key – although incomplete – variable of regional environmental change,” Grau explains. 

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The tropical Andes are one of the most biologically rich and diverse regions in the world.

Perceptions of change

The researchers created annual land-cover / land-use maps using MODIS satellite data at a resolution of 250 m pixels, and then divided these maps into 9,274 hexagons of 115.47 km2 each. They then calculated the cover of woody vegetation (trees and shrubs) within these hexagons for the period 2001-2014 in order to determine whether there was a statistically significant 14-year linear trend of gain or loss. The results of this were then assessed and validated in a second step, as Grau explains. “A particular component of this analysis was the input of regional experts, which complemented the remote sensing data – and for which Mountain Research Initiative was instrumental.”

In 2017, the MRI funded a synthesis workshop that brought together experts on different aspects of land science and ecosystem services in the Andes of Venezuela, Colombia, Perú, Bolivia, and northern Argentina. During this workshop, the results of the MODIS satellite data analysis were compared with overall perceptions of change among these local experts, changes documented in literature, and changes identified in local analyses based on other images of better spatial resolution. “The MRI was the main funding source for this workshop with national experts, which provided the local knowledge needed to assess the validity of the remote sensing analysis,” says Grau. “The MRI also provided financial support to make our paper publicly available.”

Growth and decline

Based on the MODIS satellite data analysis, the researchers identified 1,308 hexagons with significant trends – among which 36.6 percent lost forests and 63.4 percent gained forests. Overall, they estimated a net gain of approximately 500,000 hectares of woody vegetation. Patterns of woody cover change varied along the elevation gradient and among countries, with forest loss generally found to be more prevalent in the 1,000–1,499 metre elevation zone, while forest gain dominated above 1,500 metres. The most significant transitions were forest loss to pastures and croplands at lower elevations, forest gain in abandoned pastures and cropland in mid‐elevation areas, and shrub encroachment into highland grasslands as temperatures increase.

The expert validation conducted during the synthesis workshop confirmed these observed trends. However, some areas of apparent forest gain were associated with new shade coffee, pine, or eucalyptus plantations. In addition, after controlling for elevation and country, forest gain was associated with a decline in the rural population.

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Agriculture and secondary forests are key players in the land use dynamics of the tropical Andes. Pictured above, a coca plantation in the Bolivian Yungas (Photo: Ricardo Grau).

Future studies, future impacts

Despite documenting an overall gain in forest cover, the researchers stress that a recent reversal in forest gains in Colombia demonstrates that these coupled natural‐human systems are highly dynamic – and that there is therefore urgent need for a regional real‐time land‐use, biodiversity, and ecosystem services monitoring network.

Looking to future research in this area, Grau highlights the importance of further detailed analysis. “This is a descriptive study of the elevational and transnational patterns of land cover change, but it needs to be complemented with field-based assessments of socioeconomic and other drivers of these changes, and also more detailed analysis of their ecological consequences.”

“The Andes are undergoing changes that are important for biodiversity, peoples’ livelihoods, and ecosystem services,” concludes Grau. “This paper provides the first regional scale comprehensive analysis, and opens avenues for more detailed research on the causes, consequences, and local particularities of land cover change in one of the most important mountain systems globally in terms of natural and cultural value.”

READ MORE: Aide T. M., Grau H. R., Graesser J., et al. ‘Woody vegetation dynamics in the tropical and subtropical Andes from 2001 to 2014: Satellite image interpretation and expert validation.’ Global Change Biology (2019):

This project was funded by a grant from the Dynamics of Coupled Natural and Human Systems program of the U.S. National Science Foundation (# 0709598) to TMA and by a grant to H. Ricardo Grau and CONDESAN from the Mountain Research Initiative to fund the Andean expert workshop. The expert workshop received extra funding from the EcoAndes Project, implemented by CONDESAN and funded by the Global Environmental Fund, UN Environment, and the Swiss Agency for Development and Cooperation.

Cover image: Alejandro Miranda

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