Abstract
Agricultural expansion has resulted in both land use and land cover change (LULCC) across the tropics. However, the spatial and temporal patterns of such change and their resulting impacts are poorly understood, particularly for the presatellite era. Here, we quantify the LULCC history across the 33.9million ha watershed of Tanzania's Eastern Arc Mountains, using geo-referenced and digitized historical land cover maps (dated 1908, 1923, 1949 and 2000). Our time series from this biodiversity hotspot shows that forest and savanna area both declined, by 74% (2.8million ha) and 10% (2.9million ha), respectively, between 1908 and 2000. This vegetation was replaced by a fivefold increase in cropland, from 1.2million ha to 6.7million ha. This LULCC implies a committed release of 0.9PgC (95% CI: 0.4-1.5) across the watershed for the same period, equivalent to 0.3MgCha(-1) yr(-1) . This is at least threefold higher than previous estimates from global models for the same study area. We then used the LULCC data from before and after protected area creation, as well as from areas where no protection was established, to analyse the effectiveness of legal protection on land cover change despite the underlying spatial variation in protected areas. We found that, between 1949 and 2000, forest expanded within legally protected areas, resulting in carbon uptake of 4.8 (3.8-5.7) MgCha(-1) , compared to a committed loss of 11.9 (7.2-16.6) MgCha(-1) within areas lacking such protection. Furthermore, for nine protected areas where LULCC data are available prior to and following establishment, we show that protection reduces deforestation rates by 150% relative to unprotected portions of the watershed. Our results highlight that considerable LULCC occurred prior to the satellite era, thus other data sources are required to better understand long-term land cover trends in the tropics.
Highlights
Land cover is part of a constantly evolving dynamic anthropogenic-environment system with numerous complex drivers and impacts
We focus on the Tanzanian watershed of the Eastern Arc Mountains, which cover 33.9 million ha (Figure 1; see Swetnam et al (2011) for further details)
Twentieth Century Land Use/Land Cover Change Between 1908 and 2000, we estimate that 4.7 million hectares of forest and savanna vegetation within the Eastern Arc watershed (14% of total area) was converted to other land cover types, overwhelmingly to croplands, predominantly maize, the main staple, and cash crops like tobacco and coffee and tea (Börjeson, 2004)
Summary
Land cover is part of a constantly evolving dynamic anthropogenic-environment system with numerous complex drivers and impacts. Quantification of LULCC remains highly uncertain, across large spatial and temporal scales (Grainger, 2008). Remote sensing provides LULCC data of the last few decades, with Landsat constituting the longest temporal record (1972 onwards (Hansen & Loveland, 2012, Hansen et al, 2013)). Little is known about LULCC prior to the satellite era, in tropical regions, anthropogenic actions have resulted in landscape-scale changes for hundreds of years (Lewis & Maslin, 2015). Historical records in the tropics are rare so where, when and why past LULCC occurs is very uncertain for low latitude regions of the world (Kay & Kaplan, 2015)
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