[A]round 1,000 years ago, both stalagmites' calcium carbonate composition shifted suddenly and completely, from carbon isotope ratios typical of trees and shrubs, to those more consistent with grassland, within just 100 years.From here.
The paper is Stephen J. Burns, et al., "Rapid human-induced landscape transformation in Madagascar at the end of the first millennium of the Common Era.", Quaternary Science Reviews (2016).
UPDATE March 2, 2016: The abstract to the paper reveals that the dates are actually a bit earlier than 1,000 years ago, which helps to reconcile the archaeological record which favors a date somewhat earlier than 1000 CE for Austronesian arrival in Madagascar and this new data point. The abstract states:
The environmental impact of the early human inhabitants of Madagascar remains heavily debated. We present results from a study using two stalagmites collected from Anjohibe Cave in northwestern Madagascar to investigate the paleoecology and paleoclimate of northwestern Madagascar over the past 1800 years. Carbon stable isotopic data indicate a rapid, complete transformation from a flora dominated by C3 plants to a C4 grassland system. This transformation is well replicated in both stalagmites, occurred at 890 CE and was completed within one century. We infer that the change was the result of a dramatic increase in the use of fire to promote the growth of grass for cattle fodder. Further, stalagmite oxygen isotope ratios show no significant variation across the carbon isotope excursion, demonstrating that the landscape transformation was not related to changes in precipitation. Our study illustrates the profound impact early inhabitants had on the environment, and implies that forest loss was one trigger of megafaunal extinction.It also isn't implausible to think that the use of fire to cover forest to grassland wouldn't have begun immediately upon their arrival. They would have used the modest amount of existing grassland on the island at first and would have cleared more land only after their growing herds strained the carrying capacity of the existing grasslands.