Study site

The study site is located in a large Amazonian rainforest fragment in Vera Cruz farm (14.833° S 52.168° W; plot code VCR-02 in the RAINFOR forest inventory network), Nova Xavantina, Mato Grosso, Brazil. The plot is located 0.8 km from the nearest forest edge. The vegetation of the site located at the forest-savanna ecotone, is a transitional Amazonian rainforest (Marimon et al., 2014) that is mostly (over 80%) characterised by typical Amazonian tree species but also containing some species more commonly found in the Cerrado biome. (see Morandi et al. (2016) for floristic description). Mean canopy height is 13.6 m, but the height of some trees can exceed 25 m. The soil is dystrophic, acidic and shallow plinthosol.
The site is at the southernmost dry limit of the Amazon rainforest, is highly seasonal and is characterised by an average annual rainfall of 1369 mm/year (recent 20-year average). Most of the rain falls during a six–month wet period (mid–October until April), which also experiences lower mean temperatures compared to the six–month long dry season (May–October). The peak of the dry season (August-October) coincides with the hottest time of the year, where maximum air temperatures (Tmax ) frequently exceed 40°C. The daily maximum temperatures have risen considerably over the last two decades (Figure 1a). The recent decade recorded 19 heatwave events (consecutive 3-day spells where maximum daily air temperatures exceed the 90th percentile level for the entire 1999-2008 period) compared to 13 events during the previous decade (Figure 1b); maximum air temperatures have exceeded 40°C 95 times during 2009–2018 compared to 29 times during the 1999–2008 period. The absolute highest Tmax recorded was 43.9°C (15 October 2017, coinciding with the hot-dry sampling period reported in this study)