## Methods
### Field Data  
Twenty-seven vegetation plots (25 m \(\times\) 25 m, 0.0625 ha) with North-South orientation were set out at SAFE in 2010 using a Garmin GPSMap60 device. Vegetation plots were established according to a hierarchical sampling design based around a triangular pattern to assess regional forest attributes [34]. Plots were located at roughly equal altitude = 400 m and oriented to minimise potentially confounding factors such as slope. Oil palm is generally grown on fairly flat surfaces under Roundtable on Sustainable Palm Oil guidelines, recommendations from the Malaysian Palm Oil Board, and Standards for Oil Palm Production. On top of that, production on flat surfaces is higher. Therefore, these conditions will commonly be shared in oil palm plantations across SE Asia and are therefore valid. Plots were distributed among three oil palm plantation blocks of two different ages (OP1 and OP2 planted in 2006 and OP3 planted in 2000), henceforth referred to as 8-year-old and 14-year-old plots. In each 0.0625 ha plot, all oil palm individuals had their stem height measured in 2013 and remeasured in 2015; we used the average height between these years (Figure 2). The average height and standard deviation (in metres) was 6.1 \(\pm\) 2.3 for OP1, 5.4 m \(\pm\) 1.5 for OP2 and 8.5 \(\pm\) 2.0 for OP3. Palms have vascular bundles embedded in their trunks, in contrast to most tree species which have meristem cells just beneath their bark, so trunks do not increase in diameter as the trees grow taller. Thus, as secondary thickening does not occur in palm trees, we did not include tree diameter in our field measurements or biomass equation.  
A 1-ha plot was created that was centred on each 0.0625-ha plot, to expand the plot size for aboveground carbon density calculations and minimize plot size effects on LiDAR model performance [15]. We then used Earth Imaging data from the Pleiades satellite constellation (EADS Astrium), acquired over the SAFE landscape in June 2016, to visually count the tree density within each 1-ha plot. Pleiades data comprises a 0.5-m resolution panchromatic band, and four spectral bands (blue, green, red, and near infrared) with a 2.8 m spatial resolution, resampled to 2 m (Figure 3a). The panchromatic band has sufficiently high resolution to distinguish the rows of trees in oil palm stands, as interrow spacing is approximately 8 m. The average height of each 1-ha plot was assumed to be the same as in the 0.0625 ha plots, embedded at the centre. We recognise that spatial autocorrelation magnitude could impact the 1-ha plot data; however, we emphasize that the plots are not being used for landscape sampling, rather they are intended for calibration and validation purposes only. Areas within oil palm are all planted at the same time and managed in the same way, and are therefore likely to be extremely homogeneous.  
Figure 1: Panel (**a**) shows the location of the Stability of Altered Forest Ecosystem (SAFE) landscape within Sabah, Malaysia and (**b**) the LiDAR Canopy Height Model (greener shades corresponding to higher heights in metres) of the SAFE Project landscape including 27 plots located in oil palm plantations.