Modelling above-ground carbon dynamics using multi-temporal airborne lidar: insights from a Mediterranean woodland

Abstract

Abstract. Woodlands represent highly significant carbon sinks globally, though could lose this function under future climatic change. Effective large-scale monitoring of these woodlands has a critical role to play in mitigating for, and adapting to, climate change. Mediterranean woodlands have low carbon densities, but represent important global carbon stocks due to their extensiveness and are particularly vulnerable because the region is predicted to become much hotter and drier over the coming century. Airborne lidar is already recognized as an excellent approach for high-fidelity carbon mapping, but few studies have used multi-temporal lidar surveys to measure carbon fluxes in forests and none have worked with Mediterranean woodlands. We use a multitemporal (5-year interval) airborne lidar data set for a region of central Spain to estimate above-ground biomass (AGB) and carbon dynamics in typical mixed broadleaved and/or coniferous Mediterranean woodlands. Field calibration of the lidar data enabled the generation of grid-based maps of AGB for 2006 and 2011, and the resulting AGB change was estimated. There was a close agreement between the lidarbased AGB growth estimate (1.22 Mg ha −1 yr −1 ) and those derived from two independent sources: the Spanish National Forest Inventory, and a tree-ring based analysis (1.19 and 1.13 Mg ha −1 yr −1 , respectively). We parameterised a simple simulator of forest dynamics using the lidar carbon flux measurements, and used it to explore four scenarios of fire occurrence. Under undisturbed conditions (no fire) an accelerating accumulation of biomass and carbon is evident over the next 100 years with an average carbon sequestration rate of 1.95 Mg C ha −1 yr −1 . This rate reduces by almost a third when fire probability is increased to 0.01 (fire return rate of 100 years), as has been predicted under climate change. Our work shows the power of multi-temporal lidar surveying to map woodland carbon fluxes and provide parameters for carbon dynamics models. Space deployment of lidar instruments in the near future could open the way for rolling out wide-scale forest carbon stock monitoring to inform management and governance responses to future environmental change.

Dynamics of a human‐modified tropical peat swamp forest revealed by repeat lidar surveys

“…Repeat lidar survey is increasingly used to measure biomass change in different forest types, including those in boreal (Bollandsås et al, 2018; Bollandsås, Gregoire, Næsset, & Øyen, 2013; McRoberts et al, 2015; Næsset, Bollandsås, Gobakken, Gregoire, & Ståhl, 2013; Økseter, Bollandsås, Gobakken, & Næsset, 2015), temperate (Skowronski, Clark, Gallagher, Birdsey, & Hom, 2014), Mediterranean (Simonson et al, 2015) and tropical (Boehm, Liesenberg, & Limin, 2013; Cao et al, 2016; Dubayah et al, 2010; Englhart et al, 2013; Meyer et al, 2013; Réjou‐Méchain et al, 2015; Silva et al, 2017) regions, but changes in sensor and flight specifications between surveys make change detection a challenge. Our data thinning is effective at removing biases in canopy height estimation (Næsset, 2009; Simonson et al, 2015). Ideally, the ground points from both acquisitions are combined to create a common DTM (Réjou‐Méchain et al, 2015), allowing the digital surface models to normalized against the same DTM, but even without using that approach, field versus lidar estimates of biomass are close (Figure 4a).…”

“…the same spatial resolution as the field plots) using the raster package in R, and the regression model was used to calculate the AGB of each plot, applying the Baskerville correction to deal with non‐normality of errors (Baskerville, 1972). AGB was mapped with the 2011 and 2014 imagery using the same equation, and AGB change was calculated for each virtual 32 × 32 m plot as (AGB 2014 − AGB 2011 )/3 (Simonson et al, 2015).…”

“…Repeat lidar survey is increasingly recognized as a powerful approach for monitoring forest dynamics over tropical landscapes, and understanding the drivers of change (Englhart et al, 2013). Repeated lidar surveys are starting to be used to study AGB dynamics in forests (Dubayah et al, 2010; Meyer et al, 2013; Réjou‐Méchain et al, 2015; Simonson, Ruiz‐Benito, Valladares, & Coomes, 2015), and few efforts have been made to relate AGB changes to complex logging and fire disturbance history (Andersen et al, 2014; Englhart et al, 2013; Sato et al, 2016). Studies focused on AGB dynamics have generally relied on one of two main approaches.…”

Improving Individual Tree Crown Delineation and Attributes Estimation of Tropical Forests Using Airborne LiDAR Data

“…Individual tree-based mapping can act as an excellent tool for this. Secondly, tracking the dynamics of the largest trees can account for a large carbon fraction in tropical forests that are especially vulnerable to drought, acting as an early warning signal to climate change [62,63]. Frequent LiDAR surveys could be valuable for this, but only the area-based approach has been used to date [63,64].…”

Fusion of NASA Airborne Snow Observatory (ASO) Lidar Time Series over Mountain Forest Landscapes

“…Its ability to characterize fine scale features such as tree counting or low vegetation density have been demonstrated over different ecosystems [8,9]. However, the high cost of airborne lidar acquisition has been a limitation to surveying large areas with reasonable temporal resolution in order to study forest disturbance and regeneration [10][11][12][13][14][15][16].…”