Airborne and Spaceborne Lidar Reveal Trends and Patterns of Functional Diversity in a Semi-Arid Ecosystem

Abstract

Assessing functional diversity and its abiotic controls at continuous spatial scales are crucial to understanding changes in ecosystem processes and services. Semi-arid ecosystems cover large portions of the global terrestrial surface and provide carbon cycling, habitat, and biodiversity, among other important ecosystem processes and services. Yet, the spatial trends and patterns of functional diversity in semi-arid ecosystems and their abiotic controls are unclear. The objectives of this study are two-fold. We evaluated the spatial pattern of functional diversity as estimated from small footprint airborne lidar (ALS) with respect to abiotic controls and fire in a semi-arid ecosystem. Secondly, we used our results to understand the capabilities of large footprint spaceborne lidar (GEDI) for future applications to semi-arid ecosystems. Overall, our findings revealed that functional diversity in this ecosystem is mainly governed by elevation, soil, and water availability. In burned areas, the ALS data show a trend of functional recovery with time since fire. With 16 months of data (April 2019-August 2020), GEDI predicted functional traits showed a moderate correlation (r = 41–61%) with the ALS predicted traits except for the plant area index (PAI) (r = 11%) of low height vegetation (<5 m). We found that the number of GEDI footprints relative to the size of the fire-disturbed areas (=< 2 km2) limited the ability to estimate the full effects of fire disturbance. However, the consistency of diversity trends between ALS and GEDI across our study area demonstrates GEDI’s potential of capturing functional diversity in similar semi-arid ecosystems. The capability of spaceborne lidar to map trends and patterns of functional diversity in this semi-arid ecosystem demonstrates its exciting potential to identify critical biophysical and ecological shifts. Furthermore, opportunities to fuse GEDI with complementary spaceborne data such as ICESat-2 or the upcoming NASA-ISRO Synthetic Aperture Radar (NISAR), and fine scale airborne data will allow us to fill gaps across space and time. For the first time, we have the potential to monitor carbon cycle dynamics, habitats and biodiversity across the globe in semi-arid ecosystems at fine vertical scales.

Improving GEDI Footprint Geolocation using a High Resolution Digital Terrain Model

“…Note that GeoGEDI results are in the same range as Hancocks' waveform matching approach. After correcting v1 for geolocation, Ilangakoon [22] and Lang [11] observed 4.69 m and 3.6 m GEDI surface heights RMSE for their study sites, respectively, while v1-based GeoGEDI reached 4.47 to 6.65 m RMSEs. For ground elevations, after correcting v2, Liu [16] observed a 4.03 m RMSE value, while GeoGEDIs range from 0.79 (non-forest, Landes) to 2.59 m (forest, Vosges).…”

“…The method processes by successive footprint clusters along individual ground tracks and a corrected geolocation is assigned where correlation between simulated and actual GEDI waveforms is maximized [11], [21]. Different studies used this approach to improve either v1 [11], [22] or v2 [16] data. Lang [11] compared GEDI derived canopy heights with ALS heights, after geolocation correction, and obtained a 3.6 m root mean square error (RMSE) and a -0.3 m bias, while RMSE dropped to 2.7 m and bias to -0.1 m for 70 % most certain position predictions, i.e., highest correlations between real and simulated waveforms.…”

Future Direction of Fuels Management in Sagebrush Rangelands

“…A wealth of new satellite sensors, including spaceborne lidar (GEDI, ICESat-2) and thermal (ECOSTRESS), are available to quantify ecological structure and function. A recent study suggests a relationship between ecological structure and functional diversity is driven by elevation, soil, and water availability in the sagebrush steppe using GEDI ( Ilangakoon et al 2021 ). Poulos et al (2021) relates postfire evapotranspiration with vegetation recovery and suggests that ECOSTRESS may be used for understanding water balance and other postfire assessments in dry lands.…”

Postfire recovery of western US conifer forests (1984-2017) using space-borne lidar data

“…Most forest ecosystems recover their vegetation/carbon within decades following a disturbance event [11]. However, disturbance legacies can persist longer, causing ecosystem state transitions with altered vegetation structure, composition, and functioning [12]- [14]. Thus, quantifying postfire vegetation state changes and recovery rates is therefore essential to understand how quickly a fire-disturbed ecosystem can initiate vegetation regeneration and recovery to its initial state, the possibility of ecosystem state transitions [14], [15], and how fires influence the global C budget [16], as well as forest structure and functioning.…”

“…Active remote sensing techniques such as lidar have been used to characterize postfire vegetation structure, aboveground biomass, and carbon stocks at various spatial scales [13], [22]- [24] . In such studies canopy structural metrics such as canopy height, cover, and plant area index (PAI) have been used either with allometric equations to calculate biomass or as a proxy to represent the carbon potential [25].Further, these vegetation structural metrics have been used to infer postfire recovery by measuring the changes in the structure after a fire with respect to the preburn state [14]. In this study, we use canopy structural metrics from the first ever highresolution spaceborne lidar sensor in the international space station, Global Ecosystem Dynamics Investigation (GEDI) [26] along with three fire chronosequences representing three main ecoregions (Pacific Northwest (PNW), Northern Rockies (NR), and Southern Rockies and Colorado Plateau (SR)) to answer the following questions:…”