Abstract
The hemlock woolly adelgid (HWA; Adelges tsugae) is an invasive insect infestation that is spreading into the forests of the northeastern United States, driven by the warmer winter temperatures associated with climate change. The initial stages of this disturbance are difficult to detect with passive optical remote sensing, since the insect often causes its host species, eastern hemlock trees (Tsuga canadensis), to defoliate in the midstory and understory before showing impacts in the overstory. New active remote sensing technologies—such as the recently launched NASA Global Ecosystem Dynamics Investigation (GEDI) spaceborne lidar—can address this limitation by penetrating canopy gaps and recording lower canopy structural changes. This study explores new opportunities for monitoring the HWA infestation with airborne lidar scanning (ALS) and GEDI spaceborne lidar data. GEDI waveforms were simulated using airborne lidar datasets from an HWA-infested forest plot at the Harvard Forest ForestGEO site in central Massachusetts. Two airborne lidar instruments, the NASA G-LiHT and the NEON AOP, overflew the site in 2012 and 2016. GEDI waveforms were simulated from each airborne lidar dataset, and the change in waveform metrics from 2012 to 2016 was compared to field-derived hemlock mortality at the ForestGEO site. Hemlock plots were shown to be undergoing dynamic changes as a result of the HWA infestation, losing substantial plant area in the middle canopy, while still growing in the upper canopy. Changes in midstory plant area (PAI 11–12 m above ground) and overall canopy permeability (indicated by RH10) accounted for 60% of the variation in hemlock mortality in a logistic regression model. The robustness of these structure-condition relationships held even when simulated waveforms were treated as real GEDI data with added noise and sparse spatial coverage. These results show promise for future disturbance monitoring studies with ALS and GEDI lidar data.
Highlights
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2012Atpol2o0t1o6f,tihnedvicearticnagl spormofielegorof wPAthI cinhatnhgeeofvoerrbstootrhytdheureiansgtetrhnishteimmleocpkerainodd.the red oak footprints showed that the trend in plant area index (PAI) loss in the mid-canopy is ubiquitous within the infested hemlocks of the ForestGEO plot (Figure 5)
While we focus on exploring the utility of Global Ecosystem Dynamics Investigation (GEDI) and airborne lidar scanning (ALS) for monitoring structural changes associated with hemlock woolly adelgid (HWA), the general method applied in this study could be expanded to other disturbances in temperate or tropical forests around the world
Summary
InInsseeccttininfefestsatatitoionnsscacuauseseunuinqiuqeuechcahnagnegseisninthtehsetrsutrcutuctrueroef ofof rfeosrtessotsveorvteirmteim, oep, eonpiennginugp ugpapgsaipns thine ctahneopcaynaonpdytraigngdertinriggggerroiwngth garmoowntgh uanmdeornsgtoruynsdpeercsiteosry[1].spAecctiievse r[e1m]. oAtectsievnesinregminostterusmenesnitnsg, siuncshtrausmspenactse,bsourcnhe asndspaaicrbeboornrneeliadnadr saciarnbnoirnnge (lAidLaSr)sicnasntrnuinmge(nAtsL, Sca) ninpsetrnuemtraentetsd,ecnasnepfoerneesttrcaatenodpeinesse tofoorebstecrvaneothpeiesse tsouobbtlseecrhvaenthgeseinsuvbetrlteiccahlasntrguecstuinrev.eTrthicuasl, slitdruarcthuarse.pTohteunst,ilaildtaor dheatsepctothenetsipalretoaddeatnedct tothme sopnrietoardtahnedsetvoemritoynoitfoirntsheectseinvfeersitaytioofninimsepctacintsfeasttathtieolnanimdspcaacptse astcathlee. landscape scale. Emitted light reflects off target objects and returns to the instrument, which records the distribution of returning energy over time, a waveform, to measure the range to targets. Small-footprint lidar instruments sample a small area with each beam, from the millimeter to decimeter scale. A large-footprint lidar instrument might measure a stand of trees with a single beam, capturing vertical structure in a circular footprint of tens of meters in diameter. The recent launch of GEDI, a spaceborne lidar instrument designed to measure forest structure over large areas, presents new opportunities for monitoring forest disturbances with vertical profiles of forest structure [12]. Large-footprint waveform lidar data similar to GEDI (~25 m in diameter) have proven useful for observing distinct canopy structures related to disturbances. Large-footprint data have been used to infer past hurricane damage [25], to identify developmental stages of forest plots [26,27,28], to observe phenological cycles [23], and to estimate biomass change [29,30]
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