Abstract
Forest canopy openings are a key element of forest structure, influencing a host of ecological dynamics. Light detection and ranging (LiDAR) is the de-facto standard for measuring three-dimensional forest structure, but digital aerial photogrammetry (DAP) has emerged as a viable and economical alternative. We compared the performance of LiDAR and DAP data for characterizing canopy openings and no-openings across a 1-km2 expanse of boreal forest in northern Alberta, Canada. Structural openings in canopy cover were delineated using three canopy height model (CHM) alternatives, from (i) LiDAR, (ii) DAP, and (iii) a LiDAR/DAP hybrid. From a point-based detectability perspective, the LiDAR CHM produced the best results (87% overall accuracy), followed by the hybrid and DAP models (47% and 46%, respectively). The hybrid and DAP CHMs experienced large errors of omission (9–53%), particularly with small openings up to 20m2, which are an important element of boreal forest structure. By missing these, DAP and hybrid datasets substantially under-reported the total area of openings across our site (152,470 m2 and 159,848 m2, respectively) compared to LiDAR (245,920 m2). Our results illustrate DAP’s sensitivity to occlusions, mismatched tie points, and other optical challenges inherent to using structure-from-motion workflows in complex forest scenes. These under-documented constraints currently limit the technology’s capacity to fully characterize canopy structure. For now, we recommend that operational use of DAP in forests be limited to mapping large canopy openings, and area-based attributes that are well-documented in the literature.
Highlights
Canopy openings occur regularly and at various spatial scales throughout a forest, due to numerous exogenous and endogenous factors
Light detection and ranging (LiDAR) served as the basis for the derivation of three canopy height light demteoctdioenlnagltienrgna(LtiivDeAs:RL) isDeArvRed(LaisDtAhRe _bCaHsisMf)o,rDtAhePd(DerAivPa_tiCoHn Mof),tharnede acaLnioDpAyRh-eDigAhPt hybrid model ((CHHyMbr)ida_lCteHrnMat)i.vEevs:alLuiaDtiAoRn d(LatiDa AwRas_CgeHnMer)a,teDdAdPur(DinAgPa_fiCeHldMc)a,mapnadigan.LBiDoAthRa-nDAarPeah-byabsreidd as well (Hybrida_sCaHpMoi)n. tE-vbaalsueadtiaonnaldyastias wofatshgeerneesrualtteindgdoupreinnginagfwieledrecapmerpfoarigmne.dB.oDthetaanileadrefla-obwascehdaratss wfoerllthe data as a poipnrto-bcaessesidnganuasliynsgisLoAfStthoeorlsesaureltipnrgovoipdeendining twheerSeuppeprlfeomrmenetda.ryDeMtaaitleerdiafll.owcharts for the data processing using LAStools are provided in the supplementary material
When the opening maps were evaluated against our point-based validation data (Table 4), light detection and ranging (LiDAR) was found to produce the highest overall accuracy: 94%
Summary
Canopy openings occur regularly and at various spatial scales throughout a forest, due to numerous exogenous and endogenous factors. Open areas can experience markedly different biotic and abiotic conditions, including a locally increased supply of photosynthetically active radiation (e.g., sunflecks) [9,10]. If left undisturbed, these openings constitute ecologically important patches of pronounced tree recruitment and plant growth. Canopy openings support the process of regenerating forest vegetation, and help maintain meaningful biodiversity within the forest ecosystem [11,12,13]. The altered physical structure modifies spatial movement/habitat selection of fauna [14], influences hydrological processes [6], and affects local carbon cycling [15]
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