Land cover classification of treeline ecotones along a 1100 km latitudinal transect using spectral‐ and three‐dimensional information from UAV‐based aerial imagery
Abstract:The alpine treeline ecotone is expected to move upwards in elevation with global warming. Thus, mapping treeline ecotones is crucial in monitoring potential changes. Previous remote sensing studies have focused on the usage of satellites and aircrafts for mapping the treeline ecotone. However, treeline ecotones can be highly heterogenous, and thus the use of imagery with higher spatial resolution should be investigated. We evaluate the potential of using unmanned aerial vehicles (UAVs) for the collection of ul… Show more
“…are common in the ground layer. Detailed information about the vegetation at our study sites is given by Mienna et al 34 . Figure 1 Map showing the 14 study sites located along a 500 km latitudinal gradient in northern Norway.…”
As shrubs and trees are advancing into tundra ecosystems due to climate warming, litter input and microclimatic conditions affecting litter decomposition are likely to change. To assess how the upward shift of high-latitude treeline ecotones might affect soil organic carbon stocks (SOC), we sampled SOC stocks in the surface layers of 14 mountain birch forest-tundra ecotones along a 500 km latitudinal transect in northern Norway. Our objectives were to examine: (1) how SOC stocks differ between forest and tundra soils, and (2) the relative role of topography, vegetation and climate in explaining variability in SOC stock sizes. Overall, forest soils had higher SOC stocks (median: 2.01 kg m−2) than tundra soils (median: 1.33 kg m−2). However, SOC storage varied greatly within and between study sites. Two study sites had higher SOC stocks in the tundra than in the nearby forest, five sites had higher SOC stocks in the forest, and seven sites did not show differences in SOC stocks between forest and tundra soils. Thus, our results suggest that an upwards forest expansion does not necessarily lead to a change in SOC storage at all sites. Further, a partial least-squares regression (PLSR) model indicated that elevation, temperature, and slope may be promising indicators for SOC stock size at high-latitude treelines. Precipitation and vegetation were in comparison only of minor importance.
“…are common in the ground layer. Detailed information about the vegetation at our study sites is given by Mienna et al 34 . Figure 1 Map showing the 14 study sites located along a 500 km latitudinal gradient in northern Norway.…”
As shrubs and trees are advancing into tundra ecosystems due to climate warming, litter input and microclimatic conditions affecting litter decomposition are likely to change. To assess how the upward shift of high-latitude treeline ecotones might affect soil organic carbon stocks (SOC), we sampled SOC stocks in the surface layers of 14 mountain birch forest-tundra ecotones along a 500 km latitudinal transect in northern Norway. Our objectives were to examine: (1) how SOC stocks differ between forest and tundra soils, and (2) the relative role of topography, vegetation and climate in explaining variability in SOC stock sizes. Overall, forest soils had higher SOC stocks (median: 2.01 kg m−2) than tundra soils (median: 1.33 kg m−2). However, SOC storage varied greatly within and between study sites. Two study sites had higher SOC stocks in the tundra than in the nearby forest, five sites had higher SOC stocks in the forest, and seven sites did not show differences in SOC stocks between forest and tundra soils. Thus, our results suggest that an upwards forest expansion does not necessarily lead to a change in SOC storage at all sites. Further, a partial least-squares regression (PLSR) model indicated that elevation, temperature, and slope may be promising indicators for SOC stock size at high-latitude treelines. Precipitation and vegetation were in comparison only of minor importance.
“…Mountain birch is the dominating treeline-forming species, but other tree species such as Norway spruce (Picea abies), Scots pine (Pinus sylvestris), rowan (Sorbus aucuparia), and aspen (Populus tremula) are present. The vegetation varies within and between sites, with vegetation types such as ridges, lee sides, and snowbeds typically being found across the whole transect (Mienna et al, 2022b). The mountainous areas in Norway have a long land use history, and it is likely that the current treeline positions at the sites are legacies of earlier land use (Bryn & Potthoff, 2018).…”
Section: Study Sitesmentioning
confidence: 99%
“…The mountainous areas in Norway have a long land use history, and it is likely that the current treeline positions at the sites are legacies of earlier land use (Bryn & Potthoff, 2018). More background information about the transect and study sites can be found in Thieme et al (2011), Mienna et al (2020), andMienna et al (2022b).…”
Section: Study Sitesmentioning
confidence: 99%
“…The average segment size was 1.33 m 2 (min: 0.0003 m 2 , max: 386.74 m 2 ). More information on the UAV surveys and segmentation process can be found in Mienna et al (2022b).…”
Section: Remote Sensing Data Acquisition and Processingmentioning
confidence: 99%
“…For the vegetation variables, we had two types of variables: land cover and vegetation height. We acquired land cover probabilities on segment level from Mienna et al (2022b) for the land cover classes in the ridge-snowbed gradient (five classes), tree-covered area (segments with tree stem height >2 m), wetland, and rock. To represent vegetation height, we calculated the mean and standard deviation of the normalized UAV point clouds on segment level.…”
Section: Predictors Of Tree Establishmentmentioning
The transition zone between the upper forest boundary and the tree-less
alpine vegetation (i.e., the treeline ecotone) is expected to shift
upwards in elevation with global warming. Multiple factors in addition
to climate are influencing tree establishment, but it is uncertain to
what degree. In this study, we took advantage of a unique dataset with
field measurements of mountain birch trees acquired from 32
boreal-alpine treeline ecotone sites situated along an 1,100 km
latitudinal transect in Norway to investigate the role of climate,
herbivory, topography, and vegetation on the establishment success of
trees. To represent tree establishment, we recorded the occurrence,
survival, and growth of short juvenile trees (stem height ≤1 m) at each
site in 2008, 2012, and 2018. In addition to using already available
datasets on climate and herbivory at each site, we used airborne laser
scanning (ALS) and unmanned aerial vehicle (UAV) surveyed imagery
together with field measurements to compute topography and vegetation
predictor variables. We used GLMM-LASSO regression to quantify the role
of the predictor variables on the three responses. We found that short
juvenile trees do not necessarily occur, survive, and grow within the
same conditions. In general, short juvenile tree establishment decreased
with an increase in herbivore livestock density. Winter precipitation
and the two vegetation types tree-covered area and lee side were also
important for tree establishment, but if the relationships were positive
or negative differed between tree occurrence, survival, and growth. Our
results suggest that treeline ecotones will shift towards higher
elevations in areas with little or no livestock herbivory and close to
already established trees and in lee sides. In areas with less winter
precipitation, the treeline ecotones may be more stable.
The transition zone between the upper forest boundary and the treeless alpine vegetation (i.e., the treeline ecotone) is expected to shift upward in elevation with global warming. Multiple factors in addition to climate are influencing tree establishment, but it is uncertain to what degree. In this study, we took advantage of a unique dataset with field measurements of mountain birch trees acquired from 32 boreal–alpine treeline ecotone sites situated along a 1100‐km latitudinal gradient in Norway to investigate the role of climate, herbivory, topography, and vegetation on the establishment success of trees. To represent tree establishment, we recorded occurrence, survival, and height growth of short juvenile birch trees (stem height ≤1 m) as response variables at each site in 2008, 2012, and 2018. We used already available climate and herbivory data, and airborne laser scanning (ALS) and unmanned aerial vehicle (UAV) surveyed imagery together with field measurements to compute topography and vegetation explanatory variables per site on microsite scale (approx. 1 m2). We observed a clear decrease in short juvenile birch tree establishment due to livestock herbivory, evidenced by lower occurrence, reduced survival rate, and diminished height growth in treeline ecotones with higher livestock densities. Winter precipitation and the two land cover classes, tree‐covered area and lee side vegetation, were also important for tree establishment. However, the direction and magnitude of the relationships depended on the stage of establishment. While winter precipitation can lead to higher tree occurrence, long‐term survival is not necessarily ensured, reflecting diverse requirements for optimal conditions of tree establishment. Thus, our results show that occurrence, survival, and height growth of short juvenile birch trees differ in their requirements to climate, topography, and vegetation, but tree establishment is consistently low with livestock herbivory. Overall, our results suggest that treeline ecotones will shift toward higher elevations in areas with little or no livestock herbivory and close to already established trees and in lee sides. Regarding management and conservation issues, our results suggest that free‐roaming livestock can reduce tree establishment and hinder treeline ecotone shifts into threatened alpine ecosystems, at least for birch‐dominated treelines.
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