Airborne Laser Scanning (ALS) generates accurate data for calculating forest metrics, such as canopy height, yet can be cost-prohibitive. Satellite-based stereo pair photogrammetry has the potential ...to overcome this limitation of ALS to facilitate multi-temporal change analysis when ALS data capture is unfeasible; however, it remains largely untested across Australian conservation and production eucalypt forests. This study examined root-mean-square differences (RMSD) between canopy height measurements derived from ALS, field measurements and satellite-based photogrammetry for a spotted gum (Corymbia citriodora) plantation and scribbly gum (Eucalyptus racemosa) woodland in south-east Queensland, Australia. The comparison found satellite-based photogrammetry under predicted canopy height compared to field measurements and ALS, whilst the RMSD indicated low performance for satellite-based photogrammetry across the eucalypt plantation and woodland. The open and heterogenous forest structure typical in eucalypt forests combined with low point cloud density for photogrammetry to inadequately sample the canopy and increase stereo matching errors; which was exacerbated across the open and heterogenous scribbly gum woodland. Current satellite-based photogrammetry is therefore unlikely to provide a viable alternative to ALS when analysing canopy height across eucalypt forests at high-resolution. General surface analysis across large areas of eucalypt forest at moderate resolution, or airborne photogrammetric methods, could demonstrate increased viability as an alternative to ALS.
The aim of this paper is to present a strategic forest inventory approach that has been applied on public land in Victoria, Australia. The Victorian Forest Monitoring System is an integral component ...of a monitoring and reporting program that enables Victoria to critically assess and evaluate its progress towards achieving its sustainable forest management objectives and targets. The inventory approach utilises field measurements in combination with remote sensing data. The approach is novel in that it utilises a relatively small sample size with a stratification scheme designed to examine questions around tenure and land management options. The small sample was dictated by limited resources (capacity, time and budget). The stratification scheme was designed to explore different land management approaches (e.g. National Parks versus State forest), as a more consistent and balanced approach to the management of public forests is currently being considered by most State Governments in Australia. The resultant accuracy of estimates for management and bioregion strata and associated characteristics, like above ground biomass from this small sample size were found to be sufficient for the regional monitoring goals of this study.
► We monitored hourly stem size changes for 4 years along a ∼6°C Alpine gradient. ► We isolate and investigate daily stem radius variations using a novel methodology. ► Variations in temperature, ...precipitation and sunshine modulate diurnal stem cycles. ► Spruce and larch reveal different strategies in balancing internal water reserves. ► Reversible stem fluctuations challenge interpretation of dendrometer growth records.
Climate affects the timing, rate and dynamics of tree growth, over time scales ranging from seconds to centuries. Monitoring how a tree's stem radius varies over these time scales can provide insight into intra-annual stem dynamics and improve our understanding of climate impacts on tree physiology and growth processes. Here, we quantify the response of radial conifer stem size to environmental fluctuations via a novel assessment of tree circadian cycles. We analyze four years of sub-hourly data collected from 56 larch and spruce trees growing along a natural temperature gradient of ∼6°C in the central Swiss Alps. During the growing season, tree stem diameters were greatest at mid-morning and smallest in the late evening, reflecting the daily cycle of water uptake and loss. Along the gradient, amplitudes calculated from the stem radius cycle were ∼50% smaller at the upper site (∼2200m a.s.l.) relative to the lower site (∼800m a.s.l.). We show changes in precipitation, temperature and cloud cover have a substantial effect on typical growing season diurnal cycles; amplitudes were nine times smaller on rainy days (>10mm), and daily amplitudes are approximately 40% larger when the mean daily temperature is 15–20°C than when it is 5–10°C. We find that over the growing season in the sub-alpine forests, spruce show greater daily stem water movement than larch. However, under projected future warming, larch could experience up to 50% greater stem water use, which may severely affect future growth on already dry sites. Our data further indicate that because of the confounding influences of radial growth and short-term water dynamics on stem size, conventional methodology probably overstates the effect of water-linked meteorological variables (i.e. precipitation and relative humidity) on intra-annual tree growth. We suggest future studies use intra-seasonal measurements of cell development and consider whether climatic factors produce reversible changes in stem diameter. These study design elements may help researchers more accurately quantify and attribute changes in forest productivity in response to future warming.
Mapping Canada’s forests is a significant challenge given their extent and the interprovincial differences in forest inventories. We created new sets of nationally consistent forest attribute maps ...for the years 2001 and 2011 by building upon previously published work with the objective to determine if sequential maps of forest attributes could be used to quantify changes over time. We first refined our previously published methodology of using the k nearest neighbors (kNN) prediction method and MODIS spectral reflectance data as predictive variables. The maps were generated using an improved reference dataset and a new analytical kNN workflow. We then evaluated 2001 to 2011 changes in two key attributes, aboveground biomass and percent tree cover, on pixels identified from published sources as having undergone fire, harvest, or postdisturbance regrowth during that period. For all three change types, average changes in both aboveground biomass and percent tree cover between 2001 and 2011 matched expectations relative to the dynamics of Canadian forests. Our results support the use of sequential national maps of forest attributes for evaluating regionally aggregated disturbance-related changes in forest properties. The new forest attribute maps are available from
Beaudoin et al. (2017
; doi:
10.23687/ec9e2659-1c29-4ddb-87a2-6aced147a990)
at
http://ouvert.canada.ca/data/fr/dataset/ec9e2659-1c29-4ddb-87a2-6aced147a990
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•Large trees have increased in Finland during 1921–2013.•In southern boreal subzone, the densities of large trees are today more than twice as high as in the other subzones.•Old trees ...have not increased during 1971–2013.•Old trees are far more common in the northern boreal subzone.•Densities of trees that are both large and old have increased.
Large and old trees have a vital role in preserving biodiversity in forest ecosystems. We used National Forest Inventory data from 1921 to 2013 for studying changes in densities (stems per ha) of large trees (diameter ≥40 cm) in Finland. In addition, densities of old trees (age ≥150 years) are reported from 1971 to 2013. We present results separately for the three subzones of the boreal biogeographical zone. Large trees have increased as much as 325%. The change has occurred mainly since the 1970s. On country level, old trees have become slightly less common (−4%) since the 1970s, although a decrease was actually observed only in the northern boreal subzone. The large majority of old trees in Finland are quite small in diameter, however. Trees that are both large and old show a notable increase from 1971 to 2013. During the 2010s, densities of large trees were higher in the southern boreal subzone than in the northern boreal subzone, but in the 1920s the opposite was true. Densities of old trees have been much higher in the northern boreal subzone. The observed densities of large trees are still considerably smaller than those observed in unmanaged old-growth forests in Scandinavia. High densities of large and/or old trees were observed in areas with restrictions on wood production emphasizing their role in maintaining biodiversity. The results reflect the destructive effects of former land use and the transition from dimensional cuttings to clear cuts and thinning from below after the 1940s. Proportionally larger changes were observed for southern Finland, where a higher human population density and the resulting more intensive land use had more severe detrimental effects on forests. As the densities of large trees and old trees have developed in a completely different manner in Finland, our results suggest that monitoring only the size distribution of trees will not sufficiently describe the role of old trees as constituents of biodiversity. Thereagainst, densities of large trees and large old trees developed in a similar manner.
Regression models relating variables derived from airborne laser scanning (ALS) to above-ground and below-ground biomass were estimated for 1395 sample plots in young and mature coniferous forest ...located in ten different areas within the boreal forest zone of Norway. The sample plots were measured as part of large-scale operational forest inventories. Four different ALS instruments were used and point density varied from 0.7 to 1.2 m
−
2
. One variable related to canopy height and one related to canopy density were used as independent variables in the regressions. The statistical effects of area and age class were assessed by including dummy variables in the models. Tree species composition was treated as continuous variables. The proportion of explained variability was 88% for above- and 85% for below-ground biomass models. For given combinations of ALS-derived variables, the differences between the areas were up to 32% for above-ground biomass and 38% for below-ground biomass. The proportion of spruce had a significant impact on both the estimated models. The proportion of broadleaves had a significant effect on above-ground biomass only, while the effect of age class was significant only in the below-ground biomass model. Because of local effects on the biomass–ALS data relationships, it is indicated by this study that sample plots distributed over the entire area would be needed when using ALS for regional or national biomass monitoring.
We evaluated area-based approaches (ABAs) to light detection and ranging (lidar) predictions of plot- and stand-level forest attributes (tree count, height, basal area, volume, aboveground biomass, ...broadleaf/conifer, and diameter at breast height--"diameter"). ABA methods included post-stratification (PS), ordinary least squares (OLSs) regression, k nearest neighbors (kNN), and random forest (RF). This study was conducted on the Savannah River Site in South Carolina, USA. Plot- and stand-level predictions were validated against fixed-radius 0.04 ha (0.1 acre) plots in 49 approximately equal to 2.0 ha (5 acre) stands. Our findings demonstrate that lidar can be incorporated operationally into forest inventory systems to provide stand-level inferences for a wide range of forest attributes. Volume predictions for specific diameter classes, however, often fared poorly (root mean squared error (RMSE) > 100%) for the methods we explored, especially for larger (less common) diameter trees. Stand-level results were consistently better than pixel-level results (10-200+ percentage points). kNN and RF performed similarly and better than OLS and PS, but RF was the most robust to model configurations, while kNN has practical advantages such as simultaneous predictions of many attributes.
In this paper two sampling and estimation strategies for regional forest inventory were investigated in detail and results were presented for various geographical scales. Airborne laser scanner (ALS) ...data were acquired to augment data from a systematic sample of National Forest Inventory (NFI) ground plots in Hedmark County, Norway (27,390km2). Approximately 50% of the NFI field plots were covered by the systematic ALS sample of 53 parallel flight lines spaced 6km apart. The area was stratified into eight cover classes and independent log-transformed regression models were developed for each class to predict total above-ground dry biomass (AGB). The two laser-ground estimation strategies tested were a model-dependent (MD), two-phase approach that rests on the assumption that the predictive models are correctly specified, and a model-assisted (MA) approach with a two-stage probability sampling design which utilizes design-unbiased estimators. ALS AGB estimates were reported by land cover class and compared to the NFI ground estimates. The ALS-based MA and MD mean estimates differed from the NFI AGB estimates by about 2% and 8%, respectively, for the entire County. At the county level the smallest estimated standard error (SE) for the estimates was obtained using the field data alone. However, the SEs calculated from field and ALS data were based on unequal numbers of ground plots. When considering only the NFI plots in the ALS strips, the smallest SEs were obtained using the MD framework. However, we also illustrated the sensitivity of the estimates of applying different plausible models. All the applied estimators assumed simple random sampling while the selection of flight lines as well as ground plots followed a systematic design. Thus, the estimates of SE were most likely conservative. Simulated sampling undertaken in a parallel research effort suggests that the overestimation of the SEs was probably much larger for the ALS-based estimates compared to the NFI estimates. ALS-based estimates were also derived for sub-county political units and thereby demonstrated how limited sample sizes affect the standard error of the biomass estimates.
► Above-ground biomass was estimated for a County in Norway (27,390km2). ► Separate sub-County estimates and cover type estimates were provided. ► Laser scanning used for sampling was combined with National Forest Inventory plots. ► Model-dependent and model-assisted estimators were tested and compared. ► Laser sampling was demonstrated to be a feasible method for regional estimation.
Tropical forests are often located in difficult-to-access areas, which make high-quality forest structure information difficult and expensive to obtain by traditional field-based approaches. LiDAR ...(acronym for Light Detection And Ranging) data have been used throughout the world to produce time-efficient and wall-to-wall structural parameter estimates for monitoring in native and commercial forests. In this study, we compare products and aboveground biomass (AGB) estimations from LiDAR data acquired using an aircraft-borne system in 2015 and data collected by the unmanned aerial vehicle (UAV)-based GatorEye Unmanned Flying Laboratory in 2017 for ten forest inventory plots located in the Chico Mendes Extractive Reserve in Acre state, southwestern Brazilian Amazon. The LiDAR products were similar and comparable among the two platforms and sensors. Principal differences between derived products resulted from the GatorEye system flying lower and slower and having increased returns per second than the aircraft, resulting in a much higher point density overall (11.3 ± 1.8 vs. 381.2 ± 58 pts/m2). Differences in ground point density, however, were much smaller among the systems, due to the larger pulse area and increased number of returns per pulse of the aircraft system, with the GatorEye showing an approximately 50% higher ground point density (0.27 ± 0.09 vs. 0.42 ± 0.09). The LiDAR models produced by both sensors presented similar results for digital elevation models and estimated AGB. Our results validate the ability for UAV-borne LiDAR sensors to accurately quantify AGB in dense high-leaf-area tropical forests in the Amazon. We also highlight new possibilities using the dense point clouds of UAV-borne systems for analyses of detailed crown structure and leaf area density distribution of the forest interior.
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