Magnus Ekström's research while affiliated with Umeå University and other places

Publications (31)

Article
Field inventoried data are often used as references (ground truth) in forest remote sensing studies. However, the reference values are affected by various kinds of errors, which tend to make the reported accuracies of the remote sensing-based predictions worse than they are. The more accurate the remote sensing techniques are becoming, the more pro...
Article
NASA's Global Ecosystem Dynamics Investigation (GEDI) mission offers data for temperate and pan-tropical estimates of aboveground forest biomass (AGB). The spaceborne, full-waveform LiDAR from GEDI provides sample footprints of canopy structure, expected to cover about 4% of the land area following two years of operation. Several options are availa...
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The paper explores a testing problem which involves four hypotheses, that is, based on observations of two random variables X and Y , we wish to discriminate between four possibilities: identical survival functions, stochastic dominance of X over Y , stochastic dominance of Y over X , or crossing survival functions. Four-decision testing procedures...
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Measurements of human attitudes and perceptions have traditionally used numerical point judgments. In the present study, we compared conventional point estimates of weight with an interval judgment method. Participants were allowed to make step by step judgments, successively converging towards their best estimate. Participants estimated, in grams,...
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Thin, hair‐like lichens (Alectoria, Bryoria, Usnea) form conspicuous epiphyte communities across the boreal biome. These poikilohydric organisms provide important ecosystem functions and are useful indicators of global change. We analyse how environmental drivers influence changes in occurrence and length of these lichens on Norway spruce (Picea ab...
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Forest information is requested at many levels and for many purposes. Sampling-based national forest inventories (NFIs) can provide reliable estimates on national and regional levels. By combining expensive field plot data with different sources of remotely sensed information, from airplanes and/or satellite platforms, the precision in estimators o...
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The boreal forest floor vegetation is critical for ecosystem functioning and an important part of forest biodiversity. Given the ongoing global change, knowledge on broad-scale changes in the composition and abundance of different plant species and species groups is hence important for both forest conservation and management. Here, we analyse perma...
Article
Presence‐absence sampling is an important method for monitoring state and change of both individual plant species and communities. With this method only the presence or absence of the target species is recorded on plots and thus the method is straightforward to apply and less prone to surveyor judgement compared to other vegetation monitoring metho...
Article
Inventories of plant populations are fundamental in ecological research and monitoring, but such surveys are often prone to field assessment errors. Presence/absence (P/A) sampling may have advantages over plant cover assessments for reducing such errors. However, the linking between P/A data and plant density depends on model assumptions for plant...
Article
If the survival function of a random variable X lies to the right of the survival function of a random variable Y, then X is said to stochastically dominate Y. Inferring stochastic dominance is particularly complicated because comparing survival functions raises four possible hypotheses: identical survival functions, dominance of X over Y, dominanc...
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In this paper, we consider general classes of estimators based on higher-order sample spacings, called the Generalized Spacings Estimators. Such classes of estimators are obtained by minimizing the Csiszár divergence between the empirical and true distributions for various convex functions, include the “maximum spacing estimators” as well as the ma...
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Merging available sources of information is becoming increasingly important for improving estimates of population characteristics in a variety of fields. In presence of several independent probability samples from a finite population we investigate options for a combined estimator of the population total, based on either a linear combination of the...
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Interval-censored data may arise in questionnaire surveys when, instead of being asked to provide an exact value, respondents are free to answer with any interval without having pre-specified ranges. In this context, the assumption of noninformative censoring is violated, and thus, the standard methods for interval-censored data are not appropriate...
Article
Links between lichen morphology, internal/external water storage and distribution patterns are poorly known. We compared mass- (WC, % H2O) and area-based (WHC, mg H2O cm-2) hydration traits in seven pendent or shrubby Usnea species from oceanic to continental climates. All species held more external than internal water. Internal WHC and WC increase...
Article
Plant communities are attracting increased interest in connection with forest and landscape inventories due to society's interest in ecosystem services. However, the acquisition of accurate information about plant communities poses several methodological challenges. Here we investigate the use of presence-absence sampling with the aim to monitor st...
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Large-scale surveys, such as national forest inventories and vegetation monitoring programs, usually have complex sampling designs that include geographical stratification and units organized in clusters. When models are developed using data from such programs, a key question is whether or not to utilize design information when analyzing the relati...
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Self-selected interval data arise in questionnaire surveys when respondents are free to answer with any interval without having pre-specified ranges. This type of data is a special case of interval-censored data in which the assumption of noninformative censoring is violated, and thus the standard methods for interval-censored data (e.g. Turnbull’s...
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Dead wood is a critical resource for forest biodiversity and widely used as an indicator for sustainable forest management. Based on data from the Swedish National Forest Inventory we provide baseline information and analyze trends in volume and distribution of dead wood in Swedish managed forests during 15years. The data are based on ≈30,000 sampl...
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Hair lichens are strongly influenced by forest structure at local scales, but their broad-scale distributions are less understood. We compared the occurrence and length of Alectoria sarmentosa (Ach.) Ach., Bryoria spp., and Usnea spp. in the lower canopy of > 5000 Picea abies (L.) Karst. trees within the National Forest Inventory across all product...
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A two-term Edgeworth expansion for the standardized version of the sample total in a two-stage sampling design is derived. In particular, for the commonly used stratified and cluster sampling schemes, formal two-term asymptotic expansions are obtained for the Studentized versions of the sample total. These results are applied in conjunction with th...
Article
Assume that we have a sequence of n independent and identically distributed random variables with a continuous distribution function F, which is specified up to a few unknown parameters. In this paper, tests based on sum-functions of sample spacings are proposed, and large sample theory of the tests are presented under simple null hypotheses as wel...
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There is growing evidence of adverse birth outcomes due to exposure to air pollution during gestation. However, recent negative studies are also reported. The aim of this study was to assess the effect of ozone and vehicle exhaust exposure (NO(2)) on the length of the gestational period and risk of preterm delivery. We used data from the Swedish Me...
Article
A very general measure of skewness based on the quantiles is introduced, which includes several well-known measures as special cases. Sample versions of our measure may be used as test statistics for testing the hypothesis of symmetry about an unknown value. We provide large sample theory for such a statistic and discuss the asymptotic relative eff...
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Full-text available
A procedure, based on sample spacings, is proposed for testing whether a univariate distribution is symmetric about some unknown value. The proposed test is a modification of a sign test suggested by Antille and Kersting [1977. Tests for symmetry. Z. Wahrscheinlichkeitstheorie verw. Gebiete 39, 235–255], but unlike Antille and Kersting's test, our...
Article
An alternative to the maximum likelihood (ML) method, the maximum spacing (MSP) method, is introduced in Cheng and Amin [1983. Estimating parameters in continuous univariate distributions with a shifted origin. J. Roy. Statist. Soc. Ser. B 45, 394–403], and independently in Ranneby [1984. The maximum spacing method. An estimation method related to...
Article
Most proposed subsampling and resampling methods in the literature assume stationary data. In many empirical applications, however, the hypothesis of stationarity can easily be rejected. In this paper, we demonstrate that moment and variance estimators based on the subsampling methodology can also be employed for different types of non-stationarity...
Article
A procedure, based on sample spacings, is proposed for testing whether a univariate distribution is symmetric about some unknown value. The proposed test is a modification of a sign test suggested by Antille and Kersting [1977. Tests for symmetry. Z. Wahrscheinlichkeitstheorie verw. Gebiete 39, 235-255], but unlike Antille and Kersting's test, our...
Article
We discuss a class of statistics based on spacings of increasing order and show that these statistics are almost surely consistent. Special attention is devoted to estimation of f-divergences and to tests for uniformity on the unit interval. It is shown that tests for uniformity, based on sum-functions of spacings, are strongly consistent against a...

Citations

... Our contingency approach, Generalized Hierarchical Model-Based inference (GHMB) [50,51], uses two levels of models: one linking ground data and footprint scale lidar metrics (i.e. the footprint biomass calibration models) and one linking those footprint biomass predictions to wall-to-wall ancillary data. The GHMB framework uses probability theory under the model-based paradigm to appropriately combine uncertainty from the two models, as wall-to-wall predictions form the basis of a large-area estimate of biomass [51][52][53]. Thus, the theory upon which GEDI's estimation of uncertainty is built can be extended to sensor fusion. ...
... These changes can cause cascading effects in forest food webs where, for example, an altered plant composition might result in decreased snail populations, and the lower consumption of snails by birds reduces their calcium intake, leading to lower breeding success due to more fragile eggshells (Graveland et al. 1994). The gradual decrease in nitrogen and acid deposition currently taking place across Europe has not yet translated into clear improvements in forest ecosystem functioning (Schmitz et al. 2019) or species recovery (e.g., Esseen et al. 2022) because input levels continue to exceed critical loads in many regions. Also the secondary pollutant tropospheric ozone resulting from atmospheric processes in a polluted environment continues to cause damage to the health of trees and other forest dwellers (Lin et al. 2020). ...
... The coverage of all understory vegetation groups declined with time, as observed in similar studies (Hedwall and Brunet, 2016;Hedwall et al., 2019Hedwall et al., , 2021Jonsson et al., 2021). However, in contrast to previous studies, here we investigated the development of understory vegetation composition in different forest types, and found that the pattern was consistently similar. ...
... The variances and covariances involved were determined empirically by comparing predicted values with true values in the datasets designated for model estimation. With small datasets, the use of this type of empirically determined variance may sometimes be inappropriate (e.g., [34]), but in our case, the datasets were large enough for estimating the variances and covariances with high precision. For the computations, we used the R package 'DatAssim' [35]. ...
... This translates into obtaining important ecological indications also from non-dominant scarcely present species, which, however, may be functionally relevant and powerful site indicators, for example, typical, diagnostic, or even flagship species. To some extent, these categories of species can be recorded also with vegetation plots, but the probability of not recording them during the sampling is higher, especially because not always the sampling effort (and design) is adequate due to time/resource constraints (Zhang et al. 2014;Hoffmann et al. 2019;Ståhl et al. 2020). ...
... Hwang & He, 2011). In a parallel study (Ekström et al., 2020), we develop new theory for linking presence-absence data with plant density under spatial cluster models of Neyman-Scott type. ...
... It should be noted that borderline cases may occur when the test statistic is close to the border of the decision region (respectively, a marginal p-value is close to one of the thresholds and ⋆ ). Therefore, it is advisable to report the conclusion of the test together with the marginal p-values p 1 , p 2 and the thresholds , ⋆ (see Angelov et al. 2019b). ...
... The MSP method as introduced by Ranneby (1984) is based on minimizing an approximation of the Kullback-Leibler information. Generalizations to approximations of other information measures can be found in Ranneby and Ekström (1997), Ekström (2001), Ghosh and Jammalamadaka (2001) and Ekström et al. (2020). Generalizations to multivariate observations can be found in Ranneby (1996), Kuljus and Ranneby (2015), Kuljus and Ranneby (2020) and Jimenez and Yukich (2002). ...
... Alectoria and Usnea grows better in wetter and warmer climates and may expand northwards unless light and dispersal are limiting. Alectoria grows faster than Usnea across continental to oceanic macroclimates (Phinney et al., 2021) due to a larger internal water storage pool (Eriksson et al., 2018;Esseen et al., 2015), extending hydration and carbon gain. Bryoria, in contrast, has large external water storage pool (Esseen et al., 2017) causing suprasaturation depression of photosynthesis, and in situ decomposition if exposed to prolonged hydration (Coxson & Coyle, 2003). ...
... Hence, the interval judgment method used in this study is conceptually similar to the one describing the physicists' measurements of speed of light which became more and more exact over the years. We used an interval judgment method, called self-selected intervals [2,3], and compared the result with traditional point judgments. The judgment task used for comparisons of the judgment methods was a weight judgment task. ...