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Fungi play a crucial role in terrestrial Arctic ecosystems as symbionts of vascular plants and nutrient recyclers in soil, with many species persistently or temporarily inhabiting the phyllosphere of the vegetation. In this study we apply high-throughput sequencing to investigate the mycobiome of 172 samples of fresh (current year) and aged (3 year old) needles of Picea glauca from three sites over a distance of 500 km in Alaska (USA). We analysed Illumina-generated ITS2 sequences to relate mycobiome data with phenotypic tree traits, measures of genetic variation and climate variables obtained from long-term monitoring of the sites. Alpha-diversity declined with increasing environmental stress/climate harshness. Fungal communities differed in richness and taxonomic composition between sites, with a pronounced difference in the relative abundance of OTUs assigned to species of the rust genus Chrysomyxa , plant pathogens which seem to have been in an outbreak at two sites at the time of sampling. Beside climate parameters, needle age was the second strongest explanatory variable of the mycobiome composition, whereas we found no effect of tree genetic variation, indicating that environmental and tree trait specific variables mainly determined individual white spruce mycobiomes at Alaska's treelines. [ABSTRACT FROM AUTHOR]

Competition effects and management related disturbances can contribute to strong non-climatic signals in ring-width trends of trees from closed canopy forests. Removing this noise comes at the price of losing a considerable amount of (climatic) information on decadal and centennial time scales. Alternatively, open grown solitary trees, which never competed for light or other resources could function as less biased climate proxies. To explore this potential we analysed individual growth trends and climate-growth relationships of solitary oaks, potentially representative of former, open forests, and compared it to those of trees growing in adjacent closed canopy forests. Solitary oaks show differing tree allometry with bigger crowns and lower absolute heights. Their radial growth was significantly higher compared to forest trees. Contrary to our expectations, these solitary oaks show highly individualistic decadal growth trends. Their inter-annual variations are nevertheless in good agreement between individuals and stronger compared to trees from closed canopy forests. Climate –growth analysis revealed a significantly higher drought sensitivity of solitary oaks. Given the typical open medieval forest structure of large trees with extensive crowns growing in a mosaic of grazed woodlands and grasslands, it is likely that these trees contribute to the historical part of tree-ring collections. When trees from today's denser forests forming the recent parts of the collections are used to calibrate reconstruction models, the differing drought sensitivity might therefore lead to an overestimation of historical droughts. [ABSTRACT FROM AUTHOR]

Annually resolved and absolutely dated Maximum Latewood Density (MXD) and Blue Intensity (BI) measurements are frequently used for reconstructing summer temperature variability over the last centuries to millennia. A direct comparison of the outcome of both methods using similar material is needed due to how quickly this method is being adopted. The application of slightly different measuring systems (hardware) and analysis tools (software) in tandem with different wood samples and preparation procedures further challenges any straightforward assessment. Here we process 26 Norway spruce samples from the upper timberline in the Polish Tatra Mountains with the six most frequently used MXD and BI applications. Although offset is found in the raw MXD and BI data (0.04–0.13 g/cm 3 and 0.45–1.58 dimensionless blue intensity), interannual and longer-term fluctuations are significantly (p < 0.01) positively correlated between all MXD and BI time-series. Our results emphasize the potential of faster and cheaper, as well as overall more user-friendly techniques to generate reliable MXD surrogates for high-frequency dendroclimatological studies. Although the correlations between MXD and BI were lower than within MXD and BI, the results of growth-climate response performed for both proxies show only marginal differences. The obtained level-offset further questions the suitability of joining different density surrogates for developing long-term composite chronologies to reconstruct low-frequency climate variability. [ABSTRACT FROM AUTHOR]

Arctic shrubs have a strong potential for climate and environmental reconstructions in the chronically understudied regions of the high northern latitudes. The climate dynamics of these regions are important to understand because of large-scale feedbacks to the global climate system. However, little is known about other factors influencing shrub ring growth, possibly obscuring their climate signal. For example, as of yet we are not able to differentiate between herbivory or climatically induced growth depressions. Here, we use one of the most common Arctic shrubs, Alnus viridis as a test case to address this question. We sampled Alnus in Kobbefjord, Greenland, measured shrub-ring width and cell wall thickness and built site chronologies of each parameter. We analysed climate-growth relationships, tested their stability over time and employed a pointer-year analysis to detect growth depressions. We employed bootstrapped transfer function stability tests (BTFS) to assess the suitability of our shrub chronologies for climate reconstruction. Correlations with climate data showed strong significantly positive and stable correlations between summer temperature and ring-width with the exception of the recent decade. A climate reconstruction model failed stability tests, when the complete period of record was used for calibration and verification. Wood anatomy analysis uncovered the occurrence of unusual cell structure (very thin cell walls) in the exceptionally narrow ring of 2004, a recorded insect outbreak year in other parts of Greenland. When excluding the affected ring and a recovery period, the reconstruction model passed all tests, suggesting that the unusual 2004 ring was not climate driven, but rather the result of an insect attack. When combining anatomical analysis with traditional ring-width measurements, we move a step further in potentially distinguishing small rings caused by insect attacks from small rings formed in climatically challenging years. While this study does not provide unambiguous evidence, it does provide potential useful methodological combinations to enable more robust climate reconstructions in areas where climatic records are extremely sparse. [ABSTRACT FROM AUTHOR]

In recent years an increasing number of studies have shown shrubs to be reliable proxies of environmental conditions in regions where Trees − due to harsh climate conditions − are absent. Although many shrubs are monoecious, some are dioecious, which poses certain questions related to gender-specific growth as observed for trees in previous studies. Here, we address the questions whether dioecious shrubs, similar to trees, show growth differences between male and female plants, and − if so − whether this difference needs to be considered in terms of sample selection. We chose Juniperus communis. L., the most widely distributed woody plant, and a common and well-studied dioecious shrub species in the northern hemisphere, especially in the Boreal, Subarctic tundra and Alpine regions. Our samples were collected from four sites − three from the Ural Mountains and one site from Kirkenes in Norway. To see if there were differences in radial growth between sexes we performed four different analyses. First, we used multivariate explorative statistics to see if there were gender biased sub-populations and generally found no differences. Secondly, to compare growth over the lifetime of shrubs we computed cumulative annual increments of basal area which revealed no gender-specific growth patterns. Thirdly, to test if differences in radial growth between male and female shrubs affect the resulting site-chronology, we compared individual shrub chronologies with the site-chronology and found a significant differentiation between normalized correlations of gender-specific chronologies to the site-chronology. This significant difference was restricted to an overall comparison, but not evident at individual site-level. Lastly, we compared correlations of gender-specific chronologies and a mean site-chronology with monthly climate records to find only very few meaningful differences in their responses. In summary, we could not detect any clear gender-specific growth pattern in Juniperus communis but observed a trend towards more non-climatic signals in female junipers which may affect the resulting site-chronology. [ABSTRACT FROM AUTHOR]

Arctic and alpine shrubs are valuable for future dendro-ecological and dendro-climatological studies in regions where trees are sparse or absent. A commonly accepted procedure of sampling shrub stem disks is at the root collar. However many shrub studies report low inter-series correlations in radial measurements as compared to trees. Many studies also report cross-dating difficulties with radial measurements from shrubs within a stand and commonly attribute this to differential growth along the length of the stem. So does one stem disk entirely represent the environmental parameters the shrub might be reacting to? Does change in sampling location of the stem disk affect the subsequent ring-width chronologies and climate sensitivity? To tackle these questions, we investigated Juniperus communis L. – a species wide spread in the circumpolar arctic – across a latitudinal gradient in the Ural Mountains. Based on traditional radial ring-width measurements we assessed growth synchronicity along the length of shrub stems. We also compared ring width chronologies representing different stem heights with respect to their relationships with temperature and the standardized precipitation evapotranspiration index (SPEI). Growth patterns often varied significantly among stems disks of the same shrubs, resulting in dissimilar climate-growth relationships of stem disk chronologies. For correlations with temperature, stem disks at 20 cm distance from the root collar captured the best signal. For correlations with SPEI data we could not find any specific stem disk chronology with highest sensitivity. At least in our dataset, no “perfect sampling height” with high climate sensitivity exists and our results thus highlight that a single stem disk from a shrub may not completely represent the shrub’s growth response to climate parameters. [ABSTRACT FROM AUTHOR]

Dendrometers are useful tools to analyze intra-annual variation of radial growth in trees, but have rarely been applied in marginal environments. Our aim in this study was to explore stem radial variation (SRV) of Scots pines ( Pinus sylvestris L.) growing in a marginal environment on top of a peatland and compare it with stem radial variation of Scots pines growing in a nearby forest. We compared high-resolution (30 min) tree-growth of the peatland and forest pines in two consecutive years in two ways. First, we modeled raw SRV using site and weather parameters as predictors, to determine if and in what way stem radial variation depends on the site type. Second, we split the SRV signal into sub-series of varying length to test for differences between the time-series pattern of peatland and forest SRV with clustering methods and classifier models. We found indications that site type is influencing raw stem radial variation as: 1) an intercept, i.e. forest trees tended to grow more than peatland trees (as expected); 2) an interaction factor with structural and weather parameters, i.e. response of the forest trees to changing environmental parameters was different than the response of the peatland trees. Conversely, with regard to the temporal pattern of the stem radial variation, we found that the conditions within one year, e.g. weather patterns, were more important than site conditions, especially at short time scales. However, with increasing length of the sub-series the relative accuracy of the classifier models increased. Our results indicate that the site type was important for the raw SRV (amplitude) but not for the SRV pattern, which might be important to consider when comparing intra-annual signals from multiple sites. [ABSTRACT FROM AUTHOR]

Northern and elevational treelines are classic sites for dendroclimatological studies. At these marginal sites only one climate parameter is usually considered growth limiting and trees from these sites are therefore used to reconstruct that parameter back in time. Marginal sites are also those sites within a species range, where clonal reproduction is most frequent. Clonal growth can ensure plant species survival and growth under stressful conditions or if the environmental conditions do not allow sexual reproduction, e.g. by layering, by stems resprouting after damage, or through the exchange of resources between different clone ramets (“stems”). We literally stumbled across clonal and non-clonal growth forms of White spruce growing intermingled with each other at two Alaskan treeline sites. The two growth forms could not be distinguished a priori in the field. After sampling and detection of clones we thus asked whether clonal ramets and non-clonally grown trees (singletons) showed similar growth patterns. Clones were identified by identical multilocus genotypes in a SSR microsatellite genotyping analysis and radial growth was analyzed using traditional tree ring width methods High-frequency growth patterns were very similar between singletons and clonal ramets in Alaskan treeline White spruce, thus posing no problem in including both reproductive strategies in a classic dendroclimatological investigation. [ABSTRACT FROM AUTHOR]

The uniformitarian principle is one of the most important foundations of all dendro- and paleo-sciences. Without it, no inferences about the past can be made. However, the use of this principle in our community is not consistent and partially incorrect, with the main confusion relating to the understanding of the “uniformitarian principle” as somehow implying a stable relationship between climate and tree growth. To solve this, we look briefly at the history of the term, show how we teach this principle in our textbooks, give some examples of incorrect applications of this principle in the recent literature and close with a simple, logical and straightforward interpretation of this principle to the dendro-community. Applying the principle of aggregate tree growth we show that instable climate-growth relationships and the “no-analogue problem” are not a violation of the uniformitarian principle, but rather reflect our incomplete understanding of tree growth processes. Simply stated: The “uniformitarian principle” is an a priori assumption of spatial and temporal invariance of law’s describing nature’s processes. Applied to the dendro-sciences it means that the principle of aggregate tree growth is valid in time and space. [ABSTRACT FROM AUTHOR]

Dendrometers are measurement devices proven to be useful to analyze tree water relations and growth responses in relation to environmental variability. To analyze dendrometer data, two analytical methods prevail: (1) daily approaches that calculate or extract single values per day, and (2) stem-cycle approaches that separate high-resolution dendrometer records into distinct phases of contraction, expansion and stem-radius increment. Especially the stem-cycle approach requires complex algorithms to disentangle cyclic phases. Here, we present an R package, named dendrometeR, that facilitates the analysis of dendrometer data using both analytical methods. By making the package freely available, we make a first step towards comparable and reproducible methods to analyze dendrometer data. The package contains customizable functions to prepare, verify, process and plot dendrometer series, as well as functions that facilitate the analysis of dendrometer data (i.e. daily statistics or extracted phases) in relation to environmental data. The functionality of dendrometeR is illustrated in this note. [ABSTRACT FROM AUTHOR]

Shrub dendrochronological investigations are recently gaining more and more importance within the dendro-scientific community. As being a rather young discipline, many means of shrub dendrochronology lean on established methods that have been developed for trees. Although shrubs as trees are woody plants, it seems likely that they express differing growth characteristics due to their often multi-stemmed and prostrate stature. Yet, the majority of shrub dendrochronological investigations have measured shrub ring widths along two radii within one (sometimes several) stem disk(s) per individual. To our knowledge only one study so far has undertaken the approach to measure complete area increments (e.g. basal area increments, if applied to the basal stem disk of a shrub), however not focusing in detail on a comparative evaluation of this new approach with respect to radial measurements. To fill this knowledge gap our study focuses on the comparison of stem disk area increment measurements with radial measurements in the context of shrub growth representation and response- and transfer-function analyses. Our results indicate that for eccentric shrubs a minimum of four radial measurements per stem disk should be obtained for a good representation of the average stem disk growth. Inter-stem-disk comparisons showed that growth differences between individuals were often misestimated when only based on one or two radial measurements per stem disk. Response- and transfer-function analyses suggested, that the investigated shrubs reflect different environmental signals within different sectors of stem discs. This implies to carefully select radial measurements and individuals to increase the strength of environmental signals within transfer functions. [ABSTRACT FROM AUTHOR]

Cell-anatomical studies are a fast growing branch of dendrochronology, since they promise additional environmental proxy information with high temporal resolution. It is unclear, however, how many radial files of tracheids have to be considered to establish reliable wood anatomical time series for conifer species. Here, we investigate this question for four common cell-anatomical variables (cell-wall thickness, lumen area, lumen diameter and cell diameter) in Scots pine using examples from three Scandinavian sites. Cell-anatomical time series averaged from six measurement paths can reach the 90% confidence level of ten measurement paths at all sites. While lumen area generally required the most measurement paths in other cell-anatomical variables, cell-wall thickness required the least measurement paths. [ABSTRACT FROM AUTHOR]

The application of stable isotopes is an approach to identify pathways of methanogenesis, methane (CH4) oxidation and transport in peatlands. We measured the stable C isotopic characteristics (δ13C) of CH4 in peat profiles below hummocks, lawns and hollows of a Finnish mire to study the patterns of CHv turnover. Porewater CH4 concentrations ([CH4]; at 0.5-2 m) increased with depth below all microforms. Emissions of CHv from hummocks were the lowest, and increased with the increasing water-saturated zone, being ~10 times higher from hollows. Thus, the microtopography of the peatland did not affect the porewater [CH4] in the water-saturated part of the peat profile, but the CH4 emissions were affected due to differences in the oxidative potential of the microforms. There was a decrease in δ13C-CH4 with depth below all microforms indicating dominance of CO2-reduction over acetate cleavage pathway of methanogenesis at deep peat layers. However, estimated potential portions of transported CH4 comprised 50%-70% of the δ13C-CH enrichment on microforms at the 0.5-m depth, hereby masking the acetate cleavage pathway of methanogenesis. Stable C composition (δ13C) of CH4 proved to be a suitable (but not sufficient) tool to differentiate between types of methanogenesis in continuously water-saturated layers below microforms of a peatland. Combined flux-based and multi-isotopic approaches are needed to better understand the CH4 turnover process. [ABSTRACT FROM AUTHOR]

Abstract: Tree core samples of larch (Larix sibirica Ledeb.), spruce (Picea obovata Ledeb.) and pine (Pinus sibirica Du-Tour.) from the northern taiga of West Siberia were collected to assess their potential for summer temperature reconstructions in the Ob River region. Bootstrapped response functions showed that annual growth was mainly influenced by May to June temperatures in pine and by June to July temperatures in spruce and larch. Spruce and pine chronologies showed high positive correlations with previous October temperature. June–July temperatures were reconstructed based on spruce (1795–1996) and larch (1615–1999) tree ring chronologies. The pine chronology could not be used for a reliable temperature reconstruction, due to low values of explained May–June temperature variance (11–15%) but the species has a high potential to help clarify the May–June and October climatic influence on ring width observed in all three species. We explained the effect of the early vegetation period (May–July) and the differences in the temperature signals between spruce and larch tree ring chronologies with the influence of previous September and October temperature on tree growth with the warming effect of the Ob River and differences of the species’ photosynthetic possibilities and the activity of chloroplasts and bud meristem tissues. [Copyright &y& Elsevier]

Abstract: The Haloxylon vegetation in Central Asia has undergone vast changes throughout the last century, which has lead to a strong degradation and desertification of the desert ecosystems in Kazakhstan, Uzbekistan, and Turkmenistan. In terms of combating desertification and restoration of ecosystems, two pilot-projects recently are aiming at the reforestation of Haloxylon ecosystems. Hereby, two of the internationally discussed climate change mitigation activities may be of higher relevance: (I) to sequester CO2 through afforestation and reforestation and (II) to substitute fossil fuels by sustainable utilization of regenerative bio-fuels. With particular focus on Central Asian desert ecosystems, the questions arose: (I) to which extent Haloxylon species – the dominant shrub species – sequester CO2, (II) what the recent carbon pool related to Haloxylon ecosystems is, and (III) in which magnitude the existing and/or reforested ecosystems could be utilized for bio-fuels sustainably. Therefore, this study aims at the derivation of allometric models including carbon contents for the two Haloxylon species in Central Asian desert ecosystems. A first application to measured transect data across Central Asia is carried out to reflect the recent biomass variability of the two study species. We found, that above- and below-ground biomass for both species is best modeled when taking growth height, basal area, and canopy area of shrubs into account (0.87< r 2 <0.97 for the top-models). Regarding the transect data, Haloxylon aphyllum showed a significantly higher variability of measured and modeled variables as well as higher maximum and average biomass estimates when compared to Haloxylon persicum. As our study sites partly include regions with moderate to strong human impact, the observed biomass variability represents differing ecological conditions as well as anthropogenic effects, with the latter also being reflected in the observed shrub species compositions. [Copyright &y& Elsevier]

Abstract: Knowledge of the spatial pattern and temporal relationships between tree-growth and climatic factors are important not only for the projection of forest growth under varying climate but for dendroclimatology in general. Here, we systematically investigated tree-growth climate relationships of Picea crassifolia at upper treeline in the Qilian Mts., northwestern China. 297 trees from eleven sites, covering a large part of the natural range of this species, show increasing and partly divergent correlations with temperature in the most recent decades. The dominant signal at all sites was a strengthening of negative correlations of annual radial growth with summer temperature. In a subset of trees at six sites, a strengthening positive correlation with summer temperatures existed as well. Wetter and high altitude sites tended to show a higher percentage of trees that are positively correlated with warming temperatures, indicating that some individuals there may take advantage of warmer conditions. Divergent responses between the two sub-populations clustered by their response to climate factor were significantly stronger in the last 30 years compared to earlier time slices. In the same time frame, hydrothermal conditions of the investigation area changed to a drier and warmer combination. Drought conditions, most likely affecting the radial growth of most P. crassifolia, have been intensifying over time and expanding spatially from the middle Qilian Mts. to most of our study area during the last half century. While explanations such as methodological effects due to trend removal or human disturbance at the sampling sites might be able to explain the result at single sites, the spatial and temporal co-occurrence of large scale changes in climate and tree growth suggests a causal link between them. [Copyright &y& Elsevier]

Abstract: For north-eastern Germany regional climate models project rising temperatures in combination with decreasing summer and increasing winter precipitation. The resulting overall drier conditions during the growing season will considerably impact forest growth there. We evaluate the consequences of increasing drought on the growth of the two locally most important broadleaf tree species common beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). Three mixed forests of beech and oak were sampled along a west-east gradient of declining precipitation. In total we used 257 ring-width samples from 133 trees to build six species and site specific chronologies. Additionally, we modelled the soil water budget for each site. We performed continuous and discontinuous (pointer year) analysis of climate-tree-growth relationships with particular emphasis on inter-annual-variations and their dependence upon climatic factors (temperature, precipitation, soil moisture) and on the stability of the obtained relationships. Results of climate-growth correlations together with pointer year analysis indicate a strong dependency of growth of both species from water availability, especially during early summer (June and July). General correlation pattern between growth and climate are similar for both species, but climate sensitivity of beech is generally higher. We identified drought as the main driver of negative growth depressions in both species. Increasing drought stress along the gradient is expressed in higher correlations to climatic variables, higher sensitivity (variance) of growth, and a higher number of negative pointer years for both species. For beech we also found a significant trend of decreasing average growth rates along the gradient. Growth superiority of beech compared to oak declines with decreasing precipitation. The relationships were generally stable throughout the 20th century. A rise of sensitivity together with a higher frequency of negative pointer years during the last decades suggests that increasing climatic variability together with rising temperatures might be influencing growth of Fagus at the more humid sites. If we substitute space by time it seems that already small changes in precipitation regime can have considerable impact, especially on the growth of beech. Other, more drought tolerant species like oak might gain competitive advantages under the projected climatic changes. [Copyright &y& Elsevier]

Abstract: Divergent responses between tree growth and climate factors have been widely reported at high latitudes in the northern hemisphere. Here we show variable climate-growth relationships and divergent growth responses of Qinghai spruce (Picea crassfolia) along an elevation gradient at a mid-latitude site at the northeastern Tibetan Plateau, China. Trees from higher elevations, limited mainly by temperature, show divergent growth trends over time and two responses to climate. Some trees show increasing positive and some increasing negative responses to growing season temperature during the last decades. Trees from lower treeline show a strengthening drought stress signal over time and no divergent growth trends within sites. Our results indicate that single tree analysis might be a worthwhile tool to (1) uncover spatial–temporal changes in climate-growth relationships of trees, (2) better understand future growth performance and (3) help overcome current limitations of tree ring based climatic reconstructions. [ABSTRACT FROM AUTHOR]

Abstract: Dendroclimatological research is often based on the assumption that the relationship between tree growth and climate is not variable over time. Here we test this assumption by exploring if climate sensitivity of Picea mariana (Mill.) trees growing in open-stand lowland forest and on top of a neighboring peatland in Interior Alaska is stable or changing over time. Climate–growth correlations at the study sites are strongly dependent on microtopography and vary substantially over time. Trees growing in the open forest site generally display stronger climate–growth correlations, especially significantly negative correlations with late summer temperatures (July, August) starting in the period 1920–1970. Trees growing on the peatland site are less climate sensitive, but display positive correlations between annual growth and temperature of October and December in the early 20th century, while in the late 20th century, significant negative correlations exist with January and February temperatures. This study, thus, demonstrates a transient climate–growth response for P. mariana (Mill.) on two sites typical for lowland Interior Alaska. However, due to multiple possible explanations (e.g. changing climate, coupled with aging trees and a growing peatland surface) it is not possible at this time to pinpoint the exact cause for these changes in the climate–growth relationships. [Copyright &y& Elsevier]

Rising temperature and altered precipitation regimes will lead to severe droughts and concomitant extreme events in the future. Forest ecosystems have shown to be especially prone to climate change. In assessing climate change impacts, many studies focus on high altitude or ecological edge populations where a climate signal is supposedly most pronounced. While these studies represent only a fraction of the forest ecosystems throughout Europe, findings on climate sensitivity of lowland core populations remain comparatively underrepresented. By using tree-ring widths of a large region-wide network of European beech and Scots pine populations along a precipitation gradient in northeastern Germany, we identify main climatic drivers and spatio-temporal patterns in climate sensitivity. Further, we analyze the resistance of tree growth towards drought. Detailed data on soil characteristics was used to interpret climate-growth relationships. Beech was found to be most sensitive to summer drought during early summer at dry sites, whereas pine displayed highest sensitivity for winter temperature at wet sites. The resistance to extreme drought was lower for beech. By splitting the observation period (1964–2017) into an early and late period, we found non-stationary climate-growth relationships for both study species with beech showing an increase in drought sensitivity and pine in winter temperature sensitivity. Overall, beech populations seem to be especially endangered by prospective climate changes, whereas climate-growth relationships of pine seem more ambiguous with a possible trade-off between enhanced photosynthetic activity caused by early photosynthesis in late winter and reduced activity due to summer drought. [ABSTRACT FROM AUTHOR]

This technical note highlights a minor mistake in the calculation of Gleichläufigkeit (glk) suggested by Schweingruber (1983) . Namely, if two series that are compared with each other show a synchronous lack of growth change this will not be treated accurately. We present an accurate way of glk calculation based on Gegenläufigkeit ( Huber, 1943 ) as in Eckstein and Bauch (1969) and suggest that software-tools and methodological approaches which make use of glk should be tested for the way they compute glk. [ABSTRACT FROM AUTHOR]