Forest types and productivity
Apart from forests as a broad land cover category, forests were grouped into three broad categories: coniferous, broadleaved and mixed forests (Barbati et al. 2014). We used data from Brus et al. (2011) on the tree species types of Europe. This dataset with a spatial resolution of 1 km by 1 km provides information on the spatial distribution of the twenty most dominant tree species of Europe. These tree species were then classified into coniferous or broadleaved species based on the European Atlas of Forest Tree Species (EAFTS; San-Miguel-Ayanz et al. 2018). Following the FAO (2015) definition of monoculture forests, an 80% cover threshold was used to separate monoculture stands (coniferous or broadleaved) from mixed stands. The cells with less than 10% forest cover did not meet the definition of a forest area as outlined by (FAO 2000, 2020) and were eliminated from further analysis.
Net Primary Productivity (NPP), the difference between Gross Primary Productivity (GPP) and plant autotrophic respiration, denotes net carbon or biomass fixed by plants through the process of photosynthesis. NPP is one of the most commonly used ecological metrics to study forest ecosystems and evaluate their potential to supply goods and services (Neumann et al. 2016). NPP derived from MODIS remotely sensed data has particularly received attention in monitoring forest dynamics. MODIS has the capability to capture global land coverage after every one to two days and measures NPP at a continuous scale (Kwon & Baker 2017).
To analyze the NPP at a European regional scale, we used MOD17 product derived from the global MODIS NPP of the Numerical Terradynamic Simulation Group (NTSG). The algorithm behind MOD17 uses climate data, land cover data, leaf area index (LAI) and fraction of photosynthetically active radiation (FPAR) modelled from other MODIS products to denote estimates of GPP and NPP at a spatial resolution of approximately 1 km by 1 km (Neumann et al. 2016; Turner et al. 2006). The source of LAI and FPAR data inputs were MOD15 LAI and FPAR Collection 5, while the land cover data was MOD12Q1 Version 4 Type 2 and the climate data was E-OBS. Neumann et al. (2016) provides a detailed methodology of a European-wide scale MODIS NPP product that we used in this study. We calculated for each pixel the periodic average from 2000 to 2012 to account for interannual variations in productivity due to drought events or harvesting. Then the grids were resampled to 5 km by 5 km using bilinear resampling to match the species diversity grids.
The MOD17 algorithm incorporates light use efficiency using remotely sensed vegetation data to calculate GPP with maintenance and growth respiration to derive NPP using the following equation:
GPP = LUEmax x fTmin x fvpd x 0.45 x SWrad x FPAR
NPP = GPP - RM - RG
where: LUEmax is the maximum light use efficiency; fTim and fVPD are fluctuations in light use efficiency from low temperatures that limit the functioning of plants and deficits in high pressure that results to water stress in plants (Coops et al. 2018; Zhao & Running 2010); SWrad is the incident short wave solar radiation, of which 45% is photosynthetically active radiation absorbed by vegetation from MODIS (FPAR); RM is the maintenance respiration as measured by leaf area index; RG is the fraction of growth respiration and it accounts for about 25% of NPP.
We extracted the NPP for the total forest areas as defined by the areas indicated on the map from Brus et al. (2011). We then parsed the NPP data further by monospecific coniferous forests, monospecific broadleaved forests and mixed forests. Figure 1 shows productivity distribution across the three forest groups that we considered in this study.
Distribution patterns of biodiversity
Distribution maps of animal species that naturally occur in the territory of the EU28 were compiled by van der Sluis et al. (
2016) and a detailed description of the input species, climate data, modelling and validation approaches are provided therein. A brief summary is provided in this article for completeness. The species data was collected from European-wide atlases and predicted range maps. The choice of which source to use was dependent on data reliability. There was variation in terms of data availability and quality among and within taxa influencing the choice of the analytical methods. However, such methods were harmonized to arrive to distribution maps that are as comparable as possible.
In total, there were 169 species of mammals, 294 species of birds, 147 species of herptiles (amphibians and reptiles) and 395 species of butterflies. The dataset excluded invasive and domesticated species of mammals. Sea turtles and herptile species that occur at the fringes of the European continent but have their dominant ranges in either the African or central European continent were also excluded as they do not play a relevant role in the PBR and PBC considerations for European mainland forests ecosystems. Endemic birds were also excluded because there are very few species in the study region and this might have disproportionately large effects on the fitting of the PBR’s. For the same reason rare and very localized species of butterflies were omitted. Hence, biodiversity is characterized in this study by common European animal species (Lennon et al. 2004).
Distribution data for mammal was retrieved from and mammals from from Observado (Observation.org; http://observation.org), GBIF (www.gbif.org/), and the CKmap project (http://www.faunaitalia.it/ckmap/) .Species data for birds were provided by the European Breeding Bird Atlas (Hagemeijer & Blair 1997), herptiles by Societas Europaea Herpetologica (Sillero et al. 2014) and butterflies by the European Red List of Butterflies (van Swaay et al. 2010) These atlases cover the Pan European distribution (excluding Cyprus and the Macaronesia Islands) at a resolution of 50km by 50km resolution. Because of the coarse resolution, the extent of a species range is most likely over-estimated. Besides, the estimation of the presences was not based on a common standardized method between countries, and differed in terms of quality of field work and number of observers per country (van der Sluis et al. 2016). To lessen the effect of these issues, species modeling results were validated with several independent datasets. The main approach entailed using the 50 km by 50 km data which were downscaled to 5km by 5km using the Boosted Regression Trees (BRT) modelling technique as implemented in the BIOMOD 2 package in R for mammals and herptiles and as implemented in TRIMmaps (Hallmann et al. 2014) for birds and butterflies. As BRT constitutes a generic modelling technique, the possible differences in implementation between the two software packages will constitute only marginal differences in the results. Besides, in a relative sense, output maps are comparable, which will be sufficient for the analysis in this study. BRT was considered because it can deal with non-linear relationships and accounts for the synergistic effects between the different factors affecting a species’ distribution (Couce et al. 2013). The species models were fitted with climate, soil, nitrogen and sulfur deposition, forest management and Corine Land cover types 2013 (EEA; for birds, butterflies and herptiles), and Global Land Cover maps 2009 (JRC; for mammals) data. For each species, the models were run ten times on different random subsets. Each subset was made up of 80% training and 20% validation data of presence and absence random allocations. The distributions and accuracies were then averaged to get the model fits which were thereafter evaluated based on True Skill Statistics (TSS; (Allouche et al. 2006)). Values of TSS range between − 1 and 1, with − 1 showing a perfect inverse prediction, 0 showing a random prediction and 1 showing perfect positive prediction. TSS are normally calculated by converting predicted probability maps into presence-absence maps using a threshold. For the mammals, birds and butterflies, the threshold was based on the value where accuracy for predicted presence and predicted absence would be equal while for herptiles the threshold was based on the value where the maximum TSS would be achieved. Although maximum TSS thresholds are not necessarily the same as thresholds based on equal accuracies for absence and presence prediction, the resulting species count maps over the entire extent of Europe will show only marginal differences. Mainly at the outer fringes of a species’ range small differences would be expected. These differences might be either increasing or decreasing the total extent of a species range, and across species, this effect should average out. In the original report (van der Sluis et al. 2016) the results of these models were validated at different spatial scales (individual countries and across Europe) with independent data sources, and these showed a general consistency between species groups in terms of modelling accuracy.
SDMs provide useful information on species richness at a fine scale (Coops et al. 2018; Suttidate et al. 2019) which is more advantageous than would have been if range maps with courser resolution were used. Biodiversity was predicted by stacking presence absence predictions of individual species from the species distribution models (SDMs). SDMs are used to predict species that co-occur in a region, however, they are likely to overpredict species richness by (1) incorrectly predicting species occurrence in an environment that appears suitable but is outside the species colonizable range (Wisz et al. 2014), (2) assuming the “species capacity” of the local environment is always reached and (3) excluding biotic interactions that may control species co-occurrence (Anderson et al. 2002). Nevertheless, patterns of relative variations in species richness would still hold, and for the purpose of this study, that should suffice. Biodiversity was thus estimated for the combined taxonomic groups, and for mammals, birds, herptiles and butterflies. All biodiversity metrics, however, gave a similar distribution in biodiversity and we thus chose to report results from only species richness. In Fig. 2, each biodiversity variable is represented as the total number of species in a grid cell.
Productivity – Biodiversity relationships
The relationships were assessed between mean net primary productivity (NPP) and the five different biodiversity groups that were calculated of all forests and separately for coniferous and broadleaved forests. Given that the pixels in the map are contiguous interpolations, spatial autocorrelation can potentially negate the assumption of independent samples when all pixels from the 5 km by 5 km resolution maps are taken into account. Therefore, we first tested for spatial autocorrelation using Moran’s I statistics on a randomly selected subset of the data and found that for each combined variable there was a significant degree of spatial autocorrelation. We accounted for this spatial autocorrelation by using a randomly selected percentage of the data. In our initial regression analysis, we fitted our models on 0.75%, 2%, 5% and 25% of the data to test whether the sample size had an effect on our model performance. The models were fitted on twenty randomly selected subsets of the data and averaged to get the final correlation coefficient. We did not observe big differences in the correlation coefficient and p-values when fitting on different sized datasets, but when we used very small samples, the statistical significance of the models was lower. The sign of the correlations, or the relative differences between species groups however remained the same. Also, we compared the results with quadratic model fits to our data but there was no difference in the signs and p-values. All models were significant at p < 0.0001, regardless of the sample size or the model used, therefore, in our study, we reported on the results from only linear models with a sample size of 0.75% of all pixels for all taxa.
Spatial congruency analysis
Areas high or low in either productivity or biodiversity were labelled as ‘hotspots’ or ‘coldspots’ respectively. Hotspots were delineated by identifying the top 30% quantile of a variable and cold spots by the bottom 30% quantile of a variable (Schröter & Remme
2016). For completeness, we also delineated the remaining areas which were labelled as “medium-spots”. Schröter & Remme (
2016) and Korpilo et al. (
2018) showed that the most common thresholds for quantile ranges to determine hotspots are between 5% and 30%. We also evaluated quantile thresholds of 10% and 5% to assess the impact of setting a threshold influences the results. As the general pattern didn’t affect the overall conclusions, we provide them in the supplementary materials (see Figure S3) but do not address them further in the results.
The classified hot-, medium- and coldspot maps of productivity were overlaid with each biodiversity variable and the resulting nine congruence classes of the two variables were tabulated and presented using the color scheme as presented in the legend of Fig. 4.