Deconstructing the forest community into three structural components — tree, shrub, ground floor – reveals different origins.
Above: Forest in the lowlands of Asturias, Spain (August 2021 by Javier Loidi).
Forests have always had a special appeal to ecologists, as they represent the most complex and developed type of terrestrial ecosystem. They are composed of many interacting organisms but traditionally they have been studied preferentially from the point of view of trees, which make up most of their biomass and determine their physiognomy. However, forests are not just trees. The arboreal component has been studied extensively in forests, but the rest of the plants have been relatively neglected until now. We wondered if the different elements that make up the forest depend on the same ecological drivers and have a parallel history in terms of their origin, evolutionary and migratory past.
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Loidi, J., Chytrý, M., Jiménez-Alfaro, B., Alessi, N., Biurrun, I., Campos, J.A., Čarni, A., Fernández-Pascual, E., Font Castell, X., Gholizadeh, H., Indreica, A., Kavgacı, A., Knollová, I., Naqinezhad, A., Novák, P., Nowak, A., Škvorc, Ž., Tsiripidis, I., Vassilev, K., & Marcenò, C. (2021). Life-form diversity across temperate deciduous forests of Western Eurasia: A different story in the understory. Journal of Biogeography, 48, 2932–2945. https://doi.org/10.1111/jbi.14254
We decided to deconstruct the forest community into three structural components: tree, shrub and ground floor layers to survey the patterns of variation in floristic richness and proportion of the different plant life-forms. We focused on Western Eurasia, i.e. Europe and some areas in the Black and Caspian seas coastal areas where deciduous temperate forests have a relevant role as Potential Natural Vegetation.
For analyzing the different floristic components of the forest, we used plots in which all the vascular plant species were recorded and their abundance estimated. In Europe, thanks to the Braun-Blanquet tradition in Vegetation Science, there are large amounts of data collected in a relatively consistent way. These data were extracted from the large datasets of vegetation plots currently available (EVA and some others) and we selected 9000 plots occurring in nine European areas (1000 plots in each) covering the geographic and climatic variability of Western Eurasia.
We found that three main drivers are controlling the broad-scale distribution patterns of plant life forms and vegetation layers: current climate, topographic conditions and past climate. Considering the number of species per region in the different layers, the general trends show that the lowest richness is in the oceanic northwest and the highest in southern central Europe, where the climate has a higher annual temperature range (continentality). Southern European regions are much richer in species, as expected from their warmer climate, their abundance of mountains and the presence of Pleistocene refugia for many tree species.
Considering the most significant life-forms, we can focus on evergreen broadleaf woody plants (trees and shrubs), which are more represented in the southern regions: Italy, Balkans and Euxinian, reaching their maximal number in the Iberian Peninsula. This group is present in oceanic and warm areas with lower summer precipitation regimes, which have changed little since glacial times. This could be related to the refugial role of Mediterranean mountains during the Pleistocene which allowed the survival of elements from the ancient evergreen laurophyllous forests occupied large areas of Europe in the Tertiary and were later replaced by the current deciduous forests. Their presence could be interpreted as a heritage from the remote past, surviving in a higher proportion in areas with relatively warmer and less severe climate in the Quaternary.
While the diversity of trees is mainly linked to current climatic conditions, the shrub layer is also driven by postglacial-glacial climatic stability, measured by temperature differences between the LGM and current climate, suggesting a different origin from forest trees. Likely, the deciduous shrubs, due to their higher fitness and ecologic plasticity, survived better than trees during the dramatic climatic variations of the Pleistocene. Shrublands probably replaced the forests during the coldest periods offering, at the end of the LGM, a sort of natural ‘nursery’ for the tree recovery during the recolonization. A complementary explanation for this point is that many of the shrubs can be regarded as initially external elements to the forests. Some of them are actually heliophilous species that are non-dependent on the shady forest ecosystems but survive under the shade due to the regular seed input commonly caused by birds. Many of these under-canopy shrubs constitute sink populations of individuals of low vitality, which are maintained thanks to the constant import of propagules from external populations that receive enough light to vigorously flower and form fruits. Actually, many shrub species live currently outside forests. Likely, many shrub populations were separated from the surviving forests during the LGM, and only during the Holocene the expansion of the forests resulted in the co-occurrence with shrublands.
We have observed that deconstructing forest communities into life-forms and vegetation layers and their separate analysis can help to characterize the relationships of vegetation to current environmental conditions and history by examining whether species in these groups follow similar or different patterns.