Projecting Tree Species Composition Changes of European Forests for 2061-2090 Under RCP 4.5 and RCP 8.5 Scenarios

Abstract

Climate change poses certain threats to the World's forests. That is, tree performance declines if species-specific, climatic thresholds are surpassed. Prominent climatic changes negatively affecting tree performance are mainly associated with so-called hotter droughts. In combination with biotic pathogens, hotter droughts cause a higher tree vulnerability and thus mortality. As a consequence, global forests are expected to undergo vast changes in the course of climate change. Changed climatic conditions may on the one hand locally result in more frequent dieback of a particular tree species but on the other hand allow other-locally yet absent species-to establish themselves, thereby potentially changing local tree-species diversity. Although several studies provide valuable insights into potential risks of prominent European tree species, we yet lack a comprehensive assessment on how and to which extent the composition of European forests may change. To overcome this research gap, we here project future tree-species compositions of European forests. We combine the concept of climate analogs with national forest inventory data to project the tree-species composition for the 26 most important European tree species at any given location in Europe for the period 2061-2090 and the two most relevant CMIP5 scenarios RCP 4.5 and RCP 8.5. Our results indicate significant changes in European forests species compositions. Species richness generally declined in the Mediterranean and Central European lowlands, while Scandinavian and Central European high-elevation forests were projected an increasing diversity. Moreover, 76% (RCP 4.5) and 80% (RCP 8.5) of the investigated locations indicated a decreasing abundance of the locally yet most abundant tree species while 74 and 68% were projected an increasing tree-species diversity. Altogether, our study confirms the expectation of European forests undergoing remarkable changes until the end of the 21st century (i.e., 2061-2090) and provides a scientific basement for climate change adaptation with important implications for forestry and nature conservation.

Keywords: CMIP5 climate projections; climate analogs; climate change; climate-smart forests; forest-management adaptation; tree-species vulnerability.

FIGURE 1

Schematic flow chart depicting the processing of climate projections and EU-forest inventory data to eventually obtain projected species distribution maps (here using the grid cell corresponding with the location of Munich and Scots pine projected for current climate conditions as an example). (A) For each of the 3,949 grid cells current and future analogs are computed resulting in 3,949 different climate analog regions. (B) For each species, relative abundance per grid cell is computed, resulting in 26 species-specific relative abundance maps. (C) Intersection of the current/future climate analog region with the species-specific relative abundance map allows for computing the mean relative abundance of a given species within the corresponding climate analog region which is taken as local relative abundance probability of the corresponding grid cell. (D) Computing local relative abundance probabilities for each of the 3,949 grid cells allows for projecting the relative abundance probability of a specific species under a certain scenario across Europe. Please note the comparably good match between actual Scots pine relative abundance (B) and projected current Scots pine relative abundance probability (D).

FIGURE 2

Projected relative abundance probabilities for the four most abundant tree species Pinus sylvestris, Picea abies, Fagus sylvatica, and Quercus robur for current analogs (left panels), RCP 4.5 analogs (mid panels), and RCP 8.5 analog (right panels). Relative abundance probability increases from gray over yellow to green colors.

FIGURE 3

Mean relative abundance probabilities under current (blue) and projected future (orange = RCP 4.5, red = RCP 8.5) climate conditions. Species abbreviations: Pin.syl, Pinus sylvestris; Pic.abi, Picea abies; Fag.syl, Fagus sylvatica; Que.rob, Quercus robur; Bet.pub, Betula pubescens; Que.ile, Quercus ilex; Bet.pen, Betula pendula; Fra.exc, Fraxinus excelsior; Que.pet, Quercus petraea; Pin.pin, Pinus pinaster; Car.bet, Carpinus betulus; Pin.hal, Pinus halepensis; Ace.pse, Acer pseudoplatanus; Sor.auc, Sorbus aucuparia; Pin.nig, Pinus nigra; Pop.tre, Populus tremula; Aln.glu, Alnus glutinosa; Abi.alb, Abies alba; Pic.sit, Picea sitchensis; Cas.sat, Castanea sativa; Que.pub, Quercus pubescens; Lar.dec, Larix decidua; Sal.cap, Salix caprea; Aln.inc, Alnus incana; Que.sub, Quercus suber; Que.pyr, Quercus pyrenaica.

FIGURE 4

Proportional change of relative abundance probability of the yet dominant tree species per grid cell for RCP 4.5 (upper left) and RCP 8.5 (upper right) and a histogram depicting the frequency of features shown in the upper panels. Upper panels: black, red, and orange colors indicate losses (i.e., proportions below 1), while blue colors indicate an increasing abundance. The lower panel depicts the frequencies of relative abundance probability changes of the yet dominant tree species. Here, orange colors refer to RCP 4.5 projections, while red colors indicate RCP 8.5. The black vertical line indicates the threshold between loss and gain. The inserted values refer to the cumulative frequencies of losses (left of black vertical line) and gains (right of black vertical line) for RCP 4.5 (orange) and RCP 8.5 (red).

FIGURE 5

Absolute change in Shannon’s H for RCP 4.5 (upper left) and RCP 8.5 (upper right) and a histogram depicting the frequency of features shown in the upper panels. Upper panels: red and orange colors indicate a loss in tree-species diversity while blue colors demarcate increasing tree-species diversity. The lower panel depicts the frequencies of absolute change of Shannon’s H, i.e., tree-species diversity. Here, orange colors refer to RCP 4.5 projections, while red colors indicate RCP 8.5. The black vertical line indicates the threshold between loss and gain. The inserted values refer to the cumulative frequencies of losses (left of black vertical line) and gains (right of black vertical line) for RCP 4.5 (orange) and RCP 8.5 (red).