Gains or Losses in Forest Productivity under Climate Change? The Uncertainty of CO2 Fertilization and Climate Effects

Dominik Sperlich,Jürgen Kreuzwieser,Daniel Nadal-Sala,Rasoul Yousefpour,Carlos Gracia,Marc Hanewinkel

doi:10.3390/cli8120141

Abstract

Global warming poses great challenges for forest managers regarding adaptation strategies and species choices. More frequent drought events and heat spells are expected to reduce growth and increase mortality. Extended growing seasons, warming and elevated CO2 (eCO2) can also positively affect forest productivity. We studied the growth, productivity and mortality of beech (Fagus sylvatica L.) and fir (Abies alba Mill.) in the Black Forest (Germany) under three climate change scenarios (representative concentration pathways (RCP): RCP2.6, RCP4.5, RCP8.5) using the detailed biogeochemical forest growth model GOTILWA+. Averaged over the entire simulation period, both species showed productivity losses in RCP2.6 (16–20%) and in RCP4.5 (6%), but productivity gains in RCP8.5 (11–17%). However, all three scenarios had a tipping point (between 2035–2060) when initial gains in net primary productivity (NPP) (6–29%) eventually turned into losses (1–26%). With eCO2 switched off, the losses in NPP were 26–51% in RCP2.6, 36–45% in RCP4.5 and 33–71% in RCP8.5. Improved water-use efficiency dampened drought effects on NPP between 4 and 5%. Tree mortality increased, but without notably affecting forest productivity. Concluding, cultivation of beech and fir may still be possible in the study region, although severe productivity losses can be expected in the coming decades, which will strongly depend on the dampening CO2 fertilization effect.

Highlights

Forest productivity in Europe has generally increased over the past decades [1,2,3,4], providing that other factors were not limiting it, such as water availability, growth temperature and/or nitrogen deposition [5,6]
There exists a great deal of uncertainty regarding how growth dynamics and mortality will unfold in the future depending on the severity of climate change, the CO2 fertilization effect and the responses by forest ecosystems
Assuming a full CO2 fertilization effect, negative climate impacts were compensated for and productivity was even increased—but only until a certain tipping point after which productivity decreased towards the end of the century

Summary

Introduction

Forest productivity in Europe has generally increased over the past decades [1,2,3,4], providing that other factors were not limiting it, such as water availability, growth temperature and/or nitrogen deposition [5,6]. Climate 2020, 8, 141 affect forest productivity, for example, due to the positive effect on photosynthesis in mountains or high latitudes, which are energy limited and not water limited [13], and due to the lengthening of the growing season [14,15]. Considerable uncertainties remain regarding the impacts and interplay of temperature, precipitation and eCO2 on forest growth and productivity—especially over longer periods of time, considering the long life-spans of trees [18]. Other uncertainties stem from the different responses of forest ecosystems and underlying ecophysiological processes themselves (Figure 1)

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