Climate Change Effects on Secondary Compounds of Forest Trees in the Northern Hemisphere

Abstract

Plant secondary compounds (PSCs), also called secondary metabolites, have high chemical and structural diversity and appear as non-volatile or volatile compounds. These compounds may have evolved to have specific physiological and ecological functions in the adaptation of plants to their growth environment. PSCs are produced by several metabolic pathways and many PSCs are specific for a few plant genera or families. In forest ecosystems, full-grown trees constitute the majority of plant biomass and are thus capable of producing significant amounts of PSCs. We summarize older literature and review recent progress in understanding the effects of abiotic and biotic factors on PSC production of forest trees and PSC behavior in forest ecosystems. The roles of different PSCs under stress and their important role in protecting plants against abiotic and biotic factors are also discussed. There was strong evidence that major climate change factors, CO₂ and warming, have contradictory effects on the main PSC groups. CO₂ increases phenolic compounds in foliage, but limits terpenoids in foliage and emissions. Warming decreases phenolic compounds in foliage but increases terpenoids in foliage and emissions. Other abiotic stresses have more variable effects. PSCs may help trees to adapt to a changing climate and to pressure from current and invasive pests and pathogens. Indirect adaptation comes via the effects of PSCs on soil chemistry and nutrient cycling, the formation of cloud condensation nuclei from tree volatiles and by CO₂ sequestration into PSCs in the wood of living and dead forest trees.

Keywords: CO2; UV-B; VOCs; drought; ozone; phenolics; temperature; terpenes.

FIGURE 1

Global climate change – related abiotic and biotic stresses and their influence on types of plant secondary compounds (PSCs) in forest trees. Ecosystem level feedbacks transmitted by leaf PSCs are indicated with arrows on the left from the target tree. PSCs in foliage provide chemical defenses against herbivores and pathogens (Lämke and Unsicker, 2018). PSCs of leaf and needle litter affect mostly on tree nutrient uptake (Smolander et al., 2012) and rhizosphere organisms while PSCs in deadwood are part of important carbon storage (Pan et al., 2011) and stored PSCs could mitigate wood decay by decomposer organisms (Nerg et al., 2004; Karppanen et al., 2007) and release of CO₂ to the atmosphere. Volatile PSCs (VOCs) affect the trophic interactions in the forest ecosystem where the tree is growing (Blande et al., 2014), while reactive VOCs affect the atmosphere and may have atmosphere-biosphere level feedbacks in the surrounding ecosystems (Joutsensaari et al., 2015).

FIGURE 2

Summary of the results in comprehensive reviews and meta-analyses of plant PSCs responses in their concentration or emissions in forest trees under climate change related stresses. This tabulation covers nearly 400 original scientific articles. Type of stresses: O₃ = elevated ozone, CO₂ = elevated carbon dioxide, UV-B = elevated UV-B radiation. The tabulation gives the direction of PSC responses to single environmental factors, except for the combined O₃+CO₂ and CO₂+warming effects. Arrow direction (increase or decrease) shows the effect size (significant or highly significant effect in meta-analysis) or the amount of evidence in reviews. Horizontal bidirectional arrow indicates the studies with non-significant results and up and down arrows in the same cell indicate significant results in both directions dominate. Question mark indicates that the reviews and meta-analyses did not provide enough data. Gray arrows in CO₂, warming and CO₂+warming columns indicate woody tissue responses as specified in reference 2. Small numbers in the upper-right corner of reach cell indicate the source reference: 1 = Koricheva et al., 1998; 2 = Zvereva and Kozlov, 2006; 3 = Valkama et al., 2007; 4 = Peñuelas and Staudt, 2010; 5 = Lindroth, 2012; 6 = Julkunen-Tiitto et al., 2015; 7 = Robinson et al., 2012; 8 = Li et al., 2017. References 1–3 and 7–8 are meta-analyses and 4–6 literature reviews. Meta-analyses give the size of the effect, and the reviews give the frequency of observations in each response categories (–, 0, +).