Conifer tree species differ in traits, growth, and drought resilience

Summary

Climate and climate change affect plant species worldwide. This PhD dissertation aims to understand how climatic variation and species traits affect the growth of a wide range of 28 conifer specie. I studied the stem growth of conifer species planted in 1960’s in a common garden experiment in the Netherlands and assessed growth sensitivity to climate variation and especially drought resilience, and its two underlying components, i.e., drought resistance (reduction in stem growth during a dry year) and drought recovery (a measure of achieving pre-drought growth rate). To identify possible mechanisms that can explain species differences in growth, I also measured 43 plant traits that are important for carbon, water, and nutrient use.

In chapter 2, a dendrochronological approach was used to assess the growth sensitivity of 19 conifer species to climatic variation. The growth of conifers was most negatively affected by summer drought (significantly for 89% of species), followed by spring frost (37%) and winter cold (32%). This implies that conifer species will lose productivity in a warmer and drier future climate during the growing season.

In chapter 3, I related drought resilience in stem growth to multiple dimensions of drought (timing, duration and severity), and addressed the possible underlying hydraulic mechanisms. Droughts led to 22% reduction in stem growth for 90% of species, but most species (80%) were resilient due to high recovery. Drought resistance decreased when droughts occurred early (significant for 65% of species), lasted longer (60%) or were more intense (55%). Surprisingly, hydraulic traits could not explain drought resilience of conifer species, perhaps because species avoid drought through other traits such as deep roots, leaf shedding or early leaf and cambial activity before summer drought. This chapter highlights the importance of addressing multiple dimensions of drought, i.e., timing, duration and severity to predict species responses to climate change.

The variation in growth and drought resistance might be determined by tracheids and pits since they could affect hydraulic safety and efficiency. In chapter 4, I assessed 1) the mechanisms underlying cavitation resistance and hydraulic conductivity, and 2) the phylogenetic signal of pits and tracheids across 28 conifer species. High cavitation resistance was determined by small pit size and strong pit sealing, which restrict air seeding, and are under strong phylogenetic control, whereas all hydraulic conductivity and tracheid traits were under weak phylogenetic control. Surprisingly, none of tracheid and pit traits could predict hydraulic conductivity, probably because species varied relatively little in hydraulic conductivity. Hydraulic conductivity only decreased with the cell wall thickness, probably due to the increased flow resistance between adjacent tracheids or reduced lumen area. In sum, conifer species differ largely in cavitation resistance, the underlying traits, and hydraulic conductivity. They may therefore differ strongly in their climatic distribution and drought responses to climate change.

Relatively few studies have assessed how a comprehensive suite of traits affects the growth and drought resilience of conifer species. In chapter 5, I measured 43 functional traits for 28 conifer species and assessed how multiple leaf and stem traits were associated, and how these traits affected stem growth and drought resilience. Two trait spectra were found, reflecting a trade-off between hydraulic- and biomechanical safety versus hydraulic efficiency, and a trade-off between tough, long-lived tissues versus high carbon assimilation rate. Stem growth rate only increased with hydraulic efficiency (i.e., pit aperture and tracheid diameter), and drought resilience decreased with leaf lifespan. A longer leaf lifespan reduces drought recovery and resilience because of a reduced ability to replace drought-damaged tissues and track new climatic conditions with new, acclimated leaves. These insights may improve growth- and carbon cycling-related models and predict how trees respond to a drier future.

In sum, this thesis shows that 1) most conifer species have low resistance to early, prolonged and intense droughts, but they have a high recovery and are, therefore,  highly resilient to drought; 2) small pit size and strong pit sealing capacity facilitate cavitation resistance, whereas none of the anatomical traits (except wall thickness) can explain hydraulic conductivity; 3) stem growth rate only increased with hydraulic efficiency (i.e., pit aperture diameter) and 4) drought resilience decreased with leaf lifespan rather than hydraulic traits. Conifer tree species can adopt multiple strategies of water or carbon use, which allows them to grow fast or be highly resilient to climatic variation.