The growth of trees depends on photosynthetic carbon gain by the leaves, which in turn relies on water and nutrient acquisition by the fine roots. Because the availability of carbon, water and nutrients fluctuates, trees can adjust their leaf and fine-root functional traits to maintain their resource uptake and growth rates. Aboveground, the variation in leaf traits is closely related to light availability, light uptake and tree growth. Within species, leaves show general, plastic responses to their light environment, so that trees can still intercept light when its availability changes. Across species, leaf traits are coordinated along a leaf economics spectrum (LES), which reflects species resource strategies. On the one end of this spectrum, acquisitive species have leaves that allow fast resource uptake and therefore fast tree growth. Conversely, species with a conservative strategy acquire resources more slowly, but retain them longer, so they can tolerate low resource availability.
Belowground, the relationships between fine-root functional traits, water and nutrient availability and acquisition, and tree growth are expected to be similar to those aboveground, but are still poorly understood. Understanding these relationships is essential as tree growth results from the simultaneous uptake of above- and belowground resources. Therefore, this thesis examines how fine-root traits relate to growth, and focuses on across- and within-species variation in tree root traits.
We first tested whether plant resource strategies can explain drought effects on tree growth across 10 common tree species that ranged from acquisitive to conservative species (Chapter 2). Based on tree-ring analyses, we found that the growth rates of all species were significantly lower in years with dry summers. Although the strength of these growth responses differed, these differences were not related to species resource strategies. However, when groundwater levels receded, acquisitive species grew slower but conservative species did not, which suggests root trait differences across these species. Drought effects on tree growth may thus not always be fully explained from an acquisitive or conservative resource strategy.
We further evaluated whether a root economics spectrum (RES) parallel to a LES can explain variation in fine-root functional traits across species (Chapter 3). Our literature review shows no consistent evidence for an RES, due to three fundamental differences between fine roots and leaves. First, fine-root traits are not only aimed at increased resource uptake or conservation, but are also constrained by the soil environment. Second, the relationships between traits and function are far less clear for roots than for leaves. Third, the expected relationships between fine-root traits and resource uptake are obscured by mycorrhizal fungi. Revealing the links between fine-root traits, resource acquisition, and growth across species, therefore requires a multidimensional approach that incorporates these different interacting variables.
Chapter 4 examines intraspecific variation in fine-root traits and mycorrhizal biomass in Fagus sylvatica L. and Picea abies L. forests on a poor, sandy soil and a resource-rich clay soil in the Netherlands. Both species increased their fine-root mass and fine-root growth rates on the sandy soils compared to the clay soils, but fine-root morphology did not differ between the soil types. In the P. abies stands, ectomycorrhizal biomass was larger on sand than on clay, possibly increasing tree resource uptake. Besides the strong increase in fine-root mass observed for both species, species may thus also differ in their fine-root plasticity strategies to cope with various soil environments.
To understand tree growth from below- and aboveground trait integration, we explored the impacts of fine-root mass and morphology on nutrient acquisition and tree fitness using a whole-tree growth model (Chapter 5). More specifically, we tested which combination(s) of fine-root mass and specific root length (SRL) led to optimal fitness, based on the uptake benefits (i.e. increasing the belowground uptake area) and carbon costs (i.e. turnover and respiration) of these traits. Our results show that tree fitness increased with fine-root mass but especially through an increase in SRL. Furthermore, both a combination of high fine-root mass and low SRL, and of low fine-root mass and high SRL, resulted in similar net carbon gain, indicating that alternative strategies that may lead to similar fitness.
To conclude, trees rely on various uptake strategies to ensure belowground resource uptake and tree growth in different environments. Specific root length is often expected to be tightly linked to tree growth, but this thesis shows that there is little support for this hypothesised relationship. Consequently, the functional meaning of SRL requires further study. Instead, fine-root mass and mycorrhizal symbiosis may present more important alternatives to enhance water and nutrient uptake, both across and within species. Moreover, to cope with the highly complex soil (resource) environment, species have adopted various other uptake strategies besides fine-root mass, morphology and mycorrhiza. This thesis stresses that a multidimensional root-trait framework is needed to link fine-root traits to tree growth, that can accommodate this variety of fine-root traits and the diversity of the soil environment.