Above- and Belowground Traits are Linked Within Species, but Vary Differently Along Environmental Gradients

Plant functional traits are frequently used to compare species strategies and predict functions such as growth, reproduction, or phenology. They can also quantify intraspecific variation and thereby assess species' capacity to respond to environmental variation. While aboveground traits are well studied, comparatively little is known about belowground traits. In this study, we investigated the intraspecific response of above- and belowground plant traits to climatic and soil parameters by cultivating five perennial herbaceous species in experimental beds at five Central European botanical gardens. After up to four growing seasons we measured above- and belowground traits, and analysed their relationships with climatic conditions, soil chemistry and nutrient availability. We specifically tested whether above- and belowground trait pairs with similar functions show corresponding responses to environmental parameters. We found that above- and belowground traits are linked, with plant height negatively correlated with root branching and leaf dry matter content (LDMC) positively correlated with root dry matter content (RDMC). However, we found no evidence for a coupling between the leaf economic spectrum (LES) and the root economic spectrum (RES). Overall, intraspecific variation in response to environmental factors was mainly driven by nutrient availability, reflected in soil C/N ratios, whereas climatic differences played a less important role. Traits differed in the extent to which they were influenced by broad-scale environmental conditions versus micro-environmental variation. Our results demonstrate that even relatively small environmental differences can generate intraspecific variation in above- and belowground traits. This highlights the importance of measuring traits directly at study sites when applying trait-based approaches. Although some easily measurable aboveground traits were linked to root traits in terms of function and environmental responses, we do not recommend using aboveground traits as proxies for root traits. The observed relationships were inconsistent, difficult to predict, and may depend on habitat conditions.