Barbara Meyers, University of Freiburg in Germany, discusses her article: Soil nutrient availability rather than spatial nutrient heterogeneity shapes the intraspecific response of root architectural, morphological and mycorrhizal traits in Vaccinium myrtillus
Roots make up for a large proportion of plant biomass and play a central role in several plant functions: accessing water and nutrients and providing anchorage to the plant. Roots are characterized by various traits that define the strategies for accomplishing these functions. These traits largely vary among species, but they also vary within species with individuals responding to changing environmental conditions. Importantly, high plasticity in roots at the intraspecific level is expected to play an important role in defining how species adapt to environmental change. While large efforts have been taken to measure and find global patterns in leaf traits, we know a lot less about root traits, which are known to be highly variable at the intraspecific and local scale. This gap matters as shifts in plant strategies are expected to have large impacts on ecosystem functioning, such as nutrient and carbon cycling, or soil stability.
Therefore, the question we asked was: in temperate forests, how much do root traits change with nutrient availability and with the spatial distribution of those nutrients? To study these patterns of root trait variation at the intraspecific level, we decided to focus on a single, dominant understory species, the bilberry (Vaccinium myrtillus), in the Black Forest in southern Germany.
To answer these questions, we set up fertilization experiments across ten 1-ha forest sites. Within each site, we established 2.5 × 2.5 m subplots and applied fertilizer once a year for three years. Treatments increased nutrients either homogeneously (evenly spread) or heterogeneously (patchy). To capture root strategies comprehensively, we measured an extensive trait set, from morphological traits (those usually measured) to traits more closely related to nutrient acquisition, like mycorrhizal colonization and enzyme activity (produced to access nutrients in organic matter).
We found that increased nutrient availability with fertilization drove a change in plant trait strategies, with shorter, thicker roots and a more compact root system (higher root branching intensity). This result was combined with a lower colonization intensity of ericoid mycorrhizal fungi (associated with plants from the Ericaceae family, like the bilberry). The spatial distribution of the nutrients did not strongly affect the variability in root traits, only increasing the variability of root phosphatase activity, thereby showing a lower importance than overall availability of nutrients. Finally, when assessing how traits correlated with each other, using the core traits usually included (morphological traits and nitrogen content), we observed a similar pattern to the one observed at a global scale. However, additional traits that can be more directly related to plant nutrient acquisition did not align, highlighting the need to further explore patterns of root trait variations.
Observing the large variation at such a small scale underlines that, for roots, the scale we studied is sufficient to capture shifts in plant strategy as well as to study patterns in root trait variation. This highlights the need to include small-scale data in research on roots, an important step towards improving our understanding and modelling of how root trait variation impacts related ecosystem functions, like soil carbon storage and nutrient cycling.
