Fine-Root Strategies along an Environmental Gradient in Western Canada

Applying a trait-based approach to belowground ecology has helped researchers to identify functional characteristics of belowground plant strategies that define root resource foraging, economics strategies and plants adjustments to the abiotic environment. Yet, drivers of root trait variation remain poorly understood, which limits our ability to model and predict plant responses to global changes. Here, we quantify the responses of fine-root traits in the widely distributed, Pseudotsuga menziesii, to edaphic and climatic variations across a strong climate gradient in Western Canada. We measured key morphological, architectural, and chemical traits for each of the first three root orders separately as well as patterns of mycorrhizal colonization. We show that root morphology and nitrogen content represent one axis for root adjustments to the environment and are primarily related to mean annual temperature (MAT) whereas, architectural traits form an independent axis of variation and are related to soil available phosphorus (P). Consistent within each root order, colder climates were associated with larger root diameter, lower root tissue density and lower C:N ratio. Yet, no relationship between MAT and specific root length was found. Low soil P was closely associated with cold and dry climates and lead to a decrease in branching intensity. This was further associated with a more dichotomous branching pattern, but not related to colonization rate. Overall, we show that fine-root adjustments to cold, dry and P-depleted soils fall into the conservative end of the economics spectrum. Changes in the branching habit might allow roots to explore soil volumes more intensively while constructing less 1^st order root. Our study clarifies mechanisms of fine-root adjustment to the abiotic environment. However, as root architecture is not included in most trait studies, trade-offs between branching habit and mycorrhizal colonization and their role in root resource acquisition strategies remain unclear.