Microtopographic refugia against drought in temperate forests: Lower water availability but more extensive fine root system in mounds than in pits

Water stress is one of the primary drivers of forest mortality and, with climate change, an increase in drought duration and severity is expected. Microtopography may offer hydrologic refugia against exposure to drought but how these local conditions affect fine root development of trees, and thus their sensitivity to drought, remains, however, poorly known. We studied how microtopography and water limitations influence the fine root system of two tree species, sugar maple (Acer saccharum) and bitternut hickory (Carya cordiformis), in natural forest stands at 16 sites in the temperate forest of southern Quebec (Canada). At each site, we measured the fine root standing crop and production (length, mass, and specific root length (SRL)) of four mature trees per species, located in pits and mounds. Half of them were also exposed to a rainfall exclusion for three months. We measured soil water potential at the base of each tree throughout the growing season. Soil water availability was significantly lower in mounds (mean +/- SE: -22.5 +/- 0.9 kPa) than in pits (-16.0 +/- 1.0 kPa). We found a more extensive fine root system for trees in mounds than in pits, both in terms of length and mass; the standing crop length was 19% greater and the production length at a 20-40 cm depth was 77% greater in mounds than in pits. Fine root standing crop was slightly reduced in the rainfall exclusion treatment, whatever the microtopographical position. The sugar maple had a greater standing crop mass (but not length), and a greater fine root production at deeper soil layers (20-40 cm) than the bitternut hickory. Still, microtopography had a greater influence than tree species on fine root production at the deepest soil layer (30-40 cm). Overall, this study is a first step towards a better understanding of the interaction between micro-environmental conditions and rooting strategies. Our findings highlight the distinct acclimation to microtopography of the fine root system of trees, which could play a crucial role regarding their response to drought.