Fine roots of Scots pine and European beech respond differently to changes in nutrient availability in a mixed forest: results from a 4-year experiment

Abstract A nutrient availability experiment was carried out for four years in a mature mixed stand of Pinus sylvestris and Fagus sylvatica in the South-western Pyrenees mountains. Initial homogeneous pine regeneration (~58 years ago) was followed by uneven beech regeneration (~45 years ago), leading to the current successional stage in which areas resembling pine monospecific stands are at close distances and under comparable environmental conditions to mixed pine−beech areas. Soil and fine root samples were collected beside trees under three soil nutrient availability treatments: control, litter exclusion, and nitrogen-phosphorus-potassium (NPK) fertilization. Soil samples were collected twice a year (in May and October from 2017 to 2020), and fine root samples were collected twice (in May 2018 and May 2020). Soil fertilization had a bigger impact than litter exclusion on soil chemical parameters, leading to lower pH and NH4+ values. Also, increases in soil total nitrogen, soil available phosphorus, and microbial phosphorus were found after fertilization. Scots pine showed overall lower fine root biomass, specific root length, and root tissue density than beech, presenting also very little response to treatments. On the other hand, beech showed higher biomass and specific root length, but, when soil fertility was enhanced, lower root tissue density was recorded. A negative beech effect over pine fine root biomass was observed, as pine root presence decreased when growing near beech trees in mixed areas. Beech trees not only demonstrated an elevated capability to compete for soil resources but also showed greater root plasticity than pine, enhancing beech potential to take advantage of new soil resources. Our results do not support a complementary niche partitioning but rather a competitive interaction in which European beech has an edge on colonizing the soil volume in the detriment of Scots pine.