Combined Effects of Nitrogen Deposition and Climate on Root-to-Shoot Ratios and Root Morphology in Temperate Tree Regeneration

Abstract

Nitrogen (N) availability is known to interact with the drought tolerance in plants, but its effects on the morphological acclimation of forest trees to low precipitation have rarely been investigated. Based on a Germany-wide field-study design covering different combinations of climatic conditions and atmospheric N deposition, we harvested young trees up to 70 cm in height of Fagus sylvatica , Quercus petraea , Pseudotsuga menziesii , and Abies alba . We tested the hypothesis that high N deposition decreases the root/shoot ratio, alters fine root morphology (for example, by reducing specific root length of absorptive roots) and reduces mycorrhizal colonization rate, which are factors that would reduce the capacity for water uptake under drought. European beech showed reduced root/shoot ratios at high N deposition and reduced specific root length and mycorrhizal colonization of absorptive roots at sites with both high N deposition and low precipitation. These observations were consistent with the optimal portioning theory, as less investment in belowground biomass was required to meet the N demand at high N deposition. Sessile oak showed no effect of N deposition or mean annual precipitation on the parameters studied, indicating that the gradients studied were not large enough to cause changes. Douglas-fir and silver fir expanded their root systems with increasing N deposition, with higher specific root length in Douglas-fir and a trend towards higher root tip abundance in silver fir. Thus, in contrast to European beech, we found no indication that the tree's ability to cope with drought was reduced by high N deposition in sessile oak, Douglas-fir, and silver fir. The combined negative effect of drought and high N deposition on the root system in European beech as Central Europe’s most dominant tree species in the natural vegetation emphasizes the need for curtailing N emissions, especially from agriculture.

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