Description

In Phases I-III of SFB607 principal characteristics of space occupation and nutrient gain of ectomycorrhizal fungi (ECM) were studied in laboratory and field experiments. These approaches provided the basis for evaluating the role of ECM for nutrient acquisition of mature trees under the impact of elevated ground-level ozone [O₃].

Therefore, Phase IV of SFB607 focuses on the quantification of the nutrient availability of 60-year-old Norway spruce trees ( Picea abies (L.) Karst.) in the stand by determining space-related cost-benefit ratios of associated ectomycorrhizal fungi (ECM). The classification of ECM according to their mycelial extension (exploration types, i.e. contact, short distance, medium distance and long distance, Agerer 2001) thereby serves as approach to quantify the cost of ECM for the host tree. Space occupation and costs of ECM can then be scaled up to the whole-tree level via the root system. Space exploitation is evaluated via nutrient-related efficiency of space occupation (project B10) and enzyme activity of the mycorrhizosphere (B9). The space occupation and exploitation can be linked to carbon [C] allocation within plants (project B5) as well as to allometry of trees and stand (project B1). Including root and mycorrhiza turnover as well as costs for respiration, the cost-benefit balance can be scaled up to whole-tree and stand level, in order to assess the contribution to the fungal partner on the total C balance (projects C2, C3, C7).

Hypotheses

•  Under stress, below-ground space occupation by mycorrhizae and the efficiency in space occupation are reduced in the way that this leads to a decrease in nutrient acquisition and, therefore, reduced nutrient content and plant biomass increment of the host plant.

•  Efficiency in competitiveness and stress tolerance of the plant increase with increasing C allocation to mycorrhiza

•  Pressure of competition, resource deficit and/or pathogen infestation increase efficiency in space occupation and resource uptake in root and shoot

Methods

At the free-air O₃ fumigation facility “Kranzberger Forst” (SE-Germany), soil and mycorrhizal roots were sampled from plots (3x2.5 m; 25x25 cm sample grid) underneath spruce trees exposed to double-ambient O₃ concentrations (2xO₃) at crown level, as well as underneath control trees under ambient O₃ (1xO₃). Frequency and abundance of ECM species were studied, and space occupation of roots and mycelia was investigated depending on ECM exploration types. Nutrient concentrations in soil and roots were linked to the invested mycorrhizal biomass in order to assess costs of nutrient uptake under disturbance by elevated above-ground O₃.