Fine-Root Traits Reveal Contrasting Ecological Strategies in European Beech and Norway Spruce During Extreme Drought.

Trees adjust multiple structural and functional organ-specific characteristics, "traits", to cope with diverse soil conditions. Studies on traits are widely used to uncover ecological species adaptability to varying environments. However, fine-root traits are rarely studied for methodological reasons. We analyzed the adaptability of the fine-root systems of European beech and Norway spruce to extreme drought within species-specific tree groups at Kranzberger Forst (Germany), focusing on the seasonality of morphological, physiological, and biochemical key traits in view of carbon (C) and nitrogen dynamics. We hypothesized that fine roots of both species adjust to seasonal drought: with beech representing a "fast" (i.e. with fast C turnover), and spruce a "slow" (i.e. with long-term C retention) ecological strategy. We identified three functional fine-root categories, based on root function (absorptive or transport fine roots), and mycorrhizal status of the absorptive fine-roots (mycorrhizal or non-mycorrhizal). Solely the non-mycorrhizal absorptive roots adjusted in a species-specific manner supporting fine-root ecological strategy hypothesis. During drought, beech produced thin ephemeral (absorptive non-mycorrhizal) fine roots with high specific fine-root area and high respiratory activity, representing fast C turnover and enabling effective resource exploitation. These adjustments reflect a "fast" ecological strategy. Conversely, spruce absorptive fine roots did not respond to the soil moisture deficit by growth but instead increased root suberization. Drastically lowered respiratory activity of this functional category facilitated C retention and structural persistence during drought, indicating a "slow" ecological strategy in spruce. Absorptive mycorrhizal fine roots maintained respiration throughout the drought event in both tree species, but in spruce this was the only fine-root category with high respiration. This suggests, that spruce relies heavily on mycorrhizal associations as a method of drought resistance. Accumulation of non-structural carbohydrates and high C concentrations were observed in the transport fine roots of both species, indicating drought-induced osmotic protection of these roots. Thus, functional classification enabled us to determine that fine-root branches of each species are not tied to one sole ecological strategy. The suggested approach helps to better understand the complex interplay between structure and function belowground.

Belowground effects of enhanced tropospheric ozone and drought in a beech/spruce forest (Fagus sylvatica L./Picea abies [L.] Karst).

The effects of experimentally elevated O(3) on soil respiration rates, standing fine-root biomass, fine-root production and delta(13)C signature of newly produced fine roots were investigated in an adult European beech/Norway spruce forest in Germany during two subsequent years with contrasting rainfall patterns. During humid 2002, soil respiration rate was enhanced under elevated O(3) under beech and spruce, and was related to O(3)-stimulated fine-root production only in beech. During dry 2003, the stimulating effect of O(3) on soil respiration rate vanished under spruce, which was correlated with decreased fine-root production in spruce under drought, irrespective of the O(3) regime. delta(13)C signature of newly formed fine-roots was consistent with the differing g(s) of beech and spruce, and indicated stomatal limitation by O(3) in beech and by drought in spruce. Our study showed that drought can override the stimulating O(3) effects on fine-root dynamics and soil respiration in mature beech and spruce forests.

Vertical distribution of an ectomycorrhizal community in upper soil horizons of a young Norway spruce (Picea abies [L.] Karst.) stand of the Bavarian Limestone Alps.

The vertical niche differentiation of genera of ectomycorrhiza (ECM) was assessed in a 17-year-old Norway spruce (Picea abies [L.] Karst.) plantation on a mountainous dolomitic site (1,050 m above sea level) of the Bavarian Limestone Alps. We determined ECM anatomotypes, recorded the abundance of corresponding ECM root tips and classified them into groups of ECM exploration types, which refer to the organisation and the extent of their extramatrical mycelia. The abundance of ECM was highest in the organic soil layers, compared to the mineral soil horizon. The ordination of the ECM communities and of the exploration types revealed segregation related to soil horizon properties. While Cenococcum geophilum preferred the organic soil layers, Lactarius spp., Tomentella spp. and Craterellus tubaeformis were generally most abundant in the mineral soil horizons. Cenococcum geophilum was the predominant species, possibly based on enhanced competitiveness under the prevailing site conditions. The short-distance exploration types (e.g. C. geophilum) preferentially colonised the organic soil layer, whereas the contact types (e.g. most of the Tomentella spp., C. tubaeformis) together with medium-distance types (e.g. Amphinema byssoides) were primarily associated with the underlying A-horizons. Therefore, the soil horizons had an important effect on the distribution of ECM and on their community structure. The spatial niche differentiation of ECM genera and exploration types is discussed in regard to specific physico-chemical properties of soil horizon and the assumed ecophysiological strategies of ECM.

Above-ground space sequestration determines competitive success in juvenile beech and spruce trees.

A 2-yr phytotron study was conducted to investigate the intra- and inter-specific competitive behaviour of juvenile beech (Fagus sylvatica) and spruce (Picea abies). Competitiveness was analysed by quantifying the resource budgets that occur along structures and within occupied space of relevance for competitive interaction. Ambient and elevated CO(2) and ozone (O(3)) regimes were applied throughout two growing seasons as stressors for provoking changes in resource budgets, growth and allocation to facilitate the competition analysis. The hypothesis tested was that the ability to sequester space at low structural cost will determine the competitive success. Spruce was a stronger competitor than beech, as displayed by its higher above-ground biomass increments in mixed culture compared with monoculture. A crucial factor in the competitive success of spruce was its ability to enlarge crown volume at low structural costs, supporting the hypothesis. Interspecific competition with spruce resulted in a size-independent readjustment of above-ground allocation in beech (reduced leaf : shoot biomass ratio). The efficient use of resources for above-ground space sequestration proved to be a parameter that quantitatively reflects competitiveness.

Competition modifies effects of enhanced ozone/carbon dioxide concentrations on carbohydrate and biomass accumulation in juvenile Norway spruce and European beech.

Elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) affect primary metabolism of trees in opposite ways. We studied their potential interactions on carbohydrate concentrations and contents. Two hypotheses currently under debate were tested. (1) Stimulation of primary metabolism by prolonged exposure to elevated [CO2] does not compensate for the adverse effects of O3 on carbohydrate accumulation and biomass partitioning to the root. (2) Growth in a mixed-species planting will repress plant responses to elevated [O3] and [CO2] relative to conditions in a monoculture. To this end, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) saplings grown under conditions of intra- and interspecific competition were pre-acclimated for 1 year to ambient or elevated [CO2]. In the following 2-year phytotron study, trees were exposed to factorial combinations of ambient and elevated [O3] and [CO2]. The total carbohydrate content (sugar and starch) of spruce was greater in plants exposed to elevated [CO2] than in plants exposed to ambient [CO2]. In beech, the opposite response was observed, especially when this species was grown in combination with spruce. Overall, the data did not support Hypothesis 1, because the adverse effects of O3 were counteracted by elevated [CO2]. Support for Hypothesis 2 was species-dependent. In beech saplings, reduction of carbohydrates by elevated [O3] and stimulation by elevated [CO2] were repressed by competitive interaction with spruce. In contrast, in spruce, stimulation of carbohydrates by elevated [CO2] was similar in mono- and mixed cultures. Thus Hypothesis 2 was supported for beech but not spruce. We conclude that, in juvenile beech and spruce, a 3-year exposure to elevated [CO2] counteracts the adverse effects of O3 on carbohydrate concentrations and contents. For beech, sensitivity to elevated [CO2] and [O3] was high in monoculture but was largely repressed by interspecific competition with spruce. In contrast, the response of spruce to perturbations of atmospheric chemistry was not significantly affected by either intra- or interspecific competition.