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- Materials and Methods
The dynamics of short root emergence and survival of ectomycorrhizal roots in forest trees has not been a very well understood area of tree and soil sciences until recently. Emergence and decline of fine roots occur simultaneously during the growing season, making estimates of production and mortality by sequential destructive methods impossible. Additionally, mycorrhizal roots with small diameter, that is < 0.3 mm, through which most of the mineral and water uptake takes place (Jensén & Petersson, 1980; Yanai, 1994), are not detected using these methods (Hendrick & Pregitzer, 1993). The total surface area of roots varies as a function of number, size, and longevity (Fitter & Stickland, 1992). Like leaves, mycorrhizal roots are modular in nature, and their emergence and disappearance may be very dynamic (Hooker et al., 1995; Fitter, 1996; Majdi & Nylund, 1996).
Tree seedlings grown in observation boxes, using artificial substrates (Finlay & Read, 1986) have been employed for ectomycorrhiza studies, but no data on their dynamics (production and longevity) and branching order have been presented, supposedly because of the artificiality of the systems and the lack of methods to follow individual roots. Previous mycorrhizal studies (Kårén et al., 1997) in the present study site have been focused on species diversity as observed through fruit bodies and by PCR-RFLP (genetic fingerprinting) of root tips.
Minirhizotrons have been used to study root processes from demographic perspective, and to quantify the rates of mycorrhizal short root production and longevity (Majdi & Nylund, 1996). However, studies on the pattern of mycorrhizal root emergence in relation to branching order and nutrient availability are rare and the influence of mycorrhizal root order on life span has not been investigated earlier. The advent of the minirhizotron technique has opened up new opportunities in this field. In some previous studies (Majdi & Kangas, 1997), the dynamics of long roots (production, and longevity) have been studied, and the possibilities and limitations of this technique have been discussed (Majdi, 1996).
In this paper, we present a qualitative analysis over a 4-yr period on ethology viz emergence and life span of mycorrhizal roots. Our main objective was to investigate the emergence frequency of individual mycorrhizal roots (< 0.3 mm in diameter) and their longevity in relation to branching order and nutrient availability in a Norway spruce stand in SW Sweden.
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- Materials and Methods
Previous results from the study site showed that long roots in general have a long life span even with high nitrogen input (Majdi & Kangas, 1997), and that root production was highest in late summer. The emergence of mycorrhizal roots in relation to long root production has a seasonal variability as long root production is low from the beginning of the growing season and emergence of mycorrhizal roots from these roots is high (Fig. 2a,b).
In contrast to data obtained from sequential cores, the results from the present study show that minirhizotron technique can be used to study even the dynamics of mycorrhizal roots since the emergence and disappearance of mycorrhizal roots can be measured simultaneously. However, minirhizotron studies are time consuming especially when root longevity is high. The present results cover mainly mineral soil layers as LFH-layer only occupied 3 frames (1.12 × 1.35 cm). Yet 50% of fine roots are located in mineral soil layers (cf. Majdi & Persson, 1995).
The root order within the branched hierarchy of the roots (orders 1, 2 and 3) was clearly an important determinant of life span. The median longevity of root order 3 was much lower than other mycorrhizal root orders (Fig. 3a). In contrast Reid et al. (1993), reported the longer life span for higher order of nonmycorrhizal roots of kiwifruit.
The principal reason why roots of order 3 had a shorter life span lies in the fact that mature order 2 roots carry them, and thus they emerge later but die simultaneously with their carriers. However, the difference may also be caused by the distance to the long root, which reduces access to carbon from the long root, and also by the greater activity in terms of water and nutrient uptake and respiration (Pregitzer et al., 1997). The root orders 1 and 2 remained unsuberized and vital during the winter. These short roots ought to be important for water and nutrient retention during the winter since they occur at depths where the soil is not normally frozen, but also in early spring when trees start growing and new root production has not started.
In agreement with earlier root studies (Majdi & Kangas, 1997) the median longevity of mycorrhizal roots (across orders) decreased with depth (Fig. 3b). This effect may be related to soil temperature and water availability (Eissenstat & Yanai, 1997). The percentage of mycorrhizal root orders showed different patterns in control and IL treatments (Fig. 4a). Addition of nitrogen decreased the number of mycorrhizal root order 3 proportionally compared to other orders while in control plots with relatively less access to nitrogen and water the branched density of mycorrhizal roots (orders 2 and 3) was increased.
Our findings show that in relatively nutrient poor soils, root systems exploit the soil by increased branching. Nitrogen availability decreased the number of mycorrhizal roots (order 3) and enhanced the proportion of unbranched mycorrhizal roots (Fig. 4a). In culture experiments, Read (1991) reported that the ectomycorrhiza formation can be regarded as ecologically advantageous, facilitating the exploitation of nutrient-poor environments. Other results on effects of nitrogen addition (ammonium sulphate) indicate that neither mycorrhizal colonization nor relative amount of fungal biomass decrease (Kårén & Nylund, 1997).
The similar pattern of longevity in IL and control plots supports other observations (Kårén & Nylund, 1997) that the high nitrogen input (100 kg ha−1) does not endanger the overall mycorrhizal colonization. In addition Wiklund et al. (1995), investigated sporocarp production of ectomycorrhizal fungi at the study site and found that, sporocarp production was decreased by nitrogen addition. In spite of nitrogen addition at Skogay site, the overall impression of the stand is good health, vigorous growth, and well developed species rich ectomycorrhiza. Our previous studies (Kårén & Nylund, 1997), focused on humus-layer mycorrhiza, showed nearly 100% mycorrhiza colonisation of roots in plots treated by nitrogen. However, investigations by Kårén & Nylund (1996) showed that sporocarp surveys correlate poorly with community structure of mycorrhizal colonisation.
We conclude that the pattern of mycorrhizal root longevity depends on the branching order of mycorrhizal roots, while addition of N decreases the proportion of branched mycorrhizal roots. We conclude also that nitrogen addition does not reduce the longevity of mycorrhizal short roots and, consequently, carbon consumption for root growth and construction is likely reduced.