|EFFECTS OF THINNING TREATMENT ON AN ECTOMYCORRHIZAL SUCCESSION UNDER SCOTS PINE
P.J.A. SHAW1, G. Kibby2 & J.Mayes1
1: University of Surrey Roehampton, Whitelands College London SW15 3SN, UK
2: ‘Field Mycology’, 59 Southway, Raynes Park, London SW20 9JG, UK
We describe a sixteen year dataset of ectomycorrhizal fruitbodies under Scots pine Pinus sylvestris, starting from seedlings, and explore the effects of a 50% thinning treatment imposed in year 12. The over-riding pattern in the data was of successional development, with Paxillus involutus and Laccaria proxima in the earliest years, followed by Suillus species, while in later years Amanita and Cortinarius species became prominent. The typical pattern was for each species in turn to increase to a maximum count then gradually decline without actually disappearing from the community (an addition succession). For nine out of the thirty species recorded the time profiles were fitted significantly by a log-normal curve, although the best-fit lognormal models consistently under-estimated the peak count. Thinning increased counts of Suillus bovinus, Gomphidius roseus and Cortinarius semisanguineus, but overall its effect on the community was minor. Rainfall in September was weakly correlated with successional advancement (measured as DCA first axis scores). Mean fruiting date increased significantly as the stands aged.
It has been known since the 1980s that the community of ectomycorrhizal fungi associated with tree hosts alters as the host tree matures (Last, Dighton & Mason 1987). This is usually manifested as changes in the species composition of the fruitbodies emerging under the host each autumn. Fruitbody community composition is assumed to correspond to changes in the populations of mycelia infecting tree roots, although the evidence supporting this assumption is ambiguous (Deacon, Donaldson & Last, 1983; Mason, Wilson & Last, 1984, Termorschuizen & Schaffers 1989a, Meier et al., 1990; Shaw et al., 1992, Gardes & Bruns 1996, Bruns & Bidartondo 2002). There exists a moderately extensive body of literature about fruitbody successions (Chu-chou 1979, Ford Mason & Pelham 1980, Gibson & Deacon 1980, Chu-Chou & Grace 1981; 1982, Last et al. 1984, Dighton & Mason 1985, Gapar & Lizon 1985, Fleming 1985, Dighton, Poskitt & Howard, 1986; Last Dighton & Mason 1987, Arnolds 1988, Termorschuizen & Schaffers 1987; 89a;b, Shaw & Lankey 1994, Jumpponen Trappe & Cazares 2002) but a recurring difficulty in this line of study is the need for a protracted time scale, which has usually been bypassed using stands of different ages on different sites, confounding successional age with site-specific factors such as differences in their soils (Termorschuizen & Schaffers 1989b, Vogt et al. 1992).
A typical pattern is for young trees of many species to be infected by a small number of ectomycorrhizal fungi including culturable Laccaria and Thelephora species. As trees mature the fruitbody community diversifies, with the species composition depending on the age of the host tree, its species and other factors such as soil conditions. The fungi which only appear under mature trees are generally recalcitrant to culture, or if culturable require relatively sugar-rich media (Gibson and Deacon 1990). Pollution (especially ammonia pollution) may impact on this process, reducing the diversity of the ectomycorrhizal community (Termorschuizen & Schaffers 1987; 1989b, Kowalski et al. 1989, Schaffers & Termorschuizen 1989, Peter Ayer & Egli 2001). Liming has also been shown to have a detrimental effect on the fruiting of ectomycorrhizal fungi (Agerer 1989).
There remains a shortage of manipulative experiments intended to test hypotheses about the mechanisms underlying mycorrhizal succession. Last et al. (1979) established the existence of a relationship between host tree vigour and fruitbody production by showing that the mechanical removal of leaves from trees led to a rapid cessation of ectomycorrhizal fruiting. Fox (1986) examined the effects of trenching on the colonisation of sterile birch seedlings by mycorrhizas when planted under mature trees, and found that the saplings were generally colonised by “late-stage” fungi (presumably off the adult trees’ roots) except where the adult tree was in disturbed soil and had undergone trenching (damaging the host’s roots). Fleming (1985) explored the effects of soil sterility on ectomycorrhizal infection, and showed that fungi typical of younger trees (Hebeloma and Thelephora) could persist on birch saplings moved into unsterile soil while axenic infections of Amanita and Leccinium (associated with mature trees) failed. Ohenoja (1978) and Guillette, Fraiture & Marchal (1989) explored the effects of adding fertilizer on ectomycorrhizal communities, finding species-specific responses but no consistent patterns. The climax-stage of the succession may be manipulated by deliberate inoculation of trees with edible species, mainly Tuber melanosporum (Hall, Brown & Byars 1994, Shaw, Lankey & Jourdan 1996) but also Suillus granulatus and Lactarius deliciosus (Guimberteau et al. 1989). Inoculation of young trees (mainly Quercus spp) with the valuable late stage fungus Tuber melanosporum is an established management method with a good success rate (Hall, Brown & Byars 1994), around which a body of folklore has built up (Rocchia 1995). In France truffle oaks produce Laccaria species, followed by Inocybe jurana, followed by T. melanosporum after at least ten years (Rocchia 1995, Shaw Jourdan & Lankey 1996). I. jurana is considered to be the precursor to Tuber fruitbody production in southern France (where it is known as Le Truffier), and its re-appearance under an old tree is said to indicate the end of its useful (Tuber-producing) life (Rocchia 1995). This constitutes an example of a mycorrhizal succession being reversible as the host enters senescence, consistent with the hypothesis that only mature trees can support the high glucose demands of late-stage ectomycorrhizal fungi (Dighton & Mason 1985, Gibson and Deacon 1990). Zambonelli et al. (2000) used DNA methods to studied infection by the truffle Tuber borchii of seedlings infected with this species, Laccaria bicolor and Hebeloma sinazapens, and found no hypogeous fruitbodies produced at all but extensive replacement of other fungi by T. borchii in all treatments (including uninfected controls), consistent with the hypothesis late-stage fungi (here T. borchii) gradually displace earlier successional species. To date there is no equivalent work on the root symbionts of trees that have ceased truffle production due to old age.
We report below some results from a sixteen year study of mycorrhizal fruitbodies under seven homogenous, even aged stands of Scots pine (Pinus sylvestris), testing the first five years effects on the fungal community of a selective thinning treatment imposed in February 1997. The ectomycorrhizal community at the Liphook site is well studied, having been part of an open-air SO2 fumigation experiment from 1986-1990 (Shaw et al. 1992, Shaw & Lankey 1994). Data from this early phase did show differences between the seven plots which were unrelated to pollution treatment but correlated with soil phosphate. The over-riding trend in the data however was successional, with Paxillus involutus and Laccaria proxima on the youngest trees, being gradually supplanted by a richer community which included Cortinarius and Lactarius species. Annual visits continued to monitor the succession throughout the 1990s, but preliminary inspection of data for 1990-1996 suggested that the total counts were falling and early successional species such as Laccaria proxima were returning. By 1996 the trees were 8m high but still largely at 1m spacing, and based on these observations we hypothesised that the relative poverty of the ectomycorrhizal community was due to over-crowding among the pine trees reducing the surplus of photosynthate available to mycorrhizal fungi. The hypothesis was tested by imposing a balanced thinning treatment on one half (one subplot) of each of the seven plots. The data below describe the first five years post-thinning data, aiming to assess the relative importances of soil chemistry, climatic conditions and host development on the ectomycorrhizal community.