The role of ectomycorrhiza in forest nurseries

343

Agriculture, Ecosystems and Environment, 28 E l s e v i e r S c i e n c e Publishers B.V., Amsterdam Printed in Czechoslovakia

THE ROLE OF ECTOMYCORRHIZA

(1989)

343-350

IN FOREST NURSERIES

Peitsa Mikola Department of Silviculture, University of Helsinki Unioninkatu 40 B, 00170 Helsinki, Finland i. Introduction The relationship between nutrient availability, tree growth, and mycorrhizal relations is illustrated in Fig. i. Accordinq to Bjorkman (1942) and Mikola and Laiho (1962), in natural forests of Scandinavia the soil fertility almost invariably is below the optimum level for mycorrhiza development and, thus( the number of healthy and vigorous mycorrhizae usually increases along with increasing site fertility (Table i.). Conditions in nurseries are different. Nursery managers want to produce healthy and vigorous seedlings, and this is possible with liberal use of fertilizers and irrigation. Then.there is a risk of increasing the nutrient level above the optimum for mycorrhiza development or even to such a degree that seedlings are totally devoid of mycorrhizae at the time of planting out. Seedlings can be big a n d dark green but their value for afforestation may be low because of both lack of mycorrhizae and an unfavorable s h o o t : r o o t ratio. In traditional bare-root nurseries excessive fertilization, in regard to mycorrhiza development, seldom has been practised. Many new techniques, however, have recently been adopted in nursery management, e.g. plastic greenhouses, artificial growth substrates, various kinds of seedling containers, regular chemical ~est control, and automatization of irrigation and fertilization. A general trend has been to produce plantable seedlings in ever shorter time, and for this aim the use of fertilizers, nitrogen in particular, has been increasing, even to such an extent that mycorrhiza formation is retarded or completely prevented. Both in the United States and Sweden, for instance, seedlings often are "spoiled" with optimal conditions in nurserles and completely lack mycorrhizae (Molina 1980; Stenstrom et al. 1986). The suppressing effect of heavy nitrogen fertilization on ~ycorrhizae was clearly demonstrated, for instance, in a Finnish study (Table 2~. 2. Fungal

population

Nursery soil fertility, as well a other soil properties, affect the species composition of the population of mycorrhizal fungi. Since both fertility and other soil properties in nurseries usually greatly differ from those

344

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o @

J @

Y

I n c r e a s i n g a v a i l a b l e N an~ P

F i g . 1. Tree growth and m y c o r r h i z a development as a f f e c t e d by i n c r e a l i n g n u t r i e n ~ a v a i l a b i l i t y l q $ 6 ).

(Bj~rkman Table

1.

Site

The number of A m y c o r r h i z a tips pr. dm 2 in m a t u r e spruce forests on d i f f e r e n t sites (Mikola and Laiho 1962) Ca!luna type

No. of A mycorrhiza

Table

Vaccinium type

Myrtillus type

1320

360

1760

OxalisMyrtl~us type

Oxalis~ e m u m type

1580

5450

The height, dry weight, and number of d i c h o t o m o u s mycorrhizal root tlps. Qf 1-year old Scots pine seedlings grown on peat ~j s u b s t r a t e in paper pots with dlfferent nitrogen fertilization (Anttila and L~hde 1977)

2.

Additional N fgrtilization,

0

10

30

90

6

i0

14

13

0.i

0.2

0.9

0.7

No. of d i c h o t o m o u s m y c o r r h i z a tips

80

i00

20

0

Type of m y c o r r h i z a

Ecto

Ectendo

Ectendo

-

Height,

cm

Dry weight,

1)

g/m 2

g

The peat had received as basic amendment 4 kg of ground limestone and 1 kg of NPK per m~ corresponding to ca. I0 g N per mZ of nursery bed.

345

of planting sites, a change of the fungal population is often observed after outplanting the seedlings (e.g. Mikola 1965). Even mycorrhizal seedlings may have difficulties in qetting established when transplanted from the nursery to forest soil. Bj~rkman (1953, 1956), for instance, suspected that the common stagnation of spruce seedlings after planting was caused by the change of the mycorrhizal population from "nursery species" to "forest species". Forest nurseries probably contain fewer ectomycorrhizal fungal species than natural forest soils (Molina and Trappe 1984). The fungal population of a nursery depends, besides fertility and other soil properties, on the former use "of the site and the sources of new infection. Some species seem to survive in the soil for a long time (Wilde 1954), although we do not know which species and how long. Other species can freely spread to nurseries from the surroundings in the form of spores. Thelephora terrestris is the best known and probably most common ectomycorrhizal fungus in forest nurseries (Hacskaylo 1965; Marx et al. 1970; Molina and Tra~pe 1984). It produces spores abundantly and colonizes seedllng roots rapidly even in fumigated soil. The early colonization of the root systems by Thelephora terrestris may preclude colonization by other fungi which produce spores later in the year (Marx 1980). It is also a frequent contaminant in nursery and greenhouse experiments. Other typical "nursery fungi" are Laccaria laccata, Inocybe lacera, Hebeloma crustuliniforme, and several species of Rhizopoqon (Gibson 1963; Ivory 1980; Molina and Trappe 1984). Sporocarps of these species are common in many nurseries, providing easy source of infection. The ectendomycorrhizal fungus Wilcoxina mikolae or related species also are characteristic of forest nurseries both in the temperate zone (Laiho 1965; Molina and Trappe 1984; Mikola 1988) and in the tropics (Mikola 1980). The dependence of the fungal population on soil fertility clearly appeared in the experiments of Anttila and Lahde (1977). At the lowest fertility level all the mycorrhizae were ectotrophic, at higher level mycorrhizae were mainly ectendotrophic, whereas no mycorrhizae were found in seedlings which received the highest nitrogen fertilization (Table 2.). The replacement of the ectendotrophic fungus (Wilcoxina) by other fungi after transplanting into forest soil has been demonstrated (Mikola 1965). Many common ectomycorrhizal fungi, e.g. species of Boletus, Suillus Amanita, Lactarius, Russula, and Cortinarius, seem ~o 5-6 rare or maybe t o ~ F F y - - a b s e n t from forest nurseries. 3. Manipulation of fungi in nurseries Since the hundreds of ectomycorrhizal fungi differ from each other in regard to ecological requirements and probably in symbiotic efficiency too, suggestions have been made to apply fungal pure cultures for nursery inoculation, instead of indiscriminate soil inocula. In spite of numerous experiments, however, progress along this line has

346

been slow (see refs. main obstacles:

in Trappe

1977).

These have been the

(i) Isolation and cultivation of ectomycorrhizal fungi is difficult. Many potential mycorrhiza formers have never been isolated and the growth of other species is extremely slow in pure culture. Production of vegetative inoculum in large quantities is possible with few species only. (2) Symbiotic efficiency of individual species is not known. Even different strains of the same species can greatly differ from each other. Because of difficulties in isolation and cultivation of fungi, comparative studies are ~ew and their results inconsistent. Probably many efficient symbionts have never been isolated. (3) Inoculation of pure cultures into nursery soil has usually failed. The fungus has not been able to compete with indigenous fungi present in the soil. Soil desinfection prior to inoculation is usually necessary. Even then s o i l can be so rapidly colonized by air-borne spores of Thelephora terrestris or other fungi that inoculation fails. Pure culture" inoculation was first practised successfully with Pinus cembra seedlings for afforestation ~ f alplne m e a d o ~ o v e present tree-line (Moser 1958, 1959). For the last two decades, the main emphasis of ectomycorrhizal manipulation has been in the use of Pisolithus tinctorius for nursery inoculation. In many respects Pisolithus tinctorius is an ideal fungus for nursery inoculation. It is easy to isolate and its growth is fairly fast in ~ure culture. It produces tremendous amounts of spores whlch can easily be collected and used for soil inoculation. It also has a wide geographic distribution and ecological range (Marx 1977). It tolerates high temperatures and therefore has been particularly recommended for nursery inoculation in the tropics (Momoh and Gbadegesin 1980). In the southern United States seedlings inoculated with Pisolithus tinctorius have shown better survival and early growth than non-inoculated seedlings on extremely adverse sites, such as coal spoils, gravel pits and other denuded soils. In the tropics, Pisolithus tinctorius inoculation has lead to better survival and early growth of pines under conditions of seasonal drought than inoculation with mixed soil population (Momoh and Gbadegesin 1980), whereas under more favorable moisture conditions some other fungi have been superior to Pisolithus (Ofosu-Asiedu 1980). SinGe Pisolithus tinctorius naturally occurs on poor sites only, it probably does not belong to the normal fungal population o f fertile nursery soils. Because of its ecological adaptability, however, nursery soil inoculation with Pisolithus is usually successful. Techniques have been developed for mass production of Pisolithus inoculum a n d inoculation of both bare-root and container-grown seedlings (Marx et al. 1982; Marx et al. 1984). For successful inoculation, soil sterilization is necessary.

347

Without soil fumigation Pisolithus is not able to compete with T h e l e p h o r a t e r r e s t r i s and other indigenous fungi. Even in fumigated soils seedling roots are rapidly infected, besides by the introduced Pisolithus, by T h e l e p h o r a and e v e n t u a l l y other air-borne spores. Likewise in studies with other fungi, T h e l e p h o r a t e r r e s t r i s has rapidly infected both inoculated and control seedlings (Stenstrom et al. 1986; Tyminska et al. 1986). P i s o l i t h u s t i n c t o r i u s is currently the only e c t o m y c o r r h i z a l fungus for which the t e c h n o l o g y exists of mass production of v e g e t a t i v e inoculum and large scale inoculation of nurserles. Basidiospores of Pisolithus and some other fungi, e.g. Rhizopogon spp. also have been s u c c e s s f u l l y used for inoculation of seedlings (Marx 1980; C a s t e l l a n o et al. 1985; C a s t e l l a n o and Trappe 1985). The main benefit from pure culture inoculation has usually been an increased survival of seedlings after t r a n s p l a n t i n g to the field. A faster growth of inoculated s e e d l i n g s is often o b s e r v e d even 2-3 years after planting, e s p e c i a l l y on poor sites or in c l i m a t i c a l l y harsh c o n d i t i o n s (Marx 1980; M o m o h and G b a d e g e s i n 1980). When seedlings are p l a n t e d in ordinary forest sites, the nursery inoculum is rapidly replaced by indigenous forest soil fungi. Even then inoculated seedlings may maintain the advantage that inoculation gave them to overcome the shock of t r a n s f e r to the new site (Stenstrom et al. 1986). 4. D i s c u s s i o n Pure culture inoculation can c u r r e n t l y be r e c o m m e n d e d for raising seedlings for extreme conditions, e.g. for reclamation of disturbed sites. For normal forest r e g e n e r a t i o n it is s u f f i c i e n t to watch root d e v e l o p m e n t in nurseries, to be sure that the seedlings are well m y c o r r h i z a l at the time of leaving the nursery. This aim is usually achieved by p r o v i d i n g sufficient aeration and irri~ation, r e g u l a t i n g pH if needed, and avoiding too heavy fertllization, particularly with nitrogen. If soil is fumigated, as is common practice in nulseries today, or ~eat or other non-forest soil is used as substrate, i n o c u l a t i o n with forest humus may be advisable to g u a r a n t e e rapid m y c o r r h i z a l infection. N u m e r o u s pesticides, both fungicides, insecticides, and herbicides, are used in modern nursery practice. S o m e t i m e s fungicides are used intentionally to destroy mycorrhizal fungi, T h e l e p h o r a t e r r e s t r i s in particular, from the soil prior to inoculation with known species. More often, however, fungicides are applied to the soil to destroy d a m p i n g - o f f and other p a t h o g e n i c fungi, and then there is a risk of killing ~ycorrhizal fungi at the same time. Thus the use of fungicides, as well as other pesticides, apparently increases the need for ectomycorrhizal inoculation. The effects of p e s t i c i d e s on m y c o r r h i z a l fungi and m y c o r r h i z a d e v e l o p m e n t is an endless field of research, since new chemicals are c o n t i n u o u s l y coming to the market, and before r e c o m m e n d i n g them to forest nursery use their e f f e c t s on m y c o r r h i z a e have to be known (cf. Trappe et al. 1984).

348

5. R e f e r e n c e s Anttila, T. and L~hde, E. 1977. E f f e c t of f e r t i l i z a t i o n on the development of containerized pine seedlings in a nursery. - Fol. For. 314. Bjorkman, E. 1942. Uber die B e d i n g u n g e n b i l d u n g bei K i e f e r und Fichte. - Symb. Bot.

der MykorrhizaUps. VI:2.

Bjorkman, E. 1949. The ecological significance of the e c t o t r o p h i c m y c o r r h i z a l a s s o c i a t i o n in forest trees. - Sv. Bot. Tidskr. 43:223-262. Bj6rkman, E. 1953. F a c t o r s a r r e s t i n g the e a r l y g r o w t h of spruce after plantation in n o r t h e r n Sweden. - Norrl. skogs.f. Tidskr., pp. 284-316. Bjorkman, E. 1956. 0bet die ~ a t u r der M y k o r r h i z a b i l d u n g unter besonderer Berucksichtigung der Waldbaume und die A n w e n d u n g in d e r f o r s t l i c h e n Praxis. - Forstw. Cbl. 75: 257-286. Castellano, M.A. and Trappe, J.M. 1985. E c t o m y c o r r h i z a l ~ o r m a t i o n a n d p l a n t a t i o n p e r f o r m a n c e of D o u g l a s - f i r n u r s e r y s t o c k i n o c u l a t e d w i t h R h i z o p o q o n spores. - C a n . J , For. Res. 15:613-617. Castellano, M.A., Trappe, J.M. and Molina, R. 1985. Inoculation of c o n t a i n e r - g r o w n D o u g l a s - f i r s e e d l i n g s with basidiospores of RhizoDoqon v i n i c o l o r and R. colossus: e f f e c t s of f e r t i l i t y and spore a p p l i c a t i o n ra~-e. - Can.J. For. Res. 15:10-13. Gibson, I.A.S. 1963. Eine Mitteilung U b e r die K i e f e r n mykorrhiza in den W a l d e r n Kenias. - In: Mykorrhiza (W. R a w a l d and H. Lyr eds.), pp. 49-51. G. Fischer, Jena. H a c s k a y l o , E. 1965. Thelephora terrestris of V i r g i n i a pine. - For. Sci. 11:401-404.

and m y c o r r h i z a

Ivory, M.H. 1980. E c t o m y c o r r h i z a l fungi of t r o p i c a l p i n e s in n a t u r a l f o r e s t s and e x o t i c p l a n t a t i o n s . - In: T r o p i c a l M y c o r r h i z a R e s e a r c h (P. M i k o l a ed.), pp. 110-117. C l a r e n d o n Press, Oxford. Laiho, O. 1965. Further mycorrhiza. - Acta Forest.

studies on the Fenn. 79.3.

ectendotrophic

Marx, D.H. 1977. T r e e host range and w o r l d d i s t r i b u t i o n the e c t o m y c o r r h i z a l fungus P i s o l i t h u s tinctorius. - Can. M i c r o b i o l . 23:217-223.

of J.

Marx, D.H. 1980. Ectomycorrhizal fungus i n o c u l a t i o n s : a tool for i m p r o v i n g f o r e s t a t i o n practices. - In: Tropical M y c o r r h i z a R e s e a r c h (P. M i k o l a ed.), pp. 1 3 - 7 2 . C l a r e n d o n Press, Oxford.

349

Marx, D.H., Bryan, W.C. and Grand, L.P. 1970. Colonization, isolation, and cultural descriptions of Thelephora terrestris and U t h e r ectomycorrhizal fungi of short leaf pine seedlings grown in fumigated soil. - Can. J. Bot. 48: 207-211. Marx, D.H., Ruehle, J.L., Kenney, D.S., Cordell, C.E., Riffle, J.W., Molina, R.J., Pawuk, W.H., Navratil, S., Tinus, R.W. and Goodwin, O.C. 1982. Commercial vegetative inoculum of Pisolithus tinctorius and inoculation techniques for development of ectomycorrhizae on containergrown tree seedlings. - For. Sci. 28:373-400. Marx, D.H., Cordell, C.E., Kenney, D.S., Mexal, J.G., Artman, J.D., Riffle, J.W. and Molina, R.J. 1984. commercial vegetative inoculum of Pisolithus tinctorius and inoculation techniques for development of ectomycorrhizae on bare-root tree seedlings. - For. Sci. 30 suppl. (Monogr. 25). Mikola, of pine.

P. 1965. Studies on the ectendotrophic - Acta Forest. Fenn. 79.2.

mycorrhiza

Mikola, P. 1980. Mycorrhiza across the frontiers. Tropical Mycorrhiza Research (P. Mikola ed.), pp. Clarendon Press, Oxford.

- In: 3-10.

Mikola, Fennica

Silva

P. 1988. 22:19-27.

Ectendomycorrhiza

of conifers.

Mikola, P. and Laiho, O. 1962. Mycorrhizal relations raw humus layer of northern spruce forests. - Comm. Forest. Fenn. 15.18.

in the Inst.

Molina, R. 1980. Ectomycorrhizal inoculation of containerized western conifer seedlings. -Pacific NW For Range Exp. Sta., Res. Note PNW-357. Molina, R. and Trappe, J.M. 1984. Mycorrhiza management in bareroot nurseries. - In: forest Nursery Manual: Production of Bareroot Seedlings (M.L. Dureya and T.D. Landis eds.) Corvallis, OR. Momoh, Z.O. and Gbadegesin, R.A. 1980. Field performance of Pisolithus tinctorius as a mycorrhizal fungus of pines in Nigeria. - In: Tropical Mycorrhiza Research (P. Mikola ed.), pp. 72-79. Clarendon Press, Oxford. Moser, M. 1958. Die k~nstliche Mycorrhizaimpfung Forstpflanzen. - Forstw. Cbl. 77:1-64.

an

Moser, M. 1959. Die kUnstliche Mykorrhizaimpfung an Forstpflanzen. III. Die Impfmethodik im Forstgarten. - Forstw. Cbl. 78:193-202. Ofosu-Asiedu, A. 1980. Field performance of Pinus caribaea inoculated with pure cultures of four m y c o r r - ~ a l fungi. - In: Tropical Mycorrhiza Research (P. Mikola ed.), pp. 8287. Clarendon Press, Oxford.

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Stenstrom, E., Ek, M. and Unestam, T. 1986. Prolonged effects of initially introduced mycorrhizae of pine plants after outplanting. - In: Physiological and Genetical Aspects of Mycorrhizae (V. Gianinazzi-Pearson & S. Gianinazzi eds.), pp. 503-506. INRA, Paris. Trappe, J.M. 1977. Selection of fungi for ectomycorrhizal inoculation in nurseries. - A n n . Rev. Phytopath. 15:203-222. Trappe, J.M., Molina, R. and Castellano, M. 1984. Reactions of mycorrhizal fungi and mycorrhiza formation to pesticides. - A n n . Rev. Phytopath. 22:331-359. Tyminska, A., Le Tacon, F. and Chadoeuf, J. 1986. Compared efficiency of three ectomycorrhizal fungi. Possible physiological explanations. - In: Physiological and Genetical Aspects of Mycorrhizae (V. Gianlnazzi-Pearson & S. Gianinazzl eds.), pp. 507-510. INRA, Paris. Wilde, S.A. 1954. Mycorrhizal fungi; their distribution and effect on tree growth. - Soil Sci. 78:23-31.

Mikola, p., 1989: The role of ectomycorrhiza in nurseries. Agric., Ecosystems Environ., 28: 343-350.

forest