The real king of the for­est is fun­gus

The Globe and Mail (Prairie Edition) - - NEWS - IVAN SEMENIUK SCIENCE RE­PORTER

Re­searchers find that fungi have sig­nif­i­cant ef­fects on tree di­ver­sity by in­flu­enc­ing the soil and pathogens be­neath them

To a ca­sual hiker, one bit of North Amer­i­can for­est may seem like any other. But look more closely and a mys­te­ri­ous patch­work of di­ver­sity emerges. Some stands of for­est are clearly dom­i­nated by a sin­gle kind of tree. Oth­ers are a di­verse mix of species.

Now, a mul­ti­year ef­fort to un­der­stand these dif­fer­ences has un­cov­ered a sur­pris­ing an­swer. What con­trols for­est di­ver­sity is not the trees but the fungi that in­ter­act with them, typ­i­cally at mi­cro­scopic scales, below ground and out of sight. The re­sult of­fers a new win­dow on the com­plex in­ter­ac­tions that un­der­lie some of the most fa­mil­iar ecosys­tems on the con­ti­nent and could lead to im­proved for­est-man­age­ment prac­tices.

“I find it quite amaz­ing that these or­gan­isms that we can’t see can have such a pro­found ef­fect,” said John Klironomos, a plant ecol­o­gist at the Univer­sity of Bri­tish Columbia who led the ef­fort.

Sci­en­tists have long known that plants and soil fungi can form sym­bi­otic re­la­tion­ships, called my­c­or­rhizas, in which plants pro­vide car­bon in ex­change for other nu­tri­ents such as phos­pho­rus or ni­tro­gen. Less clear is how these re­la­tion­ships can af­fect the larger nat­u­ral land­scape.

To ex­plore the ques­tion, Dr. Klironomos, to­gether with col­lab­o­ra­tors across Canada and the United States, in­ves­ti­gated 550 sep­a­rate for­est lo­ca­tions from Nova Sco­tia to North­ern Cal­i­for­nia.

In each lo­ca­tion they iden­ti­fied a ma­ture tree and col­lected root, seeds and soil sam­ples. The trees were cho­sen based on what type of my­c­or­rhiza they formed with fun­gus. Most tree species do so in­ter­nally, al­low­ing the fungi to col­o­nize their tis­sues. But for some species – par­tic­u­larly ev­er­greens – the my­c­or­rhiza forms an ex­ter­nal and en­cas­ing sheath around the en­tire root sys­tem and can also sprout mush­rooms. It is this dif­fer­ence that ap­pears to in­flu­ence where trees tend to grow, Dr. Klironomos said.

In gen­eral, plants of the same species do not tend to clus­ter to­gether be­cause that makes them a more at­trac­tive tar­get for diseases and par­a­sites that re­quire the same kind of plant. For this rea­son, seedlings that grow near their par­ent tree would seem to be at a dis­ad­van­tage.

The ex­cep­tions are tree species that have ex­ter­nal my­c­or­rhiza. They ap­pear to be less sus­cep­ti­ble to pathogens be­cause their roots are so well pro­tected.

“The pathogen can’t col­o­nize the root if it can’t get to the root,” said Jonathan Ben­nett, a post-doc­toral re­searcher and team mem­ber.

Such trees should be free to grow to­gether more densely, lead­ing to sec­tions of for­est that are less di­verse.

The team was able to test this ef­fect when seeds from their for- est sites were cul­ti­vated in the green­house. Seedlings of tree species with ex­ter­nal my­c­or­rhiza were sig­nif­i­cantly health­ier when grown in the soil that was found near their par­ent tree. By en­list­ing the fungi in that soil, they were less likely to have roots that were in­fected or dam­aged.

For tree species whose my­c­or­rhiza don’t cover their roots, the op­po­site was true. They did not grow as well when planted in the same soil as their par­ent tree be­cause they were ex­posed to their par­ent’s pathogens.

What emerged in the green­house also holds true in the for­est, the team found. The lo­cal di­ver­sity of tree species and there­fore the over­all ecol­ogy of the for­est ap­pears to be in­flu­enced by what is hap­pen­ing be­tween tree roots, fun­gus and pathogens – a phe­nom­e­non that Dr. Klironomos calls “plant-soil feed­back.”

The re­sult was bol­stered by a sec­ond study by a dif­fer­ent team that found a sim­i­lar ef­fect at work among wild shrubs in western Aus­tralia. Both stud­ies were pub­lished Thurs­day in the jour­nal Science.

Eti­enne Lal­ib­erté, a plant bi­ol­o­gist at the Univer­sity of Mon­treal and a co-au­thor of the Aus­tralian study, said the work demon­strated that fungi are not sim­ply pas­sen­gers but driv­ers of plant di­ver­sity. He added that plant-soil feed­back can make for a com­plex bi­o­log­i­cal tug of war in which some plants are in­hib­ited and oth­ers fa­cil­i­tated by the other plants around them de­pend­ing on their my­c­or­rhiza.

“It’s hard to pre­dict what the over­all ef­fect will be on di­versi- ty,” he said, adding that his team re­sorted to com­puter mod­els to help study the process.

Wim van der Put­ten, an ecol­o­gist at the Nether­lands In­sti­tute for Ecol­ogy, Wa­genin­gen, who was not in­volved in ei­ther study, said that the par­al­lel re­sults “show that we can only un­der­stand plant species di­ver­sity when com­bin­ing the pos­i­tive and neg­a­tive in­ter­ac­tions be­tween plants and their soil or­gan­isms.”

Dr. Klironomos said such an un­der­stand­ing could help those try­ing to re­store nat­u­ral forests in ar­eas that have been al­tered by hu­man ac­tiv­ity. It may also shed light on the suc­cess of in­va­sive species, many of which do not have my­c­or­rhiza and so are able to out­com­pete na­tive species in places where the soil has been heav­ily dis­turbed.

JOHN KLIRONOMOS

Mush­rooms of the species Bo­le­tus edulis, grow­ing be­side trees on the Falls Lake Trail near Hope, B.C., can help pro­mote for­est di­ver­sity.

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