THE MYCORRHIZAL STATUS OF DOUGLAS-FIR IN THE NETHERLANDS - ITS RELATION WITH STAND AGE, REGIONAL FACTORS, ATMOSPHERIC POLLUTANTS AND TREE VITALITY

Observations were made in 1985, 1986 and 1987 of the fruitbodies of mycorrhizal fungi appearing in autumn in 25 stands of Douglas fir. Seven, 8 and 10 of the stands were < 20, 20-40 and > 40 years old, respectively: 6, 13 and 6 were located in the north, central and south of The Netherlands, the atmosphere in the north was the least polluted and that in the south the most polluted. Mycorhizal status was assessed on soil samples taken in November-December 1987. Numbers of mycorrhizal species producing fruitbodies, and the numbers of their fruitbodies, were inversely related to the ages of the different stands; 16 species in stands < 20 years old and 8 species in stands > 40 years old. There were averages of 12.5 species in the northern region and 6.0 in the south. Numbers of fruitbodies, when accumulated over 3 seasons, were positively correlated with numbers of mycorrhizas per unit volume of soil. This being so, it is suggested that numbers of fruitbodies may be a reliable index of numbers of mycorrhizas. There were fewer mycorrhizas per unit volume of soil in old than in young stands; numbers were larger in the less polluted northern region than in the southern region. However, while numbers and frequencies of mycorrhizas and numbers of species and of fruitbodies were consistently and inversely related to loads of atmospheric pollutants, except NH3, only some of the associations were statistically significant. Whereas numbers of mycorrhizas and those of fruitbodies were inversely related to the extent of crown "transparency", loss of twigs and incidence of damaged bark, there were no significant correlations with tree vitality as determined by needle loss and discolouration, the criteria used by the Netherlands State Forest Service. There is circumstantial evidence suggesting that decreased numbers of mycorrhizas in old stands, particularly in the polluted southern region, may be related to the inability of mycorrhizal fungi to colonize, possibly as a result of changed soil conditions which did not seem to affect the production of root tips. The proportion of root tips that were mycorrhizal decreased from 41% in young stands to 12% in old stands and from an average of 36% in the north to 9% in the south.