153 



The Dying of Alders. 



Alders are most sensitive to a lowering of the ground water level and it 

 is easy to find diseased tracts of alders near newly cut canals or regulated 

 river beds. In the works of the Royal Biological Institution for Agriculture 

 and Forestry at Dahlem, near Berlin (1905), AppeF has published a study 

 of the death of alders well worth consideration. He found on the dying 

 branches a species of the genus Valsa known to attack diseased or dead 

 branches, — namely, Valsa oxystoma — and stated that the fungus is parasitic 

 only when the alders become susceptible, owing to abnormal circumstances. 

 Drought is the chief determinative factor. Other disturbances in nutrition 

 (injury to the roots, girdling, etc.) can also create a predisposition to fungus 

 attacks but, if the alders are enabled to make a healthy growth, the disease 

 disappears. When alders are found dying on apparently moist, imperme- 

 able, ferruginous soils, drought may be considered to be the cause. On such 

 soils, the alder spreads it roots only very superficially and in continued dry 

 weather there is a very marked scarcity of water in the upper soil layers, 

 which at once makes the alder foliage wither and dry. The beautiful tracts 

 of trees in the Tiergarten in Berlin, especially the oaks, unfortunately show 

 similar results from surface drought, and to an ever increasing degree. 



Naturally canal and river bed regulations do not always necessarily 

 cause the lowering of the ground water level. In the old Botanical Garden 

 in Berlin, for example, the building of the subway dried up the water in 

 the ponds and as a further result the tree crowns rapidly became blighted. 

 In other instances we found that the spread of brick-paving and clay- 

 diggings near forest tracts accelerated the death of the alders because the 

 deep clay pits had withdrawn water from the forests. 



The dangerous effects of lowering the ground water level often fail to 

 impress us sufficiently, since, in some tracts of woodland, the same tree 

 species (suffering from blight of the crowns in soils from which the water 

 has been removed) thrive in very dry places. Under such circumstances 

 the fact that the lack of water in itself does not cause the death of the trees, 

 but the abrupt transition from a previously well-watered condition to great 

 drought in the deeper layers of soil is overlooked. We may plant all our 

 trees in very dry soils and the individuals will adapt themselves to the 

 existing vegetative factors and the leaves will become small and coarse, the 

 internodes short. But a sudden great change in this condition will have 

 most serious results. If, however, such changes are unavoidable, our theory 

 gives only one line of action to preserve the plantation,— namely, to plant 

 young trees between the old ones. These will adapt themselves to the 

 changed vegetative conditions. 



Street Planting. 



The preservation of trees along the streets and small parks is of the 

 greatest importance for the hygiene of cities. The greatest injury results 



3 Vorlaufig-e Mitteilung- in d. Naturwiss. Zeitschr. f. Land- u. Forstwirtschaft. 

 2 Jahrg-. 1904. 



