34 CIRCULAR 791, IT. S. DEPARTMENT OF AGRICULTURE 



crowns become thin and ragged, and many branches die. One of the 

 surest signs that a tree will die soon is a dead tip. Oddly enough, 

 stands on poorer site swamps often maintain their appearance of thrift 

 somewhat longer than stands on better site swamps. In general, the 

 ages at which pronounced decadence in swamp stands occurs cor- 

 respond rather well to the ages when mean annual growth culminates ; 

 good site 100 years, medium site 140 years, and poor site 160 years. 



Although outward symptoms of the decline in vigor, such as thin 

 foliage, dead branches, and dead tips are easily observed, the causes 

 have not been determined. Black spruce is known to be susceptible 

 to a number of wood-decaying organisms. Lorenz and Christensen 

 have listed Fomes pini, Armillaria mellea, and Stereum sanguinolen- 

 ium as common in occurrence and Polyporus schtveinthii as oc- 

 casional. 20 



Upland black spruce trees are much shorter-lived than swamp trees. 

 Black spruce is a common associate of jack pine in northeastern Min- 

 nesota in even-aged stands up to 80 or 90 years old, but in older jack 

 pine stands black spruce of the same age as the jack pine seldom is 

 seen. The comparatively early disappearance of upland black spruce 

 is caused partly by windthrow and partly by breakage resulting from 

 butt rot. Advanced decay is common at the bases of the stems among 

 trees 60 to 80 years old, an earlier age than in swamps. A tree with 

 serious butt rot collapses at the base like a thin-walled tube when 

 subjected to wind pressure. Upland trees, and particularly those in 

 the poorly drained swamp borders, are readily uprooted by strong or 

 severe winds. 



The conclusion that butt rot becomes serious earlier in upland stands 

 than in swamp stands is in agreement with similar findings by Watson, 

 who examined black spruce stumps for evidences of decay on a wide 

 range of sites. 21 



The Effect of Insect and Disease Enemies on Growth 



Black spruce is susceptible to attack and injury by a large number 

 of insects (5), but the injuries of few of these have been found to 

 be of major economic importance. In their normal endemic condi- 

 tion, such insects cause slight reductions in growth and occasionally 

 the death of a tree here and there. The inappropriately named 

 "spruce budworm" (Cacoecia fumiferana) indirectly has caused rather 

 extensive damage to black spruce growing in mixture with balsam fir 

 in Minnesota. The budworm, by killing the balsam fir, exposed the 

 black spruce to windthrow (13). Spruce budworm feeds on black 

 spruce only to a limited extent. The European spruce sawny (Gilpinia, 

 hercyniae Htg.) , which caused widespread damage to spruce in eastern 

 Canada, thus greatly decreasing yield of merchantable timber, has 

 been found on black spruce in Minnesota within recent years but as yet 

 has shown no signs of becoming epidemic. 22 The spruce needle miner 



20 Lorenz, Rolland C, and Christensen, C. M. a survey of forest tree dis- 

 eases AND THEIR RELATION TO STAND IMPROVEMENT IN THE LAKE AND CENTRAL STATES. 



U. S. Dept. Agr., Bur. of Plant Ind., 52 pp., 11 plates. 1937. [Mimeographed.] 



21 Watson, Russell, silviculture of black spruue in Minnesota. U. S. 

 Forest Serv., North Central Region, 26 pp. 1937. [Mimeographed.] 



22 Hodson, A. C., and Christensen, C. M. Minnesota forest insect and dis- 

 ease survey for 1943. Minn. Agr. Expt. Sta. Paper No. 2194, Scientific Journal 

 Series, 9 pp. [No date, mimeographed.] 



