biologists have discovered attacking alder in Col- 

 orado, Valsa mchmodiscm. 



In spite of the alder canker, it's still possible to trace 

 the normal forest succession along the Tanana. 

 We pass by young sandbars thick with knee-high 

 willow, and only an occasional shoot of alder. If the 

 alder remains canker-free, it will grow like lilac 

 bushes and crowd out the willow, dominating the 

 small, sandy islands. Eventually, if the forest grows 

 unimpeded, balsam poplar — also known as cotton- 

 wood in the Lower 4S — shades over the alder and 

 replaces it. Balsam poplar sheds its seecis in fluffy 

 blossoms that litter the forest floor; our sampling 

 sites are full of them, along with thickets of prick- 

 ly rose and sprigs of pyrola, or wintergreen. 



At another island upstream, we enter a fragrant 

 cathedral of 200-year-old white spruce: the next 

 stage of the forest succession. The brushy ground- 

 cover of the balsam sites has given way to green 

 lichens, cushiony mosses, and more legroom. The 

 recent rain has prompted a show of mushrooms, too. 

 We find the wild relative of the common white but- 

 ton mushroom ( Lycoperdon) and a burnt-marshmal- 

 low look-alike (Sarcodon imbricatum). 



Up and across the river we sample the bacteria that 



Forest success/on (left to right) along the Tanana River 

 begins as the first colonizers, willow and then alder bushes, 

 get nitrogen with the help of Frankia bacteria that live 

 among the alders' roots. Balsam poplar moves in over time, 

 and after about 200 years white spruce follows. A few cen- 

 turies later, black spruce dominates. Individual islands have 

 their own soil chemistry and microbial communities, which 

 are just beginning to be studied in detail. 



live in the final stage of forest succession. It is the 

 coolest stage, both literally and figuratively. Black 

 spruce trees jut like pipe cleaners amid an aromatic 

 groundcover of Labrador tea, low-bush cranberry, 

 and more mosses. The accumulation of feathery moss 

 has by now insulated the ground, preventing it from 

 warming during summer, and so a permafrost layer 

 begins as little as a foot beneath the surface. We pull 

 samples of fibrous soil the color of chocolate cake out 

 of the ground, as cold to the touch as if it came out 

 of a cooler, rich with the complex smells of soil. The 

 smells themselves are signals of bacteria: the vapors 

 of volatile compounds released by Streptomyces bac- 

 teria, the source of streptomycin and other antibi- 

 otics. A billion bacterial cells, representing thousands 

 of different strains, can live in a teaspoon of the soil, 

 along with perhaps dozens of fungal strains. 



Stored in coolers, the soil samples get shipped to 

 Jo Handelsman's microbiology laboratory at the 

 University of Wisconsin-Madison. There, a small 

 crew of investigators starts in on the work. Most of 

 the soil ends up being processed into "libraries" of 

 DNA, the better to explore and analyze the vast 

 world of yet-uncultured microbial diversity. 



To create the libraries, pinches of soil are tucked 

 into inch-deep tubes. Minute synthetic beads are 

 added to break open the bacterial cells as the 

 tubes spin in a centrifuge. Solutions are used to 

 wash the burst DNA and separate it from the rest 

 of the compounds in the 



a clear solu- stiSi?' I 



