out of your clothes fast, before they fill with the silt 

 and drag you under. 



But mountain silt brings life as well. Deposited 

 onto sandbars along the river, the silt creates fresh 

 real estate for water-loving willows and thin-leaf 

 alder. Old and new islands in the river channel hold 

 patches of the boreal forest at 

 all stages of succession. Hop- 

 ping from one island to the 

 next, you can trace the evolu- 

 tion of forest life from the first 

 colonizing plants to the oldest 

 trees in the forest [see illustra- 

 tion on next two pages]. 



Growing a forest on islands 

 of bare silt takes unusual 

 chemistry and unique bacte- 

 ria. Plants get carbon in the 

 form of carbon dioxide from 

 the air, but the rest of the 

 nutrients they need come 

 through their roots. The river 

 washes some of these nutrients 

 onto the sandbars and fertil- 

 izes the seedlings that blow in, 

 wash in, or hitchhike ashore. 

 But not all nutrients are avail- 

 able or present in a chemical 

 form the plants can use directly. That's where bac- 

 teria, along with root-associated fungi, come in. In 

 the soil, on or near the plants' roots, microorganisms 

 retrieve and transform certain elements, thus en- 

 abling growth in nutrient-poor places. 



Like many a backyard garden, the sandbars along 

 the Tanana are poor in nitrogen — an element all 

 plants need to build protein. Nitrogen is abundant 

 in air, of course, but its atmospheric form (the mol- 

 ecule N 2 ) is useless to plants. Certain bacteria in the 

 genus Frankia, however, make enzymes that enable 

 the bacteria to retrieve molecular nitrogen in the air. 

 Those bacteria are symbiotic with alder, living in 

 nodules on the plants' roots [see images on opposite 

 page] . The bacteria provide the alder — and ultimately 

 the forest — with usable nitrogen in exchange for sug- 

 ar synthesized by the plant. As our guide and col- 

 laborator, Roger Ruess, an ecologist at the Univer- 

 sity of Alaska in Fairbanks, puts it: "The plant has to 

 support the drug habits of Frankia." 



Both the alder plant and Frankia seek another el- 

 ement that is in short supply: phosphorus. The 

 element is needed to make DNA, among other mol- 

 ecules, and limits how much nitrogen the Frankia 

 can fix, or change into a form usable by the plant. 

 When phosphate — a molecule in which one phos- 



Cankers pepper the trunk of an alder. The cankers 

 are the result of a fungal infection that eventually 

 kills plants by inhibiting nitrogen fixation. 



phorus atom is bound to four oxygen atoms — is ex- 

 perimentally added to a site, the rate of nitrogen fix- 

 ation shoots up, along with the plant's growth. Phos- 

 phorus is naturally present in the soil, but, like at- 

 mospheric nitrogen, it is locked up in chemical forms 

 plants cannot use. 



Yet the bacteria living along 

 the Tanana manage to obtain 

 some of the otherwise un- 

 available phosphorus. William 

 W. Metcalf, a microbiologist at 

 the University of Illinois in 

 Urbana, and his students have 

 assessed Tanana soil bacteria 

 for certain DNA sequences 

 and discovered that a high pro- 

 portion of them possess en- 

 zymes — only recently discov- 

 ered — that can metabolize the 

 locked-up forms of phospho- 

 rus. Most organisms must get 

 their phosphorus from phos- 

 phate, its most common state, 

 but some bacteria we collect- 

 ed on the Tanana can convert 

 "reduced" forms of phospho- 

 rus, such as the phosphite mol- 

 ecule (one phosphorous atom 

 bound to three oxygen atoms), into phosphate. Fed 

 only those reduced forms of phosphorus in the lab- 

 oratory, bacteria from the river site grow just fine. 



By growing with the captured phosphorus, the 

 collaborating alder plants, bacteria, and fungi help 

 open the way for other forest denizens. Investiga- 

 tors find that alder improves the nutrition of neigh- 

 boring plants and soon attracts wildlife. Hare and 

 moose have obviously browsed the willow bushes 

 on the riverbank where we pause for our lunch. 



Animals rarely like to eat alder, but lately it's be- 

 come lunch for something else. Last year investiga- 

 tors noticed that cankers, caused by a fungal patho- 

 gen, were appearing on the alders [see photograph 

 above]. Now they are watching intently to see how 

 the lesions may affect the forest at this pivotal, early 

 stage of its development. The fungal infection de- 

 creases the rate of nitrogen fixation and eventually 

 kills the alder. Will enough of the population suc- 

 cumb to alter the normal succession of the forest? No 

 one knows, but at some sites along the Tanana. 

 cankers have appeared on as many as SO percent 

 of the alders. 



We see pimply bark, signs of the canker disease, 

 at our first sampling site and elsewhere as w e motor 

 up and down the river to the various islands. The 

 responsible pathogen may be the same fungus that 



April 2006 NATUKAl 1 1 1 n I 1 > K \ 



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