Cottonwood and the River of Time: On Trees, Evolution, and Society

5. Perpetuate and Proliferate!

5 / PeRPetUate and PRolifeRate! C ottonwood’s developmental flexibility is also called into action to serve two additional purposes: to perpetuate the individual, and to make more of itself. we have already gained an appreciation of how challenging the fluctuating environment is in which this tree is growing. it seems that in its dependence on water, the tree has totally committed itself to the caprice of this vital element and its associated processes of transport, erosion, deposition, and accretion. at the mercy of hydraulic forces, cottonwood seems to make the best of it, doing so by retaining a remarkable phenotypic plasticity throughout its entire life cycle. Cloning is at the heart of this process. it is not uncommon to see a tree that was downed by a winter flood resprout in spring. most likely it will still have some of its root connections maintained. But a bit of digging will also show a plethora of young 33 34 PerPeTuaTe and PrOliferaTe! roots sinking into the substrate under the prostrate trunk. what then happens is that the branches on top of the trunk will become strong sinks and develop into a row of new stems, so where there was originally one tree there may eventually be a dozen of them. all of them will be clonal copies of the original, as all share the same genotype. and just like identical twins they will closely resemble each other in many traits, such as stem form, branch angle, leaf size, and sex. most strikingly, they will all leaf out in spring in perfect synchrony with one another. in this they will stand out from neighboring trees that typically vary in the timing of bud burst.This clonal row of trees may remind us of a notuncommon sight in old-growth conifer stands of the northwest, namely a row of western hemlocks, nicely lined up and all of similar size. Park naturalists love to point them out and explain the role of nurse logs, fallen trees that gradually rot and then serve as a rich substrate for the next generation of seedlings to grow up on. a celebration of natural recycling, the process is often given an exaggerated spin having to do with the way a parental generation sacrifices itself for the benefit of its offspring, an anthropomorphic notion. Young hemlocks and western red-cedars are especially proficient in exploiting these logs as new nursery sites.The seedling population thus generated is, nevertheless, a collection of genetically diverse offspring from neighboring trees, often even from different species than the nurse log itself. in cottonwood, by contrast, we are looking at the perpetuation and amplification of the original individual—the ultimate in selfishness. of course, not all downed trees remain connected to their root system . Some get washed away by the river and end up in a pile of logs. all they will leave behind is a stump and some exposed roots.Yet here again we will see sprouts emerge, primarily around the base of the stump, the root collar, and also from surface roots radiating from the stump. as long as the tree was intact the strong sinks of the upper crown were dominating shoot growth and suppressed any such activity near the base of the trunk. But once the stem is gone, existing buds and adventitious buds at the base are released and able to exercise their meristematic potential. Thus, the original tree may extend its existence by one or more copies sprouting at the original location. a clump of trees, joined at the base, will often have originated in this fashion, especially if all members of the clump show clonal similarity as described above. PerPeTuaTe and PrOliferaTe! 35 Beavers take full advantage of this resprouting capacity. after having felled a sapling or tree and removed its desirable parts, they often come back in successive years and feast on the regrown shoots. it is not uncommon to find cottonwood stumps with a history of four to five or more episodes of such subsequent harvests. and moose, elk, deer, and rabbits too depend on such a perpetual source of browse. But why not use the river as a dispersal agent? as it turns out, black cottonwood is perfectly suited for such a fragmentation strategy, in two specific ways. Some of its twigs and smaller branches, when broken off during a winter storm, may get transported downstream by the water. if they become...