Pando is a single tree, albeit a tree that is a forest of growing international reputation. Five or six species of aspen (upland Populus) reach around the entire northern hemisphere between about 30°-65° north latitude. They harbor vast amounts of biodiversity. As goes aspen, so goes dependent plants and animals; a cascading effect that may be exacerbated by a warming climate. Can the lessons learned – and answers derived – at Pando shine a light on global-scale aspen conservation?
The second part of Paul C. Rogers’ Pando series is about the growth ability of pando and the added value of its existence for science and for humanity in general:
How did this giant clone survive for thousands of years?
The ability to proliferate from existing root stock is crucial to understanding how large aspen clones thrive. After mature trees, or entire stands, of aspen are injured or die, their predominant mechanism for self-preservation is to reproduce profusely from asexual root suckering. This process involves a hormonal signal (auxin), which suppresses most primordial root buds from suckering until that signal is interrupted by tree damage or death. Reduction in auxins is closely related to increases in root cytokinins that promote new shoot growth. Dramatic examples depicting thousands of new aspen suckers following fire-, disease-, or tree cutting-mortality are common in the western US. However, this process of waiting only for new growth to emerge as a result of catastrophic disturbance paints an incomplete picture. In aspen forests where such stand-replacing disturbances are less common, there is also continuous regrowth where resources, such as water and sunlight, are available in modest measures.
Stable aspen qualities
Nearly a century ago the forester Frederick S. Baker spoke of aspen’s “two races.” He was referring to what we now call aspen “functional types” : one being very dependent on intense disturbance for regeneration and another that favors small-scale and continuous new stem growth. The latter of these is also found in nearly pure composition—without competing conifers—that provides modest protection from wildfire. Such monotypic aspen forests are called “stable aspen” due to their propensity for remaining in a single-species cover for many generations. Pando exhibits stable aspen qualities. To understand how this giant clone survived for millennia one must invoke knowledge of long-term asexual reproduction within the context of continuous tree replacement, rather than the single-aged stem cohorts commonly found in forests subjected to abrupt upheaval such as wildfire. What I refer to as ‘traditional aspen ecology’ focuses almost solely on aspen intermixed with conifer species and, through a process called succession, transitions over decades-to-centuries from early aspen domination to crowding out by longer-lived conifers. Returning to Pando, we may consider some ideas about competition, sans significant conifer encroachment, which may favor one clone over another.
A little big mystery
What do we really know about aspen growth and survival in the context of giant clones such as Pando? It turns out, very little. We may speculate, however, using our current understanding of potential environmental factors that likely influenced Pando’s success. If you look at the pattern of aspen genotypes on the ground today you see that the giant is surrounded by a few moderate-size clones and many small clones. Thus, we theorize that perhaps Pando may have simply outcompeted adjacent clones. The moniker “I spread” draws from this line of thinking, but there may be more to the story. Conceivably it does just grow faster, or is more environmentally suited to conditions there, or has characteristics of persistence that allow it to endure the wide range of climatic epochs that have occurred during its life (i.e., more effectively than others). Another hypothesis asserts important plant-animal evolutionary traits. As a triploid (having three chromosome sets), Pando appears to have a more rapid growth ability that favors some young stems reaching beyond browse height before ungulates nip their apical buds. Additionally, as much as 30% of leaf matter is comprised of chemically defended phenolic glycosides that “taste bad” to them. It may be that Pando, in an evolutionary trade-off, has invested energy in growth capacity at the expense of chemical defense. While this may not have impeded growth for most of its long life, 20th century human disruptions in the form of predator eradication, road building, livestock introduction, recreational impacts, and forest cutting seem to have disrupted the balance in this system; a balance that may be difficult to recover without additional interventions.
A barometer of sorts for broader problems
As a global citizen, is it important to invest time and resources in the fate of a single clone, even one as singular as Pando? Why fixate on the “biggest” or “oldest” of anything? After all, nature is not a competition; a cataloging of world records, is it? Why not just focus on the everyday business of conservation? Do we need such icons as Pando to grab the headlines or shake our neighbors from indifference? These questions are not unreasonable given the overwhelming array of environmental issues swamping us; not least of which, a human-induced warming planet, poses serious threats to our collective future. The short answer to these concerns was voiced succinctly by the American conservationist, Aldo Leopold, some 70 years ago: “If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts? To keep every cog and wheel is the first precaution of intelligent tinkering.” If a natural feature is very large, for example, perhaps it brings needed attention to many other issues of import; a barometer of sorts for broader problems.
What Pando teaches us…
Though small at the global scale, Pando illustrates how we might first observe, measure, understand, then take actions in a manner more compatible with the planet. For instance, aspen forests worldwide are known for their outsized influence on regional biodiversity: in the Rocky Mountains they harbor more species than the dominant conifer forests and in Europe many Red List species are aspen obligates. If we can help Pando, dependent species will benefit and so on at regional and even global scales. If our monitoring and tinkering at Pando can preserve this iconic grove, such lessons may be shared around the world for the benefit of the earth’s biota as a whole. To achieve such a lofty goal, we must start at a manageable scale, meticulously measure, take restorative actions, carefully monitor forest responses, make additional adjustments, and share what we’ve learned with others.
To be continued…
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