A couple of years ago I blogged about generalized Darwinism in a post called Occam’s Selection. This is the idea that principles of variation, selection, and preservation or retention are applicable to development and evolution of many different phenomena. The GD label is most common in evolutionary economics, but the notion is constantly being reinvented in many different fields. 

A recent example is Selection for Gaia Across Multiple Scales, published in Trends in Ecology and Evolution. The issue is how biological natural selection, which operates at the level of individuals, could result in evolutionary trends at ecosystems and broader scales, including the self-regulating biotic/abiotic coupling of the global Earth system. 

During some recent fieldwork doing forest biogeomorphology with colleagues in the Czech Republic, the idea of biogeomorphic equivalents came up. A biogeomorphic ecosystem engineer organism has a biogeomorphic equivalent if another species can potentially do the same biogeomorphic job. For example, bacteria that consume iron are important agents of weathering. There exist numerous species of iron-eating microbes, so if one is eliminated for whatever reason, another takes its place. Thus these Fe-processing bacteria have biogeomorphic equivalents.

Acidophilous iron-oxidizing bacteria (USGS photo).

On the other hand, there exists no biogeomorphic equivalent for the stream-damming effects of beavers. The disappearance of Castor canadensis from a landscape means the loss of their biogeomorphic effects, as no other organism (save humans, of course), dams up streams.

Wyoming beaver dam (photo: Wildlife Conservation Society).

Where forests grow on thin soils over bedrock, the effects of individual trees (as well as the effects of forest cover and litter) may work to deepen or thicken the soil. This occurs due to root penetration of bedrock joints and fractures. This in turn facilitates weathering and funnels moisture into the rock. Uprooting of trees may “mine” bedrock encircled by roots, and leave a locally thicker mound as rootwads deteriorate. If trees do not uproot, as stumps rot away the depressions—often extending deeper than surrounding soil—fill with soil, sediment, and organic matter. This thickening of the soil is a form of direct, positive ecosystem engineering in that it increases habitat suitability for the engineer organisms (the trees). 

Chinquapin Oak growing in limestone, Mercer County, Kentucky.

Eventually, however, the average soil or regolith thickness may increase such that tree roots no longer contact bedrock. Then the biogeomorphic ecosystem engineering effects of the trees on soil thickness ceases. In effect, you have self-limited biogeomorphic ecosystem engineering. 

In a 2009 article I introduced the concept of a geomorphological niche, defined as the resources available to drive or support a particular geomorphic process (the concept has not caught on). The niche is defined in terms of a landscape evolution space (LES), given by

where H is height above a base level, rho is the density of the geological parent material, g is the gravity constant, and A is surface area. The k’s are factors representing the inputs of solar energy and precipitation, and P_grepresents the geomorphically significant proportion of biological productivity (see this for the  background and justification).

Just published in Geomorphology:

Samonil, P., Danek, P., Adam, D., Phillips, J.D. 2017. Breakage or uprooting: how tree death affects hillslope processes in old-growth temperate forests. Geomorphology 299: 276-284. 

The abstract is below:

Posted 14 November 2017

This is a follow-up to my previous post on emergent ecosystem engineering in epikarst, so I won't repeat much of the background or analytical details. There I argued that interactions among rock weathering, moisture flux, biological effects (particularly roots and their symbionts) and soil operate such that if weathering is moisture-limited, and biota are limited by water availability and below-ground space, the system is dynamically unstable. Positive feedbacks dominate so as to reinforce or accelerate dissolution, joint/fracture widening, root growth, and soil accumulation. The net effect is to develop the epikarst as increasingly hospitable habitat. This continues, according to the analysis, until weathering becomes reaction-limited and subsurface space and moisture are no longer significant limiting factors for plant growth. Under the latter circumstances the system is dynamically stable, implying resilience to relatively small changes or disturbances and slower change.

Epikarst is defined as the uppermost zone of dissolution in karst, including whatever soil cover exists. The purpose of this analysis is to explore some of the interactions among geological controls, weathering, biota, moisture flux and soil accumulation in the regolith or critical zone of karst systems.

Epikarst exposed by gullying, Bowman's Bend, Kentucky

Figure 1 shows the interactions among geological controls (joints, fractures, bedding planes), weathering, subsurface biological activity, moisture flux, and soil accumulation the earlier stages of soil development in epikarst. The system is dominated by positive feedbacks because in early stages of epikarst development there is limited space for biological activity (e.g., roots), and moisture fluxes are limited by the size of joints, fractures, and incipient conduits. The other positive feedbacks reflect well established relationships among chemical weathering, enlargement of joints, etc., water availability, and organisms. I assume some external (to the system shown) limitations on biological activity and moisture flux.

Tree uprooting in forests has all sorts of ecological, pedological, and geomorphological impacts. Those are not just related to disturbance--because of the time it takes uprooted trees to decompose, and the distinctive pit-mound topography created, those impacts may last decades to centuries (and sometimes even longer).  One discussion I've often had with colleagues who study this sort of thing has to do with ecosystem engineering and niche construction. Obviously uprooting is a major biogeomorphic process. Obviously it has important impacts on habitat. But do these impacts favor either the engineer species (i.e., the tipped over tree) or some species? Or are they more or less neutral, in the sense of modifying habitat but not necessarily in such a way as to systematically favor any given species?

Uprooted Norway spruce.