Graphs that don't exist: state factors and shade

Shade, drought, and nutrient scarcity are three resource stresses that constrain vegetation globally. Each of these are influenced by state factors as well as other more proximal controls on ecosystem function.

At least theoretically. We've actually never tested these ideas, which constrains our ability to explain and predict a lot about how ecosystems work.

Take shade. Plants produce leaves, which creates shade beneath them. Yet, the amount of shade in different stands varies tremendously. Theoretically, sites that are more limited by water should be able to produce less leaf area, leaving more light to the understory and removing a constraint on the growth of understory vegetation.

Despite decades of light measurements and hemispherical photographs of canopies, the data has never been synthesized to generate a global map of shade that can be analyzed in terms of determinants. Do dry ecosystems have a lower potential for producing shade than wet ecosystems? And how does that vary with temperature? Do forests in colder regions cast less shade, all other things equal?

Part of this echoes Peter Grubb's assertion that higher fertility sites should generate more shade and have more slow-growing shade-tolerant species, which still hasn't been tested directly as far as I can tell.

What holds for shade also holds for nutrient availability and water potentials. We just don't know the basic drivers of resource availability and hence don't know how global change factors like warming will affect the availability of the most limiting resources.

Global change and limitation

Ternary diagram showing the inverse relationships among low resource stresses and how global change factors influence the likelihood of resource limitations.

One foundation of ecology lies in the concept of state factors. Borrowed from soil science, state factors are the properties of ecosystems that are independent of the properties of the ecosystems [see earlier post]. The bedrock under a stand of plants is largely independent of whether a forest is there or we cut it down.

Ecosystems are not entirely deterministic from state factors and there are more proximal "interactive controls" like disturbance that are co-influenced by the vegetation. The probability of fire depends on what species are present as well as state factors like macroclimate.

One deficiency in our application of state factors and interactive controls has been to map the influence of state factors and interactive controls onto limitation. What factors and controls promote water-limitation over nutrient-limitation?

I've been trying to diagram this. Terry in his ecosystem book used a flow-control diagram to map the relative influences of state factors, interactive controls, and other controls on processes.



I've tried this approach for generating limitation and my diagrams come out looking like spaghetti. Or fettuccine. Some type of long, linear pasta. At least not fusilli thankfully.

The best I can come up with is a ternary diagram. In short, we want to show that drought, shade, and nutrient scarcity are all inversely related. And that a given global change factor promotes one or two stresses over the other. Precipitation promotes shade and nutrient scarcity over drought stress. Warmer temperatures promote drought and shade over nutrient scarcity. Disturbances reduce resource stress overall (inset).

There are dependencies for the diagram, e.g. chronic vs. catastrophic fire or scarcity of N vs. P.

Still I think this encapsulates the concepts we have about how global change factors alter limitation.

Mapping out other specific state factors is still a challenge. I get more spaghetti diagrams when I do this. Well, diagrams that look like I spilled dried spaghetti.

Modification of Classic State Factors

Modified State Factors and Interactive Controls Diagram to account for external supplies of resources.

The general framework to understand ecosystem properties has been the State Factor framework. Created by Hans Jenny to understand differences in soils, it has been applied to ecosystem properties such as plant species composition and stand structure. Essentially, with Jenny's approach, soils were determined by the climate, the organisms present, the landscape relief or topography, parent material and time since a major disturbance.

Terry Chapin and others modified the state factors approach to understanding ecosystems to include interactive controls of ecosystem properties. Interactive factors are not independent of the ecosystem properties, but sit somewhere in between. For example, the macroclimate is independent of what species are present at a site, but the microclimate can be influenced by species composition. These interactive controls include disturbances, microclimate, resource availability, and species composition.

With these state factors and interactive controls, one can have a better framework for understanding how ecosystems are structured and function. For example, the amount of biomass in a forest is not just a function of the state factors, but is influenced by interactive controls such as disturbance and the species that are present in a stand, which is influenced but not determined solely by state factors.

The state factor/interactive control approach is a big improvement in our conceptual framework, but it's generally left out external supplies of resources. Atmospheric CO2 concentrations, dust inputs, and nitrogen deposition can have profound influences on ecosystem properties, but are not adequately included in the current conceptual framework. These factors are somewhat influenced by ecosystem properties. The amount of dry deposition is influenced by canopy structure, for example. Yet, it's probably better to consider these state factors. N deposition plumes are wide and more similar to climate than disturbance regimes.

It's a minor tweak in many respects, but a likely necessary improvement to a long-tenured conceptual framework.