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.

Understanding limitation by N and P

One of the most fundamental questions to understanding the adaptations of plants to low resource availability has been to understand the covariation in resource availability across sites. Do environments that have relatively low P availability have an excess of N relative to plant demand? Are plants that are limited by water also limited by N, or is N in excess for those plants and available for secondary purposes like defense?

I’m in the middle of an experiment to begin to understand the covariation in N and P limitation across grasslands in the US. Generally, researchers have attacked this question by doing field factorial fertilizations and comparing results across sites. Instead of this approach, I asked people to send me soils from 100 grasslands across the US. I grew the same species of grass (Schizachyrium scoparium) in all 100 soils with a factorial fertilization: control, +N, +P, +NP. After 10 weeks, the plants were harvested.

The most notable result at this point as the patterns of the response of biomass to N addition relative to soil P availability. As seen below, soils with low P availability had the least response to N addition, while soils with high P availability had the greatest. Yet, there was no relationship between P availability and the response to P addition. One interpretation of the data is that 1) P availability limits N response, and 2) P availability by itself does not limit P response. My guess is that plants in low P soils are not just limited by N, but by N and P. Any N supplied in excess in plant demands is lost from the ecosystem, whereas P isn’t lost when supplied in excess. Hence, I would guess that over evolutionary time scales, grassland plants would either be limited by N, or N and P, but not by P alone. Field fertilizations seem to bear this out as no grassland has ever been found to be limited by P alone.

We’re still collecting data on the plants and soils, but likely will be working the data up soon.