Countdown #1: The five resource strategies of wild plants

If there is one central element to RSWP, it is not about how to quantify importance, the nature of resource limitation, or the mechanisms of competition. These fundamental questions all serve a higher purpose in the book: to understand the resource strategies of wild plants.

The broadest theories of plant strategies have differed on whether there was a common general strategy to succeeding when resources were low, or whether there were fundamentally different strategies associated with success when different resources are low. As I describe in RSWP, the most parsimonious conclusion is that there are four major strategies for growth when limiting resources are supplied uniformly over time. There is less support for the theory of a general “low-resource” strategy with variations associated with limitation by different resources than separate strategies for succeeding when water, nutrients, light, or CO2 are strongly limiting. The availabilities of resources are independent enough from one another and there are physiological and evolutionary tradeoffs in producing traits for success for each resource availability. Consequently, there is no one general strategy that covers low availability of all resources. Being built to perform well under low light precludes being competitive for nutrients which precludes acquiring water when soil water potential is low. All of these strategies might share a low maximal relative growth rate, but this appears to be a consequence of convergence. A fifth strategy is associated with success when the availabilities of all resources are high. 

The five strategies I outline in RSWP are the most fundamental and widespread with regards to resources, yet it is important to recognize that no one set of traits works best across all environments that share having low availability of a given resource. For example, although both limited by nutrients, phosphorus limitation in the fynbos of South Africa has selected for plants that fundamentally different from those that dominate nitrogen-limited grasslands in Minnesota. How the multitude of environmental stresses and disturbances have shaped the world's flora are some of the most subtle questions about the forces that make our complex world beautiful. Yet, the skill of the ecologist is to appreciate the complex while seeking the simple. When we collapse the diversity of the world into its most fundamental units, we are left with the five resource strategies of wild plants.

Countdown#2: Competition and supply preemption

Dense root system of Hordeum pusillum. 

Previously, I discussed how competition selects for suboptimal allocation patterns—at least suboptimal in the absence of competition. If this wasn’t a regressive list, I would have been getting slightly ahead of myself.

As I said before Tilman made a great advance in linking competitive success to resource availability reduction. Although his work might have been theoretically pure, its application to terrestrial systems was conceptually flawed. Plants do outcompete one another by reducing the availability of resources. Yet, the availability of nutrients in soil is not best conceptualized as the solution concentrations. Instead, it’s supplies. When nutrients are limiting, plants outcompete one another by reducing the supplies of nutrients to neighbors. Independent of reducing mineralization rates, supply reduction comes from supply preemption. Because of diffusion limitation, the plant with the most root length per unit volume of soil acquires the majority of the nutrients supplied. Each unit of root length produced reduces nutrient supplies to neighboring plants. The best competitor for nutrients is the plant that can produce the most root length.

I develop at length in RSWP the shift from concentration reduction hypotheses to supply preemption hypotheses and how it changes our outlook on plant interactions. For example, changes in soil moisture would affect prediction of competitive superiority by altering soil solution concentrations, but do not affect predictions based on supply preemption. There still is more theoretical work switching from R* models to SL* (supply per unit length), but the concepts are now more consistent with our understanding of soil nutrient dynamics, even if the are not as theoretically pure as possible. Better understanding of how plants compete for nutrients not only help us understand how competition has altered the evolution of plants, but sets us up to ultimately better understand the resource strategies of wild plants.

Future of natural abundance 15N research for terrestrial plants

Natural abundance 15N is the Afghanistan of ecosystem ecology—it is all but impossible to conquer. In short, the N cycle is so complex that plant 15N becomes a single response with two many drivers. There is no consistent way to interpret any one difference in signatures between contrasts.

Although seemingly intractable, few other biogeochemical cycles rival N in their importance in determining how ecosystems function. Understanding the patterns of N cycling is so important that we have to continue to improve our ability to interpret natural abundance 15N patterns.

A number of coauthors and I just got a global review of 15N patterns published in New Phytologist. Details aside, it took a long time for this to happen. Although unfortunate, it gave us the opportunity to work with a number of editors and reviewers to understand the intellectual landscape better. Below I’ll post some summary thoughts on what needs to happen next in the discipline. In a later post, I’ll summarize what we learned in the paper.

1) We need CENTURY for 15N--a general model of the N cycle coupled with a stable isotope simulator to explore scenarios. 15N will not be raised up into the pantheon of ecological isotopes until the theoretical basis for patterns is worked out.

2) We need a survey to measure 15N in roots. Within-plant fractionation is the third major hypothesis for determining patterns of foliar 15N. Only 3 studies have compared 15N for leaves and roots. We'll need a lot more data to evaluate this hypothesis.

3) We need global maps of the N cycle. For all the N cycle has been measured, we do not have global, state-factor relationships with organic N uptake, N mineralization, nitrification, or gaseous N loss. We cannot make a global map of these fundamental processes. For example, time and time again reviewers choked on the idea that denitrification could be higher in sites with lower precipitation. A global synthesis would clear this up.

4) We need better measurements of the signature of available N. Measurements of soil inorganic 15N need to be as commonplace as available NH4+ and NO3-. Knowing these values are an incredible constraint on the key processes.

5) Mycorrhizal ecologists have been slow to assimilate 15N patterns into their understanding of the role of mycorrhizal fungi in plant N nutrition. For example, in Smith and Read's 3rd edition of Mycorrhizal Symbiosis, it was clear that the authors had gotten the story wrong. The attributed the higher 15N of ectomycorrhizal fungi to their reliance on enriched sources, and failed entirely to mention that ectomycorrhizal plants are depleted in 15N as much as the fungi are enriched. There is a lot of work that needs to be done to fully integrate mycorrhizal fungi into plant N nutrition. My guess is that one pressure point will be measuring 15N signatures of fungal mass and plant material in the field.

6) We need a central depository for 15N data that is better than my Macbook. Researchers being able to compare their values to global datasets quickly aids them in interpreting their data while facilitating new syntheses. Nothing new here.

Countdown #3: Competition selects for the sub-optimal

A typical eucalyptus: lots of wood, not a lot of leaves. Why aren't more plants this efficient? 

In economic and ecological theory, selection for efficiency among competing firms or organisms is a ruling paradigm. Yet, competition, among firms or plants, does not always select for the most efficient. All one needs to see this in nature is to take a shovel to their front lawn or take a walk in a forest.

When nutrients are limiting, a plant competes for a limiting nutrient supply by attempting to preempt the nutrient supply from other plants. As such, the key to acquiring the majority of a given nutrient supply is root length dominance. A consequence of supply preemption being the mechanism of competition for nutrients and root length dominance the key to preempting nutrient supplies is that plants face an evolutionary tragedy of the commons in their allocation patterns to roots. When the nutrient supply is limiting and a given plant is grown in isolation, a relatively low root length density optimizes growth. Yet, in the presence of competitors, plants that can maintain higher root length densities than are optimal in the absence of competition are able to acquire a larger fraction of the total nutrient supply and therefore would have been favored by natural selection. As such, high root length densities are more evolutionarily stable than the lower root length densities that would optimize growth in the absence of competition. This easy to see by taking a shovel to most temperate lawns. Most temperate grasses appear to have an order of magnitude more roots than is optimal for growth in the absence of competition.

The supply of light differs fundamentally from nutrient supplies in that it is largely supplied directionally. Yet, competition for light can select for plants that have canopies that are suboptimal in the absence of competition. The trunks of trees do not help a forest acquire more light—they only help an individual acquire more light than a competitor. As I discuss in RSWP, research has also shown that many plants also hold more leaves than is optimal and the leaves are held too flat. Plants that are optimizing canopy photosynthesis would hold their leaves at a higher angle in order to allow more light to penetrate deeper into the canopy where it can be used more efficiently. What would a tree with an efficient canopy look like? Probably something like a eucalyptus with its pendulous leaves and sparse canopies. It probably is no coincidence that eucalyptus are known by foresters to produce wood at some of the highest rates.

Resource Strategies of Wild Plants released

I guess I didn't time the countdown right. RSWP was released last week and available to order from Princeton University Press and retailers like Amazon.