One of the keys to understanding the evolution of plants as well as the functioning of ecosystems is understanding the geographic patterns of resource limitation. This means maps. With maps of resource limitation, we can examine patterns of dominant species and infer adaptations to differences in resource limitation. Yet, largely, these maps do not exist. We have good maps of climate, fairly good maps of soil resources, but poor maps of resource limitation.
Why is this? Why can't we open up a map that shows us where nitrogen is the most limiting? There are a few fundamental reasons.
1) We do not understand patterns of limitation at any one site. In part, this can be ascribed to limitation being variable over time and small spatial scales. Yet, the multi-factor resource addition experiments that are necessary to fully describe resource limitation are but a handful.
2) We have not developed proximal indicators of resource limitation. Outside of adding the resources directly and examining responses, there are no reliable indicators of resource limitation. For example, plant N:P ratios have long been thought to be good indicators of the relative limitation of N and P in ecosystems. Yet, for a number of reasons, such as species not having a narrow range of "optimal" N:P ratios, these don't work.
3) We don't have the networks in place to make maps. I've written on this before (Bioscience, 2007), but outside of Europe, there is virtually no infrastructure to make extensive measurements. If it can't be measured with a satellite, we just don't measure it on any meaningful geographic scale. For example, I and coauthors have been analyzing global patterns of foliar N isotopes, which is our best easy index of N availability. Over the past 30 years, there have been a little more than 10,000 measurements of foliar N isotopes. It might sound like a big number, but that's only about $100,000 in analytical costs and there are large regions of the world for which no data have been collected. The map shown above looks extensive, but only because of extrapolation from relationships with climate data.
Resource limitation is critically important for understanding the evolution of plants, but also the ecology of ecosystems. Predicting responses to climate change, nitrogen deposition, and invasive species; estimating carbon sequestration, planning for species conservation...these all ultimately depend on resource limitation. Certainly, there are a number of questions close at hand that we must answer, but global maps of resource limitation has to be the point to which we fix our gaze on the horizon.