Sometimes species are boring- how much does Picea mariana really say about a scraggly looking tree with sharp needles? Sometimes it can be more interesting to describe plants by their characteristics. If you are an ecologist- this approach is called ‘functional ecology’, and if you are a biogeographer, well... there isn't a name for it yet, but that hasn't stopped researchers from asking whether there are systematic ways that the traits of plants vary across the globe.
A lot of research has focused on two plant traits called leaf nitrogen content and specific leaf area (SLA). The first is pretty much what it sounds like, the percentage of a leaf’s dry weight that is from nitrogen. In leaves, nitrogen is mainly used for proteins involved in photosynthesis, so leaves with higher nitrogen content generally can make sugars more quickly. Within a single plant, the leaves that get the most light usually have more nitrogen because that’s where chloroplasts are most useful.
Specific leaf area is the ratio of a leaf’s area to how much it weighs when dried. Basically, it gives an estimate of how much effort a plant puts into making leaves- plants with thick hard leaves (like a rhododendron) have high SLA and consequently hold onto them for longer since they have invested so much raw material in them. Alternatively, birch trees have light thin leaves with low SLA and shed them each winter.
These two traits vary between different species, but they also vary between plants growing in different environments because the strategy that a plant uses to divvy up its limited carbon and nitrogen will depend on how much water it has access to, how long temperatures are warm enough for it to grow, and whether it will be better off growing quickly or growing more slowly. The question is, is there more variation in SLA and leaf nitrogen amongst the plant species that live together in an alpine meadow or between plants living in the meadow and those living in a temperate forest?
Because these traits are easy to measure and lots of plants ecologists have done so, this question is actually not that hard to answer- provided scientists are willing to share their data. This was the goal of Gregoire Freschet and seventeen other scientists when they pooled their data from 58 sites distributed among nine different biomes around the world. What they found surprised me- about half of the variation in SLA and leaf nitrogen found among all the species from around the world could be found within a single site. But, the average levels of these traits also differed between the different biomes that they examined; wet temperate forests in New Zealand had the lowest average leaf nitrogen and SLA (those trees just don’t put much into their leaves!) while the alpine temperate ecosystems in the Caucasas had the highest.
What I found most interesting about this paper was how the authors explained their results. Rather than try to squish all of their observations into one cohesive theory, they acknowledged that the reasons for particular levels of SLA and leaf nitrogen and the variability of these traits within communities likely differed between biomes and then proceeded to talk about each of them in turn. In the last decade, numerous groups of researchers have taken the “let’s all share and discover something global” approach. But in the excitement of forging unified global patterns, it is easy to gloss over what can be learned from how places differ. These authors do an admirable job of balancing commonalities with idiosyncrasy, showing that collaborative science truly has the ability to turn one person’s ecology into global biogeography.
You can find this paper at: