By Jane Zelikova
I love it when things come together. This is exactly what happened to me just the other day. To start at the beginning, I took an Organization for Tropical Studies (OTS) graduate tropical biology course in 2003. It was a great course, I learned a lot, and I made some great friends. While a lot of my memories from the course are a bit murky (many many late nights working on projects, writing papers, and drinking cervesas has put a fog on the course), looking through the course book has brought some things back into focus. Turns out, my personal claim to fame was extreme organismal fidelity. You know those people who always ask a question related to their study organism, even if the subject being discussed is completely unrelated? For example, the seminar might be about jaw bone development in fish and the person I’m talking about asks about plant secondary chemicals …. and receives a blank stare from the speaker. Ok, apparently, the person asking those annoying and distracting questions was me, and my inquiries always (and I mean always!) came back to ants. At the end of the course, we gave each student an award and I got “The Taxonomic Fidelity Award” for my ability to guide any scientific discussion toward ant seed dispersal. That being said, it is not a huge surprise that my ability to turn things back to the ants has stayed with me, despite my scientific departure from the world of ants.
Given my apparent taxonomic fidelity, what happened last week is not a big surprise. I’ve been working on assessing how the PHACE treatments (elevated CO2 and warming) affect patterns of plant community greenness in a short grass steppe ecosystem. The general trend is that during the early part of the growing season, when water is not as limiting, warming advances the timing of community green-up and enhances plant development. Later in the season, elevated CO2 has an effect similar to warming as water becomes more limiting. In this research site, others have shown that elevated CO2 has an overall water saving effect and my greenness and plant cover data seem to support this. With both the CO2 and warming treatments, a trade-off emerges between enhanced growth early in the season and diminished growth and potentially earlier senescence later in the season. This is where my years of extreme taxonomic fidelity came in handy. I recalled a paper by a former lab mate, Ave Bucy, where she developed a model to explain why harvester ants clear their mounds (check out Bucy and Breed 2006 paper for the full story).
By way of many shading experiments and hours spent observing ants coming and going, Ave figured out that the ants gained foraging time earlier in the year by removing vegetation around their mound and therefore allowing the mound to warm up. However, they lost foraging time during the middle of the summer, when the mound heated up so much that it prevented ants from foraging for in the middle of the day. When considered across an entire year, the ants gained more foraging time by clearing their mounds than they lost in the summer. A trade-off, to be sure, but one that benefited these ants.
Ave’s work seems relevant to what I’m thinking about within the PHACE experiment. There seems to be a trade-off between greening up earlier in the growing season (as plants in the warmed plots are doing) and losing photosynthetic ability later in the season, when water is more limiting. The thing to figure out is if the extra time gained photosynthesizing earlier in the growing season is greater than the time lost not being able to photosynthesize later in the season. Similarly, the elevated CO2 treatments appear to enable plants to save water, which ultimately extends both their photosynthetic efficiency and their ability to continue photosynthesizing under conditions when water is limiting.
What does this trade-off mean for the future function of these semi-arid grasslands? Ultimately, that is the question we want to answer as we try to predict what these systems will look like in a warmer world with more CO2. If my hunch is right, the counteracting effects of elevating CO2 and warming may actually benefit plants, as long as there is water around. The next question is … what is going to happen to precipitation?