Fire in grasslands and savannas: a trip to Afri-homa

William Bond with a Gulliver post oak at Tallgrass Prairie Preserve.  

I spent four days last week driving around Oklahoma with William Bond from South Africa. I'm not sure why we chose the state, except in many ways it is the area most analogous (ecologically) to southern Africa that we have in North America. Old soils, intermediate precipitation, native prairies and savannas, and fire.

I've spent a fair amount of time over the past decade with William learning about things I never thought cared about. This trip quickly became a study on the roles of fire, drought, and herbivory in vegetation. It's hard to summarize William's view of the world, but he spends a lot of time thinking about how ecosystems and floras have developed over the past hundred million years. It's not a narrow topic.

I'll do my best to summarize, but essentially we visited different ecosystems such as the Cross Timbers to look for evidence of the antiquity of fire in grasslands, if not the antiquity of grasslands themselves. We'd go to a place like Tallgrass Prairie Preserve and look at oaks to see whether they promote fire. We traveled to Wichita Mountains, another old landscape, to look at how trees were coping with drought. We traveled to Black Kettle grasslands to look for plants with spines as evidence of histories with browsing mammals.

You can't necessarily look at a place and see back 10 million years, but you can try. For example, you can look under a Cross Timbers canopy and try to understand whether fire would rely on grass or oak leaf litter to work through the savanna. Or look at the canopy to see whether the oaks try to shade out the understory or not. Things like this provides evidence of the evolutionary history between these plants and fire.

A place like Black Kettle isn't too far from the Cross Timbers, but it's a radically different ecosystem. Grass there is grazed short. All the woody plants are structurally defended. I'm still picking out prickers from that place. It was never likely a fire world. It was likely always an herbivore world.

Long story short, there are still some great syntheses to be made. We haven't mixed everything up enough such that an old-fashioned field trip can't provide insight into the forces that have structured our world.

Drought and stress tolerance

Comparison of photosynthetic rates for seedlings of dry- and wet-habitat tropical tree species. On average, photosynthetic rates were ~1/3rd higher for dry-habitat species. 

I wrote a bit on this just the other day, but here is a new paper that raises questions about whether low-water species should be considered "stress-tolerators". Pineda-Garcia et al. grew seedlings of 10 pairs of closely related tropical tree species and measured a suite of traits. Dry-habitat species had higher photosynthetic rates than wet-habitat species. In addition, dry-habitat species retained their leaves longer after watering was ceased.

There are always a number of ways plasticity can alter relationships. For example, I once showed that high resource species can have lower N concentrations and longer leaf longevity than low-nutrient species due to patterns of feedbacks to N cycling after establishment. Here, there are a number of mechanisms that could generate the higher photosynthetic rates and longer leaf longevity in this particular experiment that could be reversed in another. Parsimony accrues slowly.

Yet, overall, this is another example where drought-tolerant species are not necessarily following the general "stress-tolerator" syndrome. It will be interesting to begin to officially tally the evidence to see whether there is much support for the two to be linked.

Pineda-Garcia, F., H. Paz, and C. Tinoco-Ojanguren. 2011. Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest. Plant, Cell & Environment 34:1536-1547.