Stylized diagram of phenology of first flowering of different functional groups for Konza.
Global change models had often assumed categorical differences between C3 and C4 species. Because of the temperature sensitivity of photorespiration, C3 species are restricted to cooler seasons and C4 grasses to warmer seasons. The separation between C3 and C4 species, especially the grasses, was a standard categorization for plant functional types.
Yet, how much basis is there really for the separation? What role does photosynthetic pathway have to play in the phenology, if not ecology, of temperate grassland species?
At Konza, we’ve been collecting plant species when they begin to flower. It’s a rough estimate of phenology. It doesn’t capture how long they flower, or when leaves grow the most, but it’s an easily measured trait that represents phenology. We have first flowering dates for about 350 of Konza’s 550 herbaceous species.
Generalization #1: C3 grasses have an earlier phenology than the C4 grasses. The first grass to flower in 2010 was a C3 grass Poa pratensis on April 21. Yet the first C4 grass flowered just a week later. Bouteloua dactyloides flowered on April 27. Tripsacum dactyloides, another C4 grass, was just a day later—April 28. There really is little offset between C3 and C4 grasses in when they start to flower.
Generalization #2. The C3 photosynthetic pathway restricts the activity of C3 species when temperatures are high in comparison to C4 species. It is true that C4 grasses do flower later than C3 grasses. The last C3 grass to start flowering was Diarrhena obovata, a forest understory grass. It didn’t flower until June 28. Many C4 grasses do not begin to flower until July or August, when midday temperatures are routinely 30°C. Yet, C3 forbs also flower during the time when only C4 grasses are flowering. For example, Helianthus maximiliani will not flower until the first week of August.
At this point, I have a few questions.
If C4 grasses can flower as early as C3 grasses, and C3 forbs can be active during the time when C4 species should have a physiological advantage, then what are the links between photosynthetic pathway and phenology?
How much of phenology is driven by phylogeny rather than photosynthetic pathway? The Andropogoneae C4's flower mid-season, but not the Chloridoid C4's.
Why do C3 grasses not flower during the middle of the summer, while C3 forbs do? Can C3 forbs regulate their leaf temperature via transpiration to reduce photorespiration?
And why the offset for C3 and C4 grasses, if C3 species can flower mid-season? Is this an example of niche conservatism?
The topic of whole-flora analyses of phenology is complex, but some of these patterns seem clear enough to rethink some generalizations--even if they shouldn't happen based on what we know.