Table 2.

Environmental parameters hypothesized to differ by photosynthetic type in trees

Type of parameter	Parameter	Hypothesis	Supporting reference a
Climatic	Minimum Temperature of Coldest Month	C4 trees are found in areas with lower minimum monthly temperatures than C3 trees	Edwards and Smith (2010)
	Temperature of the Wettest Quarter (Growing Season)	C4 trees are found in areas with higher growing season temperatures than C3 trees	Teeri and Stowe (1976), Vogel et al. (1986)
	Precipitation of Driest Month	C4 trees are found in areas with lower minimum monthly precipitation levels than C3 trees	Pyankov et al. (2010)
	Precipitation Seasonality	C4 trees are found in areas with more seasonal precipitation than C3 trees	Edwards and Smith (2010)
	Solar Radiation	C4 trees are found in areas with higher solar radiation than C3 trees	Ehleringer (1978)
	Climatic Water Deficit	C4 trees are found in areas with higher climatic water deficits than C3 trees	Witwickiet al. (2016)
Ecological	Vegetation Cover	C4 trees are found in areas with lower vegetation cover than C3 trees	Pau et al. (2013), Still et al. (2014)

^a Supporting references are studies showing environmental distributions for C₃ and C₄ herbaceous species.