The evolution of photosynthetic capacity is associated with climate

Background/Question/Methods

Previous studies have noted a pattern of higher photosynthetic rates in areas with greater annual precipitation, and this relationship is thought to be mediated by differences in specific leaf area (SLA; cm²/g) that are associated with precipitation.  This has been used to argue that low photosynthetic rate and low SLA may be adaptive to hot, arid environments and high photosynthetic rate and SLA to moist, productive biomes.  However, there is disagreement over whether low photosynthetic rates at ambient CO₂ in arid-zone species is a result of low photosynthetic capacity inherent in species with low SLA or whether they simply reflect diffusive limitations brought about by dry conditions such as low stomatal or mesophyll conductance, limitations that may be overcome by greater CO₂ or moisture availability.  Using a literature survey, we examined the relationship of photosynthetic capacity (CO₂-saturated photosynthesis rate) to mean annual temperature and precipitation, SLA and leaf nitrogen, including phylogenetic information to identify potentially adaptive associations between traits and between traits and climate.  Photosynthetic capacity was assessed from published responses of instantaneous photosynthesis to variation in intercellular CO₂ concentration and was recorded as CO₂-saturated photosynthetic rate per unit area (A_area) and mass (A_mass).

Results/Conclusions

We recorded observations of photosynthetic capacity for 193 species and found that A_area increased with decreasing precipitation and found no significant relationship between A_area and temperature; these relationships were reflected in phylogenetic independent contrasts.  It has been noted previously that species with high photosynthetic capacity in xeric environments, particularly desert annuals, may maintain high allocation to photosynthesis to exploit periodic precipitation events and abundant solar radiation.  This hypothesis is supported in our study by a negative correlation between N_mass and precipitation indicating high leaf N in xeric environments, as well as a positive relationship between A_area and N_mass, all relationships supported by independent contrasts.  Thus, a suite of traits marked by high leaf N and potential for high A_area may be adaptive in arid-zone species that maintain low diffusive conductance to CO₂ during long, dry periods, but have high potential for photosynthesis to exploit more favorable conditions.  A_mass and SLA independent contrasts were both negatively related to mean annual temperature but not precipitation, indicating that species from hot, arid environments maintain dense, long-lived leaves with high potential for CO₂ assimilation during episodically optimal conditions.