Differential Response in Plant Taxa Morphology and Physiology During Increases in Late-Quaternary Atmospheric CO2 Concentrations Affect Plant-Climate Interactions.

The effects of changing atmospheric CO₂ on plant physiology mediate vegetation response to climate change. For example, growth chamber studies on short-lived plants show significant changes in plant morphology and physiological parameters such as changes in biomass and water-use efficiency (WUE; the amount of carbon assimilated to plant water-loss) as atmospheric CO₂ concentrations increases from ∼200 p.p.m. to modern concentrations and beyond. Many modern studies show WUE increases linearly with rising atmospheric CO₂ meaning that less water is expended for each unit of carbon assimilated. To test for the consistency of these findings with past, long-lived plants and in past communities growing under a similar range of atmospheric CO₂ levels, macrofossils of select species were analyzed from packrat (Neotoma sp.) midden chronologies gathered throughout western North America. Measurement of and analysis for the stable isotope content of these macrofossils shows greater morphological and eco-physiological differences between species than expected from study results using growth chambers. For example, isotopic analysis shows long-standing associates, Pinus edulis and Juniperus spp. have significantly different WUE during the transition from the Pleistocene to the Holocene. The WUE in Pinus edulis matches changes in atmospheric CO₂ whereas Juniperus spp. does not. Yet over the same period, changes observed in Pinus flexilis needles from trees growing in cooler habitats above the pinyon-juniper woodlands are more similar to Juniperus spp. changes compared against trends in the more closely related Pinus edulis. Morphology changes occurring during this period include increased biomass and reduced stomata. These results show taxonomic differences in the morphological and physiological adaptation to changing CO₂ concentrations. These responses need further assessment especially in light of their direct affect on plant-climate interactions.