Two tropical conifers show strong growth and water-use efficiency responses to altered CO₂concentration

Abstract

Background and Aims Conifers dominated wet lowland tropical forests 100 million years ago (MYA). With a few exceptions in the Podocarpaceae and Araucariaceae, conifers are now absent from this biome. This shift to angiosperm dominance also coincided with a large decline in atmospheric CO 2 concentration (c a ). We compared growth and physiological performance of two lowland tropical angiosperms and conifers at c a levels representing pre-industrial (280 ppm), ambient (400 ppm) and Eocene (800 ppm) conditions to explore how differences in c a affect the growth and water-use efficiency (WUE) of seedlings from these groups.Methods Two conifers (Araucaria heterophylla and Podocarpus guatemalensis) and two angiosperm trees (Tabebuia rosea and Chrysophyllum cainito) were grown in climate-controlled glasshouses in Panama. Growth, photosynthetic rates, nutrient uptake, and nutrient use and water-use efficiencies were measured.Key Results Podocarpus seedlings showed a stronger (66 %) increase in relative growth rate with increasing c a relative to Araucaria (19 %) and the angiosperms (no growth enhancement). The response of Podocarpus is consistent with expectations for species with conservative growth traits and low mesophyll diffusion conductance. While previous work has shown limited stomatal response of conifers to c a , we found that the two conifers had significantly greater increases in leaf and whole-plant WUE than the angiosperms, reflecting increased photosynthetic rate and reduced stomatal conductance. Foliar nitrogen isotope ratios (d 15 N) and soil nitrate concentrations indicated a preference in Podocarpus for ammonium over nitrate, which may impact nitrogen uptake relative to nitrate assimilators under high c a .Significance Podocarps colonized tropical forests after angiosperms achieved dominance and are now restricted to infertile soils. Although limited to a single species, our data suggest that higher c a may have been favourable for podocarp colonization of tropical South America 60 MYA, while plasticity in photosynthetic capacity and WUE may help account for their continued persistence under large changes in c a since the Eocene.

Plant–soil feedbacks and the dominance of conifers in a tropical montane forest in Borneo

“…In addition, it was recently suggested that Podocarpus guatemalensis (Podocarpaceae), a conifer species in Panama, could have a different strategy for soil N acquisition compared with broadleaf species (Dalling et al. ). Therefore, we hypothesized that podocarps associated with AMF are better at nutrient uptake than EM tree species in our study site and this hypothesis is consistent with the fact that podocarps (and other conifer species) are more abundant on N‐limited soils than on fertile soils in Borneo (Kitayama et al.…”

Gas exchange and water‐use efficiency in plant canopies

“…, ; Dalling et al . ). The increase in A / g s caused by rising CO 2 is expected to partly mitigate increasing drought stress caused by global warming in coming decades (Roderick et al .…”

Cenozoic evolution of the steppe-desert biome in Central Asia

“…The role of the atmospheric carbon dioxide concentration ( p CO 2 ) drop over the EOT ( 55 , 56 ) appears less important than temperature. Tropical conifers are more responsive to atmospheric p CO 2 than tropical angiosperms, showing improved growth and water use efficiency under higher CO 2 ( 57 ). Despite this, conifers have demonstrated range expansions during cooler periods with lower atmospheric p CO 2 ( 57 ), as is observed in Central Asia ( Fig.…”

“…This represented a catastrophic shift to a new environmental state, previously recognized in the sedimentological record as a change from a “wet” desert with ephemeral saline lakes and mudflats within the basins, to a more arid desert with no lakes in many parts of Central Asia ( 16 , 64 , 77 , 78 ). A drop in atmospheric p CO 2 would have decreased water use efficiency, thus enhancing the effect of increased aridity ( 57 ). An atmospheric p CO 2 drop could also have played an important role in differential survival of Asian plant species over the EOT, which requires further investigation.…”