OOS 45-5 - Physiological mechanisms of conifer persistence in the tropics

Few conifers persist in the tropics, yet those that do exhibit a unique flattened leaf morphology and belong almost entirely to a single family, the Podocarpaceae. Tropical flat-leaved conifers also have lower net photosynthesis (A) than needle-leaved conifers, leading to the hypothesis that shade tolerance may be important for tropical conifer persistence. Shade tolerant species frequently have wide leaves to maximize light capture, and low A due to low light availability. While studies have assessed leaf morphology and photosynthetic rates in some tropical conifers, data on respiratory traits such as basal respiration (respiration 25 °C, R25), and the short-term respiratory temperature (RT) response are extremely rare. We leveraged living collections of the New York Botanical Garden to collect RT response curves on 20 conifer species, 10 from the Podocarpaceae and 10 from the Pinaceae. Seventeen of the species we measured are tropical conifers, providing a unique opportunity to explore respiratory traits in an understudied group of tropical trees. The RT response was measured on upper-canopy foliage between 10 – 65 °C and R25 was extracted. We combined our results with published studies and global databases to create a worldwide conifer respiration database, doubling the number of conifer species available in respiratory trait databases such as GlobResp.

Podocarpaceae had significantly lower area- and mass-based R25 than Pinaceae, supporting past findings of lower A in podocarps compared to pine species. In the global database, tropical conifers had lower R25 (p < 0.05) than temperate or boreal conifers (0.31 ± 0.06 vs. 1.24 ± 0.16 or 0.88 ± 0.19 µmol m-2 s-1, respectively; means ± SE), as well as a different shape to the RT response. In particular, the change in the RT response may imply greater increases in respiration in response to future elevated or fluctuating temperatures in tropical conifers. Additionally, flat-leaved tropical conifers had lower R25 than needle-leaved tropical conifers and broad-leaved tropical angiosperm species (0.25 ± 0.05 vs. 0.48 ± 0.13 or 1.0 ± 0.07 µmol m-2 s-1, respectively). Taken together these findings suggest that tropical conifers hold some of the lowest respiration rates of all conifer species. Low respiratory costs, in addition to leaf flattening and low A, may be an important shade tolerant trait allowing tropical conifers to maintain a positive carbon balance and persist in low-light tropical forests where fast-growing angiosperms dominate.