Abstract
Tropical tree species have evolved under very narrow temperature ranges compared to temperate forest species. Studies suggest that tropical trees may be more vulnerable to continued warming compared to temperate species, as tropical trees have shown declines in growth and photosynthesis at elevated temperatures. However, regional and global vegetation models lack the data needed to accurately represent such physiological responses to increased temperatures, especially for tropical forests. To address this need, we compared instantaneous photosynthetic temperature responses of mature canopy foliage, leaf temperatures, and air temperatures across vertical canopy gradients in three forest types: tropical wet, tropical moist, and temperate deciduous. Temperatures at which maximum photosynthesis occurred were greater in the tropical forests canopies than the temperate canopy (30 ± 0.3 °C vs. 27 ± 0.4 °C). However, contrary to expectations that tropical species would be functioning closer to threshold temperatures, photosynthetic temperature optima was exceeded by maximum daily leaf temperatures, resulting in sub-optimal rates of carbon assimilation for much of the day, especially in upper canopy foliage (>10 m). If trees are unable to thermally acclimate to projected elevated temperatures, these forests may shift from net carbon sinks to sources, with potentially dire implications to climate feedbacks and forest community composition.
Highlights
Temperate and tropical forests make up much of the world’s biomass, with tropical forests alone accounting for over 60% of terrestrial global carbon [1]
High photosynthetic rates in the pooled mean of the tropical moist forest were mainly driven by high rates of O. leucoxylon, 11.5 μmol CO2 m−1 s−2, compared to the highest values by species: Castilla elastica at 9.9 μmol CO2 m−1 s−2 and Acer saccharum at 7.3 μmol CO2 m−1 s−2 (Table 2)
Optimum photosynthetic rates (Aopt ) increased with canopy height in the temperate deciduous and tropical moist forests, but not the wet forest (Figure 2a–c). This pattern could be deciduous and tropical moist forests, but not the wet forest (Figure 2a–c). This pattern could be related related to the fact that the temperate and tropical moist forests had the greatest sampling range of to the fact that the temperate and tropical moist forests had the greatest sampling range of heights, heights, though it is possible that height is not as significant for photosynthetic rates in the wet though it is possible that height is not as significant for photosynthetic rates in the wet forest, forest, potentially since D. excelsa is an evergreen species
Summary
Temperate and tropical forests make up much of the world’s biomass, with tropical forests alone accounting for over 60% of terrestrial global carbon [1]. Forests can mitigate the effects of climate change, such as elevated temperature, through carbon dioxide (CO2 ) uptake during photosynthesis. There is a theoretical thermal tipping point after which photosynthesis begins to decline while plant respiration may still be increasing. If declines in CO2 uptake are severe enough that forest-wide respiration exceeds photosynthesis, forests could become net sources of carbon to the atmosphere [4,5]. Determining where this thermal threshold exists and whether different forest types are close to shifting from carbon sources to sinks is crucial to understanding and modeling global climate feedbacks [6]
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