| 1. |
- Aloysie, Manishimwe, et al.
(författare)
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Warming Responses of Leaf Morphology Are Highly Variable among Tropical Tree Species
- 2022
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Ingår i: Forests. - : MDPI AG. - 1999-4907. ; 13:2
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Tidskriftsartikel (refereegranskat)abstract
- Leaf morphological traits vary along climate gradients, but it is currently unclear to what extent this results from acclimation rather than adaptation. Knowing so is important for predicting the functioning of long-lived organisms, such as trees, in a rapidly changing climate. We investigated the leaf morphological warming responses of 18 tropical tree species with early (ES) abd late (LS) successional strategies, planted at three sites along an elevation gradient from 2400 m a.s.l. (15.2 °C mean temperature) to 1300 m a.s.l. (20.6 °C mean temperature) in Rwanda. Leaf size expressed as leaf area (LA) and leaf mass per area (LMA) decreased, while leaf width-to-length ratio (W/L) increased with warming, but only for one third to half of the species. While LA decreased in ES species, but mostly not in LS species, changes in LMA and leaf W/L were common in both successional groups. ES species had lower LMA and higher LA and leaf W/L compared to LS species. Values of LMA and LA of juvenile trees in this study were mostly similar to corresponding data on four mature tree species in another elevation-gradient study in Rwanda, indicating that our results are applicable also to mature forest trees. We conclude that leaf morphological responses to warming differ greatly between both successional groups and individual species, with potential consequences for species competitiveness and community composition in a warmer climate. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
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| 2. |
- Dusenge, Mirindi Eric, 1986, et al.
(författare)
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Limited thermal acclimation of photosynthesis in tropical montane tree species
- 2021
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 27:19, s. 4860-4878
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Tidskriftsartikel (refereegranskat)abstract
- The temperature sensitivity of physiological processes and growth of tropical trees remains a key uncertainty in predicting how tropical forests will adjust to future climates. In particular, our knowledge regarding warming responses of photosynthesis, and its underlying biochemical mechanisms, is very limited. We grew seedlings of two tropical montane rainforest tree species, the early-successional species Harungana montana and the late-successional species Syzygium guineense, at three different sites along an elevation gradient, differing by 6.8℃ in daytime ambient air temperature. Their physiological and growth performance was investigated at each site. The optimum temperature of net photosynthesis (ToptA) did not significantly increase in warm-grown trees in either species. Similarly, the thermal optima (ToptV and ToptJ) and activation energies (EaV and EaJ) of maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport rate (Jmax) were largely unaffected by warming. However, Vcmax, Jmax and foliar dark respiration (Rd) at 25℃ were significantly reduced by warming in both species, and this decline was partly associated with concomitant reduction in total leaf nitrogen content. The ratio of Jmax/Vcmax decreased with increasing leaf temperature for both species, but the ratio at 25℃ was constant across sites. Furthermore, in H. montana, stomatal conductance at 25℃ remained constant across the different temperature treatments, while in S. guineense it increased with warming. Total dry biomass increased with warming in H. montana but remained constant in S. guineense. The biomass allocated to roots, stem and leaves was not affected by warming in H. montana, whereas the biomass allocated to roots significantly increased in S. guineense. Overall, our findings show that in these two tropical montane rainforest tree species, the capacity to acclimate the thermal optimum of photosynthesis is limited while warming-induced reductions in respiration and photosynthetic capacity rates are tightly coupled and linked to responses of leaf nitrogen.
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| 3. |
- Mujawamariya, Myriam
(författare)
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Climate Change sensitivity of Photosynthesis and Respiration in Tropical Trees
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tropical climate is getting warmer, with more pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as photosynthesis and leaf respiration, to these new conditions. However, knowledge on this in tropical tree species is currently limited due to data scarcity. In this thesis, I have studied warming and seasonal drought responses of photosynthesis and leaf dark respiration (Rd) in early-successional (ES) and late-successional (LS) species originating from Afromontane and transitional rainforest vegetation zones. My research used an elevation gradient approach with different designs in different studies: existing mature trees of four species growing at five locations at different elevation (Paper I); multispecies plantations established at three sites at different elevation and vegetation zones in an elevation experiment named Rwanda TRopical Elevation Experiment (Rwanda-TREE), using either plants freely rooted in the soil (Paper II and III) or plants growing in pots with the same soil at all sites (Paper IV). The results demonstrated that in existing mature trees leaf stomatal conductance (gs), transpiration (E) and light saturated net photosynthesis (An) decreased at warmer, lower-elevation sites during dry season, while patterns were absent (for gs and An) or opposite (for E) in the wet season. In Rwanda-TREE, I found that An under non-drought conditions decreased in trees grown at the warmest, low-elevation site, in LS but not in ES species, while An was strongly and equally reduced in ES and LS species during the dry season at the two warmer sites, but not at the high-elevation site. Rates of leaf Rd measured at 20 ℃ were strongly reduced in trees grown at the warmer sites, leading to constancy or even declines in Rd at prevailing nighttime temperatures. Drought also reduced Rd. The pot study showed that the optimum temperature of An and its underlying biochemical processes did not significantly increase in warm-grown trees, indicating limited thermal acclimation capacity of photosynthesis. The findings of this thesis have several important implications for the projection of future tropical biosphere–atmosphere interactions. Firstly, the pronounced seasonality in altitudinal patterns suggest that tropical tree water use and CO2 uptake will be substantially reduced if dry seasons become more pronounced in a warmer climate. Secondly, the strong thermal acclimation of leaf Rd observed here should be accounted for to avoid model overestimation of the impact of global warming on leaf respiration in tropical forests. Thirdly, the contrasting responses of photosynthesis to warming in ES and LS species may imply potential functional shifts in tree community composition of tropical forests in a warmer climate. Fourthly, my results also indicate that acclimation capacity of the thermal optimum of photosynthesis may be considerably weaker in tropical montane tree species compared to temperate and boreal species. With these findings, my thesis contributes to reducing the knowledge gaps regarding tropical tree responses to climate change, which is key for improving projections of future climate change responses and feedbacks of tropical forests.
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| 4. |
- Mujawamariya, Myriam, et al.
(författare)
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Complete or overcompensatory thermal acclimation of leaf dark respiration in African tropical trees
- 2021
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 229:5, s. 2548-61
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Tidskriftsartikel (refereegranskat)abstract
- © 2020 The Authors New Phytologist © 2020 New Phytologist Foundation Tropical climates are getting warmer, with pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as leaf dark respiration (Rd), to these new conditions. However, knowledge on this is currently limited due to data scarcity. We studied the impact of growth temperature on Rd and its dependency on net photosynthesis (An), leaf nitrogen (N) and phosphorus (P) contents, and leaf mass per unit area (LMA) in 16 early-successional (ES) and late-successional (LS) tropical tree species in multispecies plantations along an elevation gradient (Rwanda TREE project). Moreover, we explored the effect of drought on Rd in one ES and one LS species. Leaf Rd at 20°C decreased at warmer sites, regardless if it was expressed per unit leaf area, mass, N or P. This acclimation resulted in an 8% and a 28% decrease in Rd at prevailing nighttime temperatures in trees at the intermediate and warmest sites, respectively. Moreover, drought reduced Rd, particularly in the ES species and at the coolest site. Thermal acclimation of Rd is complete or overcompensatory and independent of changes in leaf nutrients or LMA in African tropical trees.
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| 5. |
- Mujawamariya, Myriam, et al.
(författare)
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Contrasting warming responses of photosynthesis in early- and late-successional tropical trees
- 2023
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Ingår i: Tree physiology. - : Oxford University Press (OUP). - 0829-318X .- 1758-4469. ; 43:7, s. 1104-17
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Tidskriftsartikel (refereegranskat)abstract
- The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (A(n)), maximum rates of Rubisco carboxylation at 25 degrees C (V-cmax25), stomatal conductance (g(s)) and the slope parameter of the stomatal conductance-photosynthesis model (g(1)), in 10 early successional (ES) and 8 late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8 degrees C in daytime ambient air temperature. The effect of seasonal drought on A(n) was also investigated. We found that warm climate decreased wet-season A(n) in LS species, but not in ES species. Values of V-cmax25 were lower at the warmest site across both successional groups, and A(n) and V-cmax25 were higher in ES compared with LS species. Stomatal conductance exhibited no significant site differences and g(1) was similar across both sites and successional groups. Drought strongly reduced A(n) at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of A(n) between successional groups may lead to shifts in species' competitive balance in a warmer world, to the disadvantage of LS trees.
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| 6. |
- Tarvainen, Lasse, 1977, et al.
(författare)
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Handling the heat - photosynthetic thermal stress in tropical trees.
- 2022
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Ingår i: The New phytologist. - : Wiley. - 1469-8137 .- 0028-646X. ; 233:1, s. 236-50
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Tidskriftsartikel (refereegranskat)abstract
- Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate. Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient. Heat tolerance acclimated to maximum leaf temperature (Tleaf ) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest Tleaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest Tleaf . The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling. More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.
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