| 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. |
- Cuni-Sanchez, Aida, et al.
(författare)
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High aboveground carbon stock of African tropical montane forests
- 2021
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 596:7873, s. 536-542
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Tidskriftsartikel (refereegranskat)abstract
- Tropical forests store 40–50per cent of terrestrial vegetation carbon. However, spatial variations in aboveground live tree biomass carbon (AGC) stocks remain poorly understood, in particular in tropical montane forests. Owing to climatic and soil changes with increasing elevation, AGC stocks are lower in tropical montane forests compared with lowland forests. Here we assemble and analyse a dataset of structurally intact old-growth forests (AfriMont) spanning 44 montane sites in 12 African countries. We find that montane sites in the AfriMont plot network have a mean AGC stock of 149.4megagrams of carbon per hectare (95% confidence interval 137.1–164.2), which is comparable to lowland forests in the African Tropical Rainforest Observation Network4 and about 70per cent and 32per cent higher than averages from plot networks in montane and lowland forests in the Neotropics, respectively. Notably, our results are two-thirds higher than the Intergovernmental Panel on Climate Change default values for these forests in Africa8. We find that the low stem density and high abundance of large trees of African lowland forests is mirrored in the montane forests sampled. This carbon store is endangered: we estimate that 0.8 million hectares of old-growth African montane forest have been lost since 2000. We provide country-specific montane forest AGC stock estimates modelled from our plot network to helpto guide forest conservation and reforestation interventions. Our findings highlight the need for conserving these biodiverse and carbon-rich ecosystems.
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| 3. |
- 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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| 4. |
- Gardner, A., et al.
(författare)
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Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees
- 2022
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 237:4, s. 1229-41
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Tidskriftsartikel (refereegranskat)abstract
- Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (A(net)) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO(2)), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO(2) on iWUE and its components A(net) and stomatal conductance (g(s)). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller g(s), but greater A(net), responses to eCO(2) in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO(2) in all PFTs, and that increases in A(net) had stronger effects than reductions in g(s). The USO model correctly captured stomatal behaviour with eCO(2) across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g(1)) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.
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| 5. |
- Klingberg, J., et al.
(författare)
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Polycyclic aromatic hydrocarbon (PAH) accumulation in Quercus palustris and Pinus nigra in the urban landscape of Gothenburg, Sweden
- 2022
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Ingår i: Science of the Total Environment. - : Elsevier BV. - 0048-9697 .- 1879-1026. ; 805
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Tidskriftsartikel (refereegranskat)abstract
- Trees have the potential to improve urban air quality as leaves and needles capture air pollutants from the air, but further empirical data has been requested to quantify these effects. We measured the concentration of 32 poly-cyclic aromatic hydrocarbons (PAHs) in leaves of pin oak (Quercus palustris) and needles of black pine (Pinus nigra) in the City of Gothenburg, Sweden, during the summer of 2018. Oak leaves were collected twice (June, Sep-tember), while one-year-old (C + 1) and three-year-old (C + 3) pine needles were sampled in June to study the temporal development of leaf/needle PAH concentrations. Specific leaf area (SLA) was estimated, which permit-ted calculation of leaf/needle area-based PAH content that were compared with the mass-based concentration. In addition, the air concentration of PAHs and NO2 was measured using passive samplers. There was a strong correlation between air concentrations of PAH and NO2, indicating that the pollutants to a large degree originate from the same sources. In the oak leaves there was a significant decrease in low molecular mass PAHs (L-PAH, mainly gaseous) between June and September, but a significant increase in high molecular mass PAHs (H-PAH, mainly particle-bound). There was a strong correlation between L-PAH concentration in leaves and in air indicating an influence of equilibrium processes between ambient air and leaf. In the pine needles, there was a significant increase of both L-PAH and H-PAH in three-year-old needles compared to one-year-old needles. Pine was superior to oak in accumulating PAHs from the air, especially for L-PAHs when comparing area-based content. However, H-PAH concentrations were higher in oak leaves compared to pine needles on a leaf mass basis, emphasizing the importance of how concentrations are expressed. The results from this study can contribute to the development of urban planning strategies regarding the effect of vegetation on air quality. (C) 2021 The Authors. Published by Elsevier B.V.
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| 6. |
- Klingberg, J., et al.
(författare)
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Variation in Polycyclic Aromatic Compound (PAC) Concentrations in a Norway Spruce Stand Close to a Major Traffic Route-Influence of Distance and Season
- 2023
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Ingår i: Water Air and Soil Pollution. - 0049-6979 .- 1573-2932. ; 234:9
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Tidskriftsartikel (refereegranskat)abstract
- Air concentrations of polycyclic aromatic compounds (PACs) were measured in a Norway spruce (Picea abies) stand at four different distances from a major traffic route near Gothenburg, Sweden, during summer and winter 2019, using passive samplers. In total, the concentrations of 32 polycyclic aromatic hydrocarbons (PAHs) and six dibenzothiophenes (DBTs) and NO2 were analysed. PAC air concentrations were on average 2.5 times higher during the winter compared to the summer. All investigated PAH categories and compounds as well as dibenzothiophene declined linearly with the logarithmic distance from the traffic route. This indicates that the length of pollutant pathways through vegetation is a valid proxy to assess health risks of traffic-related PAC pollutants. Concentrations of heavier PAHs declined faster (concentration at site furthest away from the road was 6% and 33% of concentration at site closest to road, for summer and winter respectively) with increasing distance from traffic compared to lighter PAHs (63% and 68% for summer and winter respectively). As a result, the mix of PAHs changed with the depth into the forest stand (i.e. distance from the road) with potential effects on the toxicity as well as on diagnostic ratios used for source attribution. This is likely caused by different deposition rates among the PAH compounds to the vegetation surfaces, although the exact mechanism needs to be further investigated. This study provides new information that can improve air quality risk assessment and limit the exposure of the population to toxic air pollutants such as PACs.
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| 7. |
- Marshall, John, et al.
(författare)
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Components explain, but do eddy fluxes constrain? Carbon budget of a nitrogen-fertilized boreal Scots pine forest
- 2023
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Ingår i: New Phytologist. - 0028-646X .- 1469-8137. ; 389:6, s. 2166-79
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Tidskriftsartikel (refereegranskat)abstract
- Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses.We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance.N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20x). Only fine-root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m(-2) yr(-1). Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon-use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m(-2) yr(-1); P < 0.01) that eddy covariance did not (19 (62) g C m(-2) yr(-1); ns).The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.
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| 8. |
- 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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| 9. |
- 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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| 10. |
- Ntawuhiganayo, E. B., et al.
(författare)
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Traits controlling shade tolerance in tropical montane trees
- 2020
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Ingår i: Tree Physiology. - : Oxford University Press (OUP). - 0829-318X .- 1758-4469. ; 40:2, s. 183-197
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Tidskriftsartikel (refereegranskat)abstract
- Tropical canopies are complex, with multiple canopy layers and pronounced gap dynamics contributing to their high species diversity and productivity. An important reason for this complexity is the large variation in shade tolerance among different tree species. At present, we lack a clear understanding of which plant traits control this variation, e.g., regarding the relative contributions of whole-plant versus leaf traits or structural versus physiological traits. We investigated a broad range of traits in six tropical montane rainforest tree species with different degrees of shade tolerance, grown under three different radiation regimes (under the open sky or beneath sparse or dense canopies). The two distinct shade-tolerant species had higher fractional biomass in leaves and branches while shade-intolerant species invested more into stems, and these differences were greater under low radiation. Leaf respiration and photosynthetic light compensation point did not vary with species shade tolerance, regardless of radiation regime. Leaf temperatures in open plots were markedly higher in shade-tolerant species due to their low transpiration rates and large leaf sizes. Our results suggest that interspecific variation in shade tolerance of tropical montane trees is controlled by species differences in whole-plant biomass allocation strategy rather than by difference in physiological leaf traits determining leaf carbon balance at low radiation.
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