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11C-autoradiographs to image phloem loading
(Frontiers in Forests and Global Change, 2019) Hubeau, Michiel; Mincke, Jens; Vanhove, Christian; Gorel, Anaïs Pasiphaé; Fayolle, Adeline; Epila, Jackie; Leroux, Olivier; Vandenberghe, Stefaan; Steppe, Kathy
Generally, tree species load photoassimilates passively into the phloem, while herbaceous species load actively. These phloem loading strategies have implications for phloem sugar concentration and growth potential. Whereas, in previous research, phloem loading identification was performed with 14C-autoradiography, we suggest 11C-autoradiography, because of its compatibility with plant-PET (positron emission tomography) scans. Because 11C-autoradiography has been hardly used in plant sciences so far, it was tested in contrasting plant species: one temperate tree species, Populus tremula L., three tropical tree species, Erythrophleum suaveolens (Guill. & Perr.) Brenan, E. ivorense A. Chev., and Maesopsis eminii Engl., and two herbaceous crop species Solanum lycopersicum L. and S. tuberosum L. Our results confirmed that P. tremula is a passive loader, and Solanum spp. are active loaders. Erythrophleum spp. and young leaves of M. eminii showed the expected passive loading strategy, but the mature leaves of M. eminii showed an uncommon pattern. Images corrected for leaf tissue thickness supported that mature leaves of M. eminii used active phloem loading, which is linked to continuous investment in growth and new leaves, supporting the lower carbon storage levels often observed in tropical tree species. With this study, we demonstrate that 11C-autoradiography is a powerful tool to acquire detailed tracer distribution in leaves to typify phloem loading strategies in plant species.
Capacitive water release and internal leaf water relocation delay drought-induced cavitation in African Maesopsis eminii
(Tree physiology, 2017) Epila, Jackie; De Baerdemaeker, Niels J.F.; Vergeynst, Lidewei L.; Maes, Wouter H.; Beeckman, Hans; Steppe, Kathy
The impact of drought on the hydraulic functioning of important African tree species, like Maesopsis eminii Engl., is poorly understood. To map the hydraulic response to drought-induced cavitation, sole reliance on the water potential at which 50% loss of xylem hydraulic conductivity (ψ50) occurs might be limiting and at times misleading as the value alone does not give a comprehensive overview of strategies evoked by M. eminii to cope with drought. This article therefore uses a methodological framework to study the different aspects of drought-induced cavitation and water relations in M. eminii. Hydraulic functioning of wholebranch segments was investigated during bench-top dehydration. Cumulative acoustic emissions and continuous weight measurements were used to quantify M. eminii’s vulnerability to drought-induced cavitation and hydraulic capacitance. Wood structural traits, including wood density, vessel area, diameter and wall thickness, vessel grouping index, solitary vessel index and vessel wall reinforcement, were used to underpin observed physiological responses. On average, M. eminii’s ψ50 (±SE) was −1.9 ± 0.1 MPa, portraying its xylem as drought vulnerable, just as one would expect for a common tropical pioneer. However, M. eminii additionally employed an interesting desiccation delay strategy, fuelled by internal relocation of leaf water, hydraulic capacitance and the presence of parenchyma around the xylem vessels. Our findings suggest that exclusive dependence on ψ50 would have misdirected our assessments of M. eminii’s drought stress vulnerability. Hydraulic capacitance linked to anatomy and leaf-water relocation behaviour was equally important to better understand M. eminii’s drought survival strategies. Because our study was conducted on branches of 3-year-old greenhouse-grown M. eminii seedlings, the findings cannot be simply extrapolated to adult M. eminii trees or their mature wood, because structural and physiological plant properties change with age. The techniques and methodological framework used in this study are, however, transferable to other species regardless of age.
Drought Effects on Photosynthesis and Implications of Photoassimilate Distribution in 11C-Labeled Leaves in the African Tropical Tree Species Maesopsis eminii Engl.
(Forests, 2018) Epila, Jackie; Hubeau, Michiel; Steppe, Kathy
Photoassimilate distribution inside leaves is less studied than photosynthesis, and yet the topic is important as it gives insights into the vital roles played by leaves in plant survival. We combined greenhouse measurements of light response curves with 11C-labelling using leaves of 3-year-old potted Maesopsis eminii Engl. trees to improve our understanding of its leaf carbon physiology. This fast-growing pioneer tree species showed low photosynthetic rates for a common tropical pioneer during well-watered reference conditions (5.0 0.7 mol m􀀀2 s􀀀1), which further decreased in response to drought. 11C-autoradiography indicated active phloem loading and/or rapid phloem transport rates. Active loading is uncommon in tree species, but might be related to deciduousness traits and continuous investment in growth, like in herbaceous active loaders. Active loading involves higher carbon allocation to growth, which might explain why low photosynthetic rates were observed in this fast-growing species. These findings suggest that examining photoassimilate distribution and transport may be critical for understanding the role tree physiology plays in terrestrial carbon cycling.
The ecology of Maesopsis eminii Engl. in tropical Africa
(African Journal of Ecology, 2017) Epila, Jackie; Verbeeck, Hans; Otim-Epila, Thomas; Okullo, Paul; Kearsley, Elizabeth; Steppe, Kathy
Maesopsis eminii is referred to as one of the most widely distributed African tree species. However, its occurrence in Africa has never been mapped and little is known as to how this species can sustain in different environments. To gain insight into Maesopsis’ ecology, we (i) made a synthesis of its functional trait data from the literature, (ii) investigated phenological patterns using data on four M. eminii trees from Yangambi, DR Congo, (iii) assessed an empirical provenance trial from Uganda on 600 Maesopsis trees and (iv) synthesized geo-referenced point location maps of Maesopsis entailing WorldClim precipitation and temperature and FAO soils, rainfall and ecological zones for Africa. We found M. eminii to straddle the equator equidistantly in terms of latitude (10.97°N and 10.98°S) covering five forest types where twenty soil types and variable rainfall regimes support complex plant biodiversity. Maesopsis eminii was, however, largely concentrated in the tropical rainforest ecosystem which contains fertile Orthic Ferralsol soils. More than 97% of the point locations were found where annual precipitation was >1000 mm, and 82% occurred where average annual temperature was 22–28°C. Its functional traits, phenology and provenance trial findings explained its occurrence in Africa.
Ecophysiological assessment of drought vulnerability of the African tropical tree species Maesopsis eminii Engl
(Ghent University, 2016) Epila, Jackie
Africa is endowed with the second largest block of nature-engineered machinery to sequester carbon: forests. Sadly, the functional traits and responses of this machinery are poorly understood, under non-drought, current drought and projected drought. Controversy surrounds the response of Africa’s forest tree species to drought. Moreover, little is known on the mechanisms or strategies they evoke to cope with drought. In this PhD study, the drought coping strategies and mechanisms of an African pioneer semi-deciduous tree species Maesopsis eminii Engl. (M. eminii) is investigated. A review of published and unpublished data provided insights in the species’ associated functional traits, phenology and provenance. The species’ occurrence niche was then characterized by combining this information and occurrence data of M. eminii in Africa with data on climate, soils and vegetation. It was found that M. eminii established itself in different ecosystems with diverse soil types and precipitation amounts. However, M. eminii mostly preferred the tropical rainforest region with fertile soils and annual precipitation exceeding 1000 mm yr-1. M. eminii’s presence in areas with pronounced seasonal precipitation prompted ecophysiological studies under natural and greenhouse settings. The aim was to determine additional traits used by this species to cope with drought. Different from most other plants, M. eminii seedlings continued to grow for a few days after the onset of drought and were also found to have nocturnal sap flow, because of low transpirational control. On the one hand, this trait seemed useful during non-drought conditions, but accelerated dehydration during drought and the eventual death of the seedlings. Unexpectedly, fast growth of M. eminii was not linked to its photosynthetic rate, as this was rather low. Instead, autoradiographs indicated a leaf role partition with older leaves actively loading sugars into the phloem compared to the expected passive loading in younger ones. Regardless of the loading mechanism, severe drought halved M. eminii’s photosynthetic rate but increased relative leaf respiration compared to a non-drought situation. This might explain why its growth eventually declined after a few days into drought. The hydraulic conduits of M. eminii were wide, which increased the likelihood of airseeding and may make this species’ xylem inherently vulnerable to drought-induced cavitation. But M. eminii was found to additionally have a considerable amount of water in its wood structure as evidenced by its low wood density, high volumetric water content and substantial hydraulic capacitance. This water probably contributed to the observed hydraulic redistribution between leaves, by which M. eminii could cope with drought for a couple of more days. During this hydraulic redistribution, a few leaves are shed while others remained hydrated, which reduced water loss but maintained limited carbon fixation. We also observed that M. eminii has low hydraulic conduits connection, which may limit the spread of drought-induced cavitation. This research has revealed different novel mechanisms (i.e., nocturnal sap flow, active sugar loading into the phloem in adult leaves, and hydraulic redistribution in the leaves) that have substantially increased the knowledge on how M. eminii copes with drought. It also shows that more attention must be paid to understand how African trees and eventually forests will deal with drought, if we aspire to more accurately predict the impact of climate change on this terrestrial ecosystem.
Forest resilience and tipping points at different spatio-temporal scales: approaches and challenges
(Journal of Ecology, 2015) Reyer, Christopher P. O.; Brouwers, Niels; Rammig, Anja; Brook, Barry W.; Epila, Jackie; Grant, Robert F.; Holmgren, Milena; Langerwisch, Fanny; Leuzinger, Sebastian; Lucht, Wolfgang; Medlyn, Belinda; Pfeifer, Marion; Steinkamp, Jorg; Vanderwel, Mark C.; Verbeeck, Hans; Villela, Dora M.
1. Anthropogenic global change compromises forest resilience, with profound impacts to ecosystem functions and services. This synthesis paper reflects on the current understanding of forest resilience and potential tipping points under environmental change and explores challenges to assessing responses using experiments, observations and models. 2. Forests are changing over a wide range of spatio-temporal scales, but it is often unclear whether these changes reduce resilience or represent a tipping point. Tipping points may arise from interactions across scales, as processes such as climate change, land-use change, invasive species or deforestation gradually erode resilience and increase vulnerability to extreme events. Studies covering interactions across different spatio-temporal scales are needed to further our understanding. 3. Combinations of experiments, observations and process-based models could improve our ability to project forest resilience and tipping points under global change. We discuss uncertainties in changing CO2 concentration and quantifying tree mortality as examples. 4. Synthesis. As forests change at various scales, it is increasingly important to understand whether and how such changes lead to reduced resilience and potential tipping points. Understanding the mechanisms underlying forest resilience and tipping points would help in assessing risks to ecosystems and presents opportunities for ecosystem restoration and sustainable forest management.
Plant measurements on African tropical Maesopsis eminii seedlings contradict pioneering water use behaviour
(Environmental and Experimental Botany, 2017) Epila, Jackie; Maesa, Wouter H.; Verbeeck, Hans; Campa, Janne Van; Okullod, John Bosco Lamoris; Steppe, Kathy
With increased drought events affecting forests globally, little is known about their future impact on Africa’s forests. In particular, we need to gain a better understanding of how key African forest species will respond to drought stress. In this study, we investigated functional traits and physiological responses to drought of the light-demanding pioneer species Maesopsis eminii Engl. The study involved an experiment on potted M. eminii seedlings with three different drought treatments in which sap flow (SF), stem diameter variation (SDV) and stomatal conductance (gs) were measured. Whereas low gs rates (39 30 mmol m 2 s 1) and pronounced SF-VPD (vapour pressure deficit) and gs-VPD hysteresis loops during well-watered conditions indicated conservative stomatal control on water loss, nocturnal sap flow implied M. eminii is not able to completely block transpiration. At the onset of drought, the pioneer seedlings retained high stem diameter growth despite highly reduced soil moisture (> 0.95 0.03 MPa) and SF rates, indicating that growth was prioritised. Contribution of stored stem water to daily water use was limited both during drought and control conditions, which was confirmed by the absence of time lags between photosynthetic active radiation (PAR)-SDV, PAR-SF and SDV-SF in all treatments. Below a soil water potential of 0.95 0.03 MPa, leaves were gradually shed, but diurnal and nocturnal SF did not turn zero, and absolute stem diameter kept decreasing for the entire experimental period (115 days), portraying that M. eminii seedlings can survive only limited drought periods. In sum, this study demonstrated that species-specific traits, as opposed to species ecological strategy, govern drought performance. For M. eminii, its leaf traits proved pivotal to its drought performance.

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