What is the effect of rising temperatures on the performance of tropical trees? Given the importance of tropical forests in the regulation of the earth’s climate it is important to understand how rising temperatures affect the capacity of tropical trees to take up and store carbon. Before we can make predictions about future carbon fluxes, however, we first need to establish the relationships between temperature and plant performance under current conditions.
I use a combination of observational and experimental approaches to determine the temperature sensitivity of net photosynthesis, the capacity for RuBP carboxylation (Vcmax) and the photosynthetic electron transport rate (Jmax), both in the short term (minutes-to-hours) and in the longer term (weeks-to-months).
Slot M, Winter K (2017) Photosynthetic acclimation to warming in tropical forest tree seedlings. Journal of Experimental Botany (accepted pending minor revisions)
Slot M, Winter K (2017) In situ temperature response of photosynthesis of 42 tree and liana species in the canopy of two Panamanian lowland tropical forests with contrasting rainfall regime. New Phytologist. In Press. doi: 10.1111/nph.14469
Reef R, Slot M, Motro U, Motro M, Motro Y, Adame MF, Garcia M, Aranda J, Lovelock CE, Winter K (2016) The effects of CO2 and nutrient fertilization on the growth and temperature response of the mangrove Avicennia germinans. Photosynthesis Research 129, 159-170. PDF
Slot M, Winter K (2016) The effects of rising temperature on the ecophysiology of tropical forest trees. In: Tropical Tree Physiology (Eds. Goldstein G, Santiago LS). Springer International Publishing, Switzerland, pp 385-412. PDF
Thermal acclimation of leaf dark respiration (see also: canopy respiration website Kaoru Kitajima)
The instantaneous temperature response of dark respiration is an important parameter for calculation of diurnal respiratory carbon fluxes, but to predict plant response to global warming longer term temperature responses need to be assessed. I have experimentally tested whether top-canopy leaves of six species of tropical forest trees and lianas can acclimate to elevated night-time temperature. Preliminary analyses indicate that after 1- 2 weeks of night-time warming by 4°C, most species respire less than control leaves, suggesting thermal acclimation. Accounting for this acclimation reduces the calculated respiratory carbon efflux from a tropical forest stand that experiences night-time temperatures 4°C above current temperature by ca. 1.5 Mg carbon ha-1 yr-1. If widespread, the positive feedback of respiration to temperature will be much smaller than predicted. The mechanism of acclimation of these species is yet to be unraveled as preliminary tests indicate no treatment effect on carbohydrate pools in the leaves. Controlled and well-replicated experiments will be needed to achieve this goal, which I’m hoping to start in the near-future.
Slot M, Rey-Sanchez C, Winter K, Gerber S, Lichstein JW, Kitajima K (2014).Thermal acclimation of leaf respiration of tropical trees and lianas: response to experimental canopy warming, and consequences for tropical forest carbon balance. Global Change Biology 20, 2915-26.
Vanderwel MC, Slot M, Lichstein JW, Reich PB, Kattge J, Atkin OK, Bloomfield K, Tjoelker M, Kitajima K (2015) Global convergence in projected leaf respiration from estimates of thermal acclimation across time and space. New Phytologist 207, 1026–1037
Foliar respiration and its temperature response in a tropical forest canopy
Temperature is expected to affect long term stand-level CO2 fluxes in tropical forest. By 2100 most of the region currently supporting tropical forest will experience mean annual temperatures that are greater than the highest mean annual temperature that supports closed-canopy forest today. However, relatively little research has been done on the ecophysiology of tropical forest trees. In particular, data on leaf respiration are scarce, compromising our ability to model carbon fluxes in tropical forests. To help establish a baseline for leaf respiration of tropical canopy trees I measured species-level temperature response curves of leaf dark respiration of top canopy leaves of a large number of species of tree and liana in a tropical forest in Panama. Respiration rates I recorded were on the high end of what has been published previously for tropical forest species, and the temperature sensitivity was greater than what is expected for tropical forests. As a result, the leaf respiratory carbon efflux I calculated for this forest stand is 10% higher than if I had used the theoretically predicted temperature sensitivity.
Slot M, Wright SJ, Kitajima K (2013) Foliar respiration and its temperature sensitivity of trees and lianas: in situ measurements in the upper canopy of a tropical forest. Tree Physiology 33, 505-515.
Slot M, Rey-Sánchez C, Winter K, Kitajima K (2014) Trait-based scaling of temperature-dependent foliar respiration in a species-rich tropical forest canopy. Functional Ecology 28, 1074–1086.
Atkin OK, et al. (including Slot M) (2015) Global variability in leaf respiration among plant functional types in relation to climate and leaf traits. New Phytologist 206, 614–636.
Shade tolerance of seedlings from tropical dry- and moist forests
In tropical forests, gradients of species distribution and diversity are associated with water availability, and plants at different positions along the precipitation gradient are adapted to different limiting resources. In a shade house experiment in Bolivia I studied the effect of deep shade on performance of seedlings of tropical tree species from dry and from moist forests in lowland Bolivia. A tendency for reduced growth and survival of dry forest species in low irradiance suggests a trade-off between drought tolerance and shade tolerance, governed by the inherent preferential investment in below ground biomass in species adapted to dry conditions.
Slot M (2004) Comparative growth analysis of seedlings of 13 wet and dry tropical forest tree species: the relative importance of physiological and morphological traits in deep shade and moderate light. Unpublished thesis University of York.
Drought tolerance of seedlings from tropical dry- and moist forests
In a dry-down experiment I further found very diverse physiological responses to drought across species, with little similarity in drought response strategies of species originating from a similar position along the rainfall gradient, suggesting that there are multiple evolutionary strategies to optimize performance in dry habitats.
Slot M, Poorter L (2007) Diversity of seedling responses to drought. Biotropica 39, 683-690.
Biomass allocation as drought avoidance strategy in Quercus pubescens
Using dendrochronology of stems and roots, I reconstructed growth and biomass allocation patterns of mature Quercus pubescens trees from a dry alpine valley in Switzerland. In contrast to what has been found for seedlings, wet years did not see preferential investment in aboveground tissues, but both stem and root increment correlated with growing season precipitation.
Slot M, Janse-ten Klooster SH, Sterck FJ, Sass-Klaassen U, Zweifel R (2012) A life-time perspective of biomass allocation in Quercus pubescens trees in a dry alpine valley. Trees-Structure and Function 26, 1661-1668.
Cold-induced photoinhibition in Scots pine seedlings in Central-Siberia
I became fascinated by plant ecophysiological responses to temperature when studying photosynthesis of evergreen trees growing in the extreme cold of central Siberia. These trees cope with extreme winter temperatures by down-regulating their metabolism through structural rearrangements within the needles. When the photosystems get reorganized in spring, they automatically start absorbing light energy. At low temperature, however, low enzyme activity limits leaf photosynthetic capacity, and the imbalance between absorbed light energy and the photochemical energy use can be harmful. Using measurements of chlorophyll fluorescence and photosynthesis I found that this cold-induced photoinhibition of photosynthesis contributes to the poor performance of Scots pine seedlings in sun-exposed areas in the forest.
Slot M, Wirth C, Schumacher J, Mohren GMJ, Shibistova O, Lloyd J, Ensminger I (2005) Regeneration patterns in boreal Scots pine glades are linked to cold-induced photoinhibition. Tree Physiology 25, 1139-1150.
Effects of environmental stress on the temperature sensitivity of dark respiration
Abiotic stress factors associated with climatic change may affect the temperature sensitivity of leaf respiration by affecting the availability of carbohydrates as respiratory substrate. At the University of York I studied the effects of drought and shade on the temperature sensitivity of leaf dark respiration in the herb Geum urbanum. While extended shading decreased temperature sensitivity, short-term drought increased the temperature sensitivity of leaf respiration. The increased respiration rate appeared to be driven by greater demand for respiratory products, needed to maintain the function of the increasingly wilted leaves. The pool of sugars within the leaves did not decrease in either the shade or the drought treatment, and respiration was supported by starch-to-sugar conversion in the leaves. This suggests that availability of carbohydrates is unlikely to control respiration rates at ambient or short-term elevated temperature, and that demand for ATP and other respiratory products may be a more important factor controlling leaf respiration rates.
Slot M, Zaragoza-Castells J, Atkin OK (2008) Transient shade and drought have divergent impacts on the temperature sensitivity of leaf dark respiration. Functional Plant Biology 35, 1135-1146.
Effects of nitrogen and phosphorus limitation on early seedling development of Leucaena leucocephala
After seed mineral reserves expire, Leucaena leucocephala seedlings form associations with N-fixing bacteria and with mycorrhizal fungi that aid in phosphorus uptake. We determined experimentally that seed nitrogen is depleted several weeks before P, so seedlings need to invest in N-fixing bacteria first, a process that requires P that seed reserves can still supply. These results are relevant for understanding nutrient-acquisition strategies during establishment of L. leucocephala, which is invasive outside its native range.
Slot M, Palow DT, Kitajima K (2013) Seed reserve dependency of Leucaena leucocephala seedling growth for nitrogen and phosphorus. Functional Plant Biology 40, 244-250.