Florian Hofhansl

Tropical Ecosystem Research

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New study published in Nature Plants

Our most recent article “Towards a unified theory of plant photosynthesis and hydraulics” has been published in Nature Plants.


Terrestrial plants remove a whopping 125 gigatons of carbon from the atmosphere every year by photosynthesis – that’s approximately the weight of a billion blue whales. While much of this carbon makes it back into the atmosphere, net carbon removal still amounts to 2.6 gigatons – a third of our annual fossil fuel emissions. The same plants together transpire 81,000 cubic km of water into the atmosphere every year – equivalent to the amount of water in the Caspian Sea. Transpiration by terrestrial forests accounts for 30% of the global water cycle – indeed, the Amazon Forest makes its own rain!

Plants have evolved different pathways by which photosynthesis can take place. A vast majority of plant species (85%) follow the C3 pathway, so named because the starting point is a molecule with three carbon atoms. This pathway dominates the plant kingdom because it is energetically most efficient (except in highly arid or hot conditions, where other pathways are favoured). Yet, it comes at a price. If C3 plants open their stomata – tiny valves on the leaf surface – to absorb CO2, they lose water through the same valves. And losing water means risking hydraulic failure and death, especially when the soil is dry. Therefore, C3 plants face a fundamental dilemma: should they open their stomata wide to absorb more CO2, or should they keep them closed to avoid hydraulic failure?

How about dynamically controlling the stomatal opening, to maximize carbon uptake under wet conditions and to avoid hydraulic failure under dry conditions? After all, 70% of global forests are prone to at least some dry periods. So, are plants using some clever survival strategies? Indeed, plants have evolved diverse strategies for controlling their stomata, suited to the environments they live in. Under normal conditions, most plants keep the stomata open, even to the extent of operating dangerously close to their hydraulic thresholds. But under severe drought, survival becomes the prime concern – stomata close, and photosynthetic machinery is scaled down.

We have set out to develop a unified theory of plant photosynthesis and hydraulics that can predict how plants with given hydraulic characteristics respond to drought. A simple optimality principle holds the key – plants simultaneously control their stomatal opening and photosynthetic capacity to maximize profit. Profit equals revenue minus losses, measured in carbon currency. Carbon revenue is from CO2 uptake, and carbon losses stem from the real costs of maintaining the photosynthetic machinery and the prospective costs of hydraulic failure. Our theory predicts how carbon assimilation rate, stomatal conductance, leaf-internal CO2 concentration, water suction pressure in the leaves, and photosynthetic capacity acclimate to climatic and soil-water conditions. The figure below shows sample predictions for two species from different habitats – Eucalyptus pilularis, found in wet coastal areas of eastern Australia, and Eucalyptus populnea, found in semi-arid regions of interior Australia. The lines are theoretical predictions, whereas the points are observed data.

We tested our theory with previously published experimental data from 18 plant species from diverse biomes. With just two parameters, the theory explains a wide range of empirical observations in a simple way. It expounds why photosynthesis and transpiration diminish with declining soil moisture. It elucidates, for the first time, why the photosynthetic capacity of leaves decreases when soils become drier. It reveals why most plants operate dangerously close to their hydraulic thresholds. It accurately predicts, for the first time, the photosynthetic responses to drying air.

Climate change may give rise to novel environmental conditions that plants have not experienced before. Being based on eco-physiological processes and optimality principles, our theory is expected to scale to such out-of-sample environmental conditions better than statistical extrapolations. The theory can readily be plugged into global land-surface models. By capturing the synergistic effects of rising CO2 concentrations and rising rainfall variability under climate change, it could significantly improve projections of the global carbon and water cycles under future climate scenarios.

Read more about our research under the following link to the article on the publisher website:

Joshi, J., Stocker, B.D., Hofhansl, F., Zhou, S., Dieckmann, U, & Prentice, I.C. (2022). Towards a unified theory of plant photosynthesis and hydraulics. Nature Plants DOI: 10.1038/s41477-022-01244-5


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    New study published in Ecology & Evolution

    Our most recent article “Mechanisms driving plant functional trait variation in a tropical forest” has been published in Ecology and Evolution.

    Results of this study were proudly presented @AGU Centennial Fall meeting in San Francisco, USA.

    In this study we have been investigating mechanism driving the variation of functional traits among neotropical tree individuals. To this end, we have used a statistical approach to separate different components of trait variation. To achieve this, we assumed that genetic differences in plant functional traits between species and genotypes increase with environmental heterogeneity and geographic distance, whereas trait variation due to plastic acclimation to the local environment is independent of spatial distance between sampling sites. We applied a so-called multiple regression on distance matrices to dissect the relative amount of variation associated with environmental heterogeneity vs. the one related to purely spatial constraints, i.e. geographic distance between sampling sites, which allowed to separate the impact of environmental driving factors from neutral processes in concert driving plant functional trait variation in hyperdiverse tropical forests. Beyond the initial study aims to separate respective components of trait variation we found that coexisting neotropical tree species responded differently to the same environmental cues, which indicates that under projected climate change endemic species with conservative ecological strategies could be even more prone to competitive exclusion than their widespread congeners.

    Read more about our research on the following link to the article on the publisher website.

    Fieldwork for this study was sometimes extremely challenging but absolutely worth it due to my fabulous field assistant.
    Not seeing the forest for the trees…?
    When the day is done…!

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    New study published in Nature Scientific Reports

    Our most recent article “Climatic and edaphic controls over tropical forest diversity and vegetation carbon storage” has been published in Nature Scientific Reports.

    In the study we have been investigating how tropical ecosystems – and thus their functioning as global carbon stores – might react to projected future climate change scenarios. Interestingly, we found stark differences in the functional composition of tropical plant species (trees, palms, lianas) across the landscape. For instance, lianas are relatively fast growing and try to reach the canopy to get to the sunlight but do not store as much carbon as a tree stem to reach the same height in the canopy. Palms share a different strategy and mostly stay in the understory while collecting water and nutrients with their bundles of palm leaves arranged upward to catch water and nutrients falling from above and thus reducing local resource limitation. As a result, we found that each functional group was associated with specific climatic conditions and distinct soil properties across the landscape. Since each of these plant functional groups (i.e. trees, palms, lianas) differ in their growth strategy, and thus the amount of C stored in their biomass, this means that we have to account for geomorphological heterogeneity and plant species composition in order to more accurately project future ecosystem responses and associated tropical ecosystem carbon storage. Our study highlighted that it is crucial to gather knowledge from multiple scientific disciplines, such as botany (identifying species), plant ecology (identifying functional strategies), and geology (identifying differences in parent material and soil types – all of which need to be considered in concert to understand the complexity of factors driving tropical forest ecosystem functioning.

    Read more about our research here: https://natureecoevocommunity.nature.com/channels/521-behind-the-paper/posts/61449-climatic-and-edaphic-controls-over-tropical-forest-diversity-and-vegetation-carbon-storage

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    Presentation at AGU 2019 fall meeting, San Francisco, USA

    IIASA researchers have participated in the American Geosciences Union Fall Meeting 2019 presenting research on climate adaptation and impacts on global ecosystems

    Background: Tropical plant communities exhibit extraordinary species richness and functional diversity in highly heterogeneous environments. Albeit the fact that such environmental filtering shapes local species composition and associated plant functional traits, it remains elusive to what extend tropical vegetation might be able to acclimate to environmental changes via phenotypic plasticity, which could be a critical determinant affecting the resistance and resilience of tropical vegetation to projected climate change.

    Methods: Based on a dataset compiled from 345 individuals and comprising 34 tropical tree species we here investigated the role of phenotypic plasticity versus non-plastic variation among key plant functional traits, i.e. wood density, maximum height, leaf thickness, leaf area, specific leaf area, leaf dry mass, nitrogen and phosphorus content. We hypothesized that trait variation due to plasticity is driven by environmental variability independently of spatial effects due to geographic distance between forest stands, whereas non-plastic variation increases with geographic distance due to adaption of the plant community to the local environment. Based on these hypotheses we partitioned total observed trait variation into phenotypic plasticity and neutral components and quantified respective amount of variation related to environmental filtering and neutral community assembly.

    Results: We found that trait variation was strongly related to spatial factors, thus often masking phenotypic plasticity in response to environmental cues. However, respective environmental factors differed among plant functional traits, such that leaf traits varied in association with light regime and soil nutrient content, whereas wood traits were related to topography and soil water content. Our results further suggest that phenotypic plasticity increased with the range size of congeneric tree species, indicating less plasticity within range restricted endemics compared to their widespread congeners.

    Conclusions: Differences in phenotypic trait plasticity affect stress tolerance and range size of tropical tree species, therefore endemic species could be especially prone to projected climate change.

    Check out the link to the website including a link to the e-poster: https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/501085

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    The role of plant functional trait variation in tropical forests

    Evaluating the role of plant functional trait variation in tropical forests we found that environmental controls and different trait components, such as phenotypic plasticity, genetic adaptation and random factors differed between neotropical tree species and plant tissues. Results suggest that leaf traits varied in association with light regime and soil nutrient content, whereas wood traits were related to topography and soil water content. We furthermore found that phenotypic plasticity increased with the range size of congeneric tree species, indicating less plasticity within range restricted endemics compared to their widespread congeners. Our findings indicate that differences in phenotypic trait plasticity affect stress tolerance and range size of tropical tree species, and therefore suggest that endemic species could be especially prone to projected climate change.

    Click on the poster to enlarge or download following the link below.

    Check out the e-poster at the conference meeting website: https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/501085

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    Amazon forest response to CO2 fertilization dependent on plant phosphorus acquisition

    New study published in Nature Geoscience reports first modelling results from the exciting CO2-fertilisation experiment conducted in the middle of the Amazon

    Current climate models suggest that trees will continue to remove manmade greenhouse gas emissions from the atmosphere, making it possible to stay within the targets set by the Paris Agreement. A study led by an international team of scientists however indicates that this uptake capacity could be strongly limited by soil phosphorus availability. The authors note that how the ecosystem reacts – whether trees will be capable of getting additional phosphorus from the soil through enzymatic processes or by forming more roots and symbiotic interactions that can provide scarce nutrients – must be further investigated. One thing is however certain, tropical rainforests are not an infinite CO2 sink and the Amazon forest reservoir must be preserved. This comes in line with response to rising international criticism over a surge in forest clearing since the beginning of the year, Brazilian President Jair Bolsonaro and officials in his administration have recently stepped up attacks on scientists at the country’s National Space Research Institute (INPE) for continuing to report transparently on deforestation in the Amazon.

    Check out the press release and media attention for the article published in Nature Geoscience, August 2019.

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    International Trait School – Porquerolles (France), 19-24 May 2019

    In May 2019, I took the great opportunity to participate in the 8th edition of the graduate thematic course, sponsored by CNRS, France and the Centre for Forest Research, Quebec Centre for Biodiversity Science. It was one of the best ecological experiences of my scientific life so far and I would definitely recommend to anyone interested in plant ecology (see website for applications and some impressions of this edition below).

    Also check out some of the islands incredible trail running routes and annual trail running events (https://www.hyeres-tourisme.com/agenda-fetes-manifestations/trail-de-porquerolles/).

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    Permakultur – Dein Garten. Deine Revolution.

    Die Permakultur orientiert sich an den Regeln der Natur. Und damit ist sie nicht allein. Wer entdeckt wie genial die Natur wirklich ist muss sich förmlich etwas von ihr abschauen. Doch was können wir noch alles von der Natur lernen.

    Permakultur ist ein großes miteinander: Beziehungen, Netzwerke, Systeme, Kooperationen, “das Leben, das Universum und der ganze Rest” (laut Douglas Adams Autor des gleichnamigen Fantasy-Romans). Die Autorin erklärt diese Zusammenhänge in Ihrem Buch und zeigt wie diese Erkenntnisse in der Permakultur umsetzbar sind. Mit einem Gast-beitrag von Florian Hofhansl.

    Link zum Buch: https://www.loewenzahn.at/buecher/2650/permakultur-dein-garten-deine-revolution/

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    Beta diversity and oligarchic dominance in the tropical forests of Southern Costa Rica

    Our article entitled “Beta diversity and oligarchic dominance in the tropical forests of Southern Costa Rica” was recently published in the journal Biotropica doi.org/10.1111/btp.12638.

    Forest community assembly is shaped by environmental factors and stochastic processes, but so far the contribution of oligarchic species to the variation of community composition (i.e., beta diversity) remains poorly known. We established 1‐ha permanent inventory plots in humid lowland tropical forests of southwestern Costa Rica to identify oligarch species characterizing changes in community composition among forest types. Based on this network of forest plots we investigate how community composition responds to differences in topography, successional stage, and distance among plots for different groups of species (all, oligarch, common and rare/very rare species). From a total of 485 species of trees, lianas and palms recorded in this study only 27 species (i.e., 6%) were nominated as oligarch species. Oligarch species accounted for 37% of all recorded individuals and were present in at least half of the plots. Plant community composition significantly differed among forest types, thus contributing to beta diversity at the landscape scale. Oligarch species was the component best explained by geographical and topographic variables, allowing a confident characterization of the beta diversity among tropical lowland forest stands.

    Please see this link to the online version of the article.