And just like that, we have already finished the first of five and a half years of the LEMONTREE project. Whilst LEMONTREE officially started on the 1st January 2021, it didn’t really get into full swing until the middle of the year due to a few administrative issues. Therefore, we have just submitted our Period 1 report to our funders (Eric and Wendy Schmidt, by recommendation of the Schmidt Futures programme) and we present here a summary of the first year.

A delayed start and the restrictions in fieldwork due to COVID-19, meant we missed the start of the 2021 growing season and so experimental work had been pushed back to year 2 and is starting now. However, despite this set-back, we have made substantial progress on the development and testing of eco-evolutionary optimality theories to describe physiological processes, at leaf level, whole plant level and ecosystem level.

The LEMONTREE research is centred around four main challenges which we aim to solve during the project. We have made great strides in the first three of these challenges already in the first year. Here are the highlights of our research.

Challenge 1: Optimality at the leaf and plant level

Aim:  to develop a comprehensive “optimal trait theory” predicting the leading dimensions of variations in the functional morphology of plants.

  • We have shown that leaf size, as measured by dry mass per unit area, and leaf lifespan co-vary in such a way as to maximise overall carbon gain under given environmental conditions (Wang et al.). Our theory explains why there can be different ratios of size and lifespan within an ecosystem, and how this ratio varies with environmental conditions. It also explains why the trends in leaf dry mass per unit area with latitude differ between deciduous and evergreen trees.
  • We have developed a model that successfully predicts the geographic patterns and trends in peak growing-season vegetation cover, based on assuming that the maximum foliage cover is determined by either available energy or available water, and used this model to examine observed recent trends in vegetation cover associated with rising CO2 and warming. We have applied this to predict spatial patterns and trends in foliage cover from eco-evolutionary optimality principles, at the regional scale of the Tibetan Plateau (Zhu et al.) and globally (Cai et al.).
  • We have shown that the characteristics that regulate plant hydraulics, e.g, water transport in trees and shrubs, are coordinated with traits that regulate photosynthesis because the uptake of carbon dioxide for photosynthesis necessarily involves water loss from the leaves. This will allow us to model optimal behaviour to maximise carbon gain and minimise water loss under different environmental conditions (Xu et al.).
  • Some woody plants recover quickly after damage by wildfires by using stored carbon to create new shoots. Our analyses of European and Australian data confirm that this behaviour is more frequent when the interval between fires is short and when fires are intense. This suggests a trade-off between the carbon costs of resprouting and the risk of damage, which should provide a more mechanistic way of modelling plant behaviour in response to wildfire (Harrison et al.).


Challenge 2: Biophysical coupling of atmosphere, water and land

Aim: to develop a comprehensive theory describing how ecosystems regulate water and energy exchanges between the atmosphere and land.

  • We have developed a simple model to predict evapotranspiration, the water flux from the land surface, based on the observed link between plant carbon uptake and water loss that is controlled by environmental conditions and generic for all types of plants. Although simpler than existing models of evapotranspiration, this model makes equally accurate predictions (Tan et al.)


Challenge 3: Ecosystem properties and biochemical cycling

Aim: To create a new model for land carbon cycling.

  • Plants which fix carbon in a form with three carbon atoms (C3 plants) differ from those which fix carbon in a form with four atoms (C4 plants) in their response to changes in atmospheric carbon dioxide concentration and sensitivity to temperature changes. We have developed a model to predict the relative abundance of C3 and C4 plants in response to environmental conditions and used this to quantify the contribution of these two plant types to global gross primary production (GPP) as a result of recent environmental changes. We have a paper in review at Global Change Biology that present the new implementation of C3/C4 competition into P model (Lavergne et al).
  • Growing seasons are becoming warmer due to anthropogenic carbon emissions. At the leaf level plants acclimate to warming by adjusting the maximum rate at which they fix carbon during photosynthesis on a timescale of weeks to months. Our analyses show that there are differences in the rate of thermal acclimation between different ecosystems, with grasslands adapting within ca 12 days while forests take several weeks to adjust.
  • The physiological processes for carbon uptake and release require nitrogen but the effect of adding nitrogen to the soil on plant growth is not well understood. Our experiments have shown that increasing soil nitrogen reduces the amount of carbon used to form roots and thus the cost of acquiring nitrogen but also leads to more efficient use of nitrogen in photosynthesis allowing plants to use less water. (Perkowski et al; Perkowski et al.; Smith et al.).


Challenge 4: From Plant function, biophysics and biogeochemistry to land-surface and climate modelling

Aim: To implement new formulations (Challenges 1-3) in a global modelling context.

Work has yet to start on this challenge.


In May 2021, the LEMONTREE team in collaboration with Tsinghua University held a joint science meeting to discuss Land system models; their known deficiencies and how the LEMONTREE project will be bringing new scientific theory to create the next generation models. Professor Sandy Harrison was co-host and introduced the LEMONTREE project to the participants. and Professor Colin Prentice presented the main topic “Land-atmosphere exchanges of carbon, energy and water: new theory and next-generation models”.

Although the project was still in its early days, several members of the team were able to present their research which is fundamental to the LEMONTREE project: Dr Wang Han, Huiying Xu, Giulia Mengoli and Dr Koen Hufkens. It was a great opportunity for the team to outline the problems with current ecosystem models and how the optimality theory, combined with biophysical constraints is contributing to a new theory of ecosystem function.

In June 2021, Dr Wang Han also presented the LEMONTREE research at the 11th International Symposium on Modern Ecology Series (ISOME).


Members of the project attended the AGU Fall meeting (Dec 2021) in New Orleans. Dr Julia Green convened session B14B- Advancing our understanding of vegetation stress and its feedbacks with energy, water and carbon fluxes II. Dr Trevor Keenan convened session B23D- Vegetation canopies: physiology structure and function I. Within this session, Dr Jiangong Liu presented his work on this task “Widespread thermal acclimation of canopy photosynthesis”. This research was also presented at the online AsiaFLUX meeting in December 2021.


In May 2022, members of the LEMONTREE team attended the annual EGU General Assembly in Vienna, Austria, and others joined remotely. Sandy Harrison convened session 3.8: Exploring the use of optimaility approaches in vegetation and land-surface models. This session, focussed on the the Eco-Evolutionary Optimality (EEO) theory which underpins LEMONTREE research. The talks in this session focussed on plant traits such as leaf area index, stomata, transpiration and the conductive efficiency of the xylem. The role of nutrient availability and photosynthetic pathways were also discussed. It was great to have a big LEMONTREE presence in the session to discuss this state-of-the-art in modelling, whether from experimental or observational data sets and thinking of ways to move EEO-based approaches forward as our knowledge grows.

EGU Optimality dinner with members of the LEMONTREE tree and associates. Back row left to right:  Remko Nijzink, Jan Lankhorst, Astrid Odé, Jaideep Joshi, Giulia Mengoli, Jaize Li, Ruijie Ding, Yunke Peng, Sandy Harrison, Colin Prentice. Front row: left to right: Chuanxin Gu, Cai Wenjia (Shirley), Alienor Lavergne, Oskar Franklin, Karin Rebel, Ning Dong.


As this goes to press, LEMONTREE members are presenting at the 8th edition of the Mathematical Models in Ecology and Evolution (MMEE) conference, Reading. Sandy Harrison is one of the organisers of this mini-symposium which aims to promote the use of mathematical approaches to answer a wide range of ecological and evolutionary questions such as the application of the optimality theory.

After this mini-symposium, we will hold the first in-person LEMONTREE meeting of the project where we hope to have most of the team present for a 2-day workshop to share recent progress, discuss the challenges to progressing with optimality theory and plan future work.


Looking forward

Our goals for Year 2 are to complete the theoretical work on leaf- and plant-level processes, and to make advances towards developing an optimality-based model at ecosystem level. Experimental field work will be conducted this summer in Colorado, Texas and in Western Australia.

We anticipate completing the development of our data tools. Also, work will begin on testing the new model components extensively both in the hydrological and land-surface modelling schemes such as JULES and ECMWF.


We look forward to sharing more about our progress on our blog over the coming year.



The project is off to a great start with 11 papers already published in peer-reviewed journals (listed on our website) and a further 23 papers either in review or in preparation. These are the articles cited in this blog (with team members in bold).

Cai, W., Zhu, Z., Zhou, B., Wang, H., Harrison, S.P., Prentice, I.C. Optimality-based predictions of global patterns and recent trends in vegetation cover. In preparation for Nature Climate Change

Harrison, S.P., Prentice, I.C., Bloomfield, K., Dong, N., Forkel, M., Forrest, M., Ningthoujam, R.K., Pellegrini, A., Shen, Y., Baudena, M., Cardoso, A.W., Huss, J.C., Joshi, J., Oliveras, I., Pausas, J.G., Simpson, K.J., 2021. Understanding and modelling wildfire regimes: an ecological perspective.  Environmental Research Letters   16: 125008.

Lavergne, A., Harrison, S.P., Atsawawaranunt, K., Dong, N., and Prentice, I.C. A semi-empirical model for primary production, isotopic discrimination and competition of C3 and C4 plants. In review at Global Change Biology.

Perkowski, EA, Frey, D.W., Goodale, C.L., Smith, N.G. Nitrogen availability increases leaf-level photosynthesis through an increase in leaf nitrogen allocation to Rubisco carboxylation in a closed canopy temperate forest. In preparation for Global Change Biology

Perkowski, EA, Terrones, J, Smith, NG. Soil nitrogen availability and nitrogen fixation reduce carbon costs to acquire nitrogen. In preparation for Journal of Experimental Botany.

Qiao, S., Wang, H., Prentice, I.C., Harrison, S.P., 2021. Optimality-based modelling of climate impacts on global potential wheat yield Environmental Research Letters 16: 114013,

Smith, N.G., Waring, E.F., McNellis, R., Perkowski, E.A., Martina, J.P., Seabloom, E.W., Wilfhart, P.A., Dong N., Prentice, I.C., Wright, I.J., Power, S.A., Hersch-Green, E.I., Ritsch, A.C., Caldeira, M.C., Nogueira, C., Chen, Q., Nutrient Network: Integrating data and theory to understand leaf-level nitrogen responses to soil nitrogen in grasslands. In preparation for Biogeosciences

Tan, S., Wang, H., Prentice, I.C., Yang, K., 2021. Land-surface evapotranspiration derived from a first-principles primary production model. Environmental Research Letters 16: 104047

Wang, HPrentice, I.C., Wright I. J.   Qiao, S., Xu, X., Kikuzawa, K., Stenset, N.C. (in review). Leaf economics explained by optimality principles. Science.

Xu, H., Wang, H., Prentice, I.C., Harrison, S.P. (in review). Environmental controls of leaf carbon and nitrogen stoichiometry. New Phytologist

Zhu, Z., Wang, H, Harrison, S.P., Prentice, I. C., Qiao, S., Tan, S. (in review). Co-limitation theory explains divergent responses of alpine vegetation to recent climate change. Global Change Biology.