The puzzle: How warming climate affects the growing season in Europe?

Warming climate is expected to have significant impacts on the growing season in Europe by 1) shifting crop phenology and lengthening the growing season due to warmer springs and longer summers, which may benefit certain crops and plant species, 2) altering crop suitability, as some regions may become more favorable for certain crops while others may face challenges from increasing drought, 3) increasing water stress, which may affect plant growth and productivity and increase irrigation needs, and 4) leading to pest and disease outbreaks, as warmer temperatures and changing precipitation patterns may create more favorable conditions for certain pests, leading to increased infestations and crop damage. All these factors together can affect ecosystem services provided by plants, such as carbon sequestration, water regulation, and habitat provision. Altered plant phenology can disrupt ecological interactions and affect the overall functioning of ecosystems.

While there is consensus about the earlier start of the growing season due to warming climate, there is disagreement about the effects of global warming on the timing of the completion of growing season. Some studies suggest a shift in the onset of dormancy, others an earlier onset (Figure 1). Therefore, in our recent publication "Continuous increase in evaporative demand shortened the growing season of European ecosystems in the last decade," we explored the often confusing and counter-intuitive effects of climate change on European growing seasons and reasoning behind that. As in a previous analysis (https://doi.org/10.1002/vzj2.20029) we could show that climate change affects the hydrological status of both water- and energy-limited sites, we were curious to see how different vegetation types react to related impacts of climate change. We focused on the length of the growing season, as an integrative indicator of vegetation functionality and an important driver for successful land management.

Figure 1 | Schematic representation of the effects of climate change on the occurrence of onset of greening (OG) and dormancy (OD) that determine growing season length (GSL). It shows the effect of increased temperature (T) in spring on early OG and the effect of increased T in summer on increased vapor pressure deficit (VPD) and decreased soil moisture (SM) and consequently on OD.

Methodology: How do we tease out changes in season length?

Our research journey began with a first great challenge: We needed to develop an algorithm that could infer the onset of greening and vegetation dormancy from Normalized Difference Vegetation Index (NDVI) data independently for each year and each pixel. This was important because classical methods typically calculate a critical long-term NDVI value for the entire period and then determine the timing for approaching such a critical value in all individual years. However, in our case, this was prone to bias because the NDVI data used came from three different products of the Global Inventory Monitoring and Modelling System (GIMMS), the Advanced Very High-Resolution Radiometer (AVHRR), and the Moderate Resolution Imaging Spectroradiometer (MODIS), each of them with different temporal resolutions (biweekly, daily, and monthly) affecting the long-term critical value. To address this challenge, we developed the logistic function derivative NDVI (LFD NDVI) method. We then performed the traditional Mann-Kendall test to identify likely trends in growing season length. This was done for the entire Europe as well as for different dominant vegetation types in Europe. Then we developed a novel method by adapting the Group Method of Data Handling (GMDH) to determine the controlling factors affecting the beginning and end of the growing season. GMDH combines the advantages of both machine learning and statistical regression analysis. To ensure robust and reliable results, we conducted rigorous data collection, including both in situ data from FLUXNET as well as reanalysis data from the NASA Global Land Data Assimilation System (GLDAS) along with the NOAH Land Surface Model, the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5- land, and the Global Land Evaporation Amsterdam Model (GLEAM).

During our research we experienced both triumphant and difficult moments. For example, to innovate the LFD NDVI curve method, we had to discover all possible shapes of the NDVI curve within the pixels under study (~20000 pixels) as well as individual years (~40 years). This required both an automated algorithm and visual inspection of hundreds of random cases. These experiences have shaped our understanding and demonstrated the importance of perseverance and commitment required in scientific research. My thanks also go to the invaluable contributions of our team members and collaborators, without whom this endeavor would not have been possible.

 Key Findings: what do the findings mean?

The efforts culminated in a series of remarkable results. We found that the trend toward lengthening the growing season in Europe has reversed over the past decade, challenging prevailing theories and opening new avenues for exploration. The discovery that the shortening of the growing season is primarily due to increased water demand in the atmosphere rather than an earlier moisture deficit in the soil provides important insights for the management of European ecosystems and opens exciting opportunities for analyzing the effects of climate change on net carbon balance and water and energy exchange with the atmosphere.

The implications of our findings are far-reaching and may have applications in European ecosystem management. We discuss how our work contributes to existing knowledge, expands the theoretical framework of climate change and its potential impacts on terrestrial ecosystems, and addresses previously unexplored aspects. In addition, we highlight areas for future research, such as similar analyses on a global scale that consider conditions different from those in Europe and that will further unravel the intricacies of how climate change impacts ecosystems and vegetation functioning.

 Engagement and Conclusion:

We invite the scientific community to participate in a lively dialogue about our research. We welcome feedback, questions, and collaborations that can deepen our understanding of climate change and its potential impacts on vegetation functionality. Our publication represents the culmination of our scientific expedition, and we hope it will inspire other researchers to embark on their own journey of discovery and uncover the hidden secrets of their field.

In conclusion, our “Behind the Paper” offers a glimpse into the untrodden path we've taken, highlighting the excitement, challenges, and importance of our research on climate change- vegetation functionality relationships. Through our relentless pursuit of knowledge, we aim to make a lasting impact on the scientific community and society at large by unraveling the mysteries of climate change impacts on food production and advancing advances in managing future disturbances and their impacts on terrestrial ecosystems.