Phenological and thermal growing season changes in the Baltics

One of the most visible signs of climate change in nature are shifting growing cycles. In northern areas, mostly energy limited environments, the growing seasonality is usually defined in 3 different ways:
- frost free period: time period between last spring frost and first autumn frost;
- thermal growing season: in the Baltic region traditionally is defined as the number of days with mean temperatures above 5°C. 
- phenological growing season: the period between leaf unfolding (BBCH11) and senescence (coloring (BBCH92) or leaf fall (BBCH93)).

In this study we examine recent trends in growing season length in the Baltic region according to these 3 definitions from 1991-2020. The duration of frost-free period and thermal growing season was calculated using the gridded daily temperature from the E-OBS data set version v24.0e (Cornes et al., 2018). The phenological growing season was examined using a recently published open source phenological observations data set (Kalvāne et al, 2021) and remote sensing leaf area index (LAI) obtained from Copernicus data store.
Changes in the growing season vary within species, for example, for Acer platanoides, the prolongation of the growth trend is highly pronounced due to earlier flowering and later onset of autumn phases. For Populus tremula on the other hand, the changes in flowering are not as significant as the changes in the autumn phases (occurring on average later, the trend of which is positive), and the trend in the duration of the growing season is neutral.  The results indicate a statistically significant trend toward pioneer species such as Alnus incana and Corylus avellana.  The data regarding the end of the growing season is less conclusive. Autumn senescence in many cases does not show a significant trend.
However rising temperatures contribute to the increasing length of the thermal growing season. These thermal shifts have already influenced the bird migratory patterns, for example later migration of  Anser anser, as well as agricultural practices - sowing of winter cereals and soil cultivation - are taking place later in the autumn than it used to (Kalvāne and Kalvāns, 2021).
This study has shown that significant seasonal changes have taken place across the Baltics landscape due to climate change.

This study was carried out within the framework of the Climate change and sustainable use of natural resources institutional research grant of the University of Latvia (No. AAP2016/B041//ZD2016/AZ03) and the Latvian Council of Science, project “Spatial and temporal prediction of groundwater drought with mixed models for multilayer sedimentary basin under climate change” (No. lzp-2019/1-0165).
 
References: Cornes, R. C., van der Schrier, G., van den Besselaar, E. J. M. and Jones, P. D.: An Ensemble Version of the E-OBS Temperature and Precipitation Data Sets, J. Geophys. Res. Atmos., 123(17), 9391–9409, doi:10.1029/2017JD028200, 2018.
Kalvāne, G., Kalvāns, A., Ģērmanis, A.: Long term phenological data set of multi-taxonomic groups and agrarian activities, abiotical parameters from Northern Europa, Latvia, Earth Syst. Sci. Data, 13, 4621–4633.,https://doi.org/10.5194/essd-13-4621-2021, 2021
Kalvāne, G. and Kalvāns, A.: Phenological trends of multi-taxonomic groups in Latvia, 1970-2018, Int. J. Biometeorol., 65, 895–904, doi:https://doi.org/10.1007/s00484-020-02068-8, 2021.