CASSIA is a daily time-step, whole tree physiological model for growth, seasonal cycle and growth allocation that has been parameterized to Scots pine and Norway spruce (Schiestl-Aalto et al. 2015, 2019). It can be used for studying source and sink interaction effects on growth. CASSIA can also be used for studying the possible variations in carbon allocation to tree organs.

Different components of CASSIA model

As the above flow chart describes, photosynthesized carbon that can be either measured or modelled (e.g. with SPP or PRELES) goes into the storage pool. From the storage pool, carbon is used for energy production (maintenance / growth respiration) or building up new tissues. Tree is divided into fine roots, leaves and xylem, and xylem is further divided to primary and secondary growth as they can take place during different parts of growing season.

Dimensional organ growth in CASSIA depends on (1) phenology, i.e., integrated environment effect, (2) direct environment, and (3) availability of stored sugars. Typically in boreal conditions, phenology is largely controlled by temperature, the direct environment effect consists of temperature and water availability, while storage is rarely limiting. Dimensional growth is converted to carbon mass with density parameter and allometric relationships following pipe theory and functional balance theory (Shinozaki et al., 1964; Mäkelä & Valentine, 2006).

CASSIA is fully explained in Schiestl-Aalto et al. (2015, 2019). The model is coded with R and is available upon request from Pauliina Schiestl-Aalto (

Mäkelä A. and Valentine H. 2006. Crown ratio influences allometric scaling in trees. Ecology 87:2967-2972

Schiestl-Aalto P, Kulmala L, Mäkinen H, Nikinmaa 465 E, Mäkelä A (2015) CASSIA–a dynamic model for predicting intra-annual sink demand and interannual growth variation in Scots pine. New Phytol. doi: 10.1111/nph.13275

Schiestl-Aalto P, Ryhti K, Mäkelä A, Peltoniemi M, Bäck J, Kulmala L. (2019). Analysis of the NSC Storage Dynamics in Tree Organs Reveals the Allocation to Belowground Symbionts in the Framework of Whole Tree Carbon Balance. Frontiers in Forests and Global Change 2. DOI=10.3389/ffgc.2019.00017
Shinozaki K, Yoda K, Hozumi K, Kira T. 1964. A quantitative analysis of plant form – the pipe model theory. I. Basic analyses. Japanese Journal of Ecology 14: 97–105.