Environmental factors have a dual effect on growth as they affect both the momentary growth rate and the rate of ontogenetic development. Photosynthesized carbon on the other hand is needed for growth, respiration and other purposes. According to opposite theories, growth rate is determined by: 1) the availability of carbon for growth (source limitation) or 2) limitation that environmental factors cause on tissue ability to grow (sink limitation). Understanding the responses of wood, needle and root growth to environmental and other factors allows us to predict changes in tree growth and carbon balance in changing climatic conditions.
The purpose of this study was to define the effects of temperature on Scots pine growth at different temporal scales and to estimate the relative importances of the source and sink effects on growth. For that, a dynamic growth model CASSIA (Carbon Allocation Sink Source InterAction) was constructed.
CASSIA was able to predict daily primary, secondary and needle growth rate variation with indirect and direct effects of temperature. In addition, warm previous late summer was observed to lead to enhanced length (in temperature accumulation units) of shoot growth period in the following year. Growth onset during spring was a continuous process determined by temperature accumulation instead of momentary temperatures.
Short-term growth variations in normal conditions were concluded to be sink limited because the within year growth was satisfyingly predicted with temperature and without direct effect of photosynthesis or stored carbon. On the other hand carbon source effect (GPP) was needed to produce the between year variation in growth.
The results suggest that growth is limited by a complex combination of sink and source effects. Furthermore, environmental factors affect growth at different time scales varying from instantaneous to delayed effects from previous year(s). More research is needed to determine carbon flows to different processes.