Tree growth patterns and diagnosis of water status based on trunk diameter fluctuations in fast-growing Populus tomentosa plantations

High resolution measurements of trunk diameter fluctuations (TDF) provide a means to study short-term and long-term stem radial variations and response to water stress of fruit trees. There is a lack of information, however, on the performance of TDF measurements on fast-growing poplar trees. To both clarify the intra-annual and inter-annual growth patterns and to diagnose water status of young poplar trees, we monitored TDF, leaf area index, soil matric potential, midday stem water potential (Ψstem) and main meteorological variables in a plantation with 3- and 4-year-old Populus tomentosa trees over two growing seasons (2016 and 2017). We had two irrigation treatments, well-watered (WW) and water-stressed (WS). For the first time for this species, we identified accurately and quantitatively the seasonal growth pattern and associated parameters (growth initiation and cessation time, growing season duration, maximum growth rate, etc.) of young P. tomentosa with four clear stages, based on the growth rate for both treatments. We identified that stage II (mid-April to mid-June), when trees showed the most active growth rate and less rainfall was recorded, is the most crucial stage for irrigation management, especially as trees grow larger and have a greater water demand. In the two experimental years, irrigation did not affect the timing of growth initiation and cessation. However, maximum growth rate was significantly higher and occurred earlier in the WW trees than in the WS trees. From the tested trunk diameter fluctuation indicators, trunk growth rate (TGR) showed a greater potential for assessing tree water stress than both maximum daily shrinkage (MDS) and daily growth (DG), being even more sensitive to the onset of water stress than midday Ψstem. We conclude that high resolution trunk dendrometers are useful to track growth and determine water stress, which make them a good candidate to schedule irrigation in fast-growing poplar plantations.