Initial growth of Brazilian pine (Araucaria angustifolia) under equal soil volumes but contrasting rooting depths

Araucaria angustifolia is a critically endangered tall tree species of valuable wood, and field observations led to the suggestion that limitations imposed to the vertical growth of its tap root system greatly restrict the height of mature individuals. However, experimental studies dealing with the effects of soil depth on the species growth are mostly lacking. This study evaluated and compared the growth responses of young plants of A. angustifolia to distinct rooting depths but same soil volumes. Seeds were planted in pots of different heights and diameters, all containing 3 liters of soil mixture. Plants were submitted to four available rooting depths: 65 (T1), 35 (T2), 20 (T3), and 10 (T4) cm. There were eight experimental units in each treatment, arranged in a randomized complete block design, each block containing two units per treatment. Contrary to what was expected, the T3 and T4 plants had accumulated more mass and attained the same height as the other two groups, after a 10-month growth period in a green house. Those plants also had thicker stems, longer shoot branches, and thicker and longer lateral roots, which were interpreted as compensatory responses to increase plant anchorage and stability. The inverse relationship between rooting depth and plant mass was attributed to a down-regulation of shoot growth because or restricted lateral space and/or poor soil aeration of the longer and narrower pots. This experiment allowed us to demonstrate that is not the possibility of the tap root to grow deep into the soil that ensures a better growth to plants of A. angustifolia: provided that the offer of soil volume and resources are the same, the vertical extension of the tap root does not result in greater growth of the plants. In fact, much greater growth impairment is expected from lateral than from vertical restriction to root growth.