Effects of irrigation and N fertilization on growth and structure of Pinus radiata stands between 10 and 29 years of age

Temporal change in stem volume, basal area, diameter, and height are reported for stands of Pinus radiata growing near Canberra, ACT, Australia between the ages of 10 and 29 years. The stands were subjected to either a single fertilization (F) at age 10 years, irrigation (I), irrigation after a single fertilization at age 10 years (IF), irrigation with continuous liquid fertilization (IL), or no treatment (Control (C)). At age 15 years all the stands were thinned. At the same time, irrigation ceased in a duplicate IF stand, and irrigation and liquid fertilization ceased in a duplicate IL stand. Stem growth responded markedly to irrigation, with a strong interactive effect when combined with N fertilization. Over a 19 year treatment period (ages 10–29 years) the percentage gross underbark volume responses, relative to the control which had an annual growth rate of 19.3 m 3 ha −1 year −1, were F (98%), I (132%), IF (184%), and IL (229%). Standing stem volumes (m 3 ha −1) at the end of the study were C (340), F (328), I (438), IF (624), and IL (776). The IF stand continued to grow at rates considerably greater than the I or F for the entire experiment, indicating efficient N cycling with increased water supply over a period of nearly 20 years. In non-irrigated stands, added N only increased growth during wet periods, and the response declined to be similar to the control within 5–8 years. Irrigation increased growth in the larger trees proportionally more than the smaller trees, resulting in an increased diameter distribution compared with the non-irrigated stands. Cessation of irrigation in combination with thinning after 5 years of treatment immediately reduced annual basal area growth to levels similar to the control for at least 2 years, but did not lead to tree mortality. Thinning reduced stand basal area increment for 2–3 years in treatments without adequate N supply, even if they were irrigated. The study demonstrates the importance of monitoring long-term growth to understand the changing interactions between water and N availability. For example, initial responses to irrigation only were markedly attenuated over time because of increasing N limitations, while irrigated and fertilized stands showed little or no change in the response pattern. Understanding the temporal change in water–N interactions has implications for modelling long-term forest growth, and forest management.