Growth and water relations of Eucalyptus marginata (jarrah) stands in response to thinning and fertilization

We studied the effects of five thinning treatments (T1 = 5.5, T2 = 11, T3 = 16.5, T4 = 22.5 and T5 = 28.5 m2 ha-1 basal area under bark) X two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha-1 N plus 229 kg ha-1 P) on growth and water relations of pole-sized Eucalyptus marginata J. Donn ex Sm. trees growing in southwestern Australia. Thinning reduced leaf area index (LAI) from 2.1 in the T4 and T5 treatments to 0.8 in the T1F0 treatment. Fertilizer had no effect on LAI in the T2, T4 or T5 treatments, but increased LAI by 45 and 20% in the T1 and T3 treatments, respectively. Thinning plus fertilizing increased diameter growth most in the fastest growing trees, from 0.4 cm year-1 for trees in the T5F0 and T5F1 treatments to 0.7 and 1.2 cm year-1 for trees in the T1F0 and T1F1 treatments, respectively. In both fertilizer treatments, stand basal area and volume growth increased with increasing stand density up to 15 m2 ha-1, and thereafter declined with increasing stand density, such that the growth rate of trees in the T5 treatment was only half of that at a stand density of 15 m2 ha-1. In response to fertilizer, growth rates of the slowest and fastest-growing trees increased from 0.35 and 3.5 m2 ha-1 year-1 (F0) to 0.56 and 5.4 m2 ha-1 year-1 (F1), respectively. Stand growth efficiency (growth per unit LAI) increased in response to thinning, and fertilizer increased stand growth efficiency at all stand densities. Throughout the dry season, T5 trees had lower predawn shoot water potentials (psi (pd) (minimum of -1.5 MPa) than T1 or T2 trees (minimum of -0.7 MPa). Fertilizer decreased psi (pd) in T5 trees (by -0.9 and -1.5 MPa, respectively, in F0 and F1), but not in T1 or T2 trees. Stand growth rate was closely related to cumulative midday water stress (CMWS) over the dry season, and volume growth rate declined sharply from 6 m3 ha-1 year-1 at a CMWS of 130 MPa days, to zero at a CMWS of 220 MPa days. Application of fertilizer to thinned stands increased LAI, stand growth efficiency and stand growth. In unthinned stands, fertilizer increased stand growth efficiency and stand growth; however, it also increased tree water stress, which limited the fertilizer-induced increases in LAI and growth. We attribute the increase in tree and stand growth in response to application of fertilizer to increased photosynthetic rates, increased allocation to stem wood, and in thinned stands also to higher LAIs.