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New Zealand Journal of Forestry (1993) 38(2): 24–28
©New Zealand Institute of Forestry

Research article
Upper Mid-Crown Yellowing (UMCY) in Pinus radiata forests

P.N. Beets , T.W. Payn and E.J. Jokela

Upper Mid-Crown Yellowing is a condition of radiata pine in which needles in the sub-apical zone of the upper crown become yellow tipped with age, needle retention is low, and crown dieback occurs.
Many factors have been suggested as possible causes of UMCY; however, ecological, physiological, and chemical evidence suggests that a nutritional imbalance involving magnesium and potassium is the most likely cause. Magnesium deficiency in young stands and UMCY in older stands occur because the effective supply of magnesium is too low to meet the needs of radiata pine. Expected changes in the relative supplies of soil Mg and K suggest that the incidence and severity of UMCY is likely to increase in the future; however monitoring of the severity of UMCY on a national scale has been inadequate to confirm this.
The available evidence suggests that deficiency symptoms appear more severely in genotypes with a predisposition to accumulate low amounts of magnesium and high amounts of potassium in their foliage. Radiata pine has an inherently low capacity to accumulate magnesium in its foliage in comparison with other species, and variation within radiata pine is also large. Foliar chemistry data from seedling trees, clones, and radiata families show that within-stand variation in foliar Mg arises from genetic differences in tree nutritional characteristics. Based on the evidence linking UMCY to nutritional traits, the heritability of UMCY is likely to be high but family differences in UMCY (narrow sense heritability) need to be determined.
Research is under way to survey the incidence and severity of UMCY, and its association with site and management factors. The effects of UMCY on individual tree and stand growth and yield are being determined, to assess the cost of UMCY. Trials are being established to determine if UMCY can be economically treated by fertilisation with magnesium. The heritability of UMCY, and the extent breeding for tolerance to UMCY impacts on growth and form are being investigated. Soil tests to predict Mg deficiency, and plant tests to screen for
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