Water stress integral--a link between short-term stress and long-term growth

Water stress integral (S(Psi)), the cumulative integral of pre-dawn leaf water potential over any chosen period of time, was estimated from measurements of pre-dawn water potential made every two weeks in a Pinus radiata D. Don plantation near Canberra, Australia. Also measured were final length of current-season needles and annual stem basal area increment. Data were gathered over a 4-year period from a control plot, a fertilized plot, an irrigated plot, and two plots that were both fertilized and irrigated. Among years and treatments, annual basal area increment varied over a threefold range. Of this variation, 91% was accounted for by variation in S(Psi) for the entire year, during every month of which stem diameter growth occurred. Of variation in annual needle elongation, 90% was accounted for by variation in S(Psi) from late August to late February, which was the period of needle growth. In dry years, the annual value of S(Psi) in non-irrigated plots was mainly determined by soil water content, but in wet years in non-irrigated plots, and in all years in irrigated plots, it was closely correlated with tree nutrient status (r(2) = 0.81).