Wood density and anatomy of water-limited eucalypts

We hypothesized that seedlings grown under water-limited conditions would develop denser wood than seedlings grown under well-watered conditions. Three Eucalyptus species (E. grandis Hill (ex Maiden), E. sideroxylon Cunn. (ex Woolls) and E. occidentalis Endl.) were grown in a temperature-controlled greenhouse for 19 weeks with watering treatments (well-watered and water-limited) applied at six weeks. The water-limitation treatment consisted of four drought cycles. Wood density increased by between 4 and 13% in the water-limited seedlings, but this increase was mainly due to extractive compounds embedded in the cell wall matrix. Once these compounds were removed, the increase was 0-9% and was significant for E. grandis only. Water-limitation significantly reduced mean vessel lumen area; however, this was balanced by a trend toward greater vessel frequency in water-limited plants, and consequently there was no difference in the proportion of stem area allocated to vessels. Conduit efficiency value was lowest in the water-limited plants, indicating that there was a cost in terms of stem hydraulic conductivity for decreasing vessel lumen area. Wood density was negatively correlated with vessel lumen fraction in well-watered plants, but this relationship broke down in the water-limited plants, possibly because of the significantly larger proportion of the stem taken up by pith in water-limited seedlings. Diurnal variation in leaf water potential was positively correlated with wood density in well-watered plants. This relationship did not hold in the water-limited plants owing to the collapse of the pressure gradient between soil and leaf. We conclude that drought periods of greater than 1 month are required to increase wood density in these species and that increases in wood density appear to result in diminished capacity to supply water to leaves.