Species differences in transpiration on a saline discharge site

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Abstract

Growth, sap velocity, tree water use and transpiration rates per unit of leaf area were compared between Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. on a moderately-saline discharge site near Wellington, NSW, Australia. These were four of the best performed species in a 7-year old trial of 36 species and provenances.

Even though all trees were the same age and had grown under identical conditions, water use per tree was four–five times greater in E. spathulata than in the other three species. This difference was due to a large difference in tree size. E. spathulata had grown faster than the other species and had a mean tree leaf area four–five times greater than the other species.

Species differences in water use per unit of leaf area were smaller, but sometimes statistically significant. During a period of cool dry weather in late winter, there were no significant differences between species in transpiration per unit of leaf area. In early summer, however, when the maximum vapour pressure deficit reached 6–7.5 kPa on some days, E. leucoxylon had a 22% lower rate of transpiration per unit of leaf area than the other three species. This difference was presumably due to a stronger stomatal response to high vapour pressure deficit in E. leucoxylon than the other species. During a period of warm humid weather in late summer, transpiration per unit of leaf area was 75% higher in E. cladocalyx compared with the other three species. The reason for this difference is not known, but it may indicate a species difference in root architecture, and hence a difference in access to ground water or soil water.

Introduction

Dryland seepage salinity is widely acknowledged as a significant and worsening problem in many areas of Australia, with over 21 000 km2 affected Australia-wide (Robertson, 1996), and has largely resulted from rises in local and regional groundwater in response to wide spread clearing of native vegetation for agriculture. Although re-establishment of deep-rooted perennial vegetation, such as trees, into recharge areas, is widely recognised as the best way to slow this rising groundwater, large areas will require revegetation and the full benefits are only likely to be realised in the long term. Large scale revegetation of recharge areas will involve considerable refocussing of agricultural practices. Planting of trees in discharge locations may provide environmental benefits in the short term (e.g. assisting with reducing soil erosion and the opportunity for water use from locally high water tables) and may enable productive use of already salinised land.

Trees planted in saline areas will need to cope with the prevailing soil salinity and water regimes as well as any increase in root-zone salinity that may result from use of saline groundwater by the trees. Thus, the salt tolerance of a particular species is crucial to its sustainable culture in a plantation or agroforestry design (Marcar et al., 1995). Faster growth rates on saline sites will provide better prospects for commercial returns as long as tree products have sufficient economic uses. Planting of fast-growing species will also allow more rapid attainment of maximum leaf area index (LAI) than slow growing species for a given planting density. In saline areas, more salt tolerant species may be able to sustain a higher LAI, and hence higher rates of growth and water use than less tolerant trees. Provided that increases in root-zone salinity can be contained below species-specific salinity thresholds, fast growing species have the potential to use more water on saline discharge sites and hence, at some sites, slow the hydrologically-driven salinisation process.

Many catchment scale experiments show that vegetation change can change the hydrological balance of a catchment. Such changes are often related to changes in rates of transpiration (Bosch and Hewlett, 1982). Whether high or low rates of transpiration are desired, it is important to understand how transpiration rates differ between tree species. There is only limited information on the relationship between water use rates of different species and their growth rates under saline and non-saline conditions. Some recent studies indicate that transpiration rates per unit of leaf area generally do not differ substantially between species growing under common environmental conditions (Hatton et al., 1998, Khanzada et al., 1998, Benyon et al., 1999). However, more information on whether the ‘constant leaf water efficiency’ principle (Hatton et al., 1998) holds in a variety of soils and climates would give modellers more confidence in excluding species considerations (and, therefore, simplifying their approach) in predicting the effects of revegetation with trees.

In this paper, we compare stem basal area growth, leaf area, sap velocities, daily water use rates and water use per unit of leaf area among four eucalypt species planted in a species evaluation trial on a saline discharge site in centralwest NSW, Australia. Our null hypothesis is that the principle of constant leaf water efficiency holds during different times of the year even on a site where trees may have access to shallow, saline groundwater, and that for trees grown under common conditions, species differences in water use are largely dependent on species differences in leaf area.

Section snippets

Site description

The trial site is located 19 km southeast of Wellington in centralwest NSW (latitude 32°42′S, longitude 149°02′E; 425 m above mean sea level).

The climate in the Wellington region is temperate, with cool to cold winters and warm to hot summers. Long-term mean monthly maximum/minimum temperatures vary from about 15°C/2°C (July) to 32°C/17°C (January). The mean annual rainfall of 656 mm is evenly distributed throughout the year. Mean annual pan evaporation is 1715 mm, with maximum mean monthly pan

Accuracy of estimates of tree leaf area based on leaf counts

Averaged over the four sample trees the difference between the two methods of estimating tree leaf area was about 9% (2.5 m 2 per tree). For the E. spathulata and E. cladocalyx trees there was no difference between the two methods. For the other two trees, the leaf count method under-estimated tree leaf area compared with the felling and weighing method (3 m2, or 10%, for E. leucoxylon and 7 m2, or 24%, for E. occidentalis). The size of the error was unrelated to tree leaf area. With only one tree

Discussion

In two instances, we rejected the null hypothesis, that the principle of constant leaf water efficiency holds during different times of the year in a plantation growing over a shallow water table. During the November/December measurement period, E. leucoxylon had lower transpiration per unit of conducting surface than the other three species (Fig. 6). In February, E. cladocalyx had a higher transpiration rate per unit of conducting surface than the other species (Fig. 6). As we discuss later,

Conclusions

To maximise water uptake in plantations grown over shallow water tables, careful selection of species to identify those best adapted for fast growth at the site is important to ensure rapid development of large, leafy crowns. While tree water use is largely related to tree leaf area, significant differences in water use per unit of leaf area can also occur for different species growing under common conditions. In this study, significant differences in transpiration rates occurred as a result of

Acknowledgements

Funding of this study by the Joint Venture Agroforestry Program (CSF-46A, Agroforestry Design Guidelines for Balancing Catchment Health with Primary Production) is gratefully acknowledged. The assistance of Russell Millard (NSW Land and Water), and Debbie Crawford and Afzal Hossain (CSIRO Forestry and Forest Products) with field data collection is also gratefully acknowledged. We thank the Campbell family, owners of the ‘Bonada’ and ‘Easterfield’ properties, for providing the land for, and

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