Water use and carbon assimilation respond differently to nitrogen supplementation in mid-rotation Eucalyptus nitens

ABSTRACT

Large amounts of nitrogen (N) fertiliser are often applied to commercial plantations in southern Australia to obtain high growth rates. The growth response to fertiliser can be realised through increased foliar N and hence improved leaf-level photosynthetic capacity, or through increased leaf area production, which in turn increases light capture and hence canopy-level photosynthesis. While extra leaf area is beneficial to a tree in this regard, extra leaves may also incur costs in terms of both carbon (C) and water. Since canopy water use scales with total leaf area, greater leaf area could cause stomatal closure in times of water shortage, reducing the C assimilation rate. Here, we used three factorial N × phosphorus fertiliser experiments in Eucalyptus nitens plantations across Tasmania, Australia, to assess the effects of 12 fertiliser treatments on C- and water-related gas exchange, foliar N and trunk shrinkage. As prior research in this system has demonstrated an increase in leaf area in response to high N, we investigated the potential physiological costs and benefits of this extra leaf area. N application appeared to have consequences for plant water use, as evidenced by greater trunk shrinkage and lower stomatal conductance. However, this varied by site and year, indicating that the response is dependent on environmental conditions. These results highlight the importance of considering site conditions when prescribing fertiliser application regimes because adding large amounts of N could have unintended consequences for growth due to increased leaf area and hence potential water use, especially in dry conditions.

KEYWORDS:

• fertiliser
• plantation
• carbon assimilation
• water-stress
• leaf nitrogen content
• water-use efficiency

Acknowledgements

The authors acknowledge and pay respect to the Traditional Custodians of the land upon which this research was conducted – the pallittorre, braylwunyer and mellukerdee people of lutrawita/Tasmania. We are grateful to Forico Pty Ltd and Reliance Forest Fibre for access to coupes and general support. We thank Meagan Porter and Hugh Fitzgerald, along with a large number of volunteers (especially Matilda Brown, Hans Ammitzboll, Melika Missen and Bernadette Brinkhoff), for their assistance with field data collection. We are grateful to Amain Contracting Services for assistance in applying the fertiliser treatments, and Tassie Tree Climbers for assistance with sample collection.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

REB, MJH, MAH and DM conceptualised and planned the study, MAH secured the funding, RB and MJH collected the data, RB and MJH performed the analyses with assistance from DM, and RB wrote the original manuscript with assistance from MJH. All authors contributed to the final version of the manuscript and approved the submitted version.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00049158.2023.2286853.

Additional information

Funding

This work was supported by the Australian Research Council Industrial Transformation Training Centre for Forest Value [ICI150100004].