Gross primary productivity and water use efficiency are increasing in a high rainfall tropical savanna

Lindsay B. Hutley, Jason Beringer, Simone Fatichi, Stanislaus J. Schymanski, Matthew Northwood

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Despite their size and contribution to the global carbon cycle, we have limited understanding of tropical savannas and their current trajectory with climate change and anthropogenic pressures. Here we examined interannual variability and externally forced long-term changes in carbon and water exchange from a high rainfall savanna site in the seasonal tropics of north Australia. We used an 18-year flux data time series (2001–2019) to detect trends and drivers of fluxes of carbon and water. Significant positive trends in gross primary productivity (GPP, 15.4 g C m2 year−2), ecosystem respiration (Reco, 8.0 g C m2 year−2), net ecosystem productivity (NEE, 7.4 g C m2 year−2) and ecosystem water use efficiency (WUE, 0.0077 g C kg H2O−1 year−1) were computed. There was a weaker, non-significant trend in latent energy exchange (LE, 0.34 W m−2 year−1). Rainfall from a nearby site increased statistically over a 45-year period during the observation period. To examine the dominant drivers of changes in GPP and WUE, we used a random forest approach and a terrestrial biosphere model to conduct an attribution experiment. Radiant energy was the dominant driver of wet season fluxes, whereas soil water content dominated dry season fluxes. The model attribution suggested that [CO2], precipitation and Tair accounting for 90% of the modelled trend in GPP and WUE. Positive trends in fluxes were largest in the dry season implying tree components were a larger contributor than the grassy understorey. Fluxes and environmental drivers were not significant during the wet season, the period when grasses are active. The site is potentially still recovering from a cyclone 45 years ago and regrowth from this event may also be contributing to the observed trends in sequestration, highlighting the need to understand fluxes and their drivers from sub-diurnal to decadal scales.

Original languageEnglish
Pages (from-to)2360-2380
Number of pages21
JournalGlobal Change Biology
Issue number7
Publication statusPublished - Apr 2022

Bibliographical note

Funding Information:
Data collection at the Howard Springs flux tower site has been supported over the last two decades by the Australian Research Council’s Discovery Grants Scheme, grants DP0344744, DP0772981, DP130101566, DP160101497 and an ARC Future Fellowship FT110100602 for JB. Since 2009, infrastructure funding support has also been provided through the Australian Government's Terrestrial Ecosystem Research Network (TERN, ) that supports the OzFlux ( ) eddy covariance network across Australia. SJS is supported by the Luxembourg National Research Fund (FNR) ATTRACT programme (A16/SR/11254288) and SF by the Singapore Ministry of Education Academic Research Fund Tier 1, through the project ‘Bridging scales from below: The role of heterogeneities in the global water and carbon budgets’.

Publisher Copyright:
© 2021 The Authors. Global Change Biology published by John Wiley & Sons Ltd.

Copyright 2021 Elsevier B.V., All rights reserved.


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