Simulating the effects of different potassium and water supply regimes on soil water content and water table depth over a rotation of a tropical Eucalyptus grandis plantation☆
Introduction
Planted forests provided 46% of the wood consumption worldwide in 2012, and 65% in tropical and subtropical regions (Payn et al., 2015). In tropical and subtropical regions, the growth of these highly productive planted forests is largely dependent on fertilization regimes (e.g. Smethurst, 2010) and their contribution to satisfying the global wood demand should increase in the future (Paquette and Messier, 2010). However, climate change is expected to exacerbate the intensity and frequency of droughts in tropical and subtropical regions (Allison et al., 2009, IPCC, 2013, Solomon et al., 2009). Fast-growing tropical plantations are particularly vulnerable to drought and changes in rainfall patterns (Allen, 2009). Consequently, the sustainability of fast-growing planted forests in a future with extended dry periods in many tropical regions will probably require a revision of management strategies to improve tree tolerance to drought (Battie-Laclau et al., 2014a, Battie-Laclau et al., 2016, Carter and White, 2009).
Water storage in deep soil layers is likely to have a major effect on tree functioning in tropical regions (Malhi et al., 2008). Indeed, water uptake by deep roots is generally considered as an efficient adaptation to drought in tropical forests to maintain transpiration rates during dry periods by withdrawing water from soil deeper than 8 to 10 m below surface (mbs) (Christina et al., 2017, Markewitz et al., 2010, Nepstad et al., 1994). Water uptake in the capillary fringe above the water table is likely to account for a substantial proportion of tree water use in eucalypt forests (Dawson and Pate, 1996, Zolfaghar et al., 2014, Eamus et al., 2015), even under relatively high rainfall regimes (approx. 1500 mm yr−1) for water tables at 10 mbs–18 mbs (Christina et al., 2017). Recent studies have shown that tree water stress and mortality are dependent on the amount of water stored in deep soil layers in Australian eucalypt forests (Harper et al., 2009, Brouwers et al., 2013, Zolfaghar et al., 2014), in the Amazonian Forest (da Costa et al., 2010, Malhi et al., 2009) and in the Brazilian savanna (Jackson et al., 1999, Oliveira et al., 2005). In consequence, modifications to current management practices in drought-prone planted forests have been proposed to decrease tree water stress during dry periods. The most common silvicultural adaptations proposed are: (i) to plant species and hybrids selected by breeding programs for their high tolerance to drought (Dutkowski and Potts, 2012, Rojas et al., 2017), (ii) to decrease the stocking densities (Mendham et al., 2011, White et al., 2009) or rotation periods, to restore soil water storage after clear-cutting (Harper et al., 2014), (iii) to reduce the amounts of fertilizer applied (Forrester et al., 2013, Battie-Laclau et al., 2014a, White et al., 2009), and (iv) to concentrate future afforestation programs on deep soils (Harper et al., 2014, Battie-Laclau et al., 2016).
Although it is well established that an adequate nutritional status helps plant tolerance to abiotic stresses (Cakmak, 2005, Reddy et al., 2004), carbon partitioning to wood production (Litton et al., 2007, Epron et al., 2012), and water-use efficiency (White et al., 2014, Battie-Laclau et al., 2016), some studies have shown that fertilization is likely to increase tree water stress during dry periods (Linder et al., 1987, White et al., 2009), for example by increasing leaf area. Measurements (Battie-Laclau et al., 2014a, Battie-Laclau et al., 2016) as well as modeling approaches (Christina et al., 2015) in a field experiment manipulating throughfall and potassium (K) supply showed that a decrease in K fertilizer relative to current practices in commercial eucalypt plantations might help reduce tree water stress during drought through lower water use and increased water storage in deep soil layers during rainy seasons.
Concerns have been raised since the new millennium about the impact of highly productive eucalypt plantations on groundwater resources and stream flow in tropical regions (Cossalter and Pye-Smith, 2003, Farley et al., 2005). In a future drier climate, management practices should be adapted to maintain wood production while limiting adverse consequences on groundwater resources. Our study aimed to gain insight into the effects of contrasting K nutrition and water supply regimes on tree water use and water seepage under highly productive Eucalyptus plantations in tropical soils. We hypothesized that: (i) a decrease in rainfall reduces tree water use and groundwater recharge, but increases tree water stress and the depth of water uptake in the soil and (ii) a decrease in K fertilization could mitigate the adverse consequences of low precipitation on tree water stress and soil water resources.
Section snippets
Site description
The experiment was conducted at the Itatinga Experimental Station of the University of São Paulo in Brazil (23° 020S; 48° 380 W). From 2010 to 2014, the mean annual precipitation was 1578 mm yr−1, with a drier year in 2014 (1189 mm yr−1) at this site. The dry season lasted from June to September with a mean monthly temperature of 15 °C, and the rainy season was from October to May, with a mean monthly temperature of 25 °C and higher overall PAR. The experiment was located on a hilltop
Comparison between predicted and measured stand transpiration and soil water content
In general the predicted canopy transpiration (TC) was a good fit to the measurements (Fig. 1). Root mean square errors (RMSE) between daily measurements and predictions ranged from 0.63 to 1.08 mm d−1 (from 4 to 16%), depending on the treatment. Over the two years of measurements, canopy transpiration was underestimated by 6% for +K+W and 16% for +K−W and was over-estimated by 9% for −K+W and 15% for −K−W.
There was good overall agreement between predicted and measured soil water content (θ)
Simulation reliability
The discrepancies between measured and predicted values were relatively small in our study for stand transpiration and soil water contents, which suggests that the effects of K nutrition and water supply regimes on soil water resources were predicted reliably. Nevertheless, canopy transpiration estimates might be affected by several sources of uncertainty such as the rough estimation of the variation of Leaf Area Index (LAI) and root depth with time, which may affect predicted transpiration,
Conclusion
Fertilization regimes strongly influence tree water use in intensively managed Eucalyptus plantations and can be used, with other management options, as a tool to decrease tree water stress through an increase in water storage in deep soil layers during rainy seasons. A decrease in annual rainfall increases the mean soil depth of water withdrawal, decreases the residual soil water content in deep soil layers and leads to a drop in the water table. The negative impact of a decrease in annual
Acknowledgements
The study was funded by Universidade de São Paulo, Centre de coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), Agence Nationale de la Recherche (MACACC project ANR-13-AGRO-0005, Viabilité et Adaptation des Ecosystèmes Productifs, Territoires et Ressources face aux Changements Globaux AGROBIOSPHERE 2013 program),. This work has benefited from the support of Agropolis Foundation as part of the program “Investissements d’avenir” (ANR-10-LabX-0001-01) and from the
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This article is part of a special issue entitled “Current advances in plant water-relations research, implications for forest management and restoration stemming from the Ecosummit 2016 conference”, published in the Journal of Forest Ecology and Management 418, 2018.