Effects of coppice and forest plantation
The analysis of the ratio between flow and monthly precipitation showed significant differences (p<0.05) between the period that the predominant land use in the catchments was coppice and the period with forest plantation (Figure 8). In the C2 catchment, the Q/P ratio was twice as high in the period with forest plantation compared to the period with coppice (from 0.03 to 0.06), and in the C3 catchment the Q/P ratio was three times higher (from 0.05 to 0.15). The baseflow index (BFI) showed the same trend, with higher values in the period with forest plantation (61% and 77% for C2 and C3, respectively) compared to the period with coppice (39% and 45% for C2 and C3, respectively).
Coppicing has been used as an alternative with lower costs of implementation, leading to lower costs for establishing the new forest (Viana, Hoeflich, Morozini, & Schwans, 2015) and even reducing the erosive effects of soil tillage (Cinnirella, Iovino, Porto, & Ferro, 1998). In this study, we observed that the water use in the coppice system was higher than in the plantation system, corroborating other studies that also reported higher water consumption by coppice management in forest plantations (Dimitriou, Busch, Jacobs, & Schmidt-Walter, 2009; Drake, Mendham, White, Ogden, & Dell, 2012).
However, it should be noted that precipitation in the regrowth period (2014-2016) was lower than in the planting period (1830 mm/year between 2014 and 2016; and 2040 mm/year, between 2017 and 2019), which might have influenced the higher flow observed in the plantation. Despite that, the difference in flow was on average 150% between the different managements, while the difference in precipitation was 11%, reinforcing the possibility of higher consumption by the regrowth regime.
The greater water use by the regrowth could be explained by the root system already developed in subsurface, which would facilitate the access to water, especially in regions with water deficit, as is the case of the study area (Laclau et al., 2013). Although the system brings economic benefits with often equivalent yields in terms of biomass production (Gonçalves, Stape, Laclau, Bouillet, & Ranger, 2008), the results indicate that, in critical regions in terms of water availability or conflicts, the abandonment of this technique can be used as an alternative to reduce the hydrological effects of plantations (Ferraz et al., 2019).
The concentrations of total suspended solids and nitrate were significantly different (p<0.05) in the periods analyzed in the two catchments. The concentrations of both total suspended solids and nitrate were higher in the period in which coppice was the main land use in the catchments (Figure 8). The concentrations were expected to be higher for the plantation regime, which has soil tillage and greater possibility of nutrient transport than in coppice (Cinnirella et al., 1998). However, it is possible that the two systems have similar effects on water quality and the differences observed might have occurred due to the higher precipitation in the plantation period, causing a reduction in nutrient concentration due to the effect of greater dilution in the period.
Upcoming challenges of continuous monitoring
This study brought the results of 3 paired catchments under different forest covers, enabling the understanding of the effects of forest management on hydrological processes. Like the classical studies conducted in Coweeta Hydrologic Laboratory (Elliott & Vose, 2011) or Hubbard Brook experimental forest (Campbell et al., 2019), the studies conducted at Itatinga Experimental Forest Station bring relevant information for the management of forest plantations in the tropical region.
The maintenance of these long-term projects, with constant investments in equipment maintenance, data collection and analysis, continues to be a challenge not contemplated in short-term research projects. On the other hand, the studies demonstrate the importance of obtaining long series of hydrological data, due to the large annual variation observed in precipitation, the interannual influence of hydrological processes and the needs for calibration, consistency adjustments and corrections of possible failures in data collection.
Thus, the results demonstrate the need for further long-term studies that include, for example, the hydrological effects of forest management at advanced ages (more than 20 years), when it is expected that there may be a reduction in water use due to the smaller annual increment in biomass (Kuczera, 1987; McCulloch, 2007; Scott & Prinsloo, 2008).
Although we observed greater flow stability in mosaic plantations of advanced ages, confirming what was proposed by (Ferraz et al., 2013), the fact that no differences were observed between the water use of plantations at ages between 2 and 7 years (Best, Zhang, McMahon, Western, & Vertessy, 2003) and even in plantations with a mean age between 10 and 16 years (this study) may indicate the need to define other strategies to reduce water use in commercial plantations in areas with water deficit.
It was also noted the information gap on inter-cycle effects of plantations because, due to the short and intense cycles used in forest plantations, the results demonstrate the possibility of interference of a cycle in the subsequent one, where the depletion of soil water by the previous cycle may intensify the hydrological effects of the following cycle (Christina et al., 2016; Rodríguez-Suárez, Soto, Perez, & Diaz-Fierros, 2011), and recommendations for longer fallow time between cycles may be a recommendation for critical regions with the aim of improving soil recharge (Christina et al., 2016).