Hydrological regime of adult plantations
The presence of forest plantations with different ages in the C1 catchment resulted in a forest mosaic with annual mean ages ranging from 10.3 to 16.2 years considering the period of 10 water years (2009 to 2019). The mean age of the catchment is higher than the cutting age normally adopted in the management of Eucalyptus planted forests in Brazil, which is 6 to 8 years (Gonçalves et al., 2013). The dynamics of the annual values of the indicators analyzed in relation to the annual mean age of forest plantations present in the C1 catchment can be observed in Figure 5.
The mosaic of forest plantations analyzed with mean ages between 10 and 16 years showed a trend of stability in relation to the hydrological regime and water quality. The results confirm the greater stability of the flow over the years in forest mosaics (Ferraz et al., 2013), resulting from the absence of clearcutting in total area of the catchment. This trend of flow stability over time could be a “buffer effect” of the presence of forest plantations at different ages (Ferraz et al., 2019). A characteristic of the mixture of ages would be to balance the water use, making the higher consumption of young plantations compensate for the low consumption of older plantations, since the higher water consumption would be observed in the first years of development (Almeida, Soares, Landsberg, & Rezende, 2007). Thus, in a forest mosaic, water consumption is expected to be lower than that observed in forest plantations with commercial rotation (less than 8 years).
It was expected to find a reduction in water use at more advanced mean ages of the forest mosaic, as observed in older forest plantations, due to the “plantation effect” (Calder, 2007; Zhang, Vertessy, Walker, Gilfedder, & Hairsine, 2007). However, the water yield observed in the C1 catchment (0.08 to 0.20) is in the upper range of variation of the values found for commercial eucalyptus plantations in Brazil, from 0.05 to 0.11 (Ferraz et al., 2019). Thus, the forest mosaic would not represent a reduction in water use by the forest plantation. However, the short period of time analyzed and the low variation of the mean age of the mosaic did not allow us to analyze the variation of water use at the catchment scale in mosaics of planted forests with more advanced ages, so specific experiments with this objective are still needed.
The C1 catchment presents a large share of the baseflow in the flow, with baseflow index (BFI) from 75% to 83% (Figure 5). The BFI observed may be one of the benefits of longer rotations on water resources compared to younger plantations (Jones & Post, 2004). In catchments with mosaic of young and adult forests, there is a greater storage of water in the soil, which would be related to the “sponge-effect-hypothesis” (Ogden, Crouch, Stallard, & Hall, 2013). This effect may be the result of greater protection of soils by forests and improvement of soil characteristics (Neary, Ice, & Jackson, 2009) in areas of forest mosaic when compared to areas that undergo clearcutting at very short intervals.
Regarding water quality, the mean annual concentration of total suspended solids ranged from 2.0 mg L-1 to 11.3 mg L-1; and the mean annual nitrate concentration ranged from 0.14 mg L-1 to 0.93 mg L-1(Figure 5). It was possible to note a trend of reduction in the concentrations of suspended sediments and nitrate, which is expected due to the absence of clearcutting in total area and forest management carried out in small portions of the catchment, a result similar to that observed in other studies (Cassiano, Salemi, Garcia & Ferraz, 2020; Ogasawara, Santos, Cassiano, Wemple, & Ferraz, 2020; Riekerk, 1983; Tremblay, Rousseau, Plamondon, Lévesque, & Prévost, 2009). Thus, despite the maintenance of water use compared to commercial plantations, the results indicate that the benefits of forest mosaic may be related to the maintenance of the hydrological processes of infiltration and percolation, which are essential for soil and water table recharge and for the control of surface runoff and erosion.