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).