4. Discussion
A growing body of literature attributes reductions in streamflow in
South America to intensive plantation forestry using non-native species
(Alvarez-Garreton, Lara, Boisier, & Galleguillos, 2019; Ferraz, Lima,
& Rodrigues, 2013, 2019; Garcia, Salemi, Lima, & Ferraz, 2018; Huber,
Iroumé, & Bathurst, 2008; Lara et al., 2009; Little, 2009, 2014). In
North America, evidence is accumulating that intensively managed
plantations of native species can reduce streamflow (Gronsdahl, Moore,
Rosenfeld, McCleary, & Winkler, 2019; Perry & Jones, 2017; Segura et
al., 2020). Drought and climate change can exacerbate these reductions
(Crampe, Segura, & Jones, in review; Iroumé et al., in review).
Consistent with studies in other locations, post-clear-cutting increases
in streamflow in absolute terms (mm) were highest and more consistent
from year to year in fall and winter, and absolute increases were low
and inconsistent in spring and summer. However, increases in streamflow
in relative terms (%) were highest in the fall and summer, although
summer streamflow was reduced in dry summers (2014-2015). Base flow
increased gradually over the restoration period. The pre-treatment
runoff ratios in the study catchments were on the high end of rates
reported from Eucalyptus and other exotic forest plantations in
south-central Chile and Brazil (e.g. Huber et al., 2008; Ferraz,
Rodrigues, Garcia, Alvares, & Lima, 2019; Iroumé et al., 2020). If the
study catchments had been replanted with Eucalyptus , we would
have expected a reduction in streamflow within one to three years after
the post-clear-cutting increase in streamflow, but under regenerating
native forests, streamflow increased, and over the longer term, base
flow also increased relative to conditions under the formerEucalyptus plantations. These findings indicate that the formerEucalyptus plantations had depleted soil moisture reservoirs in
the study catchments. Clear-cutting of Eucalyptus , its
replacement with Nothofagus native trees, and natural
regeneration of other tree species appear to have reduced transpiration,
increased soil moisture storage, and increased streamflow, except during
some dry periods.
Multiple findings, including different runoff ratios and base flow
responses among catchments, delayed responses of base flow to
restoration, and increased streamflow in an untreated catchment which
was adjacent to a clear-cut, indicate that the streams are fed in part
by delayed subsurface flow. Groundwater flows as deep as 6-8 meters
under saturated soil conditions in winter can be observed on road cuts
in the study area. Dipping bedding planes of the underlying metamorphic
rocks, which are steeper than the hillslope gradients, appear to convey
subsurface flow among these small catchments. This delayed flow is an
important contributor to base flow, especially during summer.
Relative increases in streamflow after clear-cutting were highest at the
two catchments which had the lowest runoff ratios and where summer base
flow fluctuated the most in response to seasonal and inter-annual
variation in precipitation (RC5 and RC10, Table S2). Both absolute and
relative responses were lower at the catchment which had the highest
runoff ratio and where summer base flow was insensitive to variation in
precipitation. In other words, catchments with limited streamflow
contributions from delayed flow were more responsive to restoration than
those with sustained base flow. These findings are consistent with other
studies showing that catchment hydrology can moderate streamflow
response to change in vegetation and climate (Spencer, Anderson, Silins,
& Collins, 2020; Tague, Valentine, & Kotchen, 2008; Vose et al.,
2016).
Streamflow increased following clear-cutting of Eucalyptusplantations and regrowth of planted native Nothofagus trees and
naturally regenerated native forest species over the restoration period.
The gradual recovery of annual base flow through the nine-year period of
restoration, and the pronounced increase in base flow during the last
three years of the study, despite low precipitation in the last two
years of the study, imply that native forest restoration has the
potential to restore deep soil moisture reservoirs that sustain base
flow during dry periods, and therefore may enhance the resilience of
restored catchments to drought.
The immediate post clear-cutting increase in streamflow in RC6 (aEucalyptus plantation that was not clear-cut), the delayed
response of streamflow at the adjacent clear-cut catchment RC5, and the
higher runoff ratio at RC6 compared to RC5 (Figures 3 and 4) indicate
that flow is transferred from RC5 to RC6. Yet despite the streamflow
subsidies from RC5, base flow at RC6 remained low from 2017 through
2019, whereas it increased in all the catchments under restoration
(Figure 9). In addition, the highest correlation between summer
streamflow and spring precipitation occurred at RC6. These findings
indicate that high evapotranspiration rates of the priorEucalyptus plantations had depleted deep soil moisture
reservoirs, which recovered, and restored base flow in restored
catchments over the period of study. The Eucalyptus plantation
aged from 7 to 20 years over the course of the study, whereas typical
rotations in these plantations are 12-15 years. The plantation continued
to grow and nearly doubled in basal area from 2010 to 2020 (Table 3).
Thus, in contrast to the finding that water use by Eucalyptus is
reduced in older plantations globally (Farley et al., 2005), this study
indicates that aging Eucalyptus plantations continue to
evapotranspire at high rates that prevent recharge of deep soil moisture
reserves.
The relatively high and persistent streamflow increases in catchment
RC10 (Figure 3, Figure 4, Figure 5, Figure 6) may in part be due to
native forest restoration in this catchment, which had the highest
survival of planted Nothofagus , highest density of other native
saplings and seedlings, and no Eucalyptus saplings or trees
(Table 3). However, despite similar forest restoration outcomes at RC10
and RC11, streamflow increased much less at RC11 than at RC10 during the
period of restoration (Table 2, Table 3). Nevertheless, the
substantially higher base flow at RC11 in 2016 to 2019 relative to
before clear-cutting (Figure 7, Figure 9) implies that native forest
restoration may have contributed to increased base flow even in RC11,
which had the highest runoff ratio of all study catchments (Figure 3,
Figure S4). Our results do not show a clear effect of the native forest
riparian buffer (width and percentage of the catchment covered) on
streamflow as reported earlier for this study site (Little et al.,
2014).
The magnitude of streamflow response in this long-term experiment is
roughly consistent with the streamflow responses modeled by
Alvarez-Garretón et al. (2019), who estimated 40% reductions in
streamflow in catchments with intensively managed fast-growing
plantations, relative to native forest. In addition to restoring the
hydrologic regime, forest restoration with native tree species also can
contribute to carbon sequestration for climate change mitigation efforts
(Bastin et al., 2019; Lewis, Wheeler, Mitchard, & Koch, 2019). Results
from this study indicate that the restoration of native forests might
increase carbon sequestration for climate regulation and at the same
time improve water provision, two crucial ecosystem services. This study
reinforces the tight coupling between forest management, water, and
carbon, and their relationship to Sustainable Development Goals (e.g.,
Creed et al., 2019). The findings from this study may inform policy to
address trade-offs between carbon sequestration and water yield,
relevant to the National Determined Contributions of Chile within the
United Nations Framework Convention on Climate Change.
This study also indicates that native forest restoration in areas of
former exotic forest plantations may enhance the resilience of
streamflow to climate drying in southern Chile. The study occurred
during an unprecedented mega-drought (2010 to present), although
precipitation records at the site do not show clear downward trends.
Climate models project continued drying trends throughout the present
century (Boisier, Rondanelli, Garreaud, & Muñoz, 2016; Bozkurt, Rojas,
Boisier, & Valdivieso, 2018; Garreaud et al., 2017). Hence, there is a
need for continued studies, such as this one, of forest management to
enhance streamflow resilience to drought. A broader suite of experiments
is needed to examine how native forest restoration affects water yield
along a precipitation gradient in South America. Ideally, a long-term
study program would consider the effects of different former forest
plantation species (e.g. Eucalyptus, Pinus ), stand density
and age, and soil type, as well as native forest composition, density
and diversity along this climatic gradient.