Study sites
We studied 12 second-growth forests established for 11-47 years on
former pasturelands, 13 restoration plantations of 10-61 years of age
established on pasturelands and croplands, and five old-growth,
reference forests (Fig. S1; Table S1). Second-growth forests were
sampled in the Corumbataí watershed, southeastern Brazil (Fig. S1).
Pastureland is the principal land use in this watershed (43.7% of
area), followed by sugarcane fields (29.4%), and remaining native
forest cover (12.4%; Silvio F. B. Ferraz et al., 2014). Previous land
use and the age of regenerating forests were determined based on
panchromatic aerial photographs and annual LANDSAT 5 and 8 images.
Further information about this watershed and forest patch sampling can
be found in César et al. (2017). Second-growth forests of the Corumbataí
watershed are found regenerating almost exclusively in marginal lands,
such as steep slopes and/or sandy or rocky soil, where mechanized
agriculture is not possible. Extensive cattle ranching or eucalypt
woodlots have been established in these marginal agricultural lands, and
their less intensive historical soil management, at least compared to
sugarcane or other crop production, have led to the regeneration of
secondary forests when land is abandoned (César et al., 2017). All
second-growth forests were found away from riparian areas and were
expansions of existing forest remnants.
Restoration plantations were mostly identified in the same region
(< 100 km) of second-growth forests, but some additional
plantations (three out of the four plantations older than 25 years old)
were sampled (< 300 km) to include older sites in our dataset
(Fig. S1). These plantations were established with a high diversity of
native tree species (20-100 species) and had comparable levels of tree
richness than the studied second-growth and reference forests (Fig. S2).
Plantation management consisted of planting nursery-grown seedlings in
regular spacing (usually 3 x 2 m), fertilizing the soil before (base
fertilization) and after planting (broadcast fertilization), and weeding
ruderal plants with glyphosate spraying or mowing (see details in
P.H.S.; Brancalion et al., 2019). Land use prior to plantation
establishment was determined by interviewing restoration project
managers. Only three sites were established on pasturelands, and the
rest were established on former sugarcane fields. Twelve sites were
found in riparian buffers and were established to comply with the Forest
Code (Pedro H. S. Brancalion, Garcia, et al., 2016). Nine of the 12
sites were isolated from native forest remnants, and three sites were
established within the neighborhood of existing degraded remnants.
Consequently, our analysis did not allow for a controlled comparison of
properties between tree plantations and natural regeneration because
these restoration approaches were established in quite different
biophysical conditions in the studied landscapes. Although not
appropriate for objectively comparing these restoration approachesper se and isolating uncontrolled factors (Reid et al., 2018),
our study design depicts the cost-effectiveness of restoration
approaches currently being implemented in agricultural landscapes of
Brazil’s Atlantic Forest (see Pedro H. S. Brancalion, Schweizer, et al.,
2016).
We complemented our sampling with five reference forests, represented by
old-growth remnants distributed around the Corumbataí watershed. These
remnants were selected for being the best conserved forests of the study
region, with a well-developed forest structure and some large
(> 50 cm diameter at breast height) late-successional
remnant trees (Fig. S1). These forests do not represent the
pre-disturbance carbon stocks of the original forests of this region,
since these remnants are small (<200 hectares) and are
embedded within a patchy agricultural matrix. However, they represent an
appropriate benchmark for restoration in this region, given that
restored forests will likely persist as small patches surrounded by
agriculture.
The forests sampled were selected
according to different criteria. The second-growth forests were selected
systematically based on available information on second-growth forest
age and previous land use, aiming at a representation of approximately
five forests per age class x previous land use (S. F. B. Ferraz et al.,
2014). Restoration plantations and old-growth forests were selected
based on the availability of these forest typologies with different ages
within and closer to the Corumbataí watershed, the focus area of the
study. We sampled all restoration plantation >15 years old
and old-growth forests we knew in the region due to the scarcity of
these kinds of forest, and seven restoration plantations <15
years old based on their location (forests found within the watershed or
closer to it was prioritized) and access.