Figure 5. Radar chart includes all the measured variables
scaled by the min-max method to visualize better the principal
differences among the controlled grazing sites (CG) and the open grazing
sites (OG). “*” ≤ 0.05, “**” ≤ 0.01, “***”≤0.001 according to the
two-way ANOVA results (Table 1).
4. DISCUSSION
Opposite to what has been reported elsewhere, most of the soil’s
physical properties did not respond very strongly to augmented grazing
pressure. For example, soil bulk density has been used widely as an
indicator of soil compressing effect caused by overgrazing, especially
in the topsoil; however, the evaluated sites did not display this
behavior (Donkor et al., 2002; Greenwood & McKenzie, 2001; Manzano &
Návar, 2000; Willatt & Pullar, 1984). The lack of differences in soil
bulk density (Table I) may be explained by the coarse nature of the soil
texture in these sites (Figure 2), which was dominated by sandy textures
(Sandy loam, Loamy sand). Valle and Carrasco (2018) proposed bulk
density as one of the best indicators for volcanic soils of Chile;
however, they worked with more developed andosols with relatively finer
textures. The low clay content alongside the relatively low bulk density
makes the soils in our study relatively less compressive (Etana et al.,
1997). Besides, grazing in this site is restricted mostly for the summer
season (i.e., ”veranadas” ), so at the moment of grazing
perturbations, the soil water content is generally low, so compression
potential is reduced (Sands et al.,1979).
Porosity should also respond to the higher mechanical stress under
greater grazing pressure, negatively affecting gas flux and water flow
(Dörner et al., 2012). In this study, no apparent difference in porosity
was found between grazing conditions. Beck‐Broichsitter et al. (2016)
reported similar soil pore space values for soils derived from volcanic
tephra in the southern-Andean region of Chile, showing that this soil
has low-compression (0,017 PSI m-³). This may explain
why cattle compaction is not perceptible between sites (Figure 5).
Water holding capacity is a relevant soil hydraulic property (Klute &
Dirksen, 1986) and has been proved as a useful quality indicator of
grazing-induced ecological functioning changes (Greenwood & McKenzie,
2001). However, like soil porosity, no significant differences were
found in the water holding capacity between grazing conditions.
The results for saturated hydraulic conductivity (Ksat)
are typical values for sandy loam and loamy sand textures. Higher
Ksat values in the OG suggest that controlling grazing
significantly diminished saturated conductivity (Table I). Experimental
evidence indicates that Ksat is determined by the large
pores space rather than the micropores (Alaoui et al.,2011), strongly
linked to soil structure (Dörner et al., 2012). Thus, changes in
saturated hydraulic conditions may help us understand possible changes
in pore space’s continuity and distribution (Cuevas et al., 2013). The
effect of cattle trampling causes a predisposition for finer soil
particle losses by wind and water erosion (Huang et al., 2007;
Krümmelbein et al., 2009; Su et al., 2004), causing a redistribution of
pore continuity in the topsoil thus impairing water flow and
conductivity. This effect is more likely to occur in the open grazing
site as textures tend to be coarser; however, Ksatvalues were higher in this condition. Despite this apparent
contradiction, Ksat was significantly higher under tree
cover, a more dominant cover in the CG condition. The latter implies
that even though OG had a considerably higher Ksat at
the sampling points, this condition may not be spatially dominant across
the whole ecosystem.
Like Ksat, infiltration rates were significantly faster
in the OG condition than in the CG condition. The infiltration rates (K)
were slightly slower than typical values for coarse texture soils. This
could be likely the result of a strong water repellence (hydrophobicity)
we observed in the field (S. Material 4). These soil types under
Araucaria could show a higher water repellence caused by the
accumulation of hydrophobic organic compounds as it is common for other
conifer forests (Doerr et al., 2009). This effect is notably higher in
soils with low clay percentages like the one in this study (Cuevas
Becerra, 2006; Doerr et al., 2009; Robichaud & Hungerford, 2000). The
fact that the OG site exhibits a significantly faster infiltration rate
(Table I) may be due to a lower accumulation of hydrophobic substances
because of a relatively lower amount of Araucaria trees (Figure 3) and
the lower canopy coverage (Figure 4). Even though a higher
Ksat and infiltrability are desirable functional
properties in most farmlands with fine texture soils, in this case, it
could translate into excessive percolation during snowmelt and
consequent excessive leaching of C and nutrients. This is coherent with
our chemical indicators that showed lower N and P availability and lower
carbon accretion in the OG and clearings conditions.
A consistent number of investigations have reported a reduction in
nitrogen content induced by grazing (Steffens et al., 2008; Su et al.,
2004). In our case, continuous grazing has reduced not only the total
Kjeldahl nitrogen (Table 1) but also the total phosphorus concentrations
(Figure 5). Also, our data suggest accelerated acidification in the open
grazing condition. We also found significantly lower CEC and base
saturation in both the OG and the clearing conditions. Therefore, it is
clear from this data that uncontrolled grazing has reduced soil
fertility, affecting not only commonly limiting elements like N and P
but also exchangeable bases (Ca, Mg, K and Na). This has affected
productivity, reducing C content and forest regeneration in the OG
condition (Figure 3).
The differences in tree density and ground cover between the controlled
grazing conditions and the open grazing site could explain the
differences we found on available phosphorus (Olsen P) between
conditions. The higher organic phosphorus fraction pool in the CG
condition could be considered evidence of the higher proportion of P
absorbed, cycle, and stored by vegetation. Similarly, the significantly
lower concentration of the main soil nitrogen available form in this
forest (i.e., ammonium) is also the result of higher plant uptake and
storage in vegetation, litter layers, and soil organic matter in the CG
condition. Therefore, more densely vegetated CG sites are likely more
conservative forest ecosystems with higher nutrient internal recycling
than the OG conditions.
Indicators of biologic activity such as mineralizable nitrogen and
POXC/C were not sensitive enough to found differences between sites.
Still, if we look at the differences between cover conditions, most of
our indicators (including biological) become significantly different
(Table I). The increase in shrubs and herbs under open grazing areas is
most likely due to combined historic logging activity, forest clearing
for communal grazing, and impaired tree regeneration due to intensive
grazing activity. Our analysis of coverage analysis was not able to
differentiate between grasses and shrubs. However, we observed a clear
difference in the relative abundance of grasses compared to shrub
between conditions (S. Material 2), being the contribution of grasses
much lower in the open grazing condition. Considering the higher removal
of grass in more intensive grazing conditions (Greenwood & McKenzie,
2001), we could explain the higher contribution of small unpalatable
thorny shrubs like Colletia spinosa Lam. and bare soil
conditions. The latter is direct evidence of deteriorating physical,
chemical, and biological soil quality, thus accelerating soil
degradation and reducing ecosystem services such as water, nutrient
cycling, and biodiversity maintenance.
Even though aggregate stability is the main soil physical property that
explains the OG site’s clustering away from the CG site across PC 1
(Figure 5), no significant difference was found between conditions for
this property (Table I, Figure 5). Aggregate Stability greatly depends
on soil texture and the organic matter content (Bissonnais, 1996;
Staricka & Benoit, 1995), and as we mentioned, the soil textures for
both conditions were very similar (S. Material 2) as well as soil total
carbon content (Table 1). The lack of a clear deterioration aggregate
stability could mislead us to conclude that livestock has no negative
impact. However, like other physical properties, the aggregate stability
in soils under the canopy was significantly higher than in the
clearings.
Chemical indicators allow us to explain the effect of grazing (Figure 4)
more clearly. Among all variables, total phosphorus and CEC (PC1), along
with available ammonium and available Olsen-P (PC2), are the best
indicators. These indicators are easily and routinely measured in most
soil labs, and they can be useful and specific soil quality indexes for
these coarse-textured volcanic soils. Moreover, the use of sensitive
soil quality indicators could be a valuable assessment tool for forest
managers and local communities to better plan their grazing activities
in this endangered and particularly valuable forest ecosystem.
The fact that most physical and chemical indicators were significantly
better under tree cover highlights the effects of more continuous and
intense trapping and herbivory during livestock grazing in the clearing
areas than more sporadic browsing of tree foliage and seeds under tree
patches. The latter could also be the effect of higher tree litter
inputs (i.e., exudates, root decay, litterfall, etc.) and consequential
enhanced soil carbon accretion and nutrient cycling under the
tree-covered patches than in the forest clearings. Enhancing nutrient
and water availability and the consequent effect on productivity in tree
and shrub patches has also been observed in similar semiarid mountainous
ecosystems (Escudero et al., 2004; Kerns, et al., 2003). These tree
patches present greater soil functionality (i.e., water and nutrient
cycling regulation), acting as critical refuges for tree regeneration,
particularly relevant for conserving endangered tree species likeA. araucana .
5. CONCLUSIONS
The study area holds the most septentrional population ofAraucaria araucana , making it particularly sensitive to
anthropogenic disturbances and climate change. Our results indicate that
uncontrolled livestock grazing, together with other anthropogenic
disturbances, has reduced soil quality and forest coverage, affecting
tree regeneration.
Most soil quality indicators respond to this anthropogenic forcing.
Overall, livestock overgrazing has reduced hydraulic functionality,
depleted nutrient reservoirs, acidified the soils, and increased erosion
susceptibility, suggesting that overgrazed sites may become increasingly
limiting for regeneration in coming years. Conversely, limiting grazing
(18 years) has significantly favored tree regeneration, and it has
helped maintain a more functional and healthy soil-forest system.
For these particularly coarse volcanic soils, soil physical indicators
behave differently than expected. Thus, soil quality indexes must
consider the specific expected response that soil properties and related
functions will have to anthropic disturbances in contrasting soils and
the consequential effect on the ecosystem services derived from them.
It is urgent to implement a combined forest restoration program in this
endangered forest considering a communal livestock integrative
management plan to secure the indigenous community’s livelihood and soil
health, and biodiversity conservation goals.
6. ACKNOWLEDGEMENTS
This project was funded by CONAF -Universidad de Concepción
Collaboration Project: ”Evaluación del efecto de perturbaciones
antrópicas en la regeneración de bosques cordilleranos de araucaria en
la región del Biobío.” The authors would like to sincerely thank the
administration of the Reserva Nacional Ralco for all their support and
help during sampling campaigns and the local Pehuenche communities’
Quepuca-Ralco and Ralco-Lepoy for allowing us to work on their ancestral
lands. We would also like to thank CONAF Biobío provincial office for
their support. The authors thank all students and technical personnel
that provided help during the field campaign and laboratory analysis
7. CONFLICT OF INTEREST STATEMENT
The authors have declared no conflict of interest.
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8. TABLES
Table 1. Measured soil quality indicators in relation to the
site grazing treatment (open or controlled) and soil coverage condition
(mean ± SE) followed by the respective P-value. Means followed by
different letter (s) across rows are significantly different (p
< 0.05) for site and condition.