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.