1. Introduction
High montane forests and páramo [
1], typical Andean ecosystems, provide important services such as water supply regulation and carbon storage [
2]. The hydrological services provided by Andean ecosystems have been closely related to the characteristic functions played by their soils at watershed scale [
3,
4,
5]. These characteristics include: high soil organic matter (SOM) content [
6], overall low bulk density (BD) (<0.6 g cm
−3) [
7], and high-water retention capacity [
7,
8,
9,
10,
11,
12]. Despite the important role soils play with respect to the provision of ecosystem services in the Andean region, they have been increasingly altered by anthropogenic activities since pre-Hispanic times [
13], and only a limited number of evaluations of the effects of anthropogenic land-use changes on soil properties exist.
In the Andean region, afforestation and grazing are the main anthropogenic activities affecting montane ecosystems [
12,
14,
15,
16]. Afforestation, primarily with pine, has been carried out to produce timber and reduce pressure on native ecosystems [
16,
17,
18,
19], as well as to capture carbon [
19,
20,
21]. Without focus on the possible impact on soil and water resources, the majority of pine plantations were established on sites of important hydrological and ecological value to the Andean population. Grazing in the montane Andean ecosystems has been carried out since pre-Hispanic times (XV century), and this practice results mainly from extensive to intensive animal farming (cattle, horses, and sheep) [
13]. In some cases, grazing is carried out directly in natural cover, considering tussock grass as fodder, or after burning to convert the land, whether or not accompanied with the introduction of more productive grass species [
22], into suitable pasture where cattle can obtain more nutritious fodder [
18].
Pine plantations and grazing interventions in Andean ecosystems have led to significant changes in the hydro-physical properties and SOM content, in particular of the top soil at depths lower than 30 cm [
19,
20,
23,
24]. Modifications in soil properties are causal factors of a reduction in water yield and variations in the hydrological response of montane Andean basins [
25,
26]. In the case of pine afforestation, the soils tend to dry out because of the high water absorption capacity of the trees, which favors SOM decomposition [
19,
20] and reduces the soil’s water retention capacity [
15]. Furthermore, preferential flows created by root growth increase the saturated hydraulic conductivity (K
sat) of soils [
27]. Despite existing investigations, there still exists indistinctness and contradictions about the effect of anthropogenic impacts, such as pine plantations and grazing, on the soil properties of Andean ecosystems [
28]. Some studies reported an increment in SOM in the Andosols of pine plantations [
28], while other studies found no evidence of changes [
29]. Additionally, in most studies, important factors capable of influencing changes in soil hydro-physical properties, such as previous land-use practices as evidenced by La Manna et al. [
30], have not been considered. Similarly, the effects of livestock grazing, whether or not accompanied by burning, have been interpreted in different ways. Some authors claim that greater solar radiation exposure leads to soil drought, an increase in BD, SOM loss [
19,
23,
31], and a reduction in soil water retention [
23,
32]; while a reduction of K
sat enhances water erosion [
28]. Despite previous findings, under grazing activities some authors have shown, probably due to pre-tilling activities, a reduction in the BD and an increment in SOM [
28,
33]. Such discrepancies could be due to the intensity of grazing. In some cases, extensive grazing is not considered a source of stress on high Andean ecosystems [
34].
Considering the role of soil properties in the provision of ecosystem services in Ecuador’s Andean environments, and the past and present anthropogenic pressures resulting from afforestation and grazing activities, the main objective of this study was to evaluate the impact of pine afforestation and grazing on the hydro-physical properties and SOM content of the surface horizons of high montane forest and páramo soils. The study was guided by the following research hypotheses: (1) to properly assess anthropogenic impacts it is recommended to collect information of unaltered and altered plots on neighboring comparable sites to avoid that interpretation of results is biased by local site differences and (2) the impact of land-use change on the hydro-physical soil properties is not unique and often masked by other factors such as antecedent land-use, spatial variability, texture, elevation, climate, among other site-specific factors. The evaluation of both these hypotheses will not only improve the research quality and the capability to generalize research findings, but also the effectiveness of future decision making and conservation policies of high-elevation ecosystems in the Andean region.
4. Discussion
Some studies in sites close to the study presented herein evaluated the impacts of anthropogenic activities by grouping soils by their conditions of natural undisturbed land cover, without considering their elevation and sampling depth [
19,
28,
29]. In our study, results suggest that in order to evaluate the impacts of any anthropogenic activity, it is essential to examine the spatial variability of soils under natural conditions or at least to avoid evaluating the impacts based on sampling sites located in different places. In other words, the impacts of anthropogenic activities on soils should be done in adjacent sites, which would guarantee similar geomorphological and climatic conditions [
58]. Even the spatial variability of soils is often the cause that the hydrological performance of Andean ecosystems can be extremely heterogeneous [
26,
59]. That is, the high variability of soils under natural conditions can even cause variability in the functioning of basins.
An aspect often discussed in monitoring the impact of afforestation on soils is at what distance from the trunk and what depth should samples preferably be collected. A previous study in Andean highlands suggested that changes in soil properties in pine plantations differ depending on the sampling distance from the trunk of the tree [
15]. However, in all the plantations within our study area, the hydro-physical properties did not differ between the tested sampling distances of 75 and 150 cm away from the trunk. These results are consistent with what Wilcox, Breshears & Turin [
60] and Ruiz et al. [
61] reported. Wilcox et al. [
60] stated that the K
sat of the soil under the canopy and between the canopy of the pine trees did not show significant differences. Likewise, Ruiz et al. [
61] reported that the water retention capacity and AW in the soil did not differ between 50 cm from the base of the trunk vs. under the crown of the tree. On the other hand, according to our results, the hydro-physical properties and the SOM content showed significant differences between the two sampling depths (0–10 cm and 10–25 cm) in each of the plantations, a finding in line with the results of Ghimire et al. [
62]. Our results suggest that pine afforestation effectively impacts in a different way the soil properties as a function of depth, but not as a function of the distance samples are collected from the tree trunk.
The comparisons of the properties of each plantation with their respective adjacent natural cover revealed that the intervention of the soils through, for example, pine afforestation in the Andean ecosystems mainly affects K
sat, water retention capacity between saturation (pF 0) and field capacity (pF 2.52), and SOM content, and this at both sampling depths but at a different intensity. Due to these changes, pine plantations could directly alter the ecosystem services such as water regulation and storage [
25,
26] and the carbon sequestration by soils [
63]. However, findings could not be generalized, showing dependencies mainly in the sampling depth, ecosystem type, characteristics of the plantations, and previous land-use. This implies a complexity in assessing the impacts of plantations and limits the generalization capability of changes in soil properties caused by pine plantations in high Andean ecosystems.
According to Alarcón et al. [
64], changes due to grazing were not statistically evident in the K
sat measured in the 0–10 cm soil layer in Andosols. However, our results partially contradict this because 2 (F1G and Tg5G *) of the 6 study sites showed a significant reduction in K
sat up to 70%, while in the Tg6G site a significant increase of 52.67% was registered. The reduction in K
sat at the F1G site could be due to the loss of stability of the soil aggregates and grazing density, while the reduction in K
sat at Tg5G * is likely the consequence of the frequent burning of tussock grass resulting in a drying and crusting of the soil surface [
65]. On the other hand, although the increase in K
sat was not significant at Tg7G, this percentage increase was very similar to that at the Tg6G site (
Figure 3a). The increase at both these sites is likely related to preferential flows between the clods formed by soil tillage [
66] during the preparation and sowing of pasture. However, this situation could apparently change over time due to the structural deterioration of the soil.
Despite the lack of significant evidence, it was observed that in 6 of the 7 pine plantations, BD tended to increase in the 0–10 cm soil layer, which certainly cannot be the consequence of the use of heavy machinery for maintenance since the associated high costs and the difficult topographic conditions prevent the use of machines [
67]. Rather the drying of the soil by evapotranspiration [
68] and the weight of the trees [
69] are responsible for the increase in BD, which is confirmed in our study by a greater increase in BD in the sites where the pine plantation is characterized by high SD and CD (e.g., F2Pi and F3Pi in high montane forest and Tg5Pi in páramo). The BD at 10–25 cm depth was not affected in most of the plantations; however, most plantations showed a decreasing trend ranging from 2% to 18%. This tendency is attributable to the increase of porosity generated by the pine subsurface root system. On the other hand, two plantations (F3Pi and Tg5Pi) showed an increase which is attributable to the compaction by the pressure exerted by the biomass of the plantation on the soil, due to its high SD, which translates into a greater number of trees, increasing the transpiration, and resulting in soil contraction.
The compaction of the soil by grazing is normally directly reflected in an increase of the BD [
70,
71]. However, this effect was only observed in two study sites (F2G and F3G) and the direct consequence of the greater grazing intensity at these sites (ABU Ha
−1 of 2 and 1, respectively), parallel to a decomposition of SOM. Our findings are consistent with those of Donkor et al. [
72] where an increase in compaction was directly related to a greater grazing intensity. In páramo, the trampling effect of cattle was not reflected in the measurements, which was consistent with the results of Alarcón et al. [
64] and Podwojewski et al. [
23]. Their results and our study suggest that Andosols (due to their high SOM content) have a greater resilience to compaction [
73] given the overall low-grazing intensity (ABU Ha
−1 of <0.5). Because of the lack of significant evidence, our results do not permit to conclude that grazing significantly compacted the soil at the 10–25 cm depth. This may be due to the high SOM content of the Andosols. In the case of Tg7G, where BD decreased significantly by 18.63%, this decrease is likely caused by the incorporation of tussock grass biomass during tillage.
Pine plantations alter the water retention capacity of soils according to Farley et al. [
20]. This is confirmed by the results of this study, with the difference that the alteration is not only dependent on associated changes in the SOM content, but also on elevation, ecosystem type, the development level of the plantation and land-use, as reported in other ecosystems [
56,
63,
74]. The highest reduction percentages in our study were attributed to a greater development of the plantations (DBH >18 cm, Ht >8 m and CD >5 m), together with a decrease in SOM and increase in BD. In the three plantations that were established on soils whose previous use was grazing (F4Pi, Tg5Pi and Tg7Pi), the changes in water retention capacity were very variable, observing significant increases or decreases between pF 0 and pF 2.52, notwithstanding the SOM content tended to increase. This variation in the changes could be due to a mixture of changes between the previous use of the soil and the growth of the plantation [
61,
74]. This apparent overlap of impacts [
62] hinders generalization of changes. On the other hand, our data show an increase in water retention is not necessarily always related to an increase in SOM content, but can also be the consequence of an increase in clay content due to the weathering of the soil [
75,
76]. In general, pine afforestation goes hand in hand with an increase in GW and a decrease in AW, suggesting that soils under pine plantations rapidly lose moisture after a rainfall event, which further enhances soil drying and decomposition of SOM [
77]. On the other hand, according to Buytaert et al. [
22], land use change could increase the AW of soils of volcanic origin by 30%. On the contrary, Hofstede et al. [
19] reported a decrease in AW as a consequence of pine afforestation. Nevertheless, in our study, only two pine plantations showed a significant decrease (F3Pi and Tg7Pi).
Under grazing, it has been shown that the loss of water retention is mainly a result of a reduction in SOM content. Increases in water retention capacity were explicitly related to increases in SOM content due to burning and/or the incorporation of tussock grass during soil preparation. This finding is in line with Daza et al. [
32] who reported that in their study the decrease in soil water retention was due to a loss of SOM content. However, in the F2G site the increase of the water retention capacity was the result of the easy weathering of volcanic glass, leading to the formation of montmorillonite clay [
76]. The latter indicates that in order to correctly assess changes in, for example, the hydro-physical properties in soils, it is essential to evaluate the full spectrum of soil properties such as SOM, soil texture, type of management, among others.
Notwithstanding that several studies associate the reduction of SOM content with pine afforestation [
19,
20,
58], our findings indicate that pine plantations could help the recovery of SOM in the 0–10 cm soil layer of former grazing sites, though the effect seems also to depend on the elevation of the site and the SD of the plantation. Although the SOM content did not show significant changes under grazing, a reduction in most of the study sites was detected. This slightly decreasing trend would imply that, over time, the soils under grazing in the Andean region could lose a considerable amount of SOM, and thus, reduce their capacity to retain water.
5. Conclusions
Andosols with their black Andic horizon are the predominant soils in Andean montane ecosystems. Our research clearly revealed that this horizon, with high water retention capacity and SOM content, is not that uniform under natural unaltered conditions. Differences in hydro-physical properties, such as Ksat and the water retention capacity, are related to the type of ecosystem and elevation of the terrain. Differences in properties not only occur between sites, but also within sites at different depths of sampling. As a result, higher values of Ksat, water retention capacity and SOM content were recorded in the 0–10 cm surface layer. The natural spatial variability in environmental conditions and the accompanied heterogeneity in soil properties requires that for the correct assessment of the impact of land use change, data are collected on neighboring comparable, unaltered and altered sites. Doing so will facilitate and help guarantee that a correct assessment of the causal factors that positively or negatively affect the soil hydro-physical properties by land use change, is drawn. Furthermore, the multitude of observations and their analyses clearly revealed that the impact of land-use change on the hydro-physical soil properties is not unique and often masked by other factors such as the antecedent land-use, spatial variability, pre-tilling and tilling activities, soil texture, elevation, climate, among other site-specific factors. Due to these differences, it is rather difficult to evaluate the impacts of pine plantations and grazing on properties at regional scale, and therefore any evaluation of the impacts of anthropogenic activities must be carried out in adjacent sites, which would guarantee similar geomorphological and climatic conditions. This conclusion clearly points out that generalization of findings related to the impacts of land use change on soil properties is not free of risks. Similarly, it also hinders the comparison of findings with published results.
The study further revealed that pine afforestation affects either in a positive or negative way the Ksat, the water retention capacity in the range pF 0 to 2.52 and the SOM content of the soil surface layer. The change and the order of magnitude of the change varies with sampling depth. Similarly, grazing causes positive and negative changes in Ksat and in the water retention capacity, and as in pine plantations, the recorded changes vary with sampling depth. Other controlling factors that define the impact of grazing are evidently pre-tilling and tilling activities in combination with cattle density. Soil spatial heterogeneity and diversity in local factors complicates the interpretation and extrapolation of observed phenomena to Andean montane ecosystems at a regional scale. Correct assessment of land use change impacts is not only of crucial importance for extrapolating findings, it also serves as a basis for the accurate estimation of socio-economic and ecological impacts that anthropogenic-induced changes might have on the water regulation and water storage functionalities of the Andic soils.