Plants create buffered microsites
Plant communities growing in adverse areas are expected to be structured by the stress tolerance of the individual species that comprise them (Lortie et al. 2004). Consistent with the widespread presence of fertility islands in high-mountain environments (Cavieres et al. 2006; Escudero et al. 2004), our findings confirm that the expected improved soil properties remained constant beneath vegetation-covered patches. However, the amelioration effect was not associated with the type of dominant species in the patch (cushion or herbaceous), nor did it differ significantly through the elevation gradients.
Contrasting the observations in bare soils, the spatial heterogeneity of soil properties was gradually shaped by the generations of pioneer vegetation, generally causing an improvement in the soil environment beneath vegetation patches in advanced stages of development (Fig. 5, Supplementary Table S1). A case in point is the relationship found between the increases in P content with the maturity of the patch successional stage, which may be related to a faster resource depletion by pioneer nurses within young vegetation patches compared to their late-successional counterparts, to prompt the colonization of open gaps (Grime 1977, 2002; Kazakou et al. 2006). These results provide further information on how the plant-soil interaction contributes to plant establishment in adverse environments (Li et al. 2021).
Despite differences in soil amelioration across the elevation gradient being generally non-significant, differences were greater in intermediate elevations (Fig. 5). Our results resemble the humped patterns observed by Milhoc et al. (2016) in a larger elevation gradient in the Mediterranean Andes and may be attributed to the two severity gradients in Mediterranean high-mountains: freezing stress increasing with altitude; drought stress increasing as elevation declines (Cavieres et al. 2006; Giménez-Benavides et al. 2007b). Additionally, and similar to the findings by Cavieres et al. (2002), we observed an increase in the abundance and number of species within vegetation patches proportional to elevation. These findings suggest an enhanced ameliorating role of certain species in the vegetation patches at higher elevations, possibly in response to the aggravation of the severity gradients. This increased amelioration would thus favor recruitment within these patches and plausibly improve the associated species’ performance. In general, our results suggest the high-mountain communities from Sierra de Guadarrama show endurance to current environmental-induced shifts, resembling the positive interaction reported by Lloret and Zedler (2009) and Lloret et al. (2012) in other Mediterranean habitats and plant communities.
Compared to bare soils and patches with other dominant species, the substantially richer soils found underneath F. curvifolia (Table 1; Supplementary Table S1) further support previous studies stating that F. curvifolia patches behave as “islands of fertility,” ameliorating soil conditions beneath them (Escudero et al. 2004; Pescador et al. 2014). Nevertheless, and despite the presence of these “fertility islands,” our findings further support the report by Pescador et al. (2014) indicating F. curvifolia does not act as a nurse or facilitator species in these summits. The latter occurs since it blocks the growth of other species within its clumps, exerting an allelopathic effect or “repulsion halo” (Gutiérrez Girón and Gavilán 2010; Pescador et al. 2014). Instead, this species should be considered a primary colonizer, forming monospecific bands of vegetation, thanks to its clonal growth, and improving the soil underneath to promote its expansion in austere circumstances (i.e., poor soils; (Gavilán et al. 2002). As per its allelopathic effect, the thick litter and leaf mats created by F. curvifolia may add to its dominant nature by reducing or impeding seedling emergence of alternative species (Suding and Goldberg 1999). These thick leaf mats could be counterbalancing the positive effect of the ameliorated soil conditions found beneath its canopy with considerable light limitations (Suding and Goldberg 1999).
Enhanced biological activity and soil processes can improve soil aggregation and macroporosity, which increase soil infiltrability and, consequently, the ability of vegetation patches to obstruct, capture, and store runoff water and nutrients (Ludwig et al. 2005). Altitude and vegetation must also be accounted for in high-mountain environments since they lead to differences in organic matter decomposition rates (Gutiérrez-Girón et al. 2015). In this line, other factors such as litter and root extension may also play a significant role in controlling runoff-run-on processes. The buildup of litter and substantial SOM values found in patches with Thymus penyalarensis as the dominant species (Supplementary Table S1) could be the effect of the antimicrobial properties attributed to this genre, limiting fungi or bacterial development or activity (Dorman and Deans 2000). These antimicrobial properties might be slowing down the SOM decomposition and lowering the mineralization rates, which could explain the high C/N ratio found in these patches (Dorman and Deans 2000; Kalbitz et al. 2003). Our findings relate to those by Gutiérrez-Girón et al. (2015) reporting a high soil organic carbon accretion under Juniperus shrubs that correlates with a reduced respiration rate of soil microorganisms. The high SOM content found in the understory of T. penyalarense, coupled with an absence of significant differences in the soil C/N ratio between microhabitats, could also be related to high chemical recalcitrance of C substrate (Duboc et al. 2014; Gutiérrez-Girón et al. 2015; McCulley et al. 2004). Taken as a whole, the present findings suggest a pattern of generally high recalcitrancy of the litter in these mountains, though further studies focusing on each species are required.
Chemical weathering comprises the processes of mineral dissolution, alteration, and transformation of initial mineral phases to those that are more stable at the surface of the earth (Egli et al. 2014). Wind could significantly influence critical soil-related effects at high elevations, such as soil erosion and surface drying. The common incidence of strong winds in the Sierra de Guadarrama summits and limited chemical weathering typical of old and flat topographies like the one of this mountain range could trigger the lifting of lighter substrates leading to a stripped soil surface (i.e., dry soils, deficient in nutrients (Egli et al. 2014; West et al. 2005). The incidence of these conditions could explain the lack of significant differences found between microhabitats in the present study. K is a monovalent cation abundant in the lithological material from the area that in soils with a high sand percentage wash away easily with water. Thus, chemical weathering could explain the generally lower K values found in bare soils than in those covered by vegetation. Furthermore, the high amounts of accumulated biomass found in vegetation patches (high SOM, see Supplementary Table S1) could suggest a seemly soil cover, retaining a certain amount of soil aggregates against the runoff be caused by the effect of wind, rain, and snow melting and enhanced nutrient enrichment. However, the high C/N ratios calculated in all vegetation patches and the slackened microbial activity described by Gutiérrez-Girón et al. (2015) put forward this alleged excess nutrient input may not necessarily translate into higher mineralization rates, adding to the scarce significant differences between microhabitats.
Cushion plants are well-known ecosystem engineers (Cavieres et al. 2006; Jones et al. 1994) as they strongly modify their surrounding environment and affect community assembly processes (Badano and Cavieres 2006; Schöb et al. 2012). Cushions are the dominant life-form in the Pan-Nav transect (Supplementary Table S1) inferring the observed soil amelioration results from the “nursing” effect of cushions in these patches. The lack of significant differences with elevation verifies a pattern of soil amelioration by cushions in these high-mountains which appears to be robust to the broad-scale environmental gradients and more related to site-specific conditions (i.e., neighboring species, soil microbial activity). The latter is based on the observed lack of significant differences in the C/N ratio between microhabitats indicating a slow SOM decomposition rate. The fair amount of residue (pers. obs.) and maintenance of a high C/N ratio found throughout the elevation gradients (Table 1) further indicates an overall declined SOM decomposition due to a reduced microbial activity (priorly described in these summits Gutiérrez-Girón et al. 2015) which directly associates with low mineralization rates. Thus, these findings could translate into a reduced nutrient availability for plants in these habitats that plausibly favors the enhancement of competition rather than facilitation, implying cushions exert a meager impact on ameliorating soil conditions in the Pan-Nav transect (Escudero et al. 2004; Gutiérrez-Girón et al. 2015).Species of Festuca growing in the Andean highlands (i.e., Festuca orthophylla) are characterized by a shallow rooting system covering up to 6-fold the area of the above-ground canopy, which allows the plant to access resources in a total area equivalent to 6-fold that of their canopy (Monteiro et al. 2011). Generally, tussock grasses (e.g., Festuca species) are thought of as strong competitors due to their vast above- and below-ground biomass (Grime 2002). Their root network is the most important mechanism for capturing resources in scarce environments and allowing for their distinctive high competitiveness (Bordeu et al. 2016). Modeling studies have shown how the spatial organization of tussock vegetation in semi-arid ecosystems is mediated by the competitive interactions and a reduction in aridity (Bordeu et al. 2016). Similar to the observed in F. orthophylla (Monteiro et al. 2011), we assume that F. curvifolia has an enlarged root system covering larger areas than the canopy to enable soil exploration. The latter could result from a strong root competition driven by the low nutrient conditions in the study area (i.e., poor soils), further reducing the possibility of positive effects on neighboring species. That could be why we observe scarce significant differences between soil nutrients between vegetation-covered dominated by this tussock and bare soil patches (i.e., Val-CH transect; Table 1). An additional explanation could be the possibility of a more intense competition occurring within the shallow soils in our system. This enhanced competition could be reflecting limited root segregation due to a lack of space and high species richness in these vegetation patches. Thus, this could be restricting the uptake of soil resources from the same depth as observed in other grasslands (i.e., niche overlap over the soil depth gradient; (Dornbush and Wilsey 2010; Martorell et al. 2015).
Similar to Mihoc et al. (2016) and Escudero et al. (2004), the present results suggest soil amelioration occurs irrespective of the status of the vegetation patch and elevation. These findings allow suggesting a minor effect of facilitative interactions in the studied gradient that could be related to the shallowness of the soils and their relatively poor nutrient content, compared to other habitats. In turn, we could assume a stronger competition effect occurs in these summits due to stress caused by the lack of a resource (e.g., water, low nutrient availability). Consequently, the plants growing in these sites tolerate the stress and do not require its amelioration by neighboring species (Maestre et al. 2009). Nonetheless, more studies exploring in-depth the balance between the ameliorative effects and competitive impacts in these plant associations are required to accurately describe these dynamic processes in Sierra de Guadarrama (Callaway and Walker 1997). These studies should be carried out mainly focusing on cushion species dominating in alternative patches in Sierra de Guadarrama due to their well-known facilitative nature and seeing as these interactions may vary in function of changing environmental factors in space-time, life stages of the dominant species, or shifts instigated by a dynamic and spatial organization (Holmgren et al. 1997; Meron 2012; Pescador et al. 2014; Soliveres et al. 2010).