Species-specific effects of the herbaceous layer on recruitment of woody species under different shading and precipitation conditions

Highlights

• Different herb species had distinct effects on the recruitment of woody species.

• The variation in effects depended on environmental conditions and woody species.

• The few positive herb effects mainly occurred in an intermediate level of moisture for these woody species.

• Interactions with herb species mainly occurred under shade because few woody species were able to recruit in open sites.

• Exotic herb species had mainly negative effects while the effect of natives was variable.

Abstract

Herbs may affect the recruitment of woody species. However, most studies have evaluated the impact of one particular herb species or the herbaceous layer as a whole. It is less known if different herb species have different effects on recruitment of woody species, and whether these effects depend on environmental conditions. In this paper, we evaluated the effects of different native and exotic herb species on recruitment of nine woody species in central Chile, under different conditions of shading and precipitation. The experiment was carried out in an open ruderal area in Santiago, Chile. We conducted a factorial experiment with 360 1 × 1 m pots with monospecific crops of eight herb species and a control without herbs, two levels of irrigation (245 and 534 mm) simulating two precipitation regimes, and the presence or absence of artificial shade simulating a woody canopy. Nine woody species common to the region were sown in each pot. Luminosity (PAR), soil moisture, herb volume, richness of woody species and seedling density of woody species recruited were measured. The effects of herb species on woody species richness varied from neutral in the open condition to mainly negative in the shaded condition. Differences in irrigation did not modify the effects of herbs on species richness. The effects of herb species were more variable when woody species were considered individually. Negative and neutral effects predominated, although some positive effects were observed under shaded and wetter conditions. Our results suggest that different herb species have distinct effects on recruitment of specific woody species and species richness, which mainly depend on shade conditions and secondarily on the precipitation regime, and that herb-woody species interactions are highly specific. Thus, management focused in herb controlling should be species-specific, in terms of herb and woody species, and dependent on climate and woody cover.

Introduction

Plant-plant interactions strongly influence the composition, structure and dynamic of vegetation (Maestre et al., 2009) and hence should be considered when planning management and restoration strategies (Gómez-Aparicio, 2009). Most of forest ecosystems have herbaceous layers that vary spatially in species and abundance (Gilliam, 2014). Herb species influence light, soil moisture, chemistry, and other biotic and abiotic factors (Morris et al., 1993, Davis et al., 1999, Olson and Wallander, 2002, Coll et al., 2003, Callaway et al., 2003, Curt et al., 2005), and may therefore affect the natural regeneration or reforestation processes (Gilliam, 2007, Gilliam, 2014). Most studies of the role of the herbaceous layer on woody regeneration have observed negative impacts (Facelli and Pickett, 1991, Castro et al., 2002a, Rey Benayas et al., 2003, Rey Benayas et al., 2005, Rey Benayas et al., 2007, Lawes and Chapman, 2006, Kunstler et al., 2006, Gilliam, 2007, Gilliam, 2014, Cuesta et al., 2010), although some neutral and positive effects have also been documented (Maestre et al., 2001, Tonioli et al., 2001, Becerra et al., 2011). This variability of outcomes seems to depend on the woody species, probably related to the ecological niche of each species, and the environmental conditions where interactions occur (Johnson et al., 2015, Loranger et al., 2017). A less known issue is the distinct effect of different herb species on regeneration of the same species or the same assembly of woody plants. Herb species can reach varying biomass levels and modify microenvironments differently (Montenegro et al., 1978, Figueroa et al., 2004a, Gilliam, 2014). Therefore, the effects of herbs may vary according to the species in the herbaceous layer.

Plant-plant interactions can depend on environmental conditions, for example, climate (Bertness and Callaway, 1994, Maestre et al., 2009, Holmgren and Scheffer, 2010). Therefore, the effect of herb species on the regeneration of woody plants may vary among different abiotic conditions. Although some studies have evaluated how the effect of trees or shrubs on tree regeneration varies with abiotic conditions (Castro et al., 2002b, Maestre et al., 2009), there have been few studies assessing how environmental conditions modify the effect of herb species on the regeneration of woody species. For example, vegetation canopy can facilitate regeneration of other species by enhancing soil moisture and reducing water stress in dry climates, while inhibit it under wetter conditions (Holmgren and Scheffer, 2010). However, the role of precipitation on the effect of herbs on regeneration of woody species remain unclear. For instance, Cuesta et al. (2010) observed that the effects of herb competition on seedlings is more significant in rainy years than in dry ones. Instead, an intermediate year in terms of precipitation trigged positive effects of herbs on the survival of woody seedlings in a semiarid climate, in contrast to a dry year in which herbs had neutral or negative effects on seedling survival (Becerra et al., 2011). Similarly, Maestre and Cortina (2004) found that herbs had a negative effect on shrub seedlings under xeric and extreme moisture conditions, while the effects were positive at intermediate moisture levels.

Another factor that produces different abiotic conditions is the shade from vegetation and other natural elements of ecosystems (stones, tree trunks). Shade can influence the effect of herbs because it can affect the level of stress experienced by the seedlings and seeds of woody species (Davis et al., 1999, Rey Benayas et al., 2002, Rey Benayas et al., 2005, Curt et al., 2005). Shade often results in higher soil moisture compared to open areas (Holmgren and Scheffer, 2010). If open sites are more stressful for woody seeds and seedlings due to lower soil moisture, which is typical in semiarid climates (Bertness and Callaway, 1994, Castro et al., 2002b, Becerra and Montenegro, 2013), and if a canopy of herbs increases soil moisture (e.g. Becerra et al., 2011), herbs may facilitate recruitment of woody species in open areas. In contrast, under shrub or tree shade, the increase in soil moisture produced by a canopy of herbs would not be significant compared to other site without herbs under tree shade, and hence herbs would not facilitate recruitment of woody species in this condition. As well, shade may influence the effect of herbs on woody seedlings and seeds by modifying herb species composition and/or biomass (Caldeira et al., 2014). For instance, microclimatic conditions under woody patches can induce dominance of different herb species than in open sites (Gómez-González and Cavieres, 2009, Griffith, 2010).

Like other areas with Mediterranean-type climates, central Chile is characterized by a seasonal climate with hot dry summers and cold humid winters, with significant inter-annual and spatial variability in precipitation and moisture (Armesto et al., 2007). It is also known that moist habitats and rainy years increase recruitment of woody species from the sclerophyllous forest (Holmgren et al., 2006, Fuentes-Castillo et al., 2012, Becerra et al., 2016). Vegetation in central Chile has been strongly disturbed by anthropogenic drivers like clearing, fires and raising livestock (Médail and Quézel, 1999, Myers et al., 2000, Holmgren, 2002, Armesto et al., 2007). These disturbances have changed continuous forests into mosaics of woody patches and open areas (Van de Wouw et al., 2011, Fuentes-Castillo et al., 2012), resulting in highly variable micro-environmental conditions. Both open areas and woody patches have been invaded by exotic herbs, producing a diverse mixture of native and exotic species (Montenegro et al., 1978, Holmgren et al., 2000, Figueroa et al., 2004a, Gómez-González et al., 2011). Several studies have documented that the regeneration of woody species in this semiarid region is facilitated by previously established woody species (Armesto and Pickett, 1985, Fuentes et al., 1984, Becerra and Montenegro, 2013), although under moist conditions facilitation appears to be less important for some species (Holmgren et al., 2000). Furthermore, the herbaceous layer can have different effects on the survival of woody species seedlings depending on habitat conditions of this region (Becerra et al., 2011). Thus, this region is an ideal system to evaluate species-specific effects of herbs on recruitment of woody species under different abiotic conditions. This knowledge would allow to establish management recommendations about necessities of herb control, if this control should be species-specific, and if this depends on shade and precipitation conditions.

In this paper, we evaluated the effect of different native and exotic herb species on recruitment of several woody species native to central Chile. We also examined whether the variability in the effects of herbs depends on shading and precipitation conditions.