Deploying community filters (dispersal, abiotic and biotic interactions) all together— instead of solely focusing on the biotic interactions—will offer a better restoration framework to land managers (Hulvey & Aigner, 2014). In invaded habitats, long-term agave restoration requires an understanding of the complexity and variability of favorable growth conditions for agave growth and sustenance. This greenhouse experiment was designed to test the growth responses and interaction of Palmer’s agave by modifying biotic (size, age) and abiotic growth conditions (simulated precipitation, surface mulch using dried Lehmann thatch, simulated moving by clipping Lehmann lovegrass to change the inter-species competition of agave and Lehmann). We measured net agave biomass (root and shoot) following the simulated precipitation events to capture agave growth responses to Lehmann lovegrass competition removal and thatch facilitation treatments. As we expected, agave biomass retention was significantly maximum in the treatments where Lehmann lovegrass competition was either reduced or absent with high and medium precipitation regimes and thatch facilitation. Conversely, Lehmann lovegrass growth (measured by whole plant biomass) was indifferent to Palmer’s agave presence, precipitation regimes, and facilitation by thatch, except clipping Lehmann lovegrass, where the Lehmann lovegrass biomass significantly reduced as a result of the clipping. To be a successful restored community, species need to pass through a series of ‘filters’ that narrow the pool including the dispersal, abiotic and biotic filters (Hulvey and Aigner, 2014).
First, we begin with the environmental stresses. Daehler (2003) reviewed studies that show that environmental growth conditions can influence a native plant’s performance more than the growth rate and productivity of a neighboring competitor. It is widely recognized that agave seedlings are seldom spotted in the wildlands and are particularly vulnerable to the environmental growing conditions. Unlike mature plants, agave seedlings need multiple and consecutive years of favorable conditions (especially during the first year of the growth) to establish, grow and survive (Gentry, 1972; Nobel, 1977). High surface temperatures and limited water availability account for high agave seedlings mortality rates, and suggestively, are the prime cause for stunted agave seedling growth in the Southwest U.S. (Nobel, 1979; Jordan and Nobel, 1979, 1982; FAO, 1989). Improving water and shade related needs play a crucial role in the advancement of agave species growth and establishment. Additional to ample moisture availability, desert agaves benefit naturally grow well in a matrix of existing vegetation, beside a ‘nurse’ plant or under indirect sunlight exposure. (Result related to just watering treatments) Several studies have reported that agave seedlings when facilitated by shade and straw mulch/thatch facilitate seedling emergence and growth. Jordan and Nobel (1979) reported one such example as they found higher Agave deserti growth in habitats surrounded by nurse plants under atypically exceptional precipitation years. Arizaga & Ezcurra (2002) reported that, in hot deserts, Agave macroacantha seedlings critically depend on the protective shade of the nurse plants for seedling establishment success, just as other succulents and cacti depend on nurse plants (Turner et al., 1966; Steenbergh and Lowe, 1969; Valiente-Banuet and Ezcurra, 1991).
Likewise, Pavliscak et al., (2011) demonstrated that facilitation by shade and straw (thatch in our case) operates to enhance the establishment and survival of Palmer’s agave in the presence of shade and surface mulch. Our results support these previous studies and suggest that agave biomass enhanced significantly amongst all simulated precipitation treatments and with the presence of thatch. Utilizing these minute additive strategies could help manipulate agaves\' functional traits according to the conditioning to the abiotic stresses and accommodate accordingly for these environmental stresses of temperature and moisture, regulating their traits to yield better traits and eventually greater biomass. Apart from the abiotic stresses, desert succulents—particularly agave species—are often exposed to biotic challenges from the surrounding vegetation. Agaves are naturally slow-growing, monocarpic plants that grow for about 35 years before blooming (Gentry, 1982).
In invaded habitats, Palmer’s agave persistence is further complicated by the non-uniform competitive interactions occurring among inter-and-intra-specific species in different age classes and size groups. One reason is that partial size and age symmetry among plants leads to unequal resource share, suppressing the growth of smaller plants (Schwinning and Weiner, 1997). Second, mature plants survive and establish better than juveniles when encountered with negative biotic interactions (Goldsmith, 1978; Keddy and Shipley, 1989). Third, the morphological and physiological differences favor bigger and more robust plants in acquiring more resources than smaller plants (Mangla et al., 2011). Our results support theses previous studies, we found that bigger agaves significantly gained greater biomass than the small ones. This implies that restoration practitioners should consider recruiting agaves of various age and size class to recover the agave population as well as ensure the sustenance of native plants and associated benefits continuous. This is particularly important for desert wildlands where meager resources, climatic adversity, predation and several other cause complicate restoration processes, putting ecological biodiversity, species preservation, and associated benefits at risk.
One prominent threat to the sustenance of Palmer’s agave is the dominant invasion of Lehmann lovegrass. Several studies report that Lehmann lovegrass dominates the plant community and structure by monopolizing it to a near monoculture of an invasive grass species, especially risking the native vegetation and associated ecological and economic benefits. Krzic et al. (2000) and Wilsey and Polley (2006) reported that Lehmann lovegrass suggestively inclined to compete robustly, producing a monotypic stand similar to crested wheatgrass dominance in the semi-arid western U.S. Sanchez Munoz (2009) also reported reduced species richness in plots adjacent to Lehmann lovegrass specie reduced to half, other studies support this too. Likewise, Bouteloua species in the native, uninvaded sites were replaced by Lehmann lovegrass in the invaded sites, similar to southwestern US (McClaran and Anable; 1992; McLaughlin and Bowers 2006). Our results support these studies that Lehmann lovegrass invasion was more rigorous dominant against Palmer’s agave. We found that agave presence varied Lehman lovegrass biomass insignificantly across all the singular or combination treatments, except the Lehmann lovegrass simulated mowing treatment.
Even though, agaves ultimately perform better with the abiotic growth conditions amendments, but the accommodations made were still not impactful enough for agaves to cope with the Lehmann lovegrass one-sided dominance. We recommend that an integrated approach is needed for the adaptive management of Lehmann lovegrass vegetation so that ecologically economic and threatened native plants like Palmer’s agave could be preserved and protected against the competitive non-native plants. Lastly, this study identifies some key aspects that help improve Palmer’s agave performance and response to different various biological and environmental stresses that impact native plants. Restoration in arid lands, particularly agave is complex hence we tested some specific agave functional traits to see if they could improve agave fortitude to these stresses. Moreover, an additional challenge is the inadequate and incomprehensive scientific knowledge, disconnectedness between stakeholders, contradictory policy and funding, low supply and high cost associated with native seeds and plant collection and restoration.
Cumulatively, these challenges suggest a need to reconsider for restoration, reseeding and/or transplanting native plants that requires additional understanding of managing native plants for production agriculture, ecological good and services, and other salient benefits which might include biodiversity, pollinators, wildlife habitat, soil and water conservation, and fire behavior, among others (Pimentel et al. 2005). Moreover,1) future research should focus on different native plant perform, assemble communities, and restore against invasion (Pywell et al. 2003; Martin and Wilsey 2012), and on the lack of quantitative data of the tradeoffs between native and exotic species plus the associated good and services (Scasta et al., 2015). Ultimately, the gap between the restoration of natural resources and commercial production must be abridged so we can objectively consider the socio-ecological complexities of exotic plants and their ecological economic impacts on the native plants.