The results of our analysis of the functional responses of 140 plant species using 17 functional traits, shows that the Austral Pampean grassland host communities function very differently. The communities respond differently to environmental factors that are currently impacted by global climate change and anthropogenic activities, which is creating novel climates and novel disturbance regimes. Drought and the herbivory regime were found to strongly influence plant communities in the Pampean region, in agreement with previous studies (Koerner and Collins 2014; Ratajczak and Ladwig, 2019), while changes in the historic fire record seemingly did not. Analyzing these factors together was important to be able to reveal their effects on the functional responses of Pampean communities. We now discuss how our findings provide the basis for more reliably predicting community responses to the consequences of climate change and anthropogenic activities, and how conservation and management practices could influence the emergence of unique community types in the Pampean biome.
Disturbances promote alternative functional groups in Pampean grasslands
While at first glance, Pampean grasslands may appear to be uniform as they share similar physiognomies, our analyses identified five functional groups of species with contrasted lifespans, pollination and dispersal, ability to resprout after disturbances, defenses, and drought adaptations on their leaves. Interestingly, the disturbance regime was important for the emergence of communities with distinct suites of trait and calls for conservation actions that manage the type, intensity and frequency of disturbance. Our control treatment, which corresponded to the exclusion of all disturbances, only promoted one of the five functional groups (PFG5) corresponding to perennial tussock grasses of the genera Nassella and Piptochaetium. Maintaining the high diversity of the Pampean grassy ecosystems therefore requires understanding the effect of disturbances and incorporating them in conservation actions.
Effects of drought
The most arid environments only enabled communities with small hairy leaves that produce numerous seeds germinating without dormancy (Fig. 3). Vegetation types with small hairy leaves are typically found in arid environments (Moles et al. 2020) as both traits help mitigate water losses during drought events (Seleiman et al. 2021). Species adapted to drier environments usually produce numerous seeds (Lorts et al. 2008), to secure persistence of the species by an effective dispersal that has a bigger chance of reaching wetter micro habitats. The lack of seed dormancy is not necessarily an advantage in the face of drought. Dormancy commonly helps avoid harsh seasons (dry or cold) and delays germination until conditions are more favorable for growth (Bewley et al. 2013). In the case of Pampa grasslands, the lack of dormancy of the functional group that survives well in the dry environment could be explained by seasonality, as the seeds are released during the fall wet season (Michalijos 2019) and could be one consequence of producing many seeds for which activating dormancy mechanisms could incur high costs.
Effects of herbivory
Pampean grasslands have been deeply modified by the presence of herbivores: the communities in sites exposed to herbivory are composed of a larger proportion of short annual species with spiny stalks or leaves and larger leaves. This distinct suite of traits encountered in areas exposed to mammalian herbivory is consistent with the results of previous works (e.g., Lavorel and Garnier, 2002; Borchardt et al. 2013; Lezama et al. 2014; Charles-Dominique et al. 2016; de Villalobos and Schwerdt 2018). Our analysis showed the effect of herbivory on community filtering to be particularly strong and confirmed that the presence of mammalian herbivores alters the vertical structure of the grasslands, as livestock tends to prefer taller species (Celaya et al. 2011; Zhang et al. 2020) and that the replacement of native herbivores such as guanacos by livestock can lead to major changes in composition, which, in some areas, could favor biological invasions in the Pampa biome (Chaneton et al. 2002; Loydi and Zalba, 2009; de Villalobos et al. 2011). Interestingly, understanding the effect of the novel herbivory regime in Pampa grasslands requires not only studying the effects of the density of herbivores compared to before the Hispanic herbivory regime but also the type of herbivory. Our results show that grazing by horses and cattle promoted different plant communities. Horse-grazed grasslands contain more plants with larger leaf areas (notably rosettes), whereas cattle-grazed grasslands contain spiny communities. The different communities that emerge in horse versus cattle grazed sites is probably related to the distinct preferences of the two herbivores. Horses tend to prefer perennial grasses with narrow leaves (Roger et al. 2014), thereby creating gaps that provide light for large leaf forbs and short statured woody species that require more light than grasses (Garnier et al. 2002). The promotion of forbs in grassy ecosystems by horses in mountain grasslands of the Austral Pampas was also reported by de Villalobos and Schwerdt (2018), and by Davies et al. (2019) in the Midwest of North America. Conversely, cows are more sensitive to spines and tend to select non-spiny species with higher nutritional quality, typically with a lower carbon-to-nitrogen ratio (Celaya et al., 2011; Pauler et al., 2020). The presence of native plant species with functional profiles that enable both horse and cattle herbivory, is first puzzling as it questions which herbivores could have selected for the species composing these communities in the pre-Hispanic period, and second, suggests that conservation programs in the Pampean region should find the right balance of herbivory (in terms of both the density and type of herbivore) to maintain the diversity of communities.
Reduced effect of fire
Surprisingly, fire frequency had little effect on filtering plant communities in our study sites. This suggests that all the functional groups of species in the Pampean grasslands we analyzed are resilient to fire, as even under high fire frequency, we did not observe a strong filtering effect, either at species or trait level. Concerning traits, only spinescence and seed dormancy (sub-significant) were associated with higher fire frequency, but not with other functional traits, such as resprouting capacity, which is commonly associated with fire-prone systems (Pausas et al. 2004; Clarck et al. 2013). Although seed dormancy has been suggested to promote fitness of plants in fire driven systems (Pausas and Lamont 2022) when dormancy is released by the fire generated heat and enable plants to exploit the post-fire environment with reduced competition (de Villalobos et al. 2007; Tangney et al. 2022), spinescence is not per se a fire adaptation. Spinescence is rarely expressed in fire-driven systems (Charles-Dominique et al. 2016) except in areas subjected to pyric herbivory, a disturbance regime in which both large mammalian herbivores are present and fires occur regularly, thereby promoting communities that are adapted to both disturbances (Fuhlendorf et al. 2009; Archibald et al. 2019). The fact spiny species are favored in frequently burned sites questions whether such a pyric herbivory regime could have driven Pampean communities in the past and calls for experimentation that combines the two types of disturbances to evaluate the effects of their combination. This would help design conservation programs adapted to the functioning of the Pampean flora. Alternatively, the absence of a clear functional profile of species in burnt sites could be related to species with higher levels of functional plasticity (not monitored in the present study) that, in turn, could promote persistence in areas with frequent fires (Simpson et al. 2019). The response of species to fire and their ability to resprout in the Austral Pampean grasslands may be influenced by complex interactions with a variety of factors, including fire intensity, and historical disturbance patterns. Such factors could modulate the relationship between fire and functional traits in a non-linear or context-dependent manner (Stephan et al., 2010). No differences were found between communities developing in sites with low (4-y-fire return interval) and high fire frequencies (2-y-fire return interval). Despite the increased occurrence of fires based on historical records in the study sites, it is possible that either these changes are too recent induce a community shift or that these fire frequencies fall within the expected frequency range that selected for the Pampas grassland ecosystems (Paruelo et al. 2022; Giorgi et al. 2020).
Combined effects of different factors
While our analysis did not consider interactions between factors, it is important to study simultaneously the main factors that could influence community dynamics and composition as it allows separating responses specific to a unique factor versus traits that could be promoted by varied alternative drivers. For example, our analysis showed that traits related to livestock grazing and drought conditions both favored plants with resprouting capacity and short life cycles. Several other studies showed traits similarly favoured by drought and grazing (Milchunas et al. 1988; Blumenthal et al. 2020; Irob et al. 2023): for example, deciduous leaves or short life cycles allow species to escape both drought and herbivory in the Austral Pampean grasslands. Short lived plants could complete their lifecycle swiftly during the wet season (Blonder et al. 2023) and efficient resource allocation could facilitate their regrowth and rapid reproduction after grazing (Hendrickson and Olson 2006). Blumenthal et al. (2020) also found that structural drought tolerance and avoidance traits were predictors of herbivore resistance in semi-arid shortgrass steppe and mixed grasslands in North America. Furthermore, Irob et al. (2023) reported that Namibian savannas with a higher proportion of herbivores were more resilient to drought. The co-occurrence of drought and herbivore adaptations could be related to the greater need of plants to defend themselves against herbivory when they grow in a more constraining environment where regrowth (and hence tolerance to damage caused by herbivory) is difficult (Grubb 1992).
Our study emphasizes the significance of taking functional traits into consideration when investigating plant communities in the Pampa biome. Analyzing how these traits are influenced by environmental factors and disturbances led to a more comprehensive understanding of community functioning and responses to environmental changes. This knowledge is critical to develop effective conservation and management strategies to preserve biodiversity and ecological integrity in the Pampa biome. Additionally, our findings provide insights into the dynamics of plant communities over time and their responses to environmental disturbances in the Austral Pampean grasslands. By examining the relationships among functional traits, environmental factors, and disturbances, we can conclude that the grasslands of the Austral Pampean region possess adaptive capacity to variations in disturbances, as evidenced by a diverse array of species with functional traits. These findings pave the way for more sustainable management and call for further studies aimed at promoting the long-term resilience and stability of the Austral Pampean grasslands.