Ezequiel Aráoz

Decreases in Fire Spread Probability with Forest Age Promotes Alternative Community States, Reduced Resilience to Climate Variability and Large Fire Regime Shifts

The generalization that plant communities increase in flammability as they age and invariably lead to resilient self-organized landscape mosaics is being increasingly challenged. Plant communities often exhibit rapidly saturating or even hump-shaped age-flammability trajectories and landscapes often display strong non-linear behaviors, abrupt shifts, and self-reinforcing alternative community states. This plethora of fire-landscape interactions calls for a more general model that considers alternative age-flammability rules. We simulated landscape dynamics assuming communities that (1) increase in flammability with age and (2) gain flammability up to a certain age followed by a slight and moderate loss to a constant value. Simulations were run under combinations of ignition frequency and interannual climatic variability. Age-increasing fire probability promoted high resilience to changes in ignition frequency and climatic variability whereas humpbacked-shaped age-flammability led to strong non-linear behaviors. Moderate (20%) reductions in mature compared to peak flammability produced the least resilient behaviors. The relatively non-flammable mature forest matrix intersected by young flammable patches is prone to break up and disintegrate with slight increases in ignition/climate variability causing large-scale shifts in the fire regime because large fires were able to sweep through the more continuous young/flammable landscape. Contrary to the dominant perception, fire suppression in landscapes with positive feedbacks may effectively reduce fire occurrence by allowing less flammable later stage communities composed of longer lived, obligate seeders to replace earlier stages of light demanding, often more flammable resprouters. Conversely, increases in anthropogenic ignitions, a common global trend of many forested regions may, in synergism with increased climate variability, induce abrupt shifts, and large-scale forest degradation.

Southern Annular Mode drives multicentury wildfire activity in southern South America

Significance Fire is a key ecological process affecting ecosystem dynamics and services, driven primarily by variations in fuel amount and condition, ignition patterns, and climate. In the Southern Hemisphere, current warming conditions are linked to the upward trend in the Southern Annular Mode (SAM) due to ozone depletion. Here we use tree ring fire scar data obtained from diverse biomes ranging from subtropical dry woodlands to sub-Antarctic rainforests to assess the effect of the SAM on regional fire activity over the past several centuries. Our findings reveal a tight coupling between fire activity and the SAM at all temporal scales and in all biomes, with increased wildfire synchrony and activity during the 20th century compared with previous centuries. The Southern Annular Mode (SAM) is the main driver of climate variability at mid to high latitudes in the Southern Hemisphere, affecting wildfire activity, which in turn pollutes the air and contributes to human health problems and mortality, and potentially provides strong feedback to the climate system through emissions and land cover changes. Here we report the largest Southern Hemisphere network of annually resolved tree ring fire histories, consisting of 1,767 fire-scarred trees from 97 sites (from 22 °S to 54 °S) in southern South America (SAS), to quantify the coupling of SAM and regional wildfire variability using recently created multicentury proxy indices of SAM for the years 1531–2010 AD. We show that at interannual time scales, as well as at multidecadal time scales across 37–54 °S, latitudinal gradient elevated wildfire activity is synchronous with positive phases of the SAM over the years 1665–1995. Positive phases of the SAM are associated primarily with warm conditions in these biomass-rich forests, in which widespread fire activity depends on fuel desiccation. Climate modeling studies indicate that greenhouse gases will force SAM into its positive phase even if stratospheric ozone returns to normal levels, so that climate conditions conducive to widespread fire activity in SAS will continue throughout the 21st century.

Biological and environmental effects on fine-scale seed dispersal of an invasive tree in a secondary subtropical forest

Dispersal abilities of invading species emerge from the interaction between the species and some features of the target community. Ligustrum lucidum is a tree species invading different ecosystems. Major spatial patterns of Ligustrum invasions and their ecological consequences have been analyzed, but no study addressed the dispersal process at a fine scale, assessing the effects of different biological and environmental factors. Ligustrum lucidum is an ornithochoric species. The structure of the environment determines bird movements and thus affects seed dispersal. We used inverse modeling to analyze bird-mediated dispersal of L. lucidum seeds in a secondary Yungas forest and surrounding crop-fields. We assessed the effects of egestion mode (regurgitation and defecation) and tree density (as an environment character) on seed dispersal. Seed dispersal presented different spatial patterns depending on the egestion mode. Tree density was positively associated with the number of regurgitated dispersed seeds and negatively associated with the number of defecated dispersed seeds. In both cases, dispersal distance increased in open areas, but absence of perches inhibited seed arrival. Thus, spread of L. lucidum is facilitated in open areas with some trees; inside the native forest, short distance dispersal facilitates the gradual invasion by this exotic species. Our results suggest that processes like crop abandonment and forest succession, which are active in subtropical montane systems, may facilitate L. lucidum invasion. Our seed dispersal models should be combined with actual distribution maps of L. lucidum to identify areas vulnerable to new invasions.

Environmental, land cover and land use constraints on the distributional patterns of anurans: Leptodacylus species (Anura, Leptodactylidae) from Dry Chaco

Subtropical dry forests are among the most vulnerable biomes to land transformation at a global scale. Among them, the Dry Chaco suffers an accelerated change due to agriculture expansion and intensification. The Dry Chaco ecoregion is characterized by high levels of endemisms and species diversity, which are the result of a variety of climates and reliefs, allowing a wide variety of environments. The amphibian group exhibits a high richness in the Dry Chaco, which has been barely studied in relation to land cover changes. We used ecological niche models (ENMs) to assess the potential geographic distribution of 10 Leptodactylus species (Anura, Leptodactylidae), which are mainly distributed within the Dry Chaco. We characterized these distributions environmentally, analyzed their overlap with land cover classes, and assessed their diversity of ecoregions. Also, we evaluated how these species potential distribution is affected by the transformation of land, and quantified the proportional area of the potential distribution in protected areas. We found that temperature seasonality is the main constraint to the occurrence of the species studied, whose main habitats are savannas, grasslands and croplands. The main threats to these species are the effects of climate change over spatial patterns of seasonality, which could affect their breeding and reproduction mode; the loss of their natural habitat; the exposure to contaminants used by intensive agriculture and their underrepresentation in protected areas.