Resilience of dry tropical rangelands : How native palms and trees mediate the effects of seasonal droughts

Summary

Cattle rangelands cover more than one quarter of the ice-free surface of Earth and have a big impact on the world´s climate, water, land and biodiversity resources. Livestock farming keeps expanding in response to increases in human population, socio-economic growth and the maintenance of meat eating habits, threatening forests across the global tropics and contributing significantly to global warming. At the same time, the higher temperatures and erratic rains resulting from climate change accentuate existing problems in livestock systems, and are expected to compromise pasture and cattle productivity. There is a strong need for more sustainable and resilient livestock systems to global warming that maintain productivity while also contributing to biodiversity conservation.

Increasing tree cover in rangelands could maintain the existing ecological networks and contribute to ameliorate the impacts of global warming on pasture and cattle productivity while having positive effects on biodiversity. By studying the interactions between trees and some of the organisms coexisting on cattle rangelands, this dissertation sheds light on how trees can contribute to increase the resilience of dry tropical rangelands to seasonal droughts.

Throughout this dissertation, I and different teams of collaborators combined different approaches to assess the effects of trees on the resilience of dry cattle rangelands to droughts. We focused on the effects of trees on 1) grass productivity, 2) biodiversity conservation and 3) cattle thermoregulation, considering seasonal weather variability.   

In chapter 2, we performed a global meta-analysis to evaluate the effects of tree cover on grass biomass 1) along environmental gradients in tropical and temperate pastures, 2)  during contrasting seasons, and 3) at different levels of canopy density. We concluded that the facilitative effects of trees on grass biomass are the strongest in neotropical pastures during dry seasons. These facilitative effects are more likely to occur at intermediate levels of evapotranspiration and irradiance. We noticed that the effects have been poorly studied for palms despite their widespread occurrence in tropical livestock pastures and their highly diversified local uses. Based on these results, we specifically aimed at understanding the effects of palms and trees in dry neotropical rangelands in chapters 3-5.

In chapter 3, we studied tree composition of dry cattle pastures of Yucatán, México and  assessed the effects of palms, N2-fixing-trees and non-N2-fixing-trees on their understory microsite environmental conditions, bare soil cover, and woody plants and grasses performance during the dry and wet seasons. We also analyzed the effect of the different canopy types on the impact of droughts on grass greenness and its recovery afterwards. We found that nearly half of the tree species of the surrounding native dry forests are conserved in the pastures. These trees facilitate woody plant recruitment below their canopies and ameliorated the impact of seasonal drought on grass greenness but slowed down its recovery afterwards. Palms had overall higher positive effects on grass greenness and height than trees, especially during the driest season. Our findings highlighted the importance of palms for grass productivity in dry tropical rangelands, as they are the most abundant species and had higher positive effects than trees.

In chapter 4, we explored the interactions between palms and the soil microbial community to further understand the mechanisms behind palm-grass facilitation. We assessed the influence of palm tree overstory on the microbial community structure of the grass rhizosphere in a cattle farm in Yucatán, México. Based on the massive sequencing of the 16S rRNA gene, we analyzed the bacterial diversity of the grass rhizosphere in the understory of the palm tree Sabal spp. and in the open pastures. We contrasted these two microenvironments with the rhizobiome of a legume tree Acacia spp., which harbours intrinsic relations with nitrogen-fixing bacteria, and compared all rhizosphere samples to the bulk soil microbial composition. We found that soil microbial richness, diversity and dominance is higher in the grass rhizosphere under palms than in the other microenvironments, suggesting a positive effect of the palms in the microbial community. Our novel findings provide evidence for the advantage of maintaining palm trees in dry tropical pastures as they allow for the establishment of a greater diversity of microorganisms that favor grass growth.

In chapter 5, we determined cow heat stress during the dry and wet seasons in dry tropical pastures and analyzed the effects of palms and tree shade on cattle behavior, based on behavioral observations in the field. We found that cows actively seek dense tree shade, especially during the wet season when the environmental heat stress is higher. These findings highlight the importance of tree shade for cow thermal regulation in dry tropical regions and contribute to identify ways of adapting production systems to environmental stresses.

In chapter 6, I synthesized all the results and discussed their contribution to increasing our understanding on the resilience of dry cattle rangelands. I argued that farmers’ choices for conserving native palms and trees in pastures may reflect a deeper and holistic understanding of their social-ecological systems. I suggested that intermediate levels of tree canopy density maximize pasture productivity and biodiversity conservation during droughts while providing shade for cattle when heat-stress increases. I also called attention on the poorly studied effects of palms on grass productivity and emphasized on their positive effects on pasture productivity and nature conservation in the dry tropics. I encouraged to integrate and maintain a diversity of trees in pastures and to actively manage intermediate levels of tree canopy density to maximize and diversify the observed facilitative effects of palms and trees on cattle rangelands. Lastly I pledged researchers to bridge across scientific disciplines to expand the traditional indigenous knowledge in agroecosystems and multifunctional landscapes across environmental gradients and cultures around the globe to generate effective strategies for sustainable productive systems to be better adapted to changing climate conditions.

Understanding the interactions within the ecological networks of tropical cattle dry rangelands brings us one step closer to developing resilient livestock systems that maintain productivity and conserve biodiversity as global warming progresses.