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1. Increased mortality of tropical tree seedlings during the extreme 2015–16 El Niño

   https://doi.org/10.1111/gcb.15809  

   Browne, Luke ; Markesteijn, Lars ; Engelbrecht, Bettina M. J. ; Jones, F. Andrew ; Lewis, Owen T. ; Manzané‐Pinzón, Eric ; Wright, S. Joseph ; Comita, Liza S. ( July 2021 , Global Change Biology)

   As extreme climate events are predicted to become more frequent because of global climate change, understanding their impacts on natural systems is crucial. Tropical forests are vulnerable to droughts associated with extreme El Niño events. However, little is known about how tropical seedling communities respond to El Niño–related droughts, even though patterns of seedling survival shape future forest structure and diversity. Using long‐term data from eight tropical moist forests spanning a rainfall gradient in central Panama, we show that community‐wide seedling mortality increased by 11% during the extreme 2015–16 El Niño, with mortality increasing most in drought‐sensitive species and in wetter forests. These results indicate that severe El Niño–related droughts influence understory dynamics in tropical forests, with effects varying both within and across sites. Our findings suggest that predicted increases in the frequency of extreme El Niño events will alter tropical plant communities through their effects on early life stages.

    

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2. Droughts, Wildfires, and Forest Carbon Cycling: A Pantropical Synthesis

   https://doi.org/10.1146/annurev-earth-082517-010235  

   Brando, Paulo M. ; Paolucci, Lucas ; Ummenhofer, Caroline C. ; Ordway, Elsa M. ; Hartmann, Henrik ; Cattau, Megan E. ; Rattis, Ludmila ; Medjibe, Vincent ; Coe, Michael T. ; Balch, Jennifer ( May 2019 , Annual Review of Earth and Planetary Sciences)

   Tropical woody plants store ∼230 petagrams of carbon (PgC) in their aboveground living biomass. This review suggests that these stocks are currently growing in primary forests at rates that have decreased in recent decades. Droughts are an important mechanism in reducing forest C uptake and stocks by decreasing photosynthesis, elevating tree mortality, increasing autotrophic respiration, and promoting wildfires. Tropical forests were a C source to the atmosphere during the 2015–2016 El Niño–related drought, with some estimates suggesting that up to 2.3 PgC were released. With continued climate change, the intensity and frequency of droughts and fires will likely increase. It is unclear at what point the impacts of severe, repeated disturbances by drought and fires could exceed tropical forests’ capacity to recover. Although specific threshold conditions beyond which ecosystem properties could lead to alternative stable states are largely unknown, the growing body of scientific evidence points to such threshold conditions becoming more likely as climate and land use change across the tropics. ▪ Droughts have reduced forest carbon uptake and stocks by elevating tree mortality, increasing autotrophic respiration, and promoting wildfires. ▪ Threshold conditions beyond which tropical forests are pushed into alternative stable states are becoming more likely as effects of droughts intensify. 

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3. Dry conditions and disturbance promote liana seedling survival and abundance

   https://doi.org/10.1002/ecy.2556  

   Umaña, María Natalia ; Forero‐Montaña, Jimena ; Nytch, Christopher J. ; Thompson, Jill ; Uriarte, María ; Zimmerman, Jess ; Swenson, Nathan G. ( January 2019 , Ecology)

   Species composition and community structure in Neotropical forests have been severely affected by increases in climate change and disturbance. Among the most conspicuous changes is the proliferation of lianas. These increases have affected not only the carbon storage capacity of forests but also tree dynamics by reducing tree growth and increasing mortality. Despite the importance of lianas in Neotropical forests, most of the studies on lianas have focused on adult stages, ignoring dynamics at the seedlings stage. Here, we asked whether observed increases in liana abundance are associated with a demographic advantage that emerges early in liana ontogeny and with decreased precipitation and increased disturbance. To test this, we compared patterns of growth and survival between liana seedlings and tree seedlings using a long‐term data set of seedling plots from a subtropical wet forest in Puerto Rico, USA. Then, we examined the effect of precipitation and land use history on these demographic variables. We found evidence for liana seedling survival advantage over trees, but no growth advantages. This survival advantage exhibited significant temporal variation linked with patterns of rainfall, as well as differences associated with land‐use history in the study area. Furthermore, we found that neighborhood density has a negative effect on liana survival and growth. Our results indicate that liana proliferation is likely related to a survival advantage that emerges in early stages and is influenced by climatic conditions and past disturbance. Predicted climatic changes in rainfall patterns, including more frequent and severe droughts, together with increases in disturbance, could have a significant effect on seedling tropical communities by favoring lianas.

    

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4. Do experimental drought stress and species' drought sensitivity influence herbivory in tropical tree seedlings?

   https://doi.org/10.1111/btp.13075  

   Muehleisen, Andrew J. ; Manzané‐Pinzón, Eric ; Engelbrecht, Bettina M. J. ; Jones, F. Andrew ; Comita, Liza S. ( February 2022 , Biotropica)

   In tropical forests, drought and herbivory represent two potent stresses on seedlings. Climate change is expected to increase the frequency of severe droughts in many tropical forests, which may influence seedling vulnerability to herbivores if drought stress affects seedling palatability. Furthermore, contrasting selective pressures in wetter vs drier forests could mean that species well‐adapted to herbivores are less drought resistant and vice versa. In this study, we measured seedling performance and herbivory in a common garden experiment where seedlings of 15 tree species were subjected to irrigation or rainfall exclusion treatments across two dry seasons in Panama. Water manipulation had no effects on foliar herbivory during the experiment for all species combined and for 14 of the 15 focal species when analyzed separately. There was large variation among species in herbivore damage, but no relationship between the sensitivity of species to drought and the amount of herbivory they experienced. Altogether, our findings suggest that increasing drought stress is unlikely to directly alter tropical tree seedling susceptibility to herbivore attack in this forest. Additional studies are needed to determine whether drought alters tropical plant‐herbivore interactions via other mechanisms, such as through changes in herbivorous insect communities and/or increases in fitness costs of herbivory.

   Abstract in Spanish is available with online material.

    

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5. Understory plant communities show resistance to drought, hurricanes, and experimental warming in a wet tropical forest

   https://doi.org/10.3389/ffgc.2022.733967  

   Alonso-Rodríguez, Aura M. ; Wood, Tana E. ; Torres-Díaz, Jamarys ; Cavaleri, Molly A. ; Reed, Sasha C. ; Bachelot, Benedicte ( July 2022 , Frontiers in Forests and Global Change)

   Global climate change has led to rising temperatures and to more frequent and intense climatic events, such as storms and droughts. Changes in climate and disturbance regimes can have non-additive effects on plant communities and result in complicated legacies we have yet to understand. This is especially true for tropical forests, which play a significant role in regulating global climate. We used understory vegetation data from the Tropical Responses to Altered Climate Experiment (TRACE) in Puerto Rico to evaluate how plant communities responded to climate warming and disturbance. The TRACE understory vegetation was exposed to a severe drought (2015), 2 years of experimental warming (4°C above ambient in half of the plots, 2016–2017 and 2018–2019), and two major hurricanes (Irma and María, September 2017). Woody seedlings and saplings were censused yearly from 2015 to 2019, with an additional census in 2015 after the drought ended. We evaluated disturbance-driven changes in species richness, diversity, and composition across ontogeny. We then used Bayesian predictive trait modeling to assess how species responded to disturbance and how this might influence the functional structure of the plant community. Our results show decreased seedling richness after hurricane disturbance, as well as increased sapling richness and diversity after warming. We found a shift in species composition through time for both seedlings and saplings, yet the individual effects of each disturbance were not significant. At both ontogenetic stages, we observed about twice as many species responding to experimental warming as those responding to drought and hurricanes. Predicted changes in functional structure point to disturbance-driven functional shifts toward a mixture of fast-growing and drought-tolerant species. Our findings demonstrate that the tropical forest understory community is more resistant to climatic stressors than expected, especially at the sapling stage. However, early signs of changes in species composition suggest that, in a warming climate with frequent droughts and hurricanes, plant communities might shift over time toward fast-growing or drought-tolerant species. 

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