​​​​​Fires in the Anthropocene, whether natural or human-induced, are among the main factors of deforestation, threatening forest resilience and biodiversity (Kelly et al. 2020). Fire events have also increased in occurrence and severity worldwide in the past decade (e.g. Whitman et al. 2022, Ribeiro et al. 2020). In this context, we need to better understand the links between fire occurrence and their impact on forest loss, especially in countries where such knowledge is lacking.

The work by Martinez-Batlle addresses this need as it thoroughly describes forest loss and fire patterns across the forests of the Dominican Republic (DR), and systematically tests their spatial and temporal correlations across the DR regions since 2001. To this end, the author combines two independent databases from NASA: the Global Forest Change 2000-2018 data service, and remotely sensed data on fire/hotspot occurrence. The author then provides a state-of-the-art analysis pipeline that first shows significant spatial autocorrelations in both forest loss and fire density over the whole period, and each year across the period. Detailed maps of zonal statistics across hexagonal grids also illustrate clusters of either high or low forest loss and fire points, and distinguish small or large clearings. Second, these spatial dependencies are accounted for in spatial autoregressive models, and congruent patterns of forest loss and fire density are shown across the 2001-2018 period in the DR. This is consistent with the initial working hypothesis of a link between deforestation and slash and burn agriculture. Third, detailed time-series analyses and modelling show common cyclical patterns for forest loss areas in large clearings, number of small clearings, and fire density in the first 14 years, with no increasing trends. In contrast, fire density does not predict extensive forest loss in the eastern half of the country for most years. Finally, yearly maps clearly depict uncontrolled wildfires that impacted larger areas in recent years in both the central and southern mountain ranges of the DR.

This work, therefore, provides a solid, detailed, and rigorous account of the current status of forest loss across the DR, and of its causes, either from recurrent fires due to shifting agriculture or from farming linked to tourism expansion. These results could be very useful for designing strategies adapted to each particular zone of the DR, for preventing human-induced fires or managing wildfires, and for planning post-fire reforestation. This is true, especially for core protected areas where an increasing trend of forest loss is identified in the last 8 years (up to 25% in some mountainous and inaccessible areas of the DR). In those areas, the author suggests implementing a natural regeneration program. Indeed, recent scientists’ warnings stress that fires should be accounted for when planning reforestation for climate change mitigation (Leverkus et al. 2022), with evidence in different ecosystems, that natural regeneration with local seed banks would benefit their post-fire recovery. As proposed by the author, this new knowledge for the DR should also help develop policies for managing forest fires and biodiversity, which are lacking in areas close to tourism facilities. More generally, this study offers methods and graphical representations that are likely to inspire future work with similar databases in other countries where data are scarce, on either spatial trends or temporal evolution of forest cover, or fire activities, or both.

References

Kelly LT, Giljohann KM, Duane A, Aquilué N, et al. (2020). Fire and biodiversity in the Anthropocene. Science, 370(6519), eabb0355. https://doi.org/10.1126/science.abb0355

Leverkus AB, Thorn S, Lindenmayer DB, Pausas JG (2022) Tree planting goals must account for wildfires. Science 376(6593): 588-589. https://doi.org/10.1126/science.abp8259

Martinez Batlle JR (2022) Fire and forest loss in the Dominican Republic during the 21st Century. bioRxiv, 2021.06.15.448604, ver. 4 peer-reviewed and recommended by Peer Community in Forest and Wood Science. https://doi.org/10.1101/2021.06.15.448604

Ribeiro LM, Viegas DX, Almeida M, McGee TX, et al. (2020) 2 - Extreme wildfires and disasters around the world: lessons to be learned. In F. Tedim, V. Leone, T.K. McGee (Eds.), Extreme Wildfire Events and Disasters, Elsevier Inc. 31-pp. 51. https://doi.org/10.1016/B978-0-12-815721-3.00002-3

Whitman et al. (2022) Climate-induced fire regime amplification in Alberta, Canada. Environ. Res. Lett. 17(5): 055003. https://doi.org/10.1088/1748-9326/ac60d6

Range limits of forest species are frequently imposed by spatial gradients in climatic variables. Tolerance to maximum and minimum temperatures, including short-term extremes, and tolerance to desiccation are crucial limiting factors for plant survival and often appear interrelated (Box 1995; Choat et al. 2018). Increasing temperatures, more frequent and extreme droughts and late frost events associated with global climate change will affect the dynamics of forest ecosystems and have the potential to dramatically increase plant mortality and accelerate species range shifts if plants are not able to adapt in situ to the novel climate regimes (Parmesan 2006; Choat et al., 2012). This is particularly important at species range edges, where climatic conditions may not be sufficient to impose mortality on individuals directly, but trees experience greater physiological stress, which influences such factors as dispersal, habitat selection, and subsequent reproductive fitness (Parmesan 2006). In such marginal situations, where gene flow may be also restricted (López de Heredia et al. 2010), the effectiveness of adjustment through natural selection is limited resulting in increased vulnerability to extreme climatic events and to a higher risk of mortality of trees.
Tree responses to drought and frost have been extensively studied at many scales from ecophysiology to molecular biology across a large range of species inhabiting diverse biomes (Sakai and Larcher 1987; Bréda et al. 2006). Avoiding dehydration of tissues to maintain cellular viability and function is at the basis of the plant strategy to deal with both constraints as shown by Charrier et al. (2020). These authors go one step further and discuss the impact of the interaction of drought and frost on tree water status and carbon metabolism with special emphasis on the temporal context. Plants from temperate and boreal regions show changes in their resistance to freezing temperature throughout the year (Bower and Aitken 2006) and xylem becomes more resistant to cavitation with cambial age (Rodríguez-Zaccaro et al. 2019). Including timing in this framework involves incorporating phenology. This will be a fundamental step to model species distribution limits in the face of climate change since most observations of climate-change responses have involved alterations of species’ phenologies (Parmesan et al. 2006). For example, the onset of the growing season of trees in temperate Europe is 2.3 days ahead per decade during the last 40 years (Parmesan et al. 2006). Moreover, some studies have shown that climatic constraints limit species distribution mainly because of their impact in phenology rather than their impact on drought and frost mortality (Morin et al. 2007). However, longer growing seasons along with more frequent extreme events increase the probability of long-lived organisms such as trees to experience frosts and drought during the same growing season or one of them after uncompleted recovery of the other in some latitudes. Over the last decade much attention has been devoted to the recovery of growth and ecological function after stressful events (Lloret et al. 2011). Some dendroecological studies have shown for example that resilience to extreme droughts might be constrained by having experienced more frequent droughts, thus exceeding the potential for acclimation of the tree (Bose et al. 2020). Drought can result in chronic hydraulic impairment which can last moths to years (Anderegg et al. 2015) and for some species also in cavitation fatigue, i.e. a progressive increase in vulnerability to cavitation (Hacke et al. 2001), thus increasing vulnerability to subsequent drought and frost events.
Models have proved to be useful tools for synthetizing and integrating climate and soil properties with key functional traits in order to determine desiccation dynamics, carbon metabolism and plant survival during drought of frost (Martin-StPaul et al., 2017; Charrier et al. 2018; Blackman et al., 2019). However, models are currently limited by gaps in our understanding of the fundamental physiological mechanisms that constrain species ranges. Most common approaches to studying species range shifts are related to climatic niches and overlook the processes and traits involved in drought or frost tolerance (Cheaib et al. 2012). Process-based models based on plant hydraulics seem promising providing the link between environmental cues and plant responses, although disregarding carbon metabolism will not give us predictive understanding of system changes such as those due to climate fluctuations (Mackay et al. 2015). New modeling approaches need to be developed not only for better drought prediction performance but for the interaction of drought with other factors. Charrier et al. (2020) offer a framework for improving process-based models with the aim to provide better prediction of carbon and water economy, organ development and ultimately species distribution limits in the face of warmer winters and more frequent winter droughts at high altitudes and late frosts events.

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Blackman CJ, Li X, Choat B, Rymer PD, De Kauwe MG, Duursma RA, Tissue DT, Medlyn BE (2019) Desiccation time during drought is highly predictable across species of Eucalyptus from contrasting climates. New Phytologist 224: 632-643. doi: https://doi.org/10.1111/nph.16042
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