Forest Structure Drives Fuel Moisture Response across Alternative Forest States

Climate warming is expected to increase fire frequency in many productive obligate seeder forests, where repeated high-intensity fire can initiate stand conversion to alternative states with contrasting structure. These vegetation–fire interactions may modify the direct effects of climate warming on the microclimatic conditions that control dead fuel moisture content (FMC), which regulates fire activity in these high-productivity systems. However, despite the well-established role of forest canopies in buffering microclimate, the interaction of FMC, alternative forest states and their role in vegetation–fire feedbacks remain poorly understood. We tested the hypothesis that FMC dynamics across alternative states would vary to an extent meaningful for fire and that FMC differences would be attributable to forest structural variability, with important implications for fire-vegetation feedbacks. FMC was monitored at seven alternative state forested sites that were similar in all aspects except forest type and structure, and two proximate open-weather stations across the Central Highlands in Victoria, Australia. We developed two generalised additive mixed models (GAMMs) using daily independent and autoregressive (i.e., lagged) input data to test the importance of site properties, including lidar-derived forest structure, in predicting FMC from open weather. There were distinct differences in fuel availability (days when FMC < 16%, dry enough to sustain fire) leading to positive and negative fire–vegetation feedbacks across alternative forest states. Both the independent (r2 = 0.551) and autoregressive (r2 = 0.936) models ably predicted FMC from open weather. However, substantial improvement between models when lagged inputs were included demonstrates nonindependence of the automated fuel sticks at the daily level and that understanding the effects of temporal buffering in wet forests is critical to estimating FMC. We observed significant random effects (an analogue for forest structure effects) in both models (p < 0.001), which correlated with forest density metrics such as light penetration index (LPI). This study demonstrates the importance of forest structure in estimating FMC and that across alternative forest states, differences in fuel availability drive vegetation–fire feedbacks with important implications for forest flammability.

Indirect contributions of global fires to surface ozone through ozone–vegetation feedback

Fire is an important source of ozone (O3) precursors. The formation of surface O3 can cause damage to vegetation and reduce stomatal conductance. Such processes can feed back to inhibit dry deposition and indirectly enhance surface O3. Here, we apply a fully coupled chemistry–vegetation model to estimate the indirect contributions of global fires to surface O3 through O3–vegetation feedback during 2005–2012. Fire emissions directly increase the global annual mean O3 by 1.2 ppbv (5.0 %) with a maximum of 5.9 ppbv (24.4 %) averaged over central Africa by emitting a substantial number of precursors. Considering O3–vegetation feedback, fires additionally increase surface O3 by 0.5 ppbv averaged over the Amazon in October, 0.3 ppbv averaged over southern Asia in April, and 0.2 ppbv averaged over central Africa in April. During extreme O3–vegetation interactions, such a feedback can rise to >0.6 ppbv in these fire-prone areas. Moreover, large ratios of indirect-to-direct fire O3 are found in eastern China (3.7 %) and the eastern US (2.0 %), where the high ambient O3 causes strong O3–vegetation interactions. With the likelihood of increasing fire risks in a warming climate, fires may promote surface O3 through both direct emissions and indirect chemistry–vegetation feedbacks. Such indirect enhancement will cause additional threats to public health and ecosystem productivity.

Indirect contributions of global fires to surface ozone through ozone-vegetation feedback

Fire is an important source of surface ozone (O3), which causes damage to vegetation and reduces stomatal conductance. Such processes can feed back to inhibit dry deposition and indirectly enhance surface O3. Here, we apply a fully coupled chemistry-vegetation model to estimate the indirect contributions of global fires to surface O3 through O3-vegetation feedback during 2005–2012. Fire emissions directly increase the global mean annual O3 by 1.2 ppbv (5.0 %) with a maximum of 5.9 ppbv (24.4 %) averaged over central Africa by emitting substantial number of precursors. Considering O3-vegetation feedback, fires additionally increase surface O3 by 0.5 ppbv averaged over the Amazon in October, 0.3 ppbv averaged over southern Asia in April, and 0.2 ppbv averaged over central Africa in April. During extreme O3-vegetation interactions, such feedback can rise to > 0.6 ppbv in these fire-prone areas. Moreover, large ratios of indirect-to-direct fire O3 are found in eastern China (3.7 %) and the eastern U.S. (2.0 %), where the high ambient O3 causes strong O3-vegetation interactions. With likelihood of increasing fire risks in a warming climate, fires may promote surface O3 through both direct emissions and indirect chemistry-vegetation feedbacks. Such indirect enhancement will cause additional threats to public health and ecosystem productivity.

Vegetation forcing modulates global land monsoon and water resources in a CO2-enriched climate

The global monsoon is characterised by transitions between pronounced dry and wet seasons, affecting food security for two-thirds of the world’s population. Rising atmospheric CO2 influences the terrestrial hydrological cycle through climate-radiative and vegetation-physiological forcings. How these two forcings affect the seasonal intensity and characteristics of monsoonal precipitation and runoff is poorly understood. Here we use four Earth System Models to show that in a CO2-enriched climate, radiative forcing changes drive annual precipitation increases for most monsoon regions. Further, vegetation feedbacks substantially affect annual precipitation in North and South America and Australia monsoon regions. In the dry season, runoff increases over most monsoon regions, due to stomatal closure-driven evapotranspiration reductions and associated atmospheric circulation change. Our results imply that flood risks may amplify in the wet season. However, the lengthening of the monsoon rainfall season and reduced evapotranspiration will shorten the water resources scarcity period for most monsoon regions.

Flammability Characteristics of Surface Fuels in a Longleaf Pine (Pinus palustris Mill.) Woodland

To investigate fuel flammability, we quantified burning characteristics of 21 fuel categories in a longleaf pine (Pinus palustris Mill.) woodland in central Alabama, USA. Litter was burned under controlled laboratory conditions. Flammability characteristics, including resistance to ignition, flaming duration, smoldering duration, maximum flame height, and percent consumption, were measured. The fuels were hierarchically clustered into five groups of similar flammability characteristics that explained 89% of the variance. Percent consumption and maximum flame height values ranged from 7% ± 1 standard errors (SE) and 12 cm ± 1 SE for the low flammability group (bark and charcoal), to 86% ± 1 SE and 62 cm ± 3 SE for the high flammability group (bracken fern (Pteridium latiusculum (Desv.) Hieron. ex R.E.Fr. = {syn: P. aquilinum}), grass, and fire-facilitating oak (Quercus spp.) leaves). Results support observed flammability differences between fuel types such as oak and pine (Pinus spp.) litter, and duff, and provide a previously unquantified comparison of surface fuels comprehensive of a longleaf pine community. Further, clustering analysis indicated that plant species that become abundant post-disturbance may help maintain fire-vegetation feedbacks in the absence of pine litter. Understanding flammability characteristics of surface fuels may further inform prescribed fire application in stands where fuels have been altered.

Vegetation feedbacks during drought exacerbate ozone air pollution extremes in Europe

&lt;p&gt;This study highlights a previously under-appreciated &amp;#8220;climate penalty&amp;#8221; feedback mechanism - namely, substantial reductions of ozone uptake by water stressed vegetation &amp;#8211; as a missing piece to the puzzle of why European ozone pollution episodes have not decreased as expected in recent decades, despite marked reductions in regional emissions of ozone precursors due to regulatory changes. The most extreme ozone pollution episodes are linked to heatwaves and droughts, which are increasing in frequency and intensity over Europe, with severe impacts on natural and human systems. Under drought stress, plants close their stomata to reduce water loss, consequently limiting the ozone uptake by vegetation (a component of dry deposition), leading to increased surface ozone concentrations. Such land-biosphere feedbacks are often overlooked in prior air quality projections, owing to a lack of process-based model formulations. Here, we use six decades of observations and Earth system model simulations (1960-2018) with an interactive dry deposition scheme to show that declining ozone removal by water-stressed vegetation in the warming climate exacerbate ozone air pollution over Europe. Incorporated into a dynamic vegetation land &amp;#8211; atmospheric chemistry &amp;#8211; climate model, the dry deposition scheme mechanistically describes the response of ozone deposition to atmospheric CO&lt;sub&gt;2&amp;#160;&lt;/sub&gt;concentration, canopy air vapor pressure deficit, and soil water availability. Our observational and modeling analyses reveal drought stress causing as much as 70% reductions in ozone removal by forests. Reduced ozone removal by water-stressed vegetation worsens peak ozone episodes during European mega-droughts, such as the 2003 event, offsetting much of the air quality improvements gained from regional emission controls. Accounting for vegetation feedbacks leads to a three-fold increase in high surface ozone events above 80 ppbv (8-hour average) and a 20% increase in the sensitivity of ozone pollution extremes (95&lt;sup&gt;th&amp;#160;&lt;/sup&gt;percentile) to increasing temperature. As the frequency of hot and dry summers is expected to increase in the coming decades, this ozone climate penalty could be severe and therefore needs to be considered when designing clean air policy in the European Union.&amp;#160;&lt;/p&gt;&lt;p&gt;Notes: This study is currently under review for possible publication in Nature Climate Change.&amp;#160;&lt;/p&gt;

Dispersal limitation and fire feedbacks maintain mesic savannas in Madagascar

Madagascar is regarded by some as one of the most degraded landscapes on Earth, with estimates suggesting that 90% of forests have been lost to indigenous Tavy farming. However, the extent of this degradation has been challenged: paleoecological data, phylogeographic analysis, and species diversity maps indicate that pyrogenic savannas in Central Madagascar pre-date human arrival, even though rainfall is sufficient to allow forest expansion into Central Madagascar. These observations raise a question—if savannas in Madagascar are not anthropogenic, how then are they maintained in regions where the climate can support forest? Observation reveals that the savanna-forest boundary coincides with a dispersal barrier—the escarpment of the Central Plateau. Using a stepping-stone model, we show that in a limited dispersal landscape, a stable savanna-forest boundary can form due to fire-vegetation feedbacks. This novel phenomenon, referred to as range pinning, could explain why eastern lowland forests have not expanded into the mesic savannas of the Central Highlands. This work challenges the view that highland savannas in Madagascar are derived by human-lit fires and, more importantly, suggests that partial dispersal barriers and strong non-linear feedbacks can pin biogeographical boundaries over a wide range of environmental conditions, providing a temporary buffer against climate change.