Birds and the Fire Cycle in a Resilient Mediterranean Forest: Is There Any Baseline?

This study investigates the effects of recurrent wildfires on the resilience of a typical Mediterranean ecosystem. It is based on uninterrupted monitoring over 42 years of the avifauna in a cork oak forest that burned three times during this time interval. The monitoring involved two line-transect counts in spring accompanied by the simultaneous and independent estimation of the vegetation cover profile. One of the two transects was initially designed to serve as an unburned control before it also burned during the second fire. Many forest bird species were already present from the first spring postfire due to the rapid regeneration of the canopy. Some open-habitat bird species colonized the burned area during the first 2–4 years after the fire, resulting in an initial phase of high diversity. The postfire bird succession was mainly driven by sedentary species that recolonized the burned area after the first winter, whereas most migratory species present before the fire resettled as early as the first postfire spring, probably because of site tenacity. It was found that the impact of the second fire on avifauna was lower than that of the first or third fire. The return to an avifauna and forest structure successionally equivalent to the prefire control was achieved in about 15 years, which can be considered as the recovery time. Afterwards, both vegetation and avifauna in the burned areas tended to take on more forest characteristics than in the prefire control. These findings suggest that: (i) the recurrence of fire does not necessarily result in the cumulative degradation of the ecosystem at each repetition; (ii) the asymptotic resilience model is not adapted to the case of disturbances in non-mature environments; (iii) the notion of returning to an original undisturbed baseline is illusive in an area that has been under continuous human influence since ancient times.

Fuel connectivity, burn severity, and seedbank survivorship drive the grass fire cycle in a semi-arid shrubland.

Introduced grasses can initiate novel grass-fire cycles that alter ecosystem structure and function, and threaten biodiversity. In sagebrush communities in the western United States, annual grass invasion increases the connectivity of fine fuels, which increases the size and spatial contiguity of fires. This increase in fire size and contiguity results in post-fire plant communities that are dominated by introduced annual grasses (IAG), which are themselves more likely to promote large fires and initiate a novel grass-fire cycle. But the mechanisms by which pre-fire invasion and fire occurrence are linked to higher post-fire flammability are not fully understood. Here, we investigate the successive mechanisms in a potential positive feedback that maintains the novel annual grass-fire cycle. We used total vegetation cover (TVC) as a proxy for fuel connectivity and found that pre-fire TVC increased burn severity. We then used a Bayesian joint species distribution model to examine how burn severity affected the proportion of IAG in the seed bank, and found that higher burn severity had mostly positive or neutral effects on the occurrence of IAG and other non-native species, and mostly negative or neutral relationships with native species. We found that the abundance of IAG seeds in the seedbank immediately post-fire had a positive effect on the fuel connectivity 3 years after fire, thus completing a positive feedback promoting IAG. These results suggest that measurable characteristics of ecosystem structure (e.g. TVC) and fire (dNBR) may be used to inform management actions to mitigate the negative effects of the grass-fire cycle, perhaps via targeted restoration applications or pre-fire fuel treatments.

A 500-year history of forest fires in Sala area, central Sweden, shows the earliest known onset of fire suppression in Scandinavia

The Sala fire in the Västmanland County of central Sweden that burned about 14,000 ha in 2014 has been the largest fire recorded in the modern history of Sweden. To understand the long-term fire history of this area, we dendrochronologically dated fire scars on Scots pine (Pinus sylvestris L.) trees (live and deadwood) to reconstruct the fire cycle and fire occurrence in the area affected by the 2014 fire. We identified 64 fire years, using a total of 378 pine samples. The earliest reconstructed fire dated back to 1113 AD. The spatial reconstruction extended over the period of 1480–2018 AD. Lower levels of fire activity (fire cycle, FC = 43 years, with the central 90% of the distribution limited by 35 to 57 years) dominated in the earlier period (1480–1690 AD) that was followed by a strong decrease in fire activity since 1700 (FC = 403 years, with 90% of the distribution being within 149 to 7308 years), with a fire-free period between 1756 and 2014. Sala area, therefore, features the earliest known onset of fire suppression in Scandinavia. The high demand for timber during the peak in mining activities in the study area around the 1700–1800s, accompanied by passive fire suppression policies, were possibly the main drivers of the decline in fire activity. Superposed epoch analysis (SEA) did not show significant departures in the drought proxy during the ten years with the largest area burned between 1480 and1690. It is unclear whether the result is due to the relatively small area sampled or an indication that human controls of fires dominated during that period. However, significant departures during the following period with low fire activity (1700–1756), which just preceded the last fire-free period, suggested that the climate became an increasingly important driver of fire during the onset of the suppression period. We speculate that the lack of major firebreaks, the homogenization of forests, and the lack of burned areas with low fuel loads might contribute to the occurrence of the exceptionally large 2014 fire in Sala.

Influence of Satellite Sensor Pixel Size and Overpass Time on Undercounting of Cerrado/Savannah Landscape-Scale Fire Radiative Power (FRP): An Assessment Using the MODIS Airborne Simulator

The fire radiative power (FRP) of active fires (AFs) is routinely assessed with spaceborne sensors. MODIS is commonly used, and its 1 km nadir pixel size provides a minimum per-pixel FRP detection limit of ~5–8 MW, leading to undercounting of AF pixels with FRPs of less than around 10 MW. Since most biomes show increasing AF pixel frequencies with decreasing FRP, this results in MODIS failing to detect many fires burning when it overpasses. However, the exact magnitude of the landscape-scale FRP underestimation induced by this type of AF undercounting remains poorly understood, as does its sensitivity to sensor pixel size and overpass time. We investigate these issues using both 1 km spaceborne MODIS data and 50 m MODIS Airborne Simulator (MAS) observations of the Brazilian cerrado, a savannah-like environment covering 2 million km2 (>20%) of Brazil where fires are a frequent occurrence. The MAS data were collected during the 1995 SCAR-B experiment, and are able to be spatially degraded to simulate data from sensors with a wide variety of pixel sizes. We explore multiple versions of these MAS data to deliver recommendations for future satellite sensor design, aiming to discover the most effective sensor characteristics that provide negligible pixel-area related FRP underestimation whilst keeping pixels large enough to deliver relatively wide swath widths. We confirm earlier analyses showing 1 km MODIS-type observations fail to detect a very significant number of active fires, and find the degree of undercounting gets worse away from the early afternoon diurnal fire cycle peak (~ 15:00 local time). However, the effect of these undetected fires on the assessment of total landscape-scale FRP is far less significant, since they are mostly low FRP fires. Using two different approaches we estimate that the MODIS-type 1 km data underestimates landscape scale FRP by around a third, and that whilst the degree of underestimation worsens away from the diurnal fire cycle peak the effect of this maybe less important since there are far fewer fires present. MAS data degraded to a 200 m spatial resolution provides landscape-scale FRP totals almost indistinguishable from those calculated with the original 50 m MAS observations, and still provides a pixel size consistent with a wide swath imaging instrument. Our work provides a potentially useful guide for future mission developers aiming at active fire and FRP applications, and we conclude that such missions need operate at spatial resolutions no higher than 200 m if they rely on cooled, low-noise IR detectors. Further work confirming this for fire-affected biomes beyond the savannah-type environments studied here is recommended.

Drought legacy effects on post-fire vegetation dynamics in a Mediterranean shrubland of Central Spain

<p>Post-fire regeneration often occurs under water scarcity conditions in Mediterranean ecosystems. This fact is likely to be more frequent in the future, as climate change projections for the Mediterranean show increased temperature and decreased rainfall, as well as higher fire weather danger. Therefore, studying how vegetation responds to fire and drought is critical for anticipating vegetation vulnerability to global warming. Here, we present the results of a manipulative field study in a Mediterranean shrubland of Central Spain in which 4 drought treatments were imposed before and after carrying out an experimental burning of the treated plots: natural rainfall, historical rainfall regime (2 months summer drought per year), moderate drought (ca. 25% rainfall reduction from historical records, 5 months drought), and severe drought (ca. 50% rainfall reduction, 7 months drought). We monitored the plant community during the first 4 years after fire under the drought treatments followed by additional 6 years once such treatments had ceased. We found that drought significantly reduced density, cover, and size of seeder species during the first post-fire years under treatment. In contrast, resprouter species were virtually unaffected. As a whole, the changes in woody species dynamics resulted in a community ‘herbalization’, which was richer and more diverse, but probably more prone to a new fire. Moreover, we found that the drought effects on the community, and especially on the seeders, were maintained various years after all plots started receiving natural rainfall. 10 years after the fire, the legacy effect of the drought had been diluted, although some effects on plant density or cover in certain seeder species were still present. Overall, this means that what happens during the first few years after fire is extremely important for vegetation recovery, and that the effects of a long drought can persist over various years, and likely over a whole fire cycle.</p>

Inter-comparison of four operational satellite Fire Radiative Power (FRP) products: A spatial and temporal consistency assessment.

<p> We inter-compare four remotely sensed Fire Radiative Power (FRP) products, the polar-orbiter products derived from active fires detected using the <span>Moderate Resolution Imaging Spectroradiometer data </span>(MCD14ML) and VIIRS (VNP14ML and VNP14IMGML), and geostationary products derived from data collected by Meteosat’s <span>Spinning Enhanced Visible and Infrared Imager (the LSA-SAF FRP-PIXEL product). We focus on seven years of data (January 2012 to December 2018), and </span>using the ability of the geostationary product to capture the daily fire cycle we quantify for each polar-orbiter FRP product the proportion of daily fire energy release that they capture and that which they miss, and also identify the areas where their overpass times successfully capture the diurnal fire activity peak, and where they do not. In addition, by analysing <span>frequency density (f-D) distributions of FRP at a 0.5° grid cell resolution we evaluate </span>each products minimum FRP detection limit, which typically precludes detection of a proportion of the highly numerous but individually relatively small and/or low intensity fires.<span> R</span><span>esults are summarized by biome type based on the ESA CCI Land Cover product. </span>Our inter-comparison allows for the identification and quantification of some of the key non-fire effects causing FRP underestimation in satellite FRP products: pixel size, pixel area growth off-nadir, and the low temporal resolution of polar-orbiting sensors. Our results and the methodology developed herein should serve to evaluate and cross-calibrate FRP estimates obtained by the future Copernicus Climate Change Services (C3S) FRP products, which initially at least will be based only on SLSTR data collected by the Sentinel-3 satellite.</p>

Invasive grasses increase fire occurrence and frequency across US ecoregions

Fire-prone invasive grasses create novel ecosystem threats by increasing fine-fuel loads and continuity, which can alter fire regimes. While the existence of an invasive grass-fire cycle is well known, evidence of altered fire regimes is typically based on local-scale studies or expert knowledge. Here, we quantify the effects of 12 nonnative, invasive grasses on fire occurrence, size, and frequency across 29 US ecoregions encompassing more than one third of the conterminous United States. These 12 grass species promote fire locally and have extensive spatial records of abundant infestations. We combined agency and satellite fire data with records of abundant grass invasion to test for differences in fire regimes between invaded and nearby “uninvaded” habitat. Additionally, we assessed whether invasive grass presence is a significant predictor of altered fire by modeling fire occurrence, size, and frequency as a function of grass invasion, in addition to anthropogenic and ecological covariates relevant to fire. Eight species showed significantly higher fire-occurrence rates, which more than tripled for Schismus barbatus and Pennisetum ciliare. Six species demonstrated significantly higher mean fire frequency, which more than doubled for Neyraudia reynaudiana and Pennisetum ciliare. Grass invasion was significant in fire occurrence and frequency models, but not in fire-size models. The significant differences in fire regimes, coupled with the importance of grass invasion in modeling these differences, suggest that invasive grasses alter US fire regimes at regional scales. As concern about US wildfires grows, accounting for fire-promoting invasive grasses will be imperative for effectively managing ecosystems.