Tuesday 15 August 2023

Birds & other wildlife in burned forests of the Karst

The Karst region stretching bewteen southwestern Slovenia and northeastern Italy, on the edge of the Mediterranean, has long been characterised by summer wildfires. Due to its limestone terrain with virtually no surface water, the region is prone to prolonged summer droughts and therefore to forest fires. The extensive fire from July 2022 will be remembered as the largest wildfire in Slovenia's history (more details on this event here). The fire affected a total of 4500 hectares, stretching in the transboundary area between Slovenia and Italy. In Slovenia it burned an area of 3707 hectares, of which 2902 hecteres were forests. However most of the burned forest was composed of non-native stands of black pine Pinus nigra (planted extensively throughout the Karst in the 19th century) and areas of native thermophilous scrub of relatively low biodiversity importance (bushes, young trees). Therefore the damage done to the ecosystem is relatively low, compared to the economical damage on the value of timber. Fortunately firefighters also managed to keep the flames away from the numerous villages scattered within the wildfire area.

In spring 2023 we set a research group among colleagues and friends, to monitor birds in the burned trans-boundary area. The first breeding season of censuses is over and hopefully we'll be able to monitor the situation on the study plot also in the next couple of years. Results from this year's census are not yet analysed, but here in this post, we'd like to share some first impressions we noted in the field, in the most severely burned patches of forest. During a recent visit (July 2023) to the northern slopes of Renški vrh/Fajti Hrib/Cerje (above the lower Vipava valley between Miren and Renče) we were amazed by the number of birds using the charred black pines for feeding. Actually the forest that most people would term as "dead" is very much alive!

Burned stands of black pine Pinus nigra on the northern slopes of Renški vrh/Fajti hrib, July 2023.

Along a 3,5 km stretch of forest road we made 5 stops/points where we listened and searched for (mostly) woodpeckers in the most fire-affected patches of forest. The "count" was not very methodological and accurate (also due to the time of visit & season), but still gave a rough idea of the importance of such severely burned forest for the local birds. Every point held at least two Great Spotted Woodpeckers Dendrocopos major, with the highest number of Great Spots counted on one point being four individuals. In the end we counted a total of 12 Great Spotted Woodpeckers which were utilising the burned trees for feeding. In addition we also observed two Black Woodpeckers Dryocopus martius feeding on the charred pines and found many signs of their bill strikes on the bark. Great Spotted and Black are the commonest species of woodpeckers in forests of black pines in the Karst, so their presence was expected, however local aggregations occur, when feeding conditions are particularly favourable, such in the case of a forest stand damaged by fire. Other less common species of woodpeckers expected to utilise this burned forest include Lesser Spotted Dryobates minor, Grey-headed Picus canus and perhaps even Green Woodpecker Picus virids which was heard on site (the latter usually strictly linked to deciduous stands).

The bark removed from a black pine Pinus nigra by a hungry woodpecker.
Feeding signs of woodpeckers on the charred trunks of black pines Pinus nigra.
Black pines Pinus nigra first hit by the fire and then by woodpeckers.
Great Spotted Woodpecker Dendrocopos major (juvenile) feeding on charred black pines Pinus nigra.
Black Woodpecker Dryocopus martius (female) feeding on charred black pines Pinus nigra.

Great Spotted & Black Woodpecker feeding on burned black pines (video).

Apart from woodpeckers there were also lots of other small songbirds feeding in the burned pine stands. Unusual species for this type of forest (if the forest would be unaffected by fire) were the numerous Black Redstarts Phoenicurus ochruros and Spotted Flycatchers Muscicapa striata, both taking advantage of the more open space in the forest created by the fire and the infinity of suitable hunting perches. However the most numerically common were birds in mixed "gangs" such as Great Parus major, Blue Cyanistes caeruleus and Coal Tits Periparus ater, Nuthatches Sitta europaea and Short-toed Treecreepers Certhia brachydactyla, plus locally also larger flocks of Chaffinches Fringilla coelebs feeding on the ground. In some places we also observed Marsh Poecile palustris and Crested Tits Lophophanes cristatus, while among the other interesting birds there was also a Goshawk Accipiter gentilis that took off from a burned stand of pines. The latter is a typical breeder of pine stands in the Karst and is one of the species that could have been affected by the loss of nesting habitat. Many of the observed birds were juveniles, as it's typical for this season. And of course only some of them were cooperative enough to be able to photograph them.

Spotted Flycatcher Muscicapa striata in burned pine forest.
Great Tit Parus major in burned pine forest.
Short-toed Treecreeper Certhia brachydactyla in burned pine forest.

And what were all these birds feeding on in such a "dead" forest? A closer inspection of some burned pine trees revealed that they were alive with bark beetles (Scolytinae), most probably Six-toothed Bark Beetles Ips sexdentatus. Other unusual insects were also present on the trunks, including the impressive shiny-blue bark-gnawing beeetles Temnoscheila caerulea (Trogossitidae) that feed by predating on bark beetles! For more on these interesting beetles see here & better photos here.

Six-toothed Bark Beetles Ips sexdentatus excavating in the bark of a black pine Pinus nigra.
Temnoscheila caerulea (a species of bark-gnawing beetle; family Trogossitidae)

Bark beetle outbreaks usually occur as a consequence of natural disturbances in the forest (fire, windfall, ice storm) that weaken the stands and make the trees more vulnerable to the attacks of various "pests". However, from an ecological point of view, such outbreaks are important events in the forest dynamics. All the invertebrates attacking the dead and dying trees provide an important food source for the local insectivorous birds. These in turn act as natural regulators of the pests' populations. Woodpeckers in particular, such as the Three-toed Woodpecker Picoides tridactylus (absent from the Karst and only found in mountain coniferous forests in Slovenia) are known to respond to such outbreaks by increasing their population (up to 44x) in forests affected by bark beetles. In turn Three-toed Woodpeckers can lower the populations of bark beetles even by 98% (check this article). In North America the Black-backed Woodpecker Picoides arcticus is almost completely dependant on stands of severely burned forest (see here). In Europe we don't really have such a specialised bird associated with burned forest, although woodpeckers clearly benefit from beetle-infested trees and many other species benefit from areas opened by the fire. In the Karst, this is the case for example with Natura 2000 species such as Ortolan Bunting Emberiza hortulana, Nightjar Caprimulgus europaeus, Woodlark Lullula arborea, Red-backed Shrike Lanius collurio and other species of endangered grassland habitats.

It remains to be seen (in the next breeding seasons) if the burned forests are also important as nesting places, or only as foraging areas for the local birds. However studying birds in the burned stands won't be easy, as few places will remain untouched. The Slovenian forestry is already making the most to remove the burned trees as soon as possible and to save the economical value of timber. Therefore now large areas already look like below... 

(why is salvage logging wrong from an ecological point of view in this video).

Salvage logging of the burned black pine forest above the lower Vipava valley near Nova Gorica.


Earlier in spring (May 2023), during our bird censuses on the fire sites, we also came across some exceptional blooms of wildflowers in the more open areas, where the fire burned the thermophilous scrub interspersed with patches of grassland. From the biodiversity point of view this is the most "problematic" habitat in the Karst, as the scrubbing over of once open areas (grasslands & hay meadows) leads to biodiversity impoverishment (lower species richness). In terms of birds, we are losing all those species of conservation priority associated with open habitats that we already mentioned above. At the same time vast areas of impenetrable scrub (with dense bushes of Smoke-tree Cotinus coggygria) or overgrown meadows promote the quick spread of flames in the event of wildfires. Therefore managing open areas with a traditonal, low intensity type of agriculture (extensive grazing & mowing) would be beneficial for biodiversity and for fighting wildfires too!

Dropwort Filipendula vulgaris on the burned ground near Brestovica pri Komnu. May 2023.
Dittany Dictamnus albus on the burned ground near Brestovica pri Komnu. May 2023.
White Laceflower Orlaya grandiflora on burned ground near Brestovica pri Komnu. May 2023.
Bloody Pink Dianthus sanguineus on the burned ground near Brestovica pri Komnu. May 2023.
Purple Mullein Verbascum phoeniceum on the burned ground near Brestovica pri Komnu. May 2023.
Amethyst Eryngo Eryngium amethystinum (with Trichodes apiarius) on Renški vrh in July 2023.

To learn more on the subject of fire ecology and species depending on wildfires, we recommend checking this informative website (and especially the videos) created by Dick Hutto, an emeritus professor in biology and wildlife biology at the University of Montana, who focused most of his research on conifer forests that have been restored by severe wildfires: Fire Ecology Story