Cultivated trees introduced from well beyond the range of our yards and parks require care that honors the evolutionary history and ecological context of the parent species'. I recently was introduced the Bosnian pine and could not diagnose the cause of needle browning. I'm still working on that part actually. My suspicion it is a response to transplant into a humid habitat. I could not find perithecia or other signs of fungal sporulation but that doesn't mean they are not present. I'll keep looking especially after a weeks worth of heavy rains. Here's the post I hope to get published in the Conifer Society's newsletter. Hopefully, it helps you in your care of cultivated plants.
Bosnian Pine Ecology as a Guide to Care
The Bosnian pine, Pinus heldreichii (H. Christ, 1863), syn. P. leucodermis (Ant. 1864) is a mountain pine found at high elevations throughout the Mediterranean Basin. The phylogeny of P heldreichii with P. leucodermis remains unresolved. What this means in terms of ecology is that the speciation event leading to some thriving in Italy while others are primarily Balkan, is unknown. Some define these as distinct species others as ecotypes. Regardless, the care of these pines is going to reflect their habitat types. We can use habitat information for the areas these trees evolved and reside in to determine the best care for them in our landscapes. Btw, for those with an interest in plant genetics I will add a bit more information at the end.
So, let’s explore their habitats. P. heldreichii is found in Albania and Greece at elevations of 1000-2500m. Dry, sunny, subalpine habitats (reaching the alpine timberline) are found on steep, dry, and rocky slopes. Neutral – alkaline soils that are a gravely clay loam and well drained on slopes enjoy high hummus accumulation on flatter terrains. These soils, despite relatively high pH substrate have a high cation exchange capacity due to diverse microbial assemblages. Microbial diversity is a consequence of pH but more so a consequence of high plant diversity. The climate during the growing season enjoys averages of 12C (54F) an annual rainfall of 150 – 300mm (5.9-11.8in). Winter temperatures remain below freezing for extended periods of time, but snow accumulation is low.
P leucodermis is a central to western Balkan peninsula tree. Enjoying a broader range from Bosnia to Serbia down to southern Italy at elevations of 900-2300m. Thus, the tolerance for temperature extremes is likely to be higher in this ecotype/species. Climate regimes are typified by avg annual temperatures of 6.8C (45F) with 12.C (45F) averages during the growing season; not too different between the trees. Annual rainfall during the growing season averages 702mm (28in). The geological formations leading to dominant soil types are complex. Thus, soil types are rich and provide for diverse forest plant communities. Because P leucodermis is found primarily on slopes, relatively lower humus concentration. However, note the area overall is dominated by a thick well-developed humus accumulation. The mountainous areas of its geographic range are typified by an acid brown soil (Dystric Cambisol) with a crumbly structure. Thus, soil aeration is high and water holding capacity good. Clay content is low, pH low, and low nutrient retention due to both pH, and preference for sloped terrain. The importance of a robust microbial community with which plants can mine and consume scare resources is likely high.
Both trees are resistant to pollution (e.g., NOx and sulfur dioxides), can withstand extremes in wind, snow/ice, and are pest resistant. They are also resistant to needle blights. However, this resistance assumes the trees are living in habitats with long, winters, high UV exposure, and low humidity.
Based on the above information, I recommend growing the trees with the following conditions when possible. Habitats that have at least two-weeks of below freezing temps in the winter, with good rainfall during the growing season are likely idea. Freezing temps are important for metabolic slowing in plants resulting in improved immunity during the growing season. Freezing is also important for killing of pests and the reduction of pest population densities during the growing season. Planting your trees with biofertilization will be important as well as ensuring they are part of a diverse habitat. Lots of different types of plants in your landscape will increase the microbial diversity and the likelihood these trees will have microbes they need to thrive. Probably they can accommodate a broad soil pH. If you start to see nutrient deficiency signs (needle loss, pale needles, or slow growth rate) shifting alkaline soils to a lower pH will benefit the trees and their microbial partners. Clay soils should be amended to increase soil porosity (reduce bulk density) and water filtration. Fertilization is not recommended, if fertilizer is applied, slow release is best or better yet, garden compost. These trees are not tolerant of humidity and may contract diseases they would otherwise be resistant to in moist habitats. Growing with lots of UV and wind exposure will help to keep their microclimate drier.
Plant pests include aphids, sawflies, and the possibility of fungal infections due to growth in inhospitable habitats.
Back to genetics:
Recent molecular analysis supports separation of the two species 15 000 years ago from a single maternal population. This is called allopatric speciation and is in response to geographic or spatial separation eventually leading to infertile hybrids (biological definition of speciation). For those remembering genetics class this is akin to the squirrel speciation resulting from the formation of the Grand Canyon. Both species are RED listed (IUCN threatened species) due to loss of land/habitat. Land Use Change is a leading cause of biodiversity losses world-wide and contributes to genetic bottlenecks that exacerbate population declines. The trees are showing poor genetic diversity in situ contributing to their decline.
Conservation through parks and habitat preserves are one way to counter continued biodiversity loss, as is using indigenous flora in yards, reducing the use of turf as a dominant habitat plant in urban and rural areas, increasing plant diversity in yards and urban greenways, and creating contiguous hedgerow habitats for wildlife conservation.
Bosnian Pine; Technical guidelines for genetic conservation and Use, Euforgen, www.biodiverstiyinternational.org.
Eremija et al. 2014. Arch. Biol Sci., Belgrade, 66, 299-306.
Kapovic et al. 2011. Bulletin of the Faculty of Forestry, 104, 71-80.
Farrer et al. 2019. Front. Microbiology. https://doi.org/10.3389/fmicb.2019.02887
Jankovic M. 1986. Serbian Academy of Sciences and Arts.