Scientists are investigating why some Scots pine trees are resistant to drought stress and insect pests

Scots pine forest stands in the Czech Republic face extreme natural conditions, including hot temperatures, uneven and low rainfall, and pest infestations. These factors can weaken the trees, often leading to their dying.

In Czech forests, Scots pine is the second most widespread tree species, accounting for 16% of the total tree population and growing across an area of 418 340 hectares of forest land. This species prefers bright, sunny environments and is highly intolerant of shade, except for certain mountain ecotypes. In natural stands, Scots pine is often outcompeted on more fertile soils by tree species that can tolerate shading better.

Scots pine primarily grows in its natural range on shallow, well-drained, poor sandy to stony soils, but it can also thrive on serpentines and limestones. This species typically struggles to grow in swampy or peaty soils, often resulting in stunted growth. As a result, Scots pine often becomes the dominant or even the only tree species in areas where other trees cannot survive.

Its ability to access water from deeper sources allows it to thrive even in extremely dry environments. However, since 2015, several periods of extremely high summer temperatures have been accompanied by significant precipitation deficits and declining groundwater levels. Consequently, pine forests have been weakened and are increasingly susceptible to various fungal pathogens and bark beetles, including the pine bark beetle (Ips acuminatus), the engraver beetle (Ips sexdentatus), and the pine beetle (Phaenops cyanea).

Under adverse conditions caused by abiotic and biotic factors, plants activate defense mechanisms that lead to physiological and molecular changes.

One of the main strategies that plants use to adapt to stress and cope with environmental changes is the precise regulation of gene expression. This regulation is linked to various molecular processes. The defense mechanisms involve genes related to plant stress tolerance, which encode proteins and enzymes that protect cells from stress.

(* Gene expression is the process through which the genetic information encoded in a specific gene is “translated” into a particular function.)

Currently, there are Scots pine stands where some trees are heavily infested with bark beetles while others show no signs of infestation. This raises the question: what accounts for the resistance of the healthy trees compared to those affected by the pests?

Scientists from the Department of Forest Tree Species Biology and Breeding at FGMRI (VÚLHM, v. v. i.), investigated a critical question as part of their research project NAZV QK22020062 titled „Identifikace přeživších jedinců lesních dřevin na kalamitních plochách, jejich záchrana a výzkum jejich rezistence“ (Identifying surviving forest trees in disaster areas, rescuing them, and researching their resistance.). They published their initial findings in an article Využití molekulárních analýz pro genetickou charakterizaci rezistentních jedinců borovice lesní (The use of molecular analyses for the genetic characterization of resistant Scots pine individuals), which appeared in the journal Reports of Forestry Research issue 4/2024.

The issue of weakening forest stands due to dry periods in previous years and subsequent attacks by bark beetles is receiving significant attention. The scientists’ research presents the extent to which defense mechanisms are activated in response to biotic damage, using molecular analyses of healthy and damaged Scots pine individuals located at two sites in Central Bohemia.

The starting material for the molecular analyses consisted of freshly sprouted Scots pine needles. Samples were collected from two locations of interest: Stará Boleslav and Podbrahy, which are approximately four kilometers apart. The researchers selected healthy trees that showed no signs of damage from bark beetles, as well as trees that had been damaged by bark beetle feeding. Only uninfested trees were used as controls, while the “stressed” group consisted of trees exhibiting a maximum density of 0.15–0.2 visible signs of the pest (e.g., holes) per square decimeter. At both locations, pine beetles predominantly affected the damaged trees. In June 2023, the sampling included five healthy and five damaged trees at both locations.

The scientists discovered that the damaged groups in Stará Boleslav and Podbrahy exhibited similar trends in the relative expression of seven genes related to the stress response primarily caused by the pine beetle.

At the Stará Boleslav locality, scientists observed a notable increase in the relative expression levels of the phenylalanine ammonia lyase (PAL) gene in a group of damaged Scots pine individuals compared to a control group. The activity of the PAL gene is linked to secondary plant metabolism, particularly in the biosynthesis pathways of lignin and flavonoids, which are part of the phenylpropanoid pathway. These initial findings regarding the regulation of gene activity in the studied Scots pine individuals are significant because the samples were collected directly from Scots pine stands that are continuously impacted by several factors affecting their growth and development. Additionally, there is considerable variability among the samples within the groups of both healthy and damaged individuals.

The paper Využití molekulárních analýz pro genetickou charakterizaci rezistentních jedinců borovice lesní (The use of molecular analyses for the genetic characterization of resistant Scots pine individuals) can be downloaded here.

Authors: Eva Pokorná, Pavlína Máchová, Johana Pechačová, Adam Véle, Martin Fulín; VÚLHM, v. v. i., e-mail: pokorna@vulhm.cz

Prepared according original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz

Illustrative photos: Damaged pine forests in Central Bohemia, author Jan Řezáč