Olive Trees Combat Air Pollution, New Research Shows

Olives are among the tree species that can best con­tribute to clean­ing the air, accord­ing to the results of VegPM, a research project coor­di­nated by the University of Florence. This project aimed to iden­tify the most suit­able indige­nous plants for com­bat­ing air pol­lu­tion caused by par­tic­u­late mat­ter (PM).

Launched in 2020 and sup­ported with 180.000 euros by the Cassa di Risparmio di Lucca Foundation, the VegPM project gath­ered data from four Italian munic­i­pal­i­ties in Tuscany affected by high lev­els of fine par­ti­cles: Lucca, Porcari, Capannori and Altopascio. In addi­tion to olive trees, the research team revealed that Laurel (Laurus nobilis), Privet (Ligustrum), Oleander (Nerium ole­an­der), Magnolia (Magnolia gran­di­flora) and Cherry lau­rel (Prunus lau­ro­cera­sus) could also improve air qual­ity.

Particulate mat­ter is a mix­ture of solid and liq­uid par­ti­cles – organic and inor­ganic. These par­ti­cles are dis­persed in the air and are highly dan­ger­ous for human health. Road traf­fic is the pri­mary PM source, but heat­ing sys­tems, waste man­age­ment and agri­cul­ture can also cause an excess of PM.

Particles are com­monly clas­si­fied by their diam­e­ter into three cat­e­gories: ​“coarse” (PM10), ​“fine” (PM2.5) and ​“ultra­fine” (PM0.2). The size of the par­ti­cles deter­mines how they affect the res­pi­ra­tory sys­tem and enter the blood­stream.

Prolonged expo­sure to PM10, in par­tic­u­lar, can cause severe effects, such as car­dio­vas­cu­lar and res­pi­ra­tory dis­com­fort, chronic aller­gies, and even pre­ma­ture mor­tal­ity in chil­dren. Epidemiological stud­ies have also shown that the prox­im­ity of busy roads is asso­ci­ated with chronic res­pi­ra­tory dis­eases in chil­dren and the elderly. In an urban envi­ron­ment, these harm­ful effects can be inten­si­fied by dan­ger­ous heavy met­als caused by expo­sure to oils, tires, fuel, metal­lic paints and waste.

Therefore, devel­op­ing prac­ti­cal mit­i­ga­tion actions is one of the most crit­i­cal chal­lenges for local gov­ern­ments. Many munic­i­pal­i­ties have looked into design­ing effec­tive ​“urban forests.” These require appro­pri­ate plant species to be planted along road­ways or near highly pol­luted areas. For this new strat­egy to work, the plants cho­sen should respond to water stress and con­tain CO2 lev­els.

The VegPM project makes the entire area encom­pass­ing Lucca, Porcari, Capannori and Altopascio – about 100 km² – the inno­v­a­tive urban green model test­ing loca­tion. Lucca, Porcari, Capannori and Altopascio have the high­est con­cen­tra­tions of PM10, nitro­gen diox­ide and ozone in all of Tuscany, accord­ing to the yearly regional air qual­ity map released by Arpat.

“Some plant species can act as nat­ural fil­ters of par­tic­u­late mat­ter by inter­cept­ing and retain­ing par­ti­cles on their leaf sur­faces: Our goal was to iden­tify, test and select the most promis­ing among the native species of our cli­matic niche in order make them ideal can­di­dates for under­tak­ing local actions to sig­nif­i­cantly reduce air pol­lu­tion,” says project coor­di­na­tor Federico Martinelli, asso­ciate pro­fes­sor of Genetics at the Department of Biology of the University of Florence.

“As a first step, we made an exten­sive screen­ing of the avail­able species capa­ble of adsorbing/trapping more PM, heavy met­als and ozone: We com­bined patho­phys­i­o­log­i­cal stud­ies with mol­e­c­u­lar analy­sis and geno­typ­ing tech­niques made avail­able by the sequenc­ing equip­ment avail­able in the Department of Biology of the University of Florence, so to under­stand the mol­e­c­u­lar mech­a­nisms that under­lie the mod­u­la­tion of pos­i­tive char­ac­ter­is­tics. Then, in 2021, in coop­er­a­tion with the National Research Council of Italy, we launched the exper­i­men­tal part of the project by set­ting up a net­work of six­teen con­trol units through­out the area, capa­ble of mon­i­tor­ing the main air pol­lu­tants and col­lect­ing both quan­ti­ta­tive and qual­i­ta­tive data. By inte­grat­ing the val­ues recorded by the mon­i­tor­ing cen­ters with the par­tic­u­late accu­mu­lated in the leaves of each species ana­lyzed, we were able to rank the species with the high­est PM depo­si­tion val­ues.”

Researchers picked sep­a­rate leaf sam­ples for every tree. The depo­si­tion val­ues of each PM frac­tion were com­pared and ana­lyzed against the aver­age par­tic­u­late mat­ter lev­els recorded through­out the year. Using this process, researchers could rank each species accord­ing to their abil­ity to accu­mu­late fine and ultra­fine par­ti­cles. They found that olive trees, in par­tic­u­lar, demon­strate a high accu­mu­la­tion capac­ity.

“This fea­ture, along with their abil­ity to tol­er­ate stresses such as drought and salin­ity, makes them one of the most promis­ing can­di­dates. In urban con­text, their pres­ence is even more impor­tant because they nat­u­rally absorb car­bon diox­ide and release oxy­gen, essen­tial for the life of every human being.”

Given the exper­i­men­tal results of the VegPM project, the researchers hope that more research will be under­taken and show the abil­ity of cer­tain trees to negate the harm­ful effects of liv­ing in an urban envi­ron­ment.

“For now, our stud­ies have focused only on exist­ing plants. But what would it hap­pen on new, planted plants? Would the PM con­cen­tra­tion decrease more? I hope this ques­tion could stim­u­late a fol­low-up of the project,” Martinelli adds.

• Lucca Green Project web­site
• University of Florence — Plant Stress Lab