Wood anatomical traits responses to climate in Scots pine at the rear edge of its natural distribution

Human-induced climate change has already affected tree species natural distribution and is likely to compromise several biomes, leading to a further widespread deterioration of ecosystem structure, function, resilience and adaptive capacity. Indeed, the current pace of change in environmental conditions is much faster than any change in the past, and this is particularly threatening to plant species, given their inherent slowness of movement. Consequently, in extreme cases - for taxa that cannot cope quickly enough with these rates of changes - this may imply tree species dieback, migration or extinction. Although Pinus sylvestris L. is one of the dominant conifers in the forests of the temperate and boreal zones of Eurasia - at the southern limit of its natural distribution in the northern Apennines, the species is restricted to small, scattered relict populations on skeletal soils within open coppiced oak and beech stands. In order to broaden the knowledge already acquired in other study areas - such as Spain and Turkey - core samples from two stands were collected in Emilia-Romagna region (Italy) and subsequently jointly analysed, using both classical dendrocnolological and dendroanatomical techniques aiming to verify the hypothesis of rather high Scots pine tree-ring inter-annual variability at this rear edge area. Initially, a purely descriptive approach was adopted for the identification and description of ring anomalies - frost rings (FR) and inter-annual density fluctuations (IADF) - as indicators of stand climate maladaptation to local climate conditions. Later, Pearson correlation coefficients were computed to investigate how monthly climatic variability affects xylem anatomical traits, namely: mean ring width (MRW), cell number (CNo), cell density (CD), mean lumen area (MLA), theoretical hydraulic conductivity (Kh), mean thickness of radial (CWTrad), tangential (CWTtan), and total (CWTall) cell walls. Additionally, with a main focus on lumen area (LA) and cell-wall thickness (CWA) chronologies split into sectors, 15-, 20- and 30-day moving windows were applied to assess the relationships with daily climate records in the 1940-2020 interval. Results suggested that sudden drops in temperatures in March-April induce earlywood FRs formation whereas latewood FRs are associated with autumn frost episodes in the mid- second half of November. Moreover, the co-occurrence of July-August summer drought period - when the highest daily maximum temperatures and no precipitations are recorded - followed by milder autumn conditions and greater water availability in September - affects latewood IADFs formation. Climate-growth associations on a monthly scale verifies positive correlations with winter temperatures and summer precipitation, whereas no negative r-values are recorded for parameters other than CWA. The correlations between LA and CWA and the monthly climate variables show associative clusters with similar patterns but opposite correlative significance, especially in summer and winter. Analyses with a 20-day moving window confirmed with a better detail the divergent responses between LA and CWA. The inter-annual variability in drought-prone Scots pine forests - where precipitation is the limiting factor for growth and the future is increasingly uncertain - provide a general picture of the ontogenetic dynamics and climatic sensitivity of this species. I recommend these key insights to be the basis for a future climate-smart management at the rear edge of the species natural distribution.