Pine processionary moth outbreaks cause longer growth legacies than drought and are linked to the North Atlantic Oscillation

Blue is the fashion in Mediterranean pines: New drought signals from tree-ring density in southern Europe

Long-term records of tree-ring width (TRW), latewood maximum density (MXD) and blue intensity (BI) measurements on conifers have been largely used to develop high-resolution temperature reconstructions in cool temperate forests. However, the potential of latewood blue intensity (LWBI), less commonly used earlywood blue intensity (EWBI), and delta (difference between EWBI and LWBI, dBI) blue intensity in Mediterranean tree species is still unexplored. Here we developed BI chronologies in moist-elevation limits of the most southwestern European distribution of Pinus nigra subsp. salzmanii Arnold. We tested whether BI variables derived from tree rings of black pine are better proxies than ring-width variables to reconstruct long-term changes in climatic factors and water availability. For this we applied correlations and regression analyses with daily and monthly climate data, a spatial and temporal drought index (Standardized Precipitation-Evapotranspiration Index-SPEI) and Vapour Pressure Deficit (VPD), as well as atmospheric circulation patterns: North Atlantic Oscillation (NAO), Southern Oscillation Index (SOI) and Western Mediterranean Oscillation (WeMO). We found a positive relation between black pine growth (RW) and temperature during the winter preceding the growing season. Among all variables LWBI and dBI were found to be more sensitive than TRW to SPEI at low-elevation site, with EWBI series containing an opposite climatic signal. LWBI and dBI were significantly related to June and September precipitation at high-elevation site. Winter VPD was related with higher EWI and LWI series, whereas dBI and EWBI were related with January SOI and February NAO. We confirm the potential of long-term dBI series to reconstruct climate in drought-prone regions. This novel study in combination with other wood anatomical measurements has wide implications for further use of BI to understand and reconstruct environmental changes in Mediterranean conifer forests.

Threshold Responses of Canopy Cover and Tree Growth to Drought and Siberian silk Moth Outbreak in Southern Taiga Picea obovata Forests

The consecutive occurrence of drought and insect outbreaks could lead to cumulative, negative impacts on boreal forest productivity. To disentangle how both stressors affected productivity, we compared changes in tree canopy cover and radial growth after a severe outbreak in Siberian spruce (Picea obovata) southern taiga forests. Specifically, we studied the impacts of the 2012 severe drought followed by a Siberian silk moth (Dendrolimus sibiricus, hereafter SSM) outbreak, which started in 2016, on spruce forests by comparing one non-defoliated site and two, nearby fully defoliated sites, using remote sensing and tree-ring data. The SSM outbreak caused total defoliation and death of trees in the infested stands. We found a sharp drop (–32%) in the normalized difference infrared index and reduced radial growth in the defoliated sites in 2018. The growth reduction due to the 2012 drought was –37%, whereas it dropped to 4% of pre-outbreak growth in 2018. Tree growth was constrained by warm and dry conditions from June to July, but such a negative effect of summer water shortage was more pronounced in the defoliated sites than in the non-defoliated site. This suggests a predisposition of sites where trees show a higher growth responsivity to drought to SSM-outbreak defoliation. Insect defoliation and drought differently impacted taiga forest productivity since tree cover dropped due to the SSM outbreak, whereas tree growth was reduced either by summer drought or by the SSM outbreak. The impacts of abiotic and biotic stressors on boreal forests could be disentangled by combining measures or proxies of canopy cover and radial growth which also allow the investigation of drought sensitivity predisposes to insect damage.