Growth Dynamics and Climate–Growth Relationships of Pedunculate Oak Before and After the Regional Appearance of the Oak Lace Bug in Spačva Basin, Croatia

European forests face increasing pressure from climate change and a growing number of invasive alien species. This study examines whether growth patterns and climate–growth relationships in pedunculate oak (Quercus robur L.) changed after the appearance of the oak lace bug (Corythucha arcuata; OLB) in the Spačva basin, Croatia. We analyzed 90 cores from 45 trees in young, middle-aged and old stands, comparing growth before and after 2013 (2000–2023). Structural change analysis identified a 2013 breakpoint in all stands, coinciding with the first regional detection of OLB. Growth trends declined significantly in young (p < 0.001) and middle-aged (p < 0.01) stands, but not in the old stand, and increased variability in tree-level slopes indicated heterogeneous individual responses. Declining mean interseries correlation (Rbar) and expressed population signal (EPS) suggested reduced growth synchrony. Positive correlations with precipitation and negative correlations with temperature strengthened in recent years, overlapping the period after OLB appearance. New earlywood signals emerged: a negative correlation with previous December precipitation and a positive correlation with previous October SPEI-3. As the breakpoint also follows the 2011–2012 drought and precedes the 2014 floods, the link to OLB remains correlative. We hypothesize that late-season drought, favoring OLB activity and limiting non-structural carbohydrate (NSC) replenishment, combined with winter waterlogging, may deplete NSC reserves and constrain growth the following year.