Climate thresholds and productivity shifts in mature Nothofagus pumilio height growth

Abstract

Understanding height growth dynamics of mature forests under changing climatic conditions is crucial for sustainable management and predicting future productivity. This study aimed to analyse the apical growth dynamics of dominant, mature Nothofagus pumilio trees in Tierra del Fuego, Argentina, over the 1970–2020 period, and to determine the effects of climate variability, including resilience thresholds. Apical growth was reconstructed from 48 felled trees across three different stands (S1, S2, S3), by performing stem analysis. Net Current Annual Increment (NCAI), inter-annual Site Index variation (ΔSI60), and 5-year Periodic Annual Increment (PAI5) were derived. These were then related to temperature and precipitation records using Pettitt’s test for change-point detection, correlation, and linear regression models. Results revealed significant initial differences in site quality (S1>S2>S3). A consistent regional warming trend was observed, with a significant shift towards warmer conditions, while annual precipitation lacked a significant long-term trend. The NCAI in the two higher-quality stands showed significant declines (S1 K = 566, p < 0.001; S2 = 549, p < 0.001) after identified change points: a 43.6 % reduction in S1 after 1994 and a 55.8 % reduction in S2 after 2005. ΔSI60 showed a significant negative relationship with temperature in S1 and S2 suggesting that warming conditions tend to reduce inferred site quality. Critical temperature thresholds for N. pumilio height growth were identified within a range of approximately 5.73–5.93◦C. Beyond this range, warmer temperatures were associated with significant declines in PAI5 (R² up to 69 % in S1), indicating a physiological optimum has been surpassed. We conclude that rising temperatures since the early 1990s have negatively impacted height growth dynamics in the most productive mature N. pumilio stands once critical thermal thresholds were exceeded. This growth reduction highlights a vulnerability to continued warming, potentially compromising long-term sustainability and productivity.