Adaptation of plantations to drought events in arid and semi-arid regions: Evidence from tree resilience

Increasing drought frequency and severity pose a significant risk to global forest plantations, particularly in arid and semi-arid regions. Developing drought-adaptive plantations is particularly important for vegetation restoration in the context of climate warming. Despite the importance of drought response and adaptation, systematic research on these aspects for major tree species remains limited. Here, we collected 467 tree ring cores from eight paired sites of Robinia pseudoacacia L. (a broad-leaved species) and Pinus tabulaeformis C. (a coniferous species) plantations across a typical semi-arid region of the Loess Plateau (LP) to discern the climate-growth patterns of both species using linear mixed models. We employed hypothesis testing to analyze the differences in tree resilience metrics, including drought resistance and recovery ability, and linear mixed models combined with partial least squares path analysis to clarify the driving factors of tree resilience between R. pseudoacacia and P. tabulaeformis. Our findings demonstrated that Palmer Drought Severity Indices is the key limiting climate factor, without spatial variation, on the growth of R. pseudoacacia and P. tabulaeformis, explaining 47.7 % and 69.8 % of the variables effect percentage, respectively. Compared to P. tabulaeformis, R. pseudoacacia is more limited by atmospheric drought stress and soil moisture, with 11.0 % and 17.9 % of the explained effect percentage, respectively. R. pseudoacacia exhibited lower drought resistance ability, evidenced by a lower resistance (the average value is 0.62) and a higher average growth reduction (0.20) than P. tabulaeformis (0.85 and 0.17, respectively). In contrast, R. pseudoacacia had higher drought recovery ability, evidenced by a higher recovery (the average value is 1.19), a shorter recovery period (1.0), and a faster average recovery rate (0.14), than P. tabulaeformis (1.07, 2.0, and 0.08, respectively). Importantly, all tree resilience metrics for both species exhibited spatial consistency across the LP. Tree characteristics had a stronger effect on drought resistance and recovery ability in R. pseudoacacia than in P. tabulaeformis. Soil properties improved R. pseudoacacia drought resistance ability and reduced its drought recovery ability, but had weaker effects on P. tabulaeformis. Additionally, tree size and age strongly influence tree resilience in R. pseudoacacia through preceding year tree growth of drought events and climate sensitivity. These findings underscore the complementary drought resilience of broad-leaved and coniferous species, emphasizing the necessity for adaptive plantations and diversified tree species plantations as a strategic response to the anticipated increase in drought frequency and severity in arid and semi-arid regions.