Legacies of La Nina: North American monsoon can rescue trees from winter drought

摘要

While we often assume tree growth-climate relationships are time-invariant, impacts of climate phenomena such as the El Nino Southern Oscillation (ENSO) and the North American Monsoon (NAM) may challenge this assumption. To test this assumption, we grouped ring widths (1900-present) in three southwestern US conifers into La Nina periods (LNP) and other years (OY). The 4 years following each La Nina year are included in LNP, and despite 1-2 year growth declines, compensatory adjustments in tree growth responses result in essentially equal mean growth in LNP and OY, as average growth exceeds OY means 2-4 years after La Nina events. We found this arises because growth responses in the two periods are not interchangeable: Due to differences in growth-climate sensitivities and climatic memory, parameters representing LNP growth fail to predict OY growth and vice versa (decreases in R-2 up to 0.63; lowest R-2 = 0.06). Temporal relationships between growth and antecedent climate (memory) show warmer springs and longer growing seasons negatively impact growth following dry La Nina winters, but that NAM moisture can rescue trees after these events. Increased importance of monsoonal precipitation during LNP is key, as the largest La Nina-related precipitation deficits and monsoonal precipitation contributions both occur in the southern part of the region. Decreases in first order autocorrelation during LNP were largest in the heart of the monsoon region, reflecting both the greatest initial growth declines and the largest recovery. Understanding the unique climatic controls on growth in Southwest conifers requires consideration of both the influences and interactions of drought, ENSO, and NAM, each of which is likely to change with continued warming. While plasticity of growth sensitivity and memory has allowed relatively quick recovery in the tree-ring record, recent widespread mortality events suggest conditions may soon exceed the capacity for adjustment in current populations.