Sequentially contingent fires, droughts and pluvials structured a historical dry forest landscape and suggest future contingencies

QuestionsDry forests throughout the world were historically influenced by fires and climatic events, evidenced by tree recruitment pulses in forest age structures, but did these influences act randomly or were recruitment pulses contingent on the type, magnitude, order and timing of events? If recruitment was random or contingent, what are the implications for future forests? LocationUnlogged, old-growth ponderosa pine landscape in Grand Canyon National Park, Arizona, US. MethodsWe spatially reconstructed and compared tree recruitment pulses evident in forest age structures within plots with tree ring reconstructions of pluvials, droughts, fires and fire quiescence (longer fire-free periods). We used chi-square analysis to test for sequential contingency of combinations and permutations of pulse influences. ResultsOf 20 recruitment pulses, 17 were influenced to some extent by fire quiescence, 13 by fires and droughts each, and 11 by pluvials. Prevalence of pulses across the landscape did not correspond to the pronounced spatial variability in fire rotation. Analysis of combinations and permutations of these influences showed that potentially mortality-inducing influences of fire and drought likely initiated pulses, whereas pluvials and quiescence, which enhance recruitment conditions, sustained 75% of pulses. ConclusionsSuccessful tree recruitment pulses in dry forests historically were sequentially contingent on mortality-inducing influences, followed by recruitment-enhancing conditions. The impacts of climate change projections, including prolonged droughts and intense fires, on dry forests may depend on the order, timing and magnitude of influences.