Structural and regenerative changes in old-growth piñon–juniper woodlands following drought-induced mortality

Abstract

Two decades of drought and rising temperatures have triggered bark beetle outbreaks and extensive mortality of Colorado piñon (Pinus edulis) and Utah juniper (Juniperus osteosperma) across the southwest US. For example from 2002 to 2005, over one-third of the piñons in Mesa Verde National Park’s old-growth woodlands were killed by the bark beetle Ips confusus and other drought-related factors. Extensive wildfires have also burned through the Park over the last two decades, reducing the extent of old-growth woodlands by nearly one-half. We compared historical (1970–1990s) and recent (2005–2011) data to gauge the effects of the 2002–2005 mortality event on canopy structure, woody surface fuels, and tree recruitment. Large, mature piñons were the most susceptible to mortality; we measured significant reductions in piñon cover and diameter. Due to the loss of large trees, woodlands that were subjected to substantial mortality had reduced structural complexity relative to nearby unaffected stands. In a longitudinal fuels comparison, 2010–2011 fuels were compared to historical 1993 values in the same plots; we could not detect a change in woody fuel load. However when the 2010–2011 fuels are compared across mortality severity classes, there was a trend toward slightly greater 1–100h, greater 1000h fuel loads, and more litter in stands with higher levels of tree mortality. However, modeled surface fire behavior did not differ across mortality severity classes. Modeled crown fires needed higher wind speeds to spread from tree to tree in stands without tree mortality, but this is likely due to the inherently more open canopy structure in stands that resisted mortality rather than due to tree mortality per se. Post-mortality piñon reproduction and recruitment were recorded from 2005 to 2011 and compared with NPS monitoring data from 1975 to 1995. Cone production in 2005 was comparable to historical trends, but recent seedling and sapling densities were significantly below historical values. Yearly seedling and sapling density in 1975–2011 was positively correlated with precipitation during the sampling year and two years prior. The largest proportion of seedlings and saplings occurred under piñon or juniper canopies, and a smaller proportion under native shrubs. Given the loss of a substantial component of the adult trees, reduced stand structural complexity, and demonstrated need for moist conditions and nurse plants for effective recruitment, these old-growth woodlands may be vulnerable to decline under future warm and dry climate projections. Management strategies focusing on maintaining structural features of both the overstory and understory could help provide conditions for sustainable recruitment and contribute to the goal of conservation of rare old-growth piñon–juniper woodlands.