Weather-induced declines in piñon tree condition and response of a declining bird species

- Average condition (vigor index) of piñon pines declined over an eight-year period.
- Annual decline in vigor increased as cool-season precipitation decreased.
- Probability of Pinyon Jays nesting was higher in areas of higher piñon vigor.
- Incremental weather-related changes in piñon condition can affect bird habitat use.

Climate impacts to piñon-juniper woodlands in the western USA include large-scale mortality, reduced canopy cover, and decreased cone production; recent climate models predict massive tree mortality in these woodlands. These habitat impacts will affect piñon-juniper wildlife, exemplified by the Pinyon Jay (Gymnorhinus cyanocephalus). Here we: (1) examine trends in an index of piñon tree condition (vigor) from 2004 to 2012, (2) assess the impact of weather and stand structure variables on annual vigor changes, and (3) investigate the relationship between patterns of piñon condition and distribution of nesting Pinyon Jays. Mean piñon vigor declined 0.12 units per year from 4.3 (maximum vigor = 5) in 2004 and bottomed out at 3.2 in 2012 (a reduction of 25.6%). Vigor changes were positive in areas of lower tree density and negative in areas of higher tree density; larger trees were more likely than smaller trees to decline in vigor. Average annual vigor decline was greater with decreasing cool-season precipitation. The probability that Pinyon Jays nested was higher in areas of higher piñon vigor in 2010-2012; this relationship became more pronounced over the three years. This example suggests that incremental, weather-influenced changes in tree condition may impact the wildlife of arid woodlands.

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