Drought years promote bark beetle outbreaks in Mexican forests of Abies religiosa and Pinus pseudostrobus

Abstract

Climate change is an important risk factor for forest ecosystems through alteration of forest disturbance regimes such as bark beetle outbreaks, which in some places now are more successfully attacking host trees weakened by hotter drought events. In Mexico, ties between climate and amplified outbreaks of bark beetles have begun to be documented, although these relationships are not entirely clear. This project aims to identify the geographic patterning and relations between climate and bark beetle outbreaks in Abies religiosa and Pinus pseudostrobus populations located in the Trans-Mexican Volcanic Belt region. We used: 1) a database of phytosanitary logging authorizations and locations issued to enable salvage logging of trees infested or killed by bark beetle outbreaks from 2009 to 2018; 2) a previously developed gridded database of contemporary suitable climatic habitat of these tree species that will be lost by 2060 to determine if outbreaks occurred in sites with climatic habitat lost; and 3) elevation values to determine if outbreaks occur at xeric limits (drought limits) of tree hosts. Climate analysis was conducted with TerraClimate data using PDSI (Palmer’s Drought Severity Index) values. We find that bark beetle outbreaks do not necessarily occur at sites where models project that both species will lose suitable climatic habitat. For A. religiosa (Sacred Fir), of the 4091 ha treated by sanitary logging, 40% occurs between 3000 and 3100 m of elevation, where the pure and relatively better-preserved Sacred Fir stands are found. For P. pseudostrobus, more than 54% of the sanitary logging area (4664 ha) ranges from 2200 to 2400 m a.s.l., which coincides with the lower elevation limit (xeric limit) of the natural distribution of the species. Regarding relationships with PDSI, for A. religiosa there was a one-year lag between the year of mortality (expressed by the year of the phytosanitary logging authorization) and the year with the most negative (driest) PDSI anomaly (one year before), while for Pinus pseudostrobus, phytosanitary logging authorizations were concentrated in the driest year. An investigation that considers more variables, especially anthropogenic factors, is necessary to be able to better understand the dynamics of forest pests and prevent unusual outbreaks induced by expected climate-change-amplified droughts.