The Fire and Tree Mortality Database, for empirical modeling of individual tree mortality after fire.

Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire and Tree Mortality (FTM) database includes records from 164,293 individual trees with records of fire injury (crown scorch, bole char, etc.), tree diameter, and either mortality or top-kill up to ten years post-fire. Data span 142 species and 62 genera, from 409 fires occurring from 1981-2016. Additional variables such as insect attack are included when available. The FTM database can be used to evaluate individual fire-caused mortality models for pre-fire planning and post-fire decision support, to develop improved models, and to explore general patterns of individual fire-induced tree death. The database can also be used to identify knowledge gaps that could be addressed in future research.

Disruptive selection maintains variable pheromone blends in the bark beetle Ips pini.

The presence of heritable variation is a prerequisite for evolution, but natural selection typically reduces genetic variation. Variation can be maintained in traits under selection through spatial or temporal variation in fitness surfaces, frequency-dependent selection, or disruptive selection. We evaluated the maintenance of variation in the enantiomeric blend of pheromones employed by the bark beetle Ips pini (Say). In natural populations, we quantified fitness surfaces for mating success and progeny production. We investigated the effects of paternal pheromone blend on offspring survival by comparing the spatial scales at which pheromone blends and larval mortality agents vary. Males with extreme pheromone blends obtained up to 1.8 times as many mates who each laid equivalent numbers of eggs, producing strong disruptive selection on male pheromone blend. In combination with imperfect assortative mating that continually produces intermediate genotypes, this fitness surface is sufficient to maintain variation in a heritable trait that is strongly linked to fitness. The ultimate explanation for female preference is unknown but could be because of selection for reduced mortality from specialist predators that prefer common prey pheromone blends. Selection is most likely occurring at the scale of small resource patches within pine stands. Selection at coarser scales (pine stands) is unlikely because pheromone blends did not vary among pine stands. Selection at finer scales (within logs) is unlikely because males of similar enantiomeric blends were not aggregated on logs, and male pheromone blend did not affect the spacing to neighboring galleries. This study documents a rare case of diversifying selection in natural populations.

Resource partitioning and overlap in three sympatric species of Ips bark beetles (Coleoptera: Scolytidae).

The bark beetles Ips pini, I. perroti, and I. grandicollis are sympatric in pine forests of the north-central United States. They share the same limited phloem resource and often coexist within the same host trees. We tested whether phloem resources are partitioned in time and space by measuring spatial and seasonal colonization of logs. Differences among species in flight phenology, development time, voltinism, and spatial colonization patterns within logs reduce, but do not eliminate, species overlap. The bark beetle species share predation by Thanasimus dubius (Cleridae) and Platysoma cylindrica (Histeridae), which exploit pheromone signals for prey location. We employed pheromone traps to test for chemical communication among bark beetle species. Heterospecific signals tend to be deterrents when they are added to conspecific signals but attractants when they are alone, indicating that the communication system can both reduce and increase species overlap in resource use depending upon relative abundance of the species. Deterrence by heterospecific signals is probably a result of selection for minimizing interspecific competition. However, individuals may sometimes benefit from joining aggregations of other species because of (1) predator swamping, (2) improved success in attacking live trees, and (3) location of suitable, recently dead, trees. These benefits should be greatest for males (which locate and colonize host trees before signalling females) and indeed males tended to be more attracted than females by heterospecific signals. Shared resources, shared predators, and heterospecific pheromone communication all contribute to species interactions in this guild of bark beetles, but predicting whether the removal of one species will tend to increase or decrease the abundance of remaining species remains difficult. Species interactions are likely conditional and coexistence is probably promoted by benefits to rare species of multispecies associations.