A Biogeographic Perspective on the Evolution of Fire Syndromes in Pine Trees (Pinus: Pinaceae)

Our goals were to explore the relationship between biogeography and the evolution of fire-adaptive syndromes in the genus Pinus. We used a previously published time-calibrated phylogeny and conducted ancestral trait reconstruction to estimate the likely timing of diversification in Pinus, and to determine when fire-adaptive syndromes evolved in the lineage. To explore trait conservation among fire syndromes and to investigate historical biogeography, we constructed ancestral state reconstructions using the program RASP and estimated the degree of conservatism for fire-adapted traits in the program BaTS. Our reconstructions suggest that the Bering land bridge, which connected North America and Asia, probably played a major role in early pine evolution. Our estimates indicated that fire-adaptive syndromes seem to have evolved more frequently in New World taxa and probably are related to the uplift of major North American mountain ranges. Our data suggest that certain geographically widespread adaptations to fire evolved repeatedly, possibly due to localized changes in climate and environment, rather than resulting from large dispersal events of pre-adapted individuals

The ecological and evolutionary effects of fire on the genus Pinus and boreal systems

Fire has a major impact on many of the world's ecosystems. In addition to being an important part of many landscapes, fire has had a long presence in natural systems. In order to persist amid fire, many plant species have evolved various strategies to deal with fire. Many members of the genus Pinus are associated with fire-prone systems and exhibit a wide range of fire strategies. In Chapter 1, we explored the parallel (or repeated) evolution of fire strategies within the genus Pinus and reconstructed the geographic center of origin within the genus. We congruently examined the reconstruction of the origin of fire strategies and geographic center of origin to discuss potential relationships between evolution of fire strategies and geographic distribution. Our analyses revealed a complex evolutionary history of fire strategies within the genus Pinus. Independent evolution of fire strategies were observed throughout the genus, especially in subgenus Pinus. Our reconstructions of geographic origin suggest that extant pines share a common North American ancestor. Major geologic events (e.g. Western Interior Seaway) had a large impact on the evolutionary trajectory of pines. These results help to elucidate both the biogeography and evolutionary history of this important genus. In fire-prone areas, plant species with adaptations that allow individuals to persist following a fire have an advantage over those species that lack these mechanisms. This observation led Mutch to propose the hypothesis that fire-adapted plants may benefit by increasing the flammability of their litter, reducing competition from non-fire-adapted neighbors. While enhanced flammability has been demonstrated to increase fitness for species that have mechanisms to persist following fire, immature individuals of fire-adapted species generally do not posess these persistence mechanisms. In Chapter 2, we present a hypothesis regarding the relationship between enhanced flammability and age among fire-adapted and non-fire-adapted species. Additionally, we were interested in exploring possible leaf traits that might explain differences across age classes. To test our hypotheses, we conducted an experiment using four North American pines, two considered fire-adapted and two lacking any adaptation to fire. We observed significant differences between fire-adapted and non-fire-adapted species. As predicted, there was a greater difference in flammability traits between mature and immature fire-adapted needles than between mature and immature needles of non-fire-adapted species. This evidence points toward a relationship between age and flammability, especially in those species that exhibit fire-adapted traits, such as thick bark and self-pruning. The effects of fire on the landscape may interact with other ecological processes to greatly affect the composition and structure of the vegetation present, especially in fire-prone areas such as the Alaskan boreal forest. In addition to fire, the presence of discontinuous permafrost on the landscape greatly influences the vegetation, causing the buildup of undecomposed organic material (O.M.). When a fire occurs on the landscape, the O.M. is combusted removing the insulation that protects permafrost from thaw. Previous work has suggested that a threshold of O.M. (between 10-15 cm) exists above which permafrost is sufficiently insulated against warmer temperatures. In Chapter 3, we test for the presence of the hypothesized threshold of O.M. depth on soil temperatures in burned areas. A manipulative experiment was conducted to manually change the depth of O.M. Additionally, an observational study was conducted in which temperature loggers were buried at different depths to record soil temperatures. In both studies differences between maximum daily and minimum daily temperatures were quantified to describe the degree of insulation provided by different depths of O.M. Results from the manipulative experiment suggest that below 14-16 cm there is little fluctuation between maximum and minimum daily temperature, which corresponds roughly to the proposed threshold found in the literature. The presence and identification of this threshold may allow researchers and managers to predict where discontinuous permafrost is more susceptible to thaw following a fire

Table S1. List of species involved in the analysis with their geographic distribution and their fire adaptations. from A biogeographic perspective on the evolution of fire syndromes in pine trees (<i>Pinus</i>: Pinaceae)

This Table provides all of the biogeographic data and fire adaptation information for all of the Pine species included in the analysis. Some Pine species exhibit multiple different fire adaptations, these are all included in the table

Table S1. List of species involved in the analysis with their geographic distribution and their fire adaptations. from A biogeographic perspective on the evolution of fire syndromes in pine trees (<i>Pinus</i>: Pinaceae)

This Table provides all of the biogeographic data and fire adaptation information for all of the Pine species included in the analysis. Some Pine species exhibit multiple different fire adaptations, these are all included in the table

Global taxes

Myriad unexplored mechanisms potentially drive ecological speciation and could help explain global variation in diversity. Here, we develop a novel hypothesis focused on variation in biotic, chemical, and physical properties of soil as a factor contributing to diversification in communities of plants and animals. The Soil Mosaic Hypothesis (SMH) suggests that differences in soil attributes can affect intraspecific variation in phytochemistry, leading to cascading ecological and evolutionary effects on higher trophic levels. To illustrate the potential importance of the SMH, we examine three underlying ideas: (1) plant species and species assemblages shift over time, exposing them to novel soil environments, which can lead to genetic differentiation(2) differences in soil properties can alter phytochemistry via plasticity and local adaptation(3) phytochemistry can drive herbivore diversification via divergent natural selection (i.e. ecological speciation). The SMH provides insight into the process of diversification in a variety of landscapes and at a variety of scales and may inform analyses of diversification at local, regional, and global scales