Browsing by Subject "Ecophysiology"
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Item Open Access Evolution and Diversification of Farinose Ferns in Xeric Environments: A Case Study Using Notholaena standleyi Maxon as a Model(2020) Kao, Tzu-TongNot all ferns grow in moist and shaded habitats. One notable example is the ecologically unusual clade of notholaenids. With approximately 40 species, the notholaenids have adapted to and diversified within the deserts of Mexico and the southwestern United States. In my dissertation, I studied the evolution and diversification of notholaenid ferns, using an approach that integrates data from multiple sources: biochemistry, biogeography, cytology, ecological niche modeling, molecular phylogeny, morphology, and physiology.
In Chapter 1, I infer a species phylogeny for notholaenid ferns using both nuclear and plastid DNA sequences, and reconstruct the evolutionary history of “farina” (powdery exudates of lipophilic flavonoid aglycones), a characteristic drought-adapted trait, that occurs on both the gametophytic and sporophytic phases of members of the the clade. Forty-nine notholaenid and twelve outgroup samples were selected for these analyses. Long (ca. 1 kb) low-copy nuclear sequences for four loci were retrieved using a recently developed amplicon sequencing protocol on the PacBio Sequel platform and a bioinformatics pipeline PURC; plastid sequences from three loci were retrieved using Sanger sequencing. Each nuclear/plastid dataset was first analyzed individually using maximum likelihood and Bayesian inference, and the species phylogeny was inferred using *BEAST. Ancestral states were reconstructed using likelihood (re-rooting method) and MCMC (stochastic mapping method) approaches. Ploidy levels were inferred using chromosome counts corroborated by spore diameter measurements. My phylogenetic analyses results are roughly congruent with previous phylogenies inferred using only plastid data; however, several incongruences were observed between them. Hybridization events among recognized species of the notholaenid clade appear to be relatively rare, compared to what is observed in other well-studied fern genera. All characters associated with farina production in the group appear to be homoplastic and have complex evolutionary histories.
In Chapter 2, I focus on the infraspecific diversification of Notholaena standleyi, a species that thrives in the deserts of the southwestern United States and Mexico and has several “chemotypes” that express differences in farina color and chemistry. Forty-eight samples were selected from across the geographic distribution of N. standleyi. Phylogenetic relationships were inferred using four plastid makers and five single/low-copy nuclear markers. Sequences were retrieved using PacBio and the PURC pipeline. Ploidy levels were inferred from relative spore size measurements calibrated with chromosome counts, and farina chemistry was compared using thin-layer chromatography (TLC). My studies of Notholaena standleyi reveal a complex history of infraspecific diversification traceable to a variety of evolutionary drivers including classic allopatry, parapatry with or without changes in geologic substrate, and sympatric divergence through polyploidization. Four divergent clades were recognized within the species. Three roughly correspond to previously recognized chemotypes: gold (G), yellow (Y), and pallid/yellow-green (P/YG). The fourth clade, cryptic (C), is newly reported here. The diploid clades G and Y are found in the Sonoran and Chihuahuan Deserts, respectively; they co-occur (and hybridize) in the Pinaleño Mts. of eastern Arizona. Clades G and Y are estimated to have diverged in the Pleistocene, congruent with the postulated timing of climatological events that divide these two deserts. Clade P/YG is tetraploid and partially overlaps the distribution of clade Y in the eastern parts of the Chihuahuan Desert. However, PY/G is apparently confined to limestone, a geologic substrate rarely occupied by members of the other clades. The newly discovered diploid clade, cryptic (C), is distributed in the southern Mexican states of Oaxaca and Puebla and is highly disjunct from the other three clades.
In Chapter 3, I study the ecological niche differentiation among the three major chemotypes––G, Y, and P/YG. Using both ordination and species distribution modelling techniques, the ecological niches for each chemotype were characterized and compared. The main environmental drivers for their distributions were identified, their suitable habitats in both geographic and environmental spaces were predicted, and their niche equivalencies and similarities were tested. My ecological niche analyses results suggest that all three chemotypes are ecologically diverged. The ecological niches of the two parapatric, sister diploid chemotypes, G and Y, are significantly different from one another. Chemotype G occupies a very extreme niche with higher solar radiation, and lower rainfall and higher temperatures in the wettest quarter. The niche space of tetraploid chemotype P/YG is similar but not equivalent to the other two chemotypes. Its distribution model is highly influenced by the high percentage of Calcids and warmer temperatures in the wet season, reflecting the fact that it is confined to limestone in areas of lower elevation/latitude.
In Chapter 4, I gather together all my other studies related to Notholaena standleyi, including: 1) morphological and anatomical observations of its desiccation-tolerant leaf, with special focus on the farina; 2) two cases of hybridization between the chemotypes, one between the diploid chemotype Y and tetraploid chemotype YG, and another between diploid chemotypes Y and G; and 3) morphological and physiological comparisons between the two diploid chemotypes Y and G. My plan is to finalize these studies and submit them for publication in the near future.
In Chapter 5, I summarize collaborative contributions that I made to other fern studies during my Ph.D.
Item Open Access Longterm Approaches to Assessing Tree Community Responses to Resource Limitation and Climate Variation(2011) Bell, David McFarlandThe effects of climate change on forest dynamics will be determined by tree responses at different life-stages and different scales -- from establishment to maturity and from individuals to populations. Studies incorporating local factors, such as natural enemies, competition, or tree physiology, with sufficient variation in climate are lacking. The importance of global and regional climate variation vs. local conditions and responses is poorly understood and may only be addressed with large datasets capturing sufficient environmental variation. This dissertation uses several large datasets to examine tree demographic and ecophysiological responses to light, moisture, predation, and climate in eastern temperate forests of North Carolina.
First, I use a 19-yr seed rain record from 13 forest plots in the piedmont, transition zone, and mountains to examine how climate-mediated seed maturation and density-dependent seed predation processes increase population reproductive variation in nine temperate tree species (Chapter 1). I address several hypotheses explaining interannual reproductive variation, such as resource matching, predator satiation, and pulse resource dynamics. My results indicate that (1) interannual reproductive variation increased as a result of seed maturation and seed predation processes, (2) seed maturation rates increased under warm, wet conditions, and (3) seed predation rates exhibited negative and positive density-dependence, depending of tree species and type of seed predator (specialist insects vs. generalist vertebrates). Because positive density-dependent seed predation dampened and negative density-dependent seed predation amplified the effects of climate-mediated maturation on reproductive variation, this study showed evaluations of tree reproduction need to incorporate both climate and seed predation.
Next, I use an 11-yr record of annual tree seedling growth and survival in 20 tree species planted in the piedmont and mountains to quantify individual tree seedling growth and survival responses to spatial variation in resources and temporal variation in climate (Chapter 2). First, I tested whether height-mediated growth provides an advantage to large individuals in all environments by amplifying responses to light and moisture or only when those resources were plentiful. Second, I tested whether allometric and survival responses differed among species based on life-history strategies. Individual height amplified tree seedling growth. However, some species exhibited amplification at moderate to high resource levels as well as depression of growth in large individuals growing in low light and moisture environments. Shade intolerant species exhibited an increasing ratio of height to diameter growth and increasing survival probability with both increasing light and moisture resources. Conversely, shade tolerant species exhibited decreasing height to diameter ratio with increasing light, possibly because of biomass allocation toward acquisition of limiting light resources. Despite relative small effects of drought and winter temperature of tree seedling demography, the results of this study indicate that individual tree seedlings sensitive to light and moisture environments, such as large seedlings and seedlings of shade intolerant species, growing in shaded or xeric sites may be particularly vulnerable to climate induced mortality.
Finally, I examine interannual and interspecific variation in canopy conductance using four years of environmental (vapor pressure deficit, above canopy light, and soil moisture) and stem sap flux data from heat dissipation probes for six co-occurring tree species. I developed a state-space modeling framework for predicting canopy conductance and transpiration which incorporates uncertainty in canopy and observation uncertainty. This approach is used to evaluate the degree to which co-occur deciduous tree species exhibited drought tolerating and drought avoiding canopy responses and whether these patterns were maintained in the face of interannual variation in environmental drivers. Comparisons of canopy conductance responses to environmental forcing across species and years highlighted the importance of tree sensitivity to moisture limitation, both in terms of high vapor pressure deficit and low soil moisture, and tree hydraulic characteristics within diverse forest communities. The state-space model produced similar parameter estimates to the more traditional boundary line analysis, performed well in terms of in-sample and out-of-sample prediction of sap flux observations, and provided for coherent incorporation of parameter, process, and observation errors in predicting missing data (i.e., gap-filling), canopy conductance, and transpiration.
Much needs to be learned about forest community responses to climate change, however these responses depend on local growing conditions (light and moisture), the life-stage being examined (seedlings, juveniles, or mature trees), and the scale of inference (individuals, canopies, or populations). Because climate change will not occur in isolation from other factors, such as stand age or disturbance, studies must characterize tree responses across multidimensional gradients in growing conditions. This dissertation addresses these challenges using large demographic and ecophysiological datasets well-suited for global change research.