RESUMO
Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7-fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re-establishment of native plant communities in this desert system. Additional research is needed to assess if repeat fires that are characteristic of invasive grass-fire cycles may limit these benefits.
RESUMO
Phenotypic variation in plant traits is strongly influenced by genetic and environmental factors. Over the life span of trees, developmental factors may also strongly influence leaf phenotypes. The objective of this study was to fill gaps in our understanding of developmental influences on patterns of phenotypic trait variation among different-aged ramets within quaking aspen (Populus tremuloides Michx.) clones. We hypothesized that phenotypic variation in leaf functional traits is strongly influenced by developmental cues as trees age. We surveyed eight aspen clones, each with eight distinct age classes ranging from 1 to 160 years in age, and selected three ramets per age class for sample collection. Leaf traits measured included photosynthesis, stomatal conductance, water use efficiency, specific leaf area, and concentrations of N, phosphorus, sucrose, starch, condensed tannins and phenolic glycosides. Using regression analysis, we examined the relationships between ramet age and expression of leaf functional traits. The data showed significant correlations between ramet age and 10 of the 12 phenotypic traits measured. Eight of the phenotypic traits demonstrated a non-linear relationship in which large changes in phenotype occurred in the early stages of ramet development and stabilized thereafter. Water relations, nutrient concentration, leaf gas exchange and phenolic glycosides tended to decrease from early to late development, whereas sucrose, condensed tannin concentrations and water use efficiency increased with ramet age. We hypothesize that ontogenetically derived phenotypic variation leads to fitness differentials among different-aged ramets, which may have important implications for clone fitness. Age-related increases in phenotypic diversity may partially underlie aspen's ability to tolerate the large environmental gradients that span its broad geographical range.
