Modeling operative temperature in desert tortoises and other reptiles: Effects imposed by habitats that filter incident radiation

We challenged the common practice of using a single mean absorptance based on unfiltered skylight spectra to model operative temperature for reptiles in filtered light habitats by examining the effects of plant canopies on light transmittance. To assess differences in light filtration over a range of microhabitats, spectra were recorded under canopies of desert plants, tropical plants, and under unfiltered skylight. Spectra were then integrated with absorptivity curves of desert reptiles to determine if differences in light quality among microhabitat types changed integrated mean absorptance. Finally, we used the desert tortoise (Gopherus agassizii) as a case study to investigate the effects of filtered microhabitats on paint choice for physical operative temperature models and determined the magnitude of error that could result from discrepancies between paint and animal absorptance. We found that light energy was partitioned similarly among microhabitats with like canopy types and that most variation was explained by differences in transmittance between the visible and near infrared wavelengths. Mean absorptance for reptiles was similar among microhabitats with the greatest differences observed between animals in unfiltered skylight and under tropical canopies. In most microhabitats paint and tortoise absorptances differed, but operative temperatures were nearly identical within microhabitats no matter the absorptance used in the model. The findings of this study support the use of a single mean absorptance in modeling operative temperature for reptiles in a variety of habitats.

Plant species effects on soil nutrients and chemistry in arid ecological zones

The presence of vegetation strongly influences ecosystem function by controlling the distribution and transformation of nutrients across the landscape. The magnitude of vegetation effects on soil chemistry is largely dependent on the plant species and the background soil chemical properties of the site, but has not been well quantified along vegetation transects in the Great Basin. We studied the effects of plant canopy cover on soil chemistry within five different ecological zones, subalpine, montane, pinyon-juniper, sage/Mojave transition, and desert shrub, in the Great Basin of Nevada all with similar underlying geology. Although plant species differed in their effects on soil chemistry, the desert shrubs Sarcobatus vermiculatus, Atriplex spp., Coleogyne ramosissima, and Larrea tridentata typically exerted the most influence on soil chemistry, especially amounts of K(+) and total nitrogen, beneath their canopies. However, the extent to which vegetation affected soil nutrient status in any given location was not only highly dependent on the species present, and presumably the nutrient requirements and cycling patterns of the plant species, but also on the background soil characteristics (e.g., parent material, weathering rates, leaching) where plant species occurred. The results of this study indicate that the presence or absence of a plant species, especially desert shrubs, could significantly alter soil chemistry and subsequently ecosystem biogeochemistry and function.