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Quantifying historical fire regimes provides important information for managing contemporary forests. Historical fire frequency and severity can be estimated using several methods; each method has strengths and weaknesses and presents challenges for interpretation and verification. Recent efforts to quantify the timing of historical high-severity fire events in forests of western North America have assumed that the \"stand age\" variable from the US Forest Service Forest Inventory and Analysis (FIA) program reflects the timing of historical high-severity (i.e. stand-replacing) fire in ponderosa pine and mixed-conifer forests. To test this assumption, we re-analyze the dataset used in a previous analysis, and compare information from fire history records with information from co-located FIA plots. We demonstrate that 1) the FIA stand age variable does not reflect the large range of individual tree ages in the FIA plots: older trees comprised more than 10% of pre-stand age basal area in 58% of plots analyzed and more than 30% of pre-stand age basal area in 32% of plots, and 2) recruitment events are not necessarily related to high-severity fire occurrence. Because the FIA stand age variable is estimated from a sample of tree ages within the tree size class containing a plurality of canopy trees in the plot, it does not necessarily include the oldest trees, especially in uneven-aged stands. Thus, the FIA stand age variable does not indicate whether the trees in the predominant size class established in response to severe fire, or established during the absence of fire. FIA stand age was not designed to measure the time since a stand-replacing disturbance. Quantification of historical \"mixed-severity\" fire regimes must be explicit about the spatial scale of high-severity fire effects, which is not possible using FIA stand age data.

I examined the seasonal and diurnal xylem pressure potentials of natural hybrids between two species of pinyon pine in Arizona, Pinus californiarum and Pinus edulis. The parent species differ mainly in the number of needles per fascicle (one versus two) and the seasonal distribution of precipitation characterizing their ranges. The hybrid trees have both single and double needles. Fewer needles per fascicle results in a reduction in the needle surface-to-volume ratio, which has been hypothesized to reduce the transpiration rate. In this study, I tested the hypothesis that single-needle fasicles can maintain higher midday xylem pressure potentials during times of water stress. A 2-year field study showed no significant differences between single and double needles. Needle morphology does not affect water relations, or the relatively high precipitation during the study did not allow significant water stress to occur.