Vegetation establishment in post‐fire Adenostoma chaparral in relation to fine‐scale pattern in fire intensity and soil nutrients

At fine spatial scales (0.1–10m), chaparral communities have been shown to be strongly influenced by canopy‐gap patterns, leading to periodicities in vegetation at 4–5 m spatial scales. Fine‐scale variations in fire behavior and post‐fire erosion can lead to changes in the patterning of viable seeds and nutrients and may alter the spatial patterning of post‐fire chaparral communities. This study deals with the relationship among fire behavior, post‐fire nutrient availabilities and vegetation patterns in a 1‐yr old, post‐fire Adenostoma fasciculatum chaparral community in the Sierra Nevada Mountains, California, USA. Variations in mineral soil exchangeable cations (Ca, Mg) and extractable phosphorus (P0₄‐P) were correlated with ash distribution. Cations and measures of ammonium and nitrate were also correlated with fire intensity, measured by the diameter of the smallest remaining A. fasciculatum twigs following fire. Fire intensity was correlated with the pattern of post‐fire vegetation establishment based on first axis DC A scores. However, ash PO4‐P was more highly correlated with sample DCA scores, local species richness and total cover (p < 0.01), suggesting that small‐scale variations in PO4‐P which correlate with ash distributions may be important in structuring this community.

Two‐ and three‐term local variance analysis revealed a maximum of pattern intensity in DCA first axis scores at 4–5 m intervals that likely corresponds to pre‐fire canopy‐gap patterns. However, total cover showed pattern at spatial scales of 8–10 m, and was correlated at this scale with patterns of ash distribution and fire intensity. Microtopographic patterns also occur at similar spatial scales. Microtopographic patterns appear important in determining post‐fire plant nutrient and water distributions and, thereby, patterns of plant establishment. Thus, the scale and intensity of post‐fire vegetation pattern may differ considerably from pre‐fire conditions.