Responses of Fendler ceanothus to overstory thinning, prescribed fire, and drought in an Arizona ponderosa pine forest

Abstract

Overstory tree thinning and prescribed fire are used to restore ecosystem structure and function in ponderosa pine ( Pinus ponderosa Laws.) forests of the southwest but little research has examined constraints on population-level responses of understory plants. In order to study growth and reproduction of a common shrub species after forest restoration treatments, we monitored Fendler ceanothus ( Ceanothus fendleri Gray) plants from 1999 to 2002 in thinned and unthinned overstory units. To study effects of tree thinning, we analyzed relationships of overstory stand density (Reineke’s SDI) and current-year branch length, number of branches, and biomass, and also evaluated the importance of mule deer ( Odocoileus hemionus) and Rocky Mountain elk ( Cervus elaphus) herbivory (Browsing) as a predictor in linear models. To study effects of prescribed fire, we examined mortality, seedling emergence, and growth response on experimentally burned and unburned plots. SDI and browsing were significantly and negatively correlated with F. ceanothus current-year branch length, biomass, and leaf area but relationships were generally weak. Across the 4 years, browsing appeared to be consistently more important than SDI in explaining variation in growth. Although SDI and browsing were significant predictors of growth in years with near normal precipitation, models failed in drought years. Burning resulted in 17–32% mortality whereas 0–5% of plants died on unburned plots. Mortality of burned plants was positively related to amount of forest floor consumed during prescribed fires. One growing season after fire, surviving burned plants responded by producing long resprouts. Current-year branches were consistently longer on burned than unburned plants only where plots were protected from large herbivores. Unburned plants had more current-year branches and greater biomass than burned plants. No seedlings emerged on unburned plots but were found on 44% of burned plots. A quadratic function represented the relationship between seedling emergence and forest floor consumption. Our results suggest that restoration treatments in ponderosa pine forests of northern Arizona can help increase abundance of F. ceanothus but population responses may be slow, particularly when constrained by ungulate herbivory, fire-related mortality of plants and dormant seeds, and drought.