Effects of thinning, prescribed burning, and burning season on the physiological performance of ponderosa pine

Low elevation ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) forests of the northern Rocky Mountains historically experienced frequent, low intensity fires that maintained open, uneven-aged stands. A century of fire suppression has led to denser ponderosa pine forests with higher competition for resources, higher tree stress, and greater risk of insect attack and stand destroying fire. Active management intended to restore historic stand conditions includes selective thiiming and prescribed fire application. Little is known about the differential effects of these management practices on the physiological performance of ponderosa pine. We examined the performance of second growth ponderosa pine trees nine years after the application of four treatments: thinning, thirming followed by spring (wet) prescribed fire, thinning followed by fall (dry) prescribed fire, and unthinned control stands. We measured stand structural characteristics, resource availability, and tree performance parameters in three replicates of each treatment at the Lick Creek Experimental Site in the Bitterroot National Forest. Thinning resulted in similar reductions in basal area in each thiimed and burned stand relative to control stands. Soil moisture content did not differ between any treatments over the field season. Soil chemical analyses revealed lower late-spring available ammonium in control stands relative to all other treatments and lower time-integrated nitrate availability in burned stands than in thirmed only or control stands. Trees of similar size and structure in the thirmed treatment and in both of the thirmed and burned treatments displayed higher maximum, leaf area-based rates of photosynthesis (Aarea) and lower levels of water stress (*F) over the course of the growing season, as well as higher rates of post-treatment diameter growth than trees in control stands. These results reflect an overall improvement in long-term physiological performance of trees in the actively managed stands relative to trees in unmanaged control stands. None of several leaf level characteristics, including specific leaf area (SLA), mass-based leaf nitrogen content (Nieaf), carbon isotope discrimination (A), and nitrogen isotope ratio (5^^) were significantly different between any of the four treatments. We found no evidence that long-term physiological performance of second growth ponderosa pine is affected positively or negatively by the application of either spring or fall prescribed fire to thiimed stands.