The role of stand density on growth efficiency, leaf area index, and resin flow in southwestern ponderosa pine forests

We examined the response of growth efficiency (GE), leaf area index (LAI), and resin flow (RF) to stand density manipulations in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests of northern Arizona, USA. The study used a 40 year stand density experiment including seven replicated basal area (BA) treatments ranging from 7 to 45 m²·ha^-1. Results were extended to the larger region using published and unpublished datasets on ponderosa pine RF. GE was quantified using basal area increment (BAI), stemwood production (NPP_s), or volume increment (VI) per leaf area (A _l) or sapwood area (A_s). GE per A_l was positively correlated with BA, regardless of numerator (BAI/A_l, NPP_s/A_l, and VI/A_l, r² = 0.84, 0.95, and 0.96, respectively). GE per A_s exhibited variable responses to BA. Understory LAI increased with decreasing BA; however, total (understory plus overstory) LAI was not correlated with BA, GE, or RF. Opposite of the original research on this subject, resin flow was negatively related to GE per A _l because A_l/A_s ratios decline with increasing BA. BAI, and to a lesser degree BA, predicted RF better than growth efficiency, suggesting that the simplest measurement with the fewest assumptions (BAI) is also the best approach for predicting RF.