Structure, production and resource use in some old-growth spruce/fir forests in the Front Range of the Rocky Mountains, USA

Abstract

Old-growth forests of Engelmann spruce ( Picea engelmannii Parry ex. Engelm.) and subalpine fir ( Abies lasiocarpa (Hook.) Nutt .) dominate much of the landscape of the Rocky Mountains. We characterized the structure, biomass and production of 18 old-growth (200–450-year-old) spruce/fir forests in Rocky Mountain National Park, Colorado, as well as the stand-level supply and use of light and nitrogen. Stands were chosen to span a broad range of elevation, aspect, and topography. Aboveground tree biomass in these old-growth forests averaged 253 Mg/ha (range 130–488 Mg/ha), with aboveground net primary production of 3700 kg ha −1 yr −1 (range from 2700 to 5200 kg ha −1 yr −1). Within stands, trees >35 cm in diameter accounted for 70% of aboveground biomass, but trees <35 cm contributed 70% of the production of woody biomass. Differences in slope and aspect among sites resulted in a range of incoming light from 58 to 74 TJ ha −1 yr −1, and tree canopies intercepted an average of 71% of incoming light (range 50–90%). Aboveground net primary production (ANPP) of trees did not relate to the supply of light or N, but ANPP correlated strongly with the amount of light and N used ( r 2=0.45–0.54, P<0.01). Uptake of 1 kg of N was associated with about 260 kg of ANPP, and one TJ of intercepted shortwave radiation produced about 78 kg of ANPP. Across these old-growth stands, stands with greater biomass showed higher rates of both ANPP and resource use; variation in aboveground biomass was associated with 24% of the variation in N use ( P=0.04), 44% of the light use ( P=0.003), and 45% of the ANPP ( P=0.002).