Rates and Mechanisms of Subalpine Forest Succession Along an Environmental Gradient

This paper examines how rates and mechanisms of succession vary spatially and temporally in xeric, subalpine forests in Colorado, United States. We reconstructed 300 years of succession from limber pine (Pinus flexilis) to Engelmann spruce (Picea engelmanii) and subalpine fir (Abies lasiocarpa) in two watersheds recovering from the same fire. More than 1850 live and dead trees were cored and mapped in 25 plots systematically spaced along a topographic gradient. We used tree ring analysis to reconstruct dates of tree establishment and death. Relative species abundances and basal areas were charted at 20-yr intervals, and three measures of ecological similarity were also used to capture different elements of change: simple Euclidean distance, Euclidean distance in ordination space, and Horn’s CH similarity index. Successional rates for mesic lower slopes and north aspects were roughly twice those on south-facing side slopes. Rates were positively correlated with soil P, N, Fe, organic matter, and Parker’s topographic moisture index, and negatively correlated with solar radiation and surface rock cover. Higher rates on mesic plots were primarily the result of heavy overstory mortality of pioneer limber pines ∼200 yr postfire, coupled with higher recruitment of spruce and fir 200–300 yr postfire. On mesic sites, successional rates peaked during this thinning phase of limber pine, but rates were stable or slowly increasing 300 yr postfire on xeric sites. Previous studies have shown that Clark’s Nutcracker catalyzes early succession by caching limber pine seeds in extensive burns, many of which germinate to form multitrunk pines. Limber pines, in turn, may act as nurse trees for spruce and fir. However, we reconstructed past spatial patterns of trees and found that limber pine mortality in midsuccession was strongly related to its clumped pattern of establishment and tendency to attract spruce and fir. Solitary pines with no spruce and fir neighbors within a 2 m radius had a >80% chance of survival, 1893–1993. In contrast, pines within clumps of six or more trees and surrounded by many spruce and fir trees had only 10–20% chance of survival, 1893–1993. The multitrunk growth form of limber pines may be selectively advantageous on harsh sites, including early postfire environments, but our results suggest that this same growth form may be disadvantageous on mesic sites with heavy spruce and fir competition during mid- to late-successional stages. We confirmed the equilibrium prediction that succession is more rapid on mesic sites, but even in this very simple system, demographic mechanisms of succession were relatively complex.