Age‐related Crown Thinning in Tropical Forest Trees

Abstract

Gap dynamics theory proposes that treefall gaps provide high light levels needed for regeneration in the understory, and by increasing heterogeneity in the light environment allow light‐demanding tree species to persist in the community. Recent studies have demonstrated age‐related declines in leaf area index of individual temperate trees, highlighting a mechanism for gradual changes in the forest canopy that may also be an important, but less obvious, driver of forest dynamics. We assessed the prevalence of age‐related crown thinning among 12 tropical canopy tree species sampled in lowland forests in Panama and Puerto Rico (total N = 881). Canopy gap fraction of individual canopy tree crowns was positively related to stem diameter at 1.3 m (diameter at breast height) in a pooled analysis, with 10 of 12 species showing a positive trend. Considered individually, a positive correlation between stem diameter and canopy gap fraction was statistically significant in 4 of 12 species, all of which were large‐statured canopy to emergent species: Beilschmiedia pendula, Ceiba pentandra, Jacaranda copaia, and Prioria copaifera. Pooled analyses also showed a negative relationship between liana abundance and canopy gap fraction, suggesting that lianas could be partially obscuring age‐related crown thinning. We conclude that age‐related crown thinning occurs in tropical forests, and could thus influence patterns of tree regeneration and tropical forest community dynamics.

Interpreting forest diversity-productivity relationships: volume values, disturbance histories and alternative inferences

“…This can be understood as a result of the initial influx and establishment of fast growing (in wet forest typically low-wood-density) early successional pioneer species, with the forest becoming more efficient at capturing light as the more shade-tolerant, later-successional species also become established. The decline likely results from the reduced efficiency (per unit area) of light interception and photosynthesis of larger (versus smaller) trees (Yoder et al 1994;Niinemets et al 2005;Nock et al 2008;Drake et al 2011;Quinn and Thomas 2015) and the proportion of energy invested in woody growth (Kaufmann and Ryan 1986;Mencuccini et al 2005;Thomas 2010).…”

Does biomass growth increase in the largest trees? Flaws, fallacies and alternative analyses

“…Informal perceptions of tree vigour indicate that the crowns on the largest oldest trees are often open and patchy with dieback long before tree death. Formal observations indicate declining foliar efficiency, photosynthetic rates and light interception (Yoder et al 1994;Niinemets, Sparrow & Cescatti 2005;Nock, Caspersen & Thomas 2008;Drake et al 2011;Quinn & Thomas 2015), and the proportion of intercepted energy invested in stem growth declines with size (Kaufmann & Ryan 1986;Mencuccini et al 2005;Thomas 2010).…”

“…; Niinemets, Sparrow & Cescatti ; Nock, Caspersen & Thomas ; Drake et al . ; Quinn & Thomas ), and the proportion of intercepted energy invested in stem growth declines with size (Kaufmann & Ryan ; Mencuccini et al . ; Thomas ).…”