Disentangling the effects of competition and climate on individual tree growth: A retrospective and dynamic approach in Scots pine

Abstract

Climate, stand structure and local site conditions are potentially important determinants of forest dynamics. Understanding the relative contributions of competition and climate to tree growth is critical to project likely stand development under different climate change scenarios. Further, current competition levels and stand structure may reflect legacies of past forest management. Here, we analyze the effects of climate, site conditions and competition on radial growth in three Scots pine plots located along an altitudinal gradient. These stands are subjected to Mediterranean climate with continental influence, i.e., growth is limited by low winter temperatures and dry summer conditions. Current stand structure and retrospective analyses of radial growth (basal area increment, BAI) were used to model changes in tree growth as a function of competition (CI) and climate at an annual resolution. Negative exponential functions characterized the CI-BAI associations, whereas linear mixed-effects models were used to model BAI and to quantify the growth response to climate of trees under low and high competition. Competition effects on growth were steady over time regardless of the elevation and tree age. High competition levels negatively affected growth, with a proportionally stronger influence in suppressed trees than in dominant trees. Sensitivity of tree growth to climate increased with decreasing competition. Altitudinal gradientrelated growth responses to climate were found only for temperature variables. Specifically, growth at high elevations was mainly limited by low winter temperatures, whilst warm spring enhanced growth at middle elevations and late summer temperatures did it at low elevations. Since growth and its sensitivity to climate is more pronounced in low competition trees, we argue that the past management of the forest overrides site conditions and climate effects through the legacies on stand structure and competition. Pro-active forest management practices should be adopted to reduce the vulnerability of previously managed Scots pine forests currently threatened by the predicted warmer and drier conditions.

Variation in the Climate Sensitivity Dependent on Neighbourhood Composition in a Secondary Mixed Forest

“…Climate and competition are the two major drivers that limit tree growth, but competition from neighbours may cause a greater reduction in growth than climate, at least in temperate forests [9,[22][23][24]. However, competition for limiting resources and climatic stress, are interrelated factors [9,23,25]. Dense stands with high competition magnify the effect of extreme weather events (e.g., drought) on growth [20,23,24,26,27].…”

Drought response of upland oak (Quercus L.) species in Appalachian hardwood forests of the southeastern USA

“…Stem size (i.e., DBH) was reconstructed using individual tree BAI values and the initial prethinning DBH value. We chose to use a measure of relative density (RD) in lieu of absolute measures of density (e.g., BA or stems/ha) because plots were of mixed-species composition and variation in species composition often confounds the effects of absolute density on growth (Roach 1977;Stout and Nyland 1986). Relative density was calculated as the quotient of stand density index (SDI) (Reineke 1933) during each inventory period and maximum SDI.…”

“…There is a growing body of literature that describes and quantifies the interacting effects of competition and climate, and drought, in particular, on individual tree growth (e.g., Kohler et al 2010;Martínez-Vilalta et al 2012;Magruder et al 2013;Keyser and Brown 2014;Sánchez-Salguero et al 2015). Thinning increases individual tree growing space and reduces competition for soil moisture (Martín-Benito et al 2010).…”

“…Retrospective analyses suggest that individual tree-and stand-level growth during drought events is modulated by stand density and intraspecific and interspecific competitive interactions (Piutti and Cescatti 1997;D'Amato et al 2013; Lebourgeois et al 2013;Sánchez-Salguero et al 2015). As such, the manipulation of stand density and associated competitive interactions via silvicultural thinning is advocated by many as an adaptation strategy that promotes the resilience of individual trees, stands, and forests to future droughtrelated decreases in growth and productivity (Bréda and Badeau 2008;Klos et al 2009;Martin-Benito et al 2010;Gómez-Aparicio et al 2011).…”

“…The authors observed that thinning to low densities at a young age (49 years) conferred increased drought resistance while the reverse was observed during a drought event occurring at an older age (76 years). Variability in the effects of density or competition on climategrowth relationships and, more specifically, response to drought have been observed in natural and plantation-origin forests in Europe (e.g., Gea-Izquierdo et al 2009;Lebourgeois et al 2014;Sánchez-Salguero et al 2015) and, although to a far lesser extent, North America (e.g., McDowell et al 2006;D'Amato et al 2013;Keyser and Brown 2014). Results from these studies suggest that the development of thinning prescriptions intended to increase resiliency to extreme climatic events, including drought, must account for variation across species, emergent stand properties, and environmental conditions.…”