Abstract
Key messageCompetitive interactions change over time and their influence on tree growth is intensified during drought events in marginal Scots pine populations.Competition is a key factor driving forest dynamics and stand structure during the course of stand development. Although the role of neighbourhood competition on stand dynamics has received increasing attention, the response of competition to environmental fluctuations and stand development remains poorly explored. We evaluated changes in competition during stand development in a dry-edge Scots pine relict population located in Central Spain. Typically, tree-to-tree interactions have been investigated through static competition measurements, which usually lack the temporal variation associated to natural forest development and environmental conditions. Here, we assessed how individual and neighbourhood components of competition evolved along a 35-year period, and we related competition dynamics to population structure and drought levels. On six plots, 508 trees were mapped and diameters at breast height (DBH) were measured. Two increment cores were taken from target trees to derive basal area increment (BAI), and neighbourhood was reconstructed back to 1980. Results provide insights into inter-annual variability in competition effects and their role on tree radial growth depending on climatic conditions. From the year 2005 onwards, both individual and neighbourhood components of competition showed a decoupled pattern over time. This effect was particularly pronounced during the extreme drought in 2012, in which the individual component decreased, whereas the neighbourhood component increased. In addition, climatic variability modulated the competition effects during stand development. This approach of evaluating competition dynamics proves to be promising for studying forest stand development and the influence of climate impacts on tree populations subjected to xeric conditions.
Highlights
Competition is a key biotic factor driving stand dynamics (Oliver and Larson 1996; Kunstler et al 2011) through its influences on light, soil water and nutrients interception (Coomes and Allen 2007; Das et al 2008)
We address the following questions: (i) What is the effect of target tree size and neighbouring trees on tree growth through time? and (ii) To what extent does climatic variability modulate such components of competition in a dry-edge Scots pine population? We hypothesize that competition will be strongly tied to structure dynamics in the sense that increasing space filling would lead to progressively stronger competitive interactions among individuals
Growth of the Scots pine trees was positively correlated with target tree size (Appendix S5a), and negatively correlated with total basal area of all neighbouring trees located within the area around a target stem (Appendix S5b)
Summary
Competition is a key biotic factor driving stand dynamics (Oliver and Larson 1996; Kunstler et al 2011) through its influences on light, soil water and nutrients interception (Coomes and Allen 2007; Das et al 2008). Different competition indices have often been applied; these indices either used distance-independent metrics at the stand level (Biging and Dobbertin 1995) or distance-dependent indices taking the spatial distribution of trees into account (Canham et al 2006; McTague and Weiskittel 2016; Tatsumi et al 2016) Most of these studies have investigated spatial interactions among individual trees using static approaches (Brooker and Callaghan 1998; Coomes and Allen 2007), which lack a temporal component and may be inadequate to represent competitive dynamics over time (Burton 1993; De Luis et al 1998). Understanding how competition evolves over time is essential to evaluate how forest functioning might be compromised under changing environmental conditions
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