Tree-architectural response to restoration thinning pattern in post-fire Aleppo pine forests – A modular view of tree plasticity

Tree-crown architecture is a critical modulator of tree performance and interactions with neighboring trees and, therefore, of forest structure and function. However, little is known about how the release of competition through restoration thinning may shape crown architecture and its modular, spatial and temporal variation. Here we investigate the tree-architecture response to thinning and to the spatial pattern of thinning in young, hyper-dense post-fire Aleppo pine ( Pinus halepensis Mill.) forests, and examine (i) whether such response is consistent at shoot, branch and tree levels, and between structural-support traits and leaf biomass; (ii) the relative within-tree and between-tree plasticity in the response of tree-architecture to thinning; and (iii) to what extent the architectural effects of thinning and thinning pattern, if any, vanish with time after treatment application. At three replicated sites in eastern Spain, we implemented the following treatments: Regular-pattern thinning, which left evenly spaced pines to a final density of 600 trees/ha, (600R); Aggregated-pattern thinning, which left clumps of ∼25 pines surrounded by clear-cut areas to a final total density of 600 trees/ha (600A); and No thinning (Control). Thinning increased structural-support trait values and leaf biomass at all organizational levels, without significant differences between the two thinning patterns. Thinning also weakened substantially the allometric relationships between tree and branch dimensions, suggesting a decoupling of their structural interdependency and a strong autonomy of branches. Branch structural traits appeared to be particularly sensitive to thinning, which led to a strong increase in higher-order branching and shoot length. The leaf-biomass/structural-support ratio at tree level showed a significant trend towards lower values for Control pines. Thinning reduced the phenotypic variation of shoot and branch traits, which showed a decreasing, trend from Control to 600R pines. However, while regular thinning similarly decreased the phenotypic variation within and between trees, aggregate thinning resulted in a smaller reduction in within-tree variation than in between-tree variation. The primary growth of shoots and the level of higher-order branching were strongly controlled by ontogeny, which contributed to the waning of thinning impact over time. Our findings shed light on the potential of thinning −and its pattern− to shape tree-crown architecture and enhance architecture-dependent functions in pine trees and forests, which can be of help to designing and guiding silvicultural practice for a variety of management objectives. • Thinning improves tree architecture at all organizational levels. • Branches displays a strong degree of modular autonomy in their response to thinning. • Thinning impact at branch level is stronger than its impact at the whole-tree level. • Aggregated thinning promotes higher within-tree than between-tree crown variation. • Ontogeny controls shoot length, while growing-season conditions control leaf biomass.