Spatial patterns and species coexistence in mixed Abies marocana-Cedrus atlantica forest in Talassemtane National Park

Abstract

Understanding spatially-explicit interactions between co-existing tree species provides valuable information about the trade-offs between facilitation and competition driving species coexistence and forest dynamics. Here, we analyzed the fine scale spatial pattern of Moroccan fir (Abies marocana) and Cedrus atlantica (Cedrus atlantica) in a mixed forest (Talassemtane National Park, northern Morocco) and discuss its consequences on intra- and inter-specific interactions and stand dynamics. All trees were mapped and measured in ten plots to investigate uni- and bivariate spatial point patterns using pair-correlation and mark correlation functions. Tree ring data was also investigated from increment cores to assess the growth and age patterns of both species. Fir and cedar trees showed similar basal area and mean diameter distributions, while A. marocana presented a higher density and regeneration. There was an overall aggregation of conspecific and heterospecific individuals at small distances up to 2 m. Univariate analysis of tree size classes supports that juveniles of both species, as well as A. marocana adult trees, show an aggregated pattern at fine scales up to 2 m, while C. atlantica adults exhibit a random pattern. Observed spatial patterns are consistent with the hypothesis of attraction mechanisms between fir and adult trees of C. atlantica. Abies marocana recruitment mainly occurred inside the canopy of dominant trees (at distances varying from c.a 1 m under cedar adults to 3 m under adult conspecifics), while C. atlantica was the dominant regeneration in small gaps. The studied A. marocana-C. atlantica mixed forests showed a transition from an initiation phase led by C. atlantica to a subsequent stem exclusion phase, where dominant trees shade out less-competitive trees and mainly fir trees can establish in the forest understory. According to the observed spatial patterns and growth rates, enhancing C. atlantica establishment in degraded areas may be a suitable proactive and adaptive management to promote long-term conservation of endemic Abies marocana under current climate change trends.