Beyond shading: Litter production by neighbors contributes to overyielding in tropical trees

Abstract

The influence of biodiversity on ecosystem functioning is now well established. However, our ability to predict the ecological consequences of biodiversity changes remains limited by our poor understanding of the mechanisms underlying biodiversity effects. We disentangled the contributions of light competition and residual neighborhood interactions in a 10‐year‐old biodiversity experiment with tropical trees that display overyielding, i.e., higher community‐level yields in mixtures compared with monocultures. We developed models of individual tree growth that partition the effects of neighboring trees into shading and residual effects assumed to reflect primarily belowground interactions. These models reject the hypothesis that reduced light competition in mixtures is the only mechanism driving overyielding. After factoring out the effects of shading, litter production by neighbors was a far better predictor of tree growth than traditional crowding indices; it contributed to overyielding by producing pairwise interactions that ranged from competitive to facilitative, but which, on average, concentrated competition within species. Consistent with litter‐mediated biodiversity effects, the magnitude of overyielding increased over time. Our results provide evidence for diversity effects extending beyond that of light and reveal the neglected role of litter‐mediated interactions among trees.

Are Mixed Tropical Tree Plantations More Resistant to Drought than Monocultures?

“…Trees growing in mixtures thus had a longer growing season and a higher biomass accumulation of those trees compared to trees growing in mono-specific stands. Further explanations for the observed over-yielding of mixed-species plots in the plantation in Panama were belowground niche differentiation [27,28] and enhanced light interception due to aboveground niche partitioning [29]. Overall, we can conclude that niche differentiation and complementarity due to functional diversity play a key role in the resilience of tropical trees to changing climatic conditions.…”

“…Between July 2007 and August 2008, a series of ecophysiological studies regarding the ecohydrology and the above-and below-ground niche separation and facilitation in the different monocultures and mixture plots were conducted (compare [26][27][28][29]46]). The ecohydrological studies included intensive assessment of annual tree transpiration rates in the different mixtures via sap flux measurements (for details see [26,46,61]) and the analysis of soil water uptake pattern with stable isotope (deuterium) measurements (for details see [27]).…”

Maximum likelihood methods and their application in neighbourhood models

“…Por ejemplo, diversos artículos han usado los modelos de vecindad como experimentos virtuales para separar la importancia relativa de la competencia sobre el suelo y bajo el suelo entre árboles vecinos , Coates et al 2009, Sapijanskas et al 2013. La metodología consiste en separar el efecto de los vecinos en dos componentes -su efecto sobre el ambiente lumínico y un efecto residual que se asume refleja fundamentalmente interacciones bajo el suelo-y comparar modelos que incluyan dichos efectos juntos o por separado.…”

“…El efecto de los distintos vecinos sobre el ambiente lumínico es fácilmente predecible midiendo variables de la geometría y copa de los árboles (Canham et al 1999, Ameztegui et al 2012. El efecto residual, por su parte, es cuantificado generalmente mediante un índice de vecindad (NI) función del tamaño y distancia a los árboles vecinos (ver ecuaciones 2 y 3), aunque trabajos recientes han encontrado que índices más específicos basados en la producción de hojarasca pueden proporcionar una mejor explicación del efecto de los vecinos sobre el crecimiento de árboles (Sapijanskas et al 2013). Estos trabajos concuerdan en concluir que la reducción en el crecimiento de adultos debido a la competencia con árboles vecinos es debida en mayor medida a competencia por los recursos del suelo que por la luz, excepto para la especies más intolerantes de la sombra.…”

Local densities connect spatial ecology to game, multilevel selection and inclusive fitness theories of cooperation

“…If we are willing to take pair densities or probabilities of identity as quantities that can be measured and not necessarily mechanistically derived, then we may be able to open up the discussion of space and cooperation to empirical application. For this purpose, we will turn to the related and empirically applied metrics -local densities -which originate in neighbourhood models of plant interaction in spatial ecology (Pacala and Silander, 1985;Sapijanskas et al, 2013).…”