Analyzing size-symmetric vs. size-asymmetric and intra-vs. inter-specific competition in beech (Fagus sylvatica L.) mixed stands

In mixed stands, inter-specific competition can be lower than intra-specific competition when niche complementarity and/or facilitation between species prevail. These positive interactions can take place at belowground and/or aboveground levels. Belowground competition tends to be size symmetric while the aboveground competition is usually for light and almost always size-asymmetric. Interactions between forest tree species can be explored analyzing growth at tree level by comparing intra and inter-specific competition. At the same time, possible causes of niche complementarity can be inferred relating intra and inter-specific competition with the mode of competition, i.e. size-symmetric or sizeasymmetric. The aim of this paper is to further our understanding of the interactions between species and to detect possible causes of competition reduction in mixed stands of beech (Fagus sylvatica L.) with other species: pine?beech, oak?beech and fir?beech. To test whether species growth is better explained by size-symmetric and/or size-asymmetric competition, five different competition structures where included in basal area growth models fitted using data from the Spanish National Forest Inventory for the Pyrenees. These models considered either size-symmetry only (Reineke?s stand density index, SDI), size-asymmetry only (SDI of large trees or SDI of small trees), or both combined. In order to assess the influence of the admixture, these indices were introduced in two different ways, one of which was to consider that trees of all species compete in a similar way, and the other was to split the stand density indices into intra- and inter-specific competition components. The results showed that in pine?beech mixtures, there is a slightly negative effect of beech on pine basal area growth while beech benefitted from the admixture of Scots pine; this positive effect being greater as the proportion of pine trees in larger size classes increases. In oak?beech mixtures, beech growth was also positively influenced by the presence of oaks that were larger than the beech trees. The growth of oak, however, decreased when the proportion of beech in SDI increased, although the presence of beech in larger size classes promoted oak growth. Finally, in fir?beech mixtures, neither fir nor beech basal area growth were influenced by the presence of the other species. The results indicate that size-asymmetric is stronger than size-symmetric competition in these mixtures, highlighting the importance of light in competition. Positive species interactions in size-asymmetric competition involved a reduction of asymmetry in tree size-growth relationships

Changes in structural heterogeneity and stand productivity by mixing Scots pine and Maritime pine

Mixed-species stands have been studied extensively due to their potentially superior productivity, multi-functionality benefits and high ecological value compared to pure stands. The higher structural heterogeneity in mixed stands that can emerge from species interactions could be linked to the relationship between species diversity and ecosystem functions. We tested whether changes in stand structure also occur in mixtures of species with similar traits and whether they explain over-yielding patterns. Based on research with 12 triplets of Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Ait.) in the northern Iberian Peninsula (Spain), we provide evidence that species mixing increased structural heterogeneity and may induce over-yielding in mixed-species stands compared to monospecific stands. In this mixture of two light-demanding species, we observed that (i) stand composition influenced the inter-specific crown allometric variation, (ii) structural heterogeneity in mixed stands was caused by both specie-specific traits and species interactions, and (iii) intraspecific and interspecific differences in both crown size plasticity and size-distribution differentiation were associated with the increased relative productivity of mixed stands. We detected that crown complementarity and vertical stratification in the canopy space is a crucial mechanism for enhancing ecosystem productivity in light-demanding species and could be related to light interception and light-use. This work improves our understanding of emerging properties in mixed stands and introduces considerations for properly scaling and tracing mixing effects at individual tree, size distribution and stand levels.Instituto Universitario de Gestión Forestal Sostenibl

Desfronde y tasa de Descomposición foliar en rebollar-pinar de repoblación con distinto grado de clara

El desfronde representa un factor clave en el funcionamiento de los ecosistemas forestales. Por un lado representa el mayor aporte de nutrientes en sistemas naturales y, por otro, su acumulación en el suelo constituye un banco de elementos esenciales que, por descomposición y mineralización, se integran en el suelo y en los ciclos biogeoquímicos. Este trabajo analiza el efecto de la clara en la producción de desfronde, la eficiencia en el uso del nitrógeno y en la tasa de descomposición de la hojarasca en una masa mixta de Pinus pinaster Ait. procedente de repoblación sobre una masa natural de Quercus pyrenaica Wild. El sitio experimental está situado en los Montes de Toledo y consta de tres tratamientos: control, clara moderada y clara fuerte con tres réplicas en un diseño de cuadrado latino. Los datos de desfronde proceden de recogida mensual en cestos y separación en fracciones para el cálculo de la materia seca. El nitrógeno se analizó en muestras compuestas cada tres meses. Para el estudio de la tasa de descomposición se utilizaron bolsas con mezcla de acículas y hojas, recolectándose y analizándose también cada tres meses. Los resultados muestran picos de desfronde en los meses de verano y en el otoño. La mayor producción de desfronde se observa en las parcelas testigo. La aplicación de las claras no supone grandes diferencias ni en el desfronde ni en la eficencia en el uso del nitrógeno. Por el contrario las claras ralentizan la tasa de descomposición de las hojas de rebollo

Efecto de la gestión forestal en la cantidad de carbono del suelo mineral en una masa de Pinus pinaster Ait.

El conocimiento del impacto de la gestión forestal sobre los diversos compartimentos de carbono existentes en una masa forestal es prioritario para la sostenibilidad del aprovechamiento y gestión de los sistemas forestales. Con la aplicación de tratamientos selvícolas hay una serie de transformaciones en la masa como la reducción de biomasa en pie, la modificación de la cantidad de residuos o la disminución de la incorporación de hojarasca, la alteración del microclima del suelo por mayor presencia de luz y su influencia en las condiciones de humedad y temperatura. Este cambio en las condiciones modifica el contenido de carbono en el suelo. En este trabajo se ha estudiado la influencia que tiene la gestión forestal en las cantidades de carbono y nitrógeno presentes en los 30 primeros cm del suelo mineral más la hojarasca y humus de una masa de Pinus pinaster Ait., a partir de la comparación entre los cantidades de carbono bajo tres intensidades de claras. A partir de los resultados obtenidos no se han encontrado diferencias significativas entre los tratamientos, tanto en la cantidad de carbono y nitrógeno almacenado en la capa orgánica como en la capa mineral del suelo, por lo cual la realización de claras fuertes no modifica sustancialmente las cantidades de carbono acumuladas

Mixing effects on growth efficiency in mixed pine forests

Producción CientíficaIncreased interest in mixed forests is due to evidence of them being more resource-use efficient and stable forest systems. However, intrinsic and extrinsic factors moderate interspecific species interactions generating different effects in productivity. Here, we explore a method to detect mixing effects in a specific mixture combination (Pinus sylvestris L. and Pinus pinaster Ait.), comparing the growth of mixed stands with that of monocultures. Combined tree and stand-level analyses also helped determine which mixing effects are most important for forest functioning and how changes at one level influence patterns at another level. Data from the Spanish National Forest Inventory were used to compare growth efficiency in mixed and pure stands; we relied on relative stand density indices to determine species-specific site occupancy. This same concept was used to evaluate competition status and inter/intra-specific competition effects as modifiers of potential growth at the tree-level. We observed that growth efficiency in both species increased with the proportion of the complementary species in the stand. At the tree-level, intraspecific competition was higher than interspecific competition in Scots pine tree growth, showing that it had benefited from the mixture. In contrast, maritime pine did not show a competitive response to the interspecific interaction, indicating that tree growth was more strongly influenced by the competition structure (size-symmetric and size-asymmetric) than by the species of the competitors. Our results highlight the importance of combining stand-level analysis with that of tree-specific competition relationships when studying mixed-species forests.Ministerio de Economía, Industria y Competitividad (projects AGL-2014-51964-C2-1-R and AGL-2014-51964-C2-2-R)Secretaría Nacional de Educación Superior, Ciencia, Tecnología e Innovación de Ecuador (SENESCYT) (grant 2013-AR3R1676

Ensayo de claras en una masa natural de "Pinus sylvestris" L. en el Sistema Central

En una masa natural regular de pino silvestre de calidad de estación media situada en el Sistema Central se ha realizado un ensayo en el que se comparan cuatro tratamientos, tres regímenes de claras en los que varía la intensidad y un tratamiento testigo (masa no aclarada), mediante un diseño experimental de bloques al azar. En este trabajo se presentan los efectos de los distintos regímenes de claras en la producción en volumen total, así como en las características del árbol medio y en la estructura de la masa. Los resultados muestran que con las claras más intensas el volumen total es ligeramente inferior, aunque esta pérdida es pequeña en las primeras edades. Por otra parte, las características del árbol medio y de la estructura de la masa son más favorables cuanto más intensas han sido las claras. Con la aplicación de claras fuertes se obtienen mayores dimensiones y se eliminan los individuos dominados y con coeficientes de esbeltez elevados, mejorando la estabilidad y el vigor de la masa junto con su valor productivo

Effect of species proportion definition on the evaluation of growth in pure vs. mixed stands

Aim of study: The aim of this paper is to compare differences in growth per hectare of species in pure and mixed stands as they result from different definitions of species proportions.Area of Study: We used the data of the Spanish National Forest Inventory for Scots pine and beech mixtures in the province of Navarra and for Scots pine and Pyrenean oak mixtures in the Central mountain range and the North Iberic mountain range.Material and Methods: Growth models were parameterized with the species growth related to its proportion as dependent variable, and dominant height, quadratic mean diameter density, and species proportion as independent variables. As proportions we use once proportions by basal area or by stand density index and once these proportions considering the species specific maximum densities.Main Results: In the pine – beech mixtures, where the maximum densities do not differ very much between species, the mixing effects are very similar, independent of species proportion definitions. In the pine – oak mixture, where the maximum densities in terms of basal area are very different, the equations using the proportions calculated without reference to the maximum densities, result in a distinct overestimation of the mixing effects on growth.Research highlights: When comparing growth per hectare of a species in a mixed stand with that of a pure stand, the species proportion must be described as a proportion by area considering the maximum density for the given species, wrong mixing effects could be introduced by inappropriate species proportion definitions.Keywords: Mixing effects; proportion by area; Stand Density Index; overyielding; Pinus sylvestris L.; Fagus sylvatica L.; Quercus pyrenaica Willd.