BUSCADOR RECOLECTA

Natural and anthropogenic factors affect forest structure worldwide, primarily affecting
forest canopy and its light properties. However, not only stand-replacing events modify canopy
structure, but disturbances of lower intensity can also have important ecological implications. To
study such effects, we analyzed long-term changes in light properties of a conifer–broadleaf mixed
forest in the Southwestern Pyrenees, placed in the fringe between the Mediterranean and Eurosi-
berian biogeographical regions. At this site, a thinning trial with different intensities (0%, 20%, and
30–40% basal area removed) took place in 1999 and 2009, windstorms affected some plots in 2009 and
droughts were recurrent during the sampling period (2003, 2005, 2011). We monitored light properties
during 14 years (2005–2019) with hemispherical photographs. We applied partial autocorrelation
functions to determine if changes between years could be attributed to internal canopy changes or
to external disturbances. In addition, we mapped the broadleaf canopy in 2003, 2008, and 2016 to
calculate broadleaf canopy cover and richness at the sampling points with different buffer areas of in-
creasing surface. We applied generalized linear mixed models to evaluate the effects of light variables
on canopy richness and cover. We found that light variables had the most important changes during
the period 2008 to 2010, reacting to the changes caused that year by the combined effects of wind and
forest management. In addition, we found that an area of 4.0 m radius around the sampling points
was the best to explain the relationship between light properties and species richness, whereas a
radius of 1.0 m was enough to estimate the relationship between light and canopy cover. In addition,
light-related variables such as diffuse light and leaf area index were related to species richness,
whereas structural variables such as canopy openness were related to canopy cover. In summary, our
study demonstrates that non stand-replacing disturbances such as windstorms, thinning, or droughts
can have an important role in modifying structural and light-related canopy properties, which in
turn may influence natural processes of stand development and ecological succession., This research was funded by the Spanish Ministry of Economy and Competitiveness, grants
numbers AGL2006-08288, AGL2009-11287, AGL2012-33465, and AGL2016-76463-P. J.R.P. was funded
by the La Caixa Foundation and Caja Navarra Foundation, under agreement LCF/PR/PR13/51080004
in the framework of the Public University of Navarre’s “Captación de Talento” program.

Management of mixedwoods is advocated as an effective adaptation strategy to increase ecosystem
resiliency in the context of climate change. While mixedwoods have been shown to have greater resource
use efficiency relative to pure stands, considerable uncertainty remains with respect to the underlying
ecological processes. We explored species interactions in Scots pine / European beech mixedwoods with
the process-based model FORECAST Climate. The model was calibrated for two contrasting forests in
the southwestern Pyrenees (northern Spain): a wet Mediterranean site at 625 m.a.s.l. and a subalpine site
at 1335 m.a.s.l. Predicted mixedwood yield was higher than that for beech stands but lower than pine
stands. When simulating climate change, mixedwood yield was reduced at the Mediterranean site (-33%)
but increased at the subalpine site (+11%). Interaction effects were enhanced as stands developed.
Complementarity dominated the Mediterranean stand but neutral or net competition dominated the
subalpine stand, which had higher stand density and water availability. Reduced water demand and
consumption, increased canopy interception, and improved water-use efficiency in mixtures compared to
beech stands suggest a release of beech intra-specific competition. Beech also facilitated pine growth
through better litter quality, non-symbiotic nitrogen fixation and above- and belowground stratification,
leading to higher foliar nitrogen content and deeper canopies in pines. In conclusion, mixtures may
improve water availability and use efficiency for beech and light interception for pine, the main limiting
factors for each species, respectively. Encouraging pine-beech mixtures could be an effective adaptation
to climate change in drought-prone sites in the Mediterranean region., Ramón y Cajal contract, Grant/Award Number:RYC‐2011‐08082; Spanish Ministry of Economy and Competitiveness, Grant/AwardNumber: AGL2012‐33465; mobility aid,Grant/Award Number: EEBB‐I‐15‐09220; Spanish Predoctoral Research Grant, Grant/Award Number: BES‐2013‐066705.

Research Highlights: Thinning and tree species alter the forest floor microclimate by modifying canopy cover, radiation, wind, and humidity. Thus, forest management can directly influence the edaphic mesofauna responsible for decomposing coarse woody debris (CWD). Background and Objectives: This research was carried out in the Southwestern Pyrenees Mountains (Northern Spain) and aimed to determine the influence of forest thinning and canopy type (pure Pinus sylvestris L. or a mix of P. sylvestris and Fagus sylvatica L.) on CWD colonization by edaphic fauna. Materials and Methods: CWD samples were collected belonging to intermediate and advanced decomposition stages, approximately 10 cm long and 5 cm in diameter. Using a design of three thinning intensities (0%, 20%, and 40% of basal area removed), with three replications per treatment (nine plots in total), four samples were taken per plot (two per canopy type) to reach 36 samples in total. Meso- and macrofauna were extracted from CWD samples with Berlese-Tullgren funnels, and individuals were counted and identified. Results: 19 taxonomic groups were recorded, the most abundant being the mesofauna (mites and Collembola). Mixed canopy type had a significant positive influence on richness, whereas advanced decay class had a positive significant influence on total abundance and richness. In addition, there were non-significant decreasing trends in richness and abundance with increasing thinning intensity. However, interactions among thinning intensity, canopy type, and decay class significantly affected mesofauna. Furthermore, some taxonomic groups showed differential responses to canopy type. CWD water content was positively correlated with total invertebrate abundance and some taxonomic groups. Our results suggest that stand composition has the potential to directly affect invertebrate communities in CWD, whereas stand density influence is indirect and mostly realized through changes in CWD moisture. As mesofauna is related to CWD decomposition rates, these effects should be accounted for when planning forest management transition from pure to mixed forests., This work has been funded through the AGL2012-33465 projects of the Ministry of Economy and Competitiveness, and the CIG-2012-326718-ECOPYREN3 project of the Marie Curie Actions of the 7th Framework Program of the European Commission. The first author of this article conducted this research during her MSc. thesis at the Public University of Navarre sponsored by the grants of the Carolina Foundation of Spain. Published with the support of the Marie Curie Alumni Association.

La gestión de bosques mixtos se ha convertido en una estrategia de adaptación para reducir los riesgos relacionados con el cambio climático. A su vez, los modelos ecológicos pueden ser una herramienta útil para el estudio del crecimiento y la productividad de dichas masas. En este trabajo se presenta una evaluación de la capacidad del modelo híbrido “FORECAST Climate” de simular el estrés hídrico y la productividad en bosques mixtos de pino silvestre y haya en Navarra (norte de España) y su interacción con distintos niveles de densidad de regenerado. En el estudio se incluyeron tres escenarios climáticos para comprobar la capacidad del modelo para simular los flujos de agua bajo condiciones de cambio climático. Las estimaciones del modelo tanto de estrés hídrico como de acumulación de biomasa se mostraron sensibles a la reducción en la densidad de regeneración inicial. Los resultados indicaron que el modelo muestra la suficiente capacidad para simular los efectos de la competencia entre especies en la mortalidad de árboles en bosques mixtos y estimar variables relacionadas con los flujos hídricos. Por un lado, los efectos más significativos de la densidad del rodal sobre la disponibilidad hídrica aparecen durante la primera etapa de desarrollo, mientras que, por otro, el estrés hídrico es mayor en el caso del haya, aunque la reducción de la competencia podría compensar dicho aumento. Las implicaciones de este trabajo para la gestión adaptativa de bosques mixtos sugieren el actual control de la densidad para que los efectos acumulativos sean significativos en próximas décadas., Management of mixed forest has become an adaptation strategy to reduce the risks related to climate change. At the same time, ecological models
can be a useful tool for studying the growth and productivity of these stands. This work introduces an assessment of the capacity of the ‘FORECAST
Climate’ hybrid model to simulate water stress and productivity in mixed Scots pine and beech forests in Navarre (northern Spain), and their interaction
with different levels of regeneration density. The study included three climate scenarios to test the model capacity to simulate water flows under
climate change conditions. Model estimations of water deficit and biomass accumulation were sensitive to the reduction in the initial regeneration
density rates. The results indicated that the model shows sufficient capacity to simulate the effects of competition between species on tree mortality
in mixedwoods, and to estimate the variables related to water flows. On the one hand, the most significant effects of stand density on water availability
occur during the first stage of development, while on the other hand, water stress is greater in the case of beech, although the reduction in competition
could offset this increase. The implications of this work for adaptive management of mixed forests suggest applying density control soon in order to
have significant cumulative effects in coming decades., Este trabajo ha sido financiado por medio de los proyectos
AGL2012-33465 del Ministerio de Economía y Competitividad,
y el proyecto CIG-2012-326718-ECOPYREN3 de las Acciones
Marie Curie del 7º Programa Marco de la Comisión Europea.

Trabajo presentado a la 1st International Electronic Conference on Forests—Forests for a Better Future: Sustainability, Innovation, Interdisciplinarity, 15–30 November 2020; Available online: https://iecf2020.sciforum.net., Background and Objectives: The forest in the Southwestern Pyrenees Mountains (Northern
Spain) is mainly composed of pure Pinus sylvestris L. or a mix of P. sylvestris and Fagus
sylvatica L. The most common forest management technique to harvest pine is the application of
forest thinning with different intensities. It promotes a change in the forest composition and structure.
Taking into consideration this region as a site specific research about this topic, we aimed to
understand the CWD invertebrate composition response to different thinning intensities and canopy
type of these tree species. Materials and Methods: CWD samples were collected belonging to
intermediate and advanced decay classes, approximately 10 cm long and 5 cm in diameter. Using a
design of three thinning intensities (0%, 20%, and 40% of basal area removed), with three replications
per treatment (nine plots in total), four samples were taken per plot (two per canopy type) to
reach 36 samples in total. Meso- and macrofauna were extracted from CWD samples with Berlese–
Tullgren funnels, and individuals were counted and identified. Results: Most of the taxonomic
groups belonged to mesofauna, mainly to Acari and Collembola orders. On the other hand, the
macrofauna represented a minimum percentage of the community composition. Our results indicated
that although thinning intensities did not significantly affect the invertebrate community, canopy
type and CWD water content influenced significantly. It is imperative to consider in forest
management the responses of canopy type and thinning intensities in CWD water content, this disturbance
could also slow down the organic matter decomposition process in the soil, thus affecting
in the long term the natural cycle of nutrients., This work has been funded through the AGL2012-33465 projects of the Ministry of Economy
and Competitiveness, and the CIG-2012-326718-ECOPYREN3 project of the Marie Curie Actions
of the 7th Framework Program of the European Commission. The first author of this article
conducted this research during her MSc. thesis at the Public University of Navarre sponsored by
the grants of the Carolina Foundation of Spain. Published with the support of the Marie Curie
Alumni Association.

Density is an important wood mechanical property and an indicator of xylem architecture
and hydraulic conductivity. It can be influenced by forest management and climate. We studied the
impact of thinning and climate variables on annual stem radial growth (ring width and ring density,
and their earlywood and latewood components) in two contrasting Scots pine (Pinus sylvestris L.)
stands in northern Spain (one continental, one Mediterranean). At each site, three thinning regimes
(control or T0, removing 20% basal area or T20, and removing 30% or T30) were randomly applied to
nine plots per site (three plots per treatment) in 1999. Thinning was repeated at the Mediterranean
site in 2009 (increasing thinning intensity in T30 to 40%). Eight trees per plot were cored in spring
2014. Second thinning at the Mediterranean site and first thinning at the continental site generally
caused significantly wider ring (RW), earlywood (EW) and latewood (LW) widths, although no
differences between T20 and T30/40 were found, supporting in part the common observation that
radial growth is enhanced following thinning as competition for water and nutrients is reduced.
At the Mediterranean site, values of latewood density (LD) and maximum density (Dmax) relative to
pre-thinning conditions were significantly lower in T0 than in T30. However, at the continental site,
relative changes of ring density (RD) and LD were significantly higher in T0 than in T20 and T30.
Climate significantly affected not only RWbut also RD, with significant RD drops during or right after
unusually warm-dry years (e.g., 2003, 2011), which were characterized by LD reductions between
5.4 and 8.0%. Such RD decreases were quickly followed by recovery of pre-drought density values.
These results indicate trees temporarily reduce LD as a way to enhance hydraulic conductivity during
dry summers. However, climate effects on wood density were site-dependent. We also detected that
the thinning effect was not intense enough to prevent drought-induced changes in wood density by
altering water availability, but it could help to reduce wood properties fluctuations and therefore
maintain more homogeneous wood mechanic features., E.G.A. was
funded through a Spanish Predoctoral Research Grant (ref. BES-2013-066705). Funding for this research has been
provided by the Spanish Ministry of Economy and Competitiveness (project AGL2012-33465), which also funded
Y.-H.L. J.A.B. was funded through a Ramón y Cajal contract (ref. RYC-2011-08082) and a Marie Curie Action
(ref CIG-2012-326718-ECOPYREN3), which also funded D.C.-P. Funds to cover the costs of publishing in open
access were provided by the OpenAIRE pilot program of the European Union 7th Framework Programme.

As afforestation programs of former farmlands take hold in Taiwan to achieve a
variety of ecological and socio-economic values, it is becoming necessary to define best
forest management. Hence, we simulated mixed stands of Cinnamomum camphora and
Fraxinus griffithii planted through a gradient of soil fertility and varying camphor/ash
density ratios, but maintaining a fixed total stand density of 1500 trees ha −1 . Total stand
productivity was slightly lower in mixed stands than the combination of both monocultures
in rich and poor sites. Maximum negative yield surpluses for 50-year old stands were
7 Mg ha −1 and 6 Mg ha −1 for rich and poor sites with a 1:1 camphor laurel/ash ratios.
Maximum stand woody biomass in rich sites was reached in camphor laurel monocultures
(120 Mg ha −1 ) and in poor sites for Himalayan ash monocultures (58 Mg ha −1 ). However,
for medium-quality sites, a small yield surplus (11 Mg ha −1 ) was estimated coinciding with
a maximum stand woody biomass of 95 Mg ha −1 for a 1:1 camphor laurel/ash density ratio.
From an ecological resilience point of view, rotation length was more important than stand
composition. Long rotations (100 years) could improve soil conditions in poor sites. In rich
sites, short rotations (50 years) should be avoided to reduce risks or fertility loss., Yueh-Hsin Lo was funded by the Spanish Ministry of Economy and Competitiveness
(Ref. AGL2012-33465). Juan A. Blanco was funded through a “Ramón y Cajal” contract
(Ref. RYC-2011-08082) and a Marie Curie Action (Ref. CIG-2012-326718-ECOPYREN3).
Shih-Chieh Chang was given a grant by the Ministry of Science and Technology
(Ref. NSC 102-2621-M-259-005)

In the southern Pyrenees, human population and therefore land uses have changed from forests to pastures, then crops, and back to pastures and secondary forests during the last two centuries. To understand what such rapid land use changes have meant for carbon (C) and nitrogen (N) stocks, we used data from two forest sites in the western Pyrenees, combined with regional data on pastures and crop production (potato, cereal), to calibrate the ecosystem-level model FORECAST. Then, we simulated 150-year of land use for each site, emulating historical changes. Our estimates show that the conversion from forests into pastures and crops created C and N deficits (378-427 Mg C ha-1, 4.0-4.6 Mg N ha-1) from which these sites are still recovering. The main ecological process behind the creation of these deficits was the loss of the ecological legacy of soil organic matter (SOM) created by the forest, particularly during conversion to farming. Pastures were able to reverse, stop or at least slow down the loss of such legacy. In conclusion, our work shows the deep impact of historical land use in ecosystem attributes, both in magnitude of removed C and N stocks and in duration of such impact. Also, the usefulness of ecological modelling in absence of historical data to estimate such changes is showcased, providing a framework for potential C and N stocks to be reached by climate change mitigation measures such as forest restoration., Funding for this research has been provided by the Spanish Ministry
of Economy and Competitiveness (project AGL2012-33465),
which also funded Yueh-Hsin Lo.Juan A. Blanco was funded through
a Ramón y Cajal contract (ref. RYC-2011-08082) and a Marie
Curie Action (ref CIG-2012-326718-ECOPYREN3). Ester González
de Andrés was funded through a Spanish Predoctoral Research
Grant (ref. BES-2013-066705).

Isolating the long-term fertilization effect of CO 2 from other climate- and site-related effects on tree
growth has been proven a challenging task. To isolate long-term effects of [CO2] on water use
efficiency at ecosystem level, we used the FORECAST Climate forest model, calibrated for Scots pine (Pinus sylvestris L.) forests in the southwestern Pyrenees, growing at a Mediterranean montane site and at a continental subalpine site. Future climate scenarios (RCP 4.5 and RCP 8.5) were generated using a battery of six climate models to estimate daily values of temperature and
precipitation in a 90-year series. A factorial experiment was designed to disentangle the importance on C pools of three growing limiting factors (nitrogen limitation, climate (temperature +
precipitation) limitation and atmospheric CO 2 concentration). The relative importance of each factor was quantified by comparing the scenario with the limitation of each individual factor turned on
with the non-limitation scenario. Positive CO 2 fertilization due to improvement in water use
efficiency was detected by the model, but its quantitative impact improving tree growth was
minimum: its average increase in ecosystem C pools ranged from 0.3 to 0.9%. At the site with
cooler climate conditions (continental), the main limitation for tree growth was climate. Such
limitation will be reduced under climate change and the ecosystem will store more carbon. At the
site with milder climate conditions (Mediterranean), N availability was the main limiting factor
albeit modulated by water availability. Such limitation could be reduced under climate change as N cycling could accelerate (higher litterfall production and decomposition rates) but also increase if droughts become more frequent and severe. In addition, the magnitude of the uncertainty related to
climate model selection was much more important than CO 2 fertilization, indicating that
atmospheric processes are more important than tree physiological processes when defining how
much carbon could be gained (or lost) in forests under climate change. In conclusion, due to the
small changes in forest C pools caused by variation of atmospheric CO 2 concentrations compared to
changes caused by other growth limiting factors (nutrients, climate), reducing uncertainty related to
climate projections seems a more efficient way to reduce uncertainty in tree growth projections than
increasing forest model complexity., Funding for this research has been provided by the Spanish Ministry of Economy and Competitiveness (projects AGL2012‐33465, AGL2016-76463-P), a Ramón y Cajal contract (ref.RYC‐2011‐08082) and Marie Curie Actions (ref CIG‐2012‐326718‐ECOPYREN3,H2020-MSCA-IF-2014-DENDRONUTRIENT).

Hoy en día, los bosques mixtos son cada vez más reconocidos como superiores a los bosques puros en cuanto a la provisión de una completa gama de bienes y servicios ecosistémicos. La gestión de los bosques en una condición mixta se considera como una estrategia de adaptación ante el cambio global debido a su mayor estabilidad ante la incertidumbre creciente que se pronostica en las condiciones ambientales. Además, los bosques mixtos pueden ser más productivos que los rodales puros. Sin embargo, todavía existen importantes vacíos de conocimiento sobre el impacto de los cambios en la disponibilidad de recursos en las interacciones entre especies y otros procesos forestales, como el uso del agua, el crecimiento de los árboles y el ciclo de nutrientes.
Los bosques mixtos de pino silvestre y haya se extienden a lo largo de Europa y alcanzan su límite de distribución suroccidental en los Pirineos. Es probable que esta región sea muy sensible al cambio climático, por lo que el estudio de estos bosques marginales puede proporcionar información valiosa para comprender los impactos a largo plazo de las condiciones más cálidas y / o más secas que pueden desarrollarse en los Pirineos antes que en las áreas centrales de su distribución. Esta tesis doctoral tiene como objetivo evaluar la sucesión ecológica natural hacia bosques mixtos de pino silvestre y haya ubicados en la región pirenaica de Navarra (norte de España) y contribuir a una mejor comprensión de las respuestas del funcionamiento de estos bosques mixtos a los efectos del cambio global utilizando un enfoque a múltiples escalas. Se estableció un conjunto de parcelas experimentales sin gestión forestal en dos bosques con distintas características climáticas y de fertilidad: un sitio sub-xérico ubicado a baja elevación con un clima mediterráneo frío y seco y alta productividad (Aspurz), y un sitio mésico situado a gran altitud caracterizado por clima continental húmedo y frío y baja productividad (Garde).
En primer lugar, a partir del estudio de los anillos de crecimiento se reconstruyeron cronologías de eficiencia intrínseca del uso del agua (iWUE) y crecimiento secundario para ambos pino silvestre y haya a escala de árbol para el período 1980-2013. Evaluamos la influencia del clima y la concentración atmosférica de CO2 sobre el crecimiento y la iWUE en relación con la competencia intra- e interespecífica, que se estimó mediante un índice de competencia espacial. Por un lado, encontramos efectos de complementariedad en el pino solo en el sitio continental probablemente debido a las interacciones con el haya relacionadas con la luz. Sin embargo, en el sitio mediterráneo propenso a la sequía, la fuerte competencia por el agua con el haya pudo anular el efecto beneficioso de la interacción en la absorción de luz para el pino, como destaca el aumento observado de la iWUE de pinos sometidos a alta competencia interespecífica. Por otro lado, la reducción de la iWUE del haya a medida que aumenta la competencia interespecífica sugiere una reducción del estrés hídrico que sufre esta especie cuando se mezcla con pino. Ambas especies muestran una relación crecimiento – iWUE opuesta. La respuesta negativa del crecimiento del pino al aumento de la iWUE puede estar relacionada con una combinación de limitaciones por agua y nutrientes. Aunque esta relación fue positiva para el haya, también encontramos un progresivo desacoplamiento entre crecimiento, iWUE y aumento de la concentración atmosférica de CO2 a lo largo del siglo XX. En segundo lugar, se calibró el modelo forestal basado en procesos a nivel de ecosistema FORECAST Climate para los dos sitios de estudio con el fin de explorar los efectos de las interacciones en bosques mixtos de pino silvestre y haya a escalas temporales más largas y para evaluar las posibles consecuencias del cambio climático. Las simulaciones a nivel de rodal apoyan los resultados del análisis dendrocronológico, aunque encontramos pequeñas diferencias entre ambos enfoques debido a cuestiones metodológicas. Se predijo que cuando crecen en rodales mixtos con haya, los pinos desarrollan copas más grandes con mayor contenido en N. Esto se debe a un mejor suministro de nutrientes debido a la mayor calidad de la hojarasca del haya, lo que mejora la absorción de luz y se traduce en un mayor crecimiento del pino. Las simulaciones de cambio climático predijeron aumentos de la acumulación de biomasa en el sitio continental húmedo (+ 11%) y reducciones en el sitio mediterráneo sub-xérico (- 33%), respaldando la hipótesis de que los beneficios relacionados con la luz quedan anulados por la competencia por agua cuando este recurso se vuelve más limitante. Con respecto al haya, la reducción prevista en la transpiración del dosel y la consecuente complementariedad cuando se mezcla con pino, especialmente bajo escenarios de cambio climático, apoyan que el efecto de las interacciones interespecíficas sobre el haya está relacionado con el agua. En general, los resultados de ambos enfoques coinciden con el marco de "complementariedad-competencia". La diferente naturaleza de los efectos de las interacciones en ambas especies podría explicar las reducciones de crecimiento del pino y las respuestas neutrales o positivas del crecimiento del haya, ya que el agua ejerce una gran limitación en la región de estudio, particularmente en el sitio mediterráneo, y se espera que esta limitación aumente. Nuestros resultados enfatizan la importancia de considerar los rasgos funcionales involucrados en una determinada mezcla y los factores específicos del sitio en lugar de solo la diversidad de especies.
Finalmente, se recolectaron muestras de producción, concentración de nutrientes y estequiometría N:P:K de hojarasca durante los meses de otoño a lo largo de 16 años en ambos sitios de estudio. Estas series temporales se descompusieron en componentes oscilatorios y tendencias siguiendo el método de ensemble empirical mode decomposition (EEMD) para evaluar la influencia climática. También evaluamos la conexión con otros procesos forestales teniendo en cuenta la estrecha vinculación entre la dinámica de la hojarasca y el retorno de nutrientes a los suelos y, por lo tanto, la productividad forestal. La series de hojarasca reflejaron el proceso de sucesión gradual hacia los bosques mixtos durante las últimas décadas, que se encuentra más avanzada en el sitio mediterráneo, y subrayaron las consecuencias para los ciclos de nutrientes de estos bosques debido a la mayor concentración de nutrientes en la hojarasca de haya. Los patrones a gran escala de circulación atmosférica-oceánica (NAO, ENSO y PDO), a través de su influencia de la disponibilidad de agua en los Pirineos, explicaron la variabilidad de las dinámicas de la hojarasca similar o incluso mayor que las variables climáticas medidas localmente. El fuerte impacto de la disponibilidad de agua en la estacionalidad y composición de la hojarasca muestra las complejas interacciones entre las limitaciones de agua y nutrientes. Las tendencias en la estequiometría de la hojarasca sugieren un aumento en la limitación por P de la descomposición especialmente en el sitio mediterráneo, con el consiguiente descenso en la liberación de nutrientes, en bosques mixtos de pino y haya de los Pirineos en gran parte relacionada con el aumento de la deposición atmosférica de N. Además, el crecimiento secundario del pino mostró una respuesta negativa a aumentos en el ratio N:P de la hojarasca caída 3 años antes, mientras que la conexión entre el ratio n:K y la iWUE pone de relieve el importante papel del K en la regulación del balance hídrico de los árboles.
Nuestros resultados enfatizan las complejas interconexiones entre las limitaciones de agua y nutrientes. El impacto negativo directo de la sequía en el crecimiento de los árboles puede empeorar si la disminución en el suministro de nutrientes reduce la capacidad de los árboles para usar el agua de manera eficiente. Esta investigación propone que el pino silvestre podría beneficiarse de las interacciones relacionadas con la luz con el haya, aunque podrían sufrir reducciones de su crecimiento en bosques mixtos con haya debido al efecto combinado de las limitaciones por agua y nutrientes si el estrés hídrico continúa en aumento. Nuestros resultados sugieren un efecto beneficioso de la mezcla con pino para el haya, especialmente en escenarios de cambio climático. Por lo tanto, los planes de gestión forestal enfocados en adaptar los bosques mixtos a los efectos del cambio global necesitan considerar la limitación simultánea por diferentes recursos, así como los usos históricos de la tierra y los factores específicos del sitio así como su impacto en las interacciones interespecíficas de una combinación de especies determinada., Nowadays, mixed-species forests are increasingly recognized as superior to pure stands regarding the provision of a full range of ecosystem goods and services. The management of forests in a mixed condition is considered as an adaptation strategy in the face of global change because of their greater stability to predicted increasing uncertainty in environmental conditions. Further, mixed forests can be more productive than pure stands. However, there are still important knowledge gaps about the impact of changes in resources availability on mixing effects and forest processes, such as water use, tree growth and nutrient cycling.
Mixed forests of Scots pine and European beech are widespread across Europe and reach their southwestern distribution limit in the Pyrenees. This region is likely to be highly sensitive to climate change so the study of these rear-edge forests can provide valuable insights towards understanding long-term impacts of warmer and/or drier conditions that can develop earlier than in core distribution areas. This research thesis aims to assess natural ecological succession within Scots pine – European beech mixed stands located in the Pyrenean region of Navarre (northern Spain) and to contribute to a better understanding of responses of the functioning of such mixtures to global change drivers using a multi-scale approach. In order to do this, a set of unmanaged experimental plots were established in two forest stands of contrasting climatic and fertility characteristics: a sub-xeric site located at low elevation with a cool and dry Mediterranean climate and high productivity (Aspurz), and a mesic site situated at high elevation characterized by cold-wet continental climate and low productivity (Garde).
Firstly, intrinsic water-use efficiency (iWUE) and secondary growth chronologies of both Scots pine and European beech were reconstructed at tree-level for the period 1980-2013 using width and stable carbon isotope composition information from tree rings. We assessed the influence of climate and atmospheric CO2 concentration on growth and iWUE in relation to intra- and interspecific competition, which was estimated by means of a distance-dependent competition index. On one hand, we found complementarity effects on Scots pine only at the continental site likely due to light-related interactions with European beech trees. However, at the drought-prone Mediterranean site strong competition for water with beech may override light-related mixing effects on Scots pine growing in mixtures, as is highlighted by the increase of iWUE of pines subjected to high interspecific competition. On the other hand, the reduction of iWUE of European beech as interspecific competition increases suggests a water stress release when pine is admixed. Both species show contrasting growth – iWUE relationships. Negative growth response of Scots pine to increasing iWUE may be related to a combination of water and nutrient limitations. Although this association was positive for European beech, we found a progressive uncoupling of growth, iWUE and rising atmospheric CO2 concentration during the 20th century.
Secondly, the process-based, ecosystem-level FORECAST Climate model was calibrated for the two study sites in order to explore mixing effects on Scots pine – European beech mixtures at longer time scales and to assess possible consequences of predicted climate change. The stand-level simulations support the results from the dendrochronological approach, although slight differences between both approaches were reported due to methodological issues. Growing in mixtures with European beech, Scots pine trees were predicted to have larger crowns with higher N contents as a result of improved nutrient supply due to the higher quality beech leaf litter, which enhanced light absorption and translated into increased pine growth. Climate change simulations predicted increases of biomass accumulation at the cold-wet continental site (+ 11 %) and reductions at the sub-xeric Mediterranean site (- 33 %), so endorsing the hypothesis of light-related benefits cancelled out by competition for water when this resource becomes more limiting. Regarding European beech, the predicted reduction in canopy transpiration and the consequent complementarity when Scots pine is admixed, especially under climate change scenarios, uphold the water-related nature of mixing effects. Overall, the results from both approaches agree with the “complementarity- competition” framework. The contrasting nature of mixing effects on both species could explain growth reductions of Scots pine and neutral or positive responses of European beech growth as water exerts a great limitation in the study region, particularly at the Mediterranean site, and this limitation is expected to increase. The results emphasize the importance of considering functional traits involved in a given mixture and site-specific factors rather than only species diversity.
Finally, a 16-year series of production, nutrient concentration and N:P:K stoichiometry of leaf litter were collected during autumn months at both study sites. Such series were split into oscillatory components and trends following the ensemble empirical mode decomposition (EEMD) method in order to evaluate the climatic influence. The connection with other forest processes was also assessed based on the close linkage between litterfall dynamics and nutrient return to soils and thus forest productivity. Leaf litter series reflected the gradual succession towards mixed forests during the last decades, which is more advanced at the Mediterranean site, and underscored the consequences for forest nutrient cycles due to the higher nutrient concentration in beech litter. Large-scale atmospheric-oceanic circulation patterns (NAO, ENSO and PDO), through their influence of water availability in the Pyrenees, accounted for similar or even higher variation of leaf litter dynamics than locally-measured climatic variables. The strong impact of water availability on leaf litter seasonality and composition shows the complex interactions between water and nutrient limitations. Trends in stoichiometry of leaf litter suggest an increasing P limitation of decomposition particularly at the Mediterranean site, with the consequent decrease in nutrient release, in Pyrenean Scots pine – European beech mixtures largely related to rising atmospheric N deposition. Furthermore, secondary growth of Scots pine negatively responded to increasing N:P ratios of litter fallen 3 years prior, while the connection between N:K ratio and pine iWUE highlights the important role of K in the regulation of tree water balance.
Our results emphasize the complex interconnections among water and nutrient limitations. The negative direct impact of drought on tree growth may be worsened if decreases in nutrient supply reduce capacity of trees to use water efficiently. This research proposes that Scots pine could benefit from light-related interactions with European beech, although growth reductions of pine trees growing in mixed forests might be expected due to the combined effect of water and nutrient limitation if water stress continues to rise. Our results suggest a beneficial effect of pine admixture on European beech, especially under climate change scenarios. It follows from the above, management plans focused on adapting mixed-species forests to the effects of global change need to consider the simultaneous limitation by different resources, as well as historical land uses and site-specific factors, and their impact on intra- and interspecific interactions of a given species combination., This work was financially supported by a grant from the Ministerio de Economía, Industria y Competitividad (AGL2012-33465) and a grant from European Commission, Marie Curie Action – Career Integration Grants (326718 - ECOPYREN3).
Ester González de Andrés has been holder of a fellowship ‘Ayudas para contratos predoctorales para la formación de doctores’ from the Ministerio de Economía, Industria y Competitividad and she has received two mobility grants from the same institution., Programa Oficial de Doctorado en Agrobiología Ambiental (RD 1393/2007), Ingurumen Agrobiologiako Doktoretza Programa Ofiziala (ED 1393/2007)

Los análisis de sensibilidad son una herramienta importante para comprender el funcionamiento de los modelos ecológicos, así como para identificar
los parámetros más importantes en su funcionamiento. Además, los análisis de sensibilidad pueden utilizarse para diseñar de forma más efectiva
planes de muestreo de campo dirigidos a calibrar los modelos ecológicos. En los estudios de ecosistemas forestales, el análisis cuantitativo de la
parte subterránea es mucho más costoso y complicado que el estudio de la parte aérea, en especial el estudio de la dinámica de producción y descomposición
de raíces gruesas y finas de los árboles. En este trabajo se muestra un ejemplo de análisis de sensibilidad del modelo forestal FORECAST
a parámetros que definen la biomasa, longevidad y concentración de nitrógeno en las raíces de los árboles. El modelo se calibró para simular
dos rodales de pino silvestre (Pinus sylvestris) en los Pirineos de Navarra. Los resultados indican que la tasa de renovación de raíces finas es el parámetro
más influyente en las estimaciones del modelo de crecimiento de los árboles, seguida de la concentración de N en las mismas, siendo la
relación biomasa subterránea/total el parámetro al cual el modelo es menos sensible. Además, el modelo es más sensible a los parámetros que definen
el componente subterráneo de la biomasa arbórea cuando simula un sitio de menor capacidad productiva y mayor limitación por nutrientes., Sensitivity analysis are an important tool to understand ecological models functioning, as well as to identify their most important parameters. In
addition, sensitivity analysis can be used to design more effective field sampling regimes to calibrate ecological models. When studying forest ecosystems,
quantitative analysis of the underground fraction is much more costly and complicated than studying the aboveground fraction, especially
when studying the dynamics of production and decomposition of large and fine tree roots. In this work we show an example of sensitivity analysis of
the forest ecosystem model FORECAST to parameters defining biomass, longevity and nitrogen concentration in tree roots. The model was calibrated
to simulate two Scots pine (Pinus sylvestris L.) stands in the Pyrenees of Navarre. Results indicate that fine root turnover is the most influential parameter
on model´s tree growth estimations, followed by fine root N concentration and the root biomass/total tree biomass ratio. Furthermore, the
model is more sensitive to root-related parameters when calibrated to simulate poor sites with low productivity capacity and higher nutrient limitation., Este trabajo ha sido financiado por medio del programa ANABASI+D
del Departamento de Educación del Gobierno de Navarra,
y el Plan Nacional de I+D (proyecto AGL2012-33465) del Ministerio
de Economía y Competitividad.

A major problem in modern dendrochronology is that the methods traditionally used for linking tree ring growth data to climate records are not well suited to reconstructing low-frequency climatic variations. In this study, we explored the alternative Ensemble Empirical Mode Decomposition to detrend tree-ring records and to extract climate signals without removing low-frequency information. Tree cores of Pseudotsuga menziesii var. glauca (Mayr.) Franco were examined in a semi-arid forest in southern interior British Columbia, western Canada. Ring width data were decomposed into five oscillatory components (intrinsic mode functions, IMFs) of increasingly longer periodicities. IMF 1 was considered white noise, IMF 2 was used to create the first diameter growth index (DGI-1), IMF 3 and IMF 4 were combined to create the second diameter growth index (DGI-2), whereas IMF 5 and the residual term together were considered as the trend term. The highest significant cross-correlations between DGI-1 and the NAOAugust, NIÑO12May, and PDOJanuary indices were found at 1-year lags. DGI-2 had positive and persistent correlations with NAOJune and PDOMay at 0 to 3 years lags, and with NAOMay at 2 and 3 years lags. Our results indicate that periods of slow growth in the tree ring record matched periods of drought in the North American Pacific Northwest. Such water limiting conditions are likely caused by oscillatory patterns in the Pacific Ocean sea surface temperatures that influence precipitation in the Pacific Northwest. These drought events are likely exacerbated by changes in winter precipitation (snowpack) related to oscillations of the Atlantic Ocean sea surface temperatures, highlighting the ecological effects of both oceans on terrestrial ecosystems. Such relationships could not be easily found by traditional tree-ring analysis that remove some of the low-frequency signal, and therefore we suggest Ensemble Empirical Mode Decomposition as an additional tool to establishing tree growth-climate relationships., Juan A. Blanco was supported by grants
from the Spanish Ministry of Economy and Competitiveness (AGL2012-33465), a Marie Curie Action
fellowship from the European Commission (CIG-2012-326718-ECOPYREN3), and a Campus Iberus grant
for mobility of research staff.

Mixed conifer-hardwood forests can be more productive than pure forests and they are increasingly considered as ecosystems that could provide adaptation strategies in the face of global change. However, the combined effects of tree-to-tree competition, rising atmospheric CO2 concentrations and climate on such mixtures remain poorly characterized and understood.2. To fill this research gap, we reconstructed 34-year series (1980-2013) of growth (basal area increment, BAI) and intrinsic water-use efficiency (iWUE) of Scots pine (Pinus sylvestris L.)-European beech (Fagus sylvatica L.) mixed stands at two climati-cally contrasting sites located in the southwestern Pyrenees. We also gathered data on tree-to-tree competition and climate variables in order to test the hypotheses that (1) radial growth will be greater when exposed to inter- than to intraspecific competition, that is, when species complementarity occurs and (2) enhanced iWUE could be linked to improved stem radial growth.3. Growth of both species was reduced when intraspecific competition increased. Species complementarity was linked to improved growth of Scots pine at the continental site, while competition overrode any complementarity advantage at the drought-prone Mediterranean site. Beech growth did not show any significant response to pine admixture likely due to shade tolerance and the highly competitive nature of this species. Increasing interspecific competition drove recent iWUE changes, which increased in Scots pine but decreased in European beech. The iWUE enhancement did not involve any growth improvement in Scots pine. However, the positive BAIiWUE relationship found for beech suggests an enhanced beech growth in drought-prone sites due to improved water use.4. Synthesis. Complementarity may enhance growth in mixed forests. However, water scarcity can constrict light-related complementarity for shade intolerant species (Scots pine) in drought-prone sites. Basal area increment-intrinsic water-use efficiency relationships were negative for Scots pine and positive for European beech. These contrasting behaviours have got implications for coping with the expected increasing drought events in Scots pine-European beech mixtures located near ecological limit of the two species. Complementarity effects between tree species should be considered to avoid overestimating the degree of future carbon uptake by mixed conifer¿broadleaf forests., Ministerio de Ciencia y Tecnología, Grant/Award Number: RYC-2011-08082; Seventh Framework Programme, Grant/Award Number: CIG-2012-326718-ECOPYREN3; Ministerio de Economía y Competitividad, Grant/Award Number: AGL2012-33465 and BES-2013-066705

Litterfall dynamics (production, seasonality and nutrient composition) are key factors influencing nutrient cycling. Leaf litter characteristics are modified by species composition, site conditions and water availability. However, significant evidence on how large-scale, global circulation patterns affect ecophysiological processes at tree and ecosystem level remains scarce due to the difficulty in separating the combined influence of different factors on local climate and tree phenology. To fill this gap, we studied links between leaf litter dynamics with climate and other forest processes, such as tree-ring width (TRW) and intrinsic water-use efficiency (iWUE) in two mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in the south-western Pyrenees. Temporal series (18 years) of litterfall production and elemental chemical composition were decomposed following the ensemble empirical mode decomposition (EEMD) method and relationships with local climate, large-scale climatic indices, TRW and Scots pine´s iWUE were assessed. Temporal trends in N:P ratios indicated increasing P-limitation of soil microbes, thus affecting nutrient availability, as the ecological succession from a pine-dominated to a beech-dominated forest took place. A significant influence of large-scale patterns on tree-level ecophysiology was explained through the impact of the North Atlantic Oscillation (NAO) and El Niño – Southern Oscillation (ENSO) on water availability. Positive NAO and negative ENSO were related to dry conditions and, consequently, to early needle shedding and increased N:P ratio of both species. Autumn storm activity appears to be related to premature leaf abscission of European beech. Significant cascading effects from large-scale patterns on local weather influenced pine TRW and iWUE. These variables also responded to leaf stoichiometry fallen three years prior to tree-ring formation. Our results provide evidence of the cascading effect that variability in global climate circulation patterns can have on ecophysiological processes and stand dynamics in mixed forests., Ministerio de Economía y Competitividad, Grant/Award Number: AGL2012-33465, AGL2016-76463-P, BES-2013-066705 and RYC-2011-08082; FP7 People: Marie-Curie Actions, Grant/Award Number: CIG-2012-326718-ECOPYREN3; H2020 Marie Skłodowska-Curie Actions, Grant/Award Number: EF-2014-656810-DENDRONUTRIENT.

Density is an important wood mechanical property and an indicator of xylem architecture and hydraulic conductivity. It can be influenced by forest management and climate. We studied the impact of thinning and climate variables on annual stem radial growth (ring width and ring density, and their earlywood and latewood components) in two contrasting Scots pine (<i>Pinus sylvestris</i> L.) stands in northern Spain (one continental, one Mediterranean). At each site, three thinning regimes (control or T0, removing 20% basal area or T20, and removing 30% or T30) were randomly applied to nine plots per site (three plots per treatment) in 1999. Thinning was repeated at the Mediterranean site in 2009 (increasing thinning intensity in T30 to 40%). Eight trees per plot were cored in spring 2014. Second thinning at the Mediterranean site and first thinning at the continental site generally caused significantly wider ring (RW), earlywood (EW) and latewood (LW) widths, although no differences between T20 and T30/40 were found, supporting in part the common observation that radial growth is enhanced following thinning as competition for water and nutrients is reduced. At the Mediterranean site, values of latewood density (LD) and maximum density (Dmax) relative to pre-thinning conditions were significantly lower in T0 than in T30. However, at the continental site, relative changes of ring density (RD) and LD were significantly higher in T0 than in T20 and T30. Climate significantly affected not only RW but also RD, with significant RD drops during or right after unusually warm-dry years (e.g., 2003, 2011), which were characterized by LD reductions between 5.4 and 8.0%. Such RD decreases were quickly followed by recovery of pre-drought density values. These results indicate trees temporarily reduce LD as a way to enhance hydraulic conductivity during dry summers. However, climate effects on wood density were site-dependent. We also detected that the thinning effect was not intense enough to prevent drought-induced changes in wood density by altering water availability, but it could help to reduce wood properties fluctuations and therefore maintain more homogeneous wood mechanic features., E.G.A. was
funded through a Spanish Predoctoral Research Grant (ref. BES-2013-066705). Funding for this research has been
provided by the Spanish Ministry of Economy and Competitiveness (project AGL2012-33465), which also funded
Y.-H.L. J.A.B. was funded through a Ramón y Cajal contract (ref. RYC-2011-08082) and a Marie Curie Action
(ref CIG-2012-326718-ECOPYREN3), which also funded D.C.-P. Funds to cover the costs of publishing in open
access were provided by the OpenAIRE pilot program of the European Union 7th Framework Programme