Climate change effects in a Mediterranean forest following 21 consecutive years of experimental drought

Research Highlights: A small, long-term decrease in the water availability in a Mediterranean holm oak forest elicited strong effects on tree stem growth, mortality, and species composition, which led to changes in the ecosystem function and service provision. Background and Objectives: Many forest ecosystems are increasingly challenged by stress conditions under climate change. These new environmental constraints may drive changes in species distribution and ecosystem function. Materials and Methods: An evergreen Mediterranean holm oak (Quercus ilex L.) forest was subjected to 21 consecutive years of experimental drought (performing 30% of rainfall exclusion resulted in a 15% decrease in soil moisture). The effects of the annual climatic conditions and the experimental drought on a tree and shrub basal area increment were studied, with a focus on the two most dominant species (Q. ilex and the tall shrub Phillyrea latifolia L.). Results: Stem growth decreased and tree mortality increased under the experimental drought conditions and in hot and dry years. These effects differed between the two dominant species: the basal area of Q. ilex (the current, supradominant species) was dependent on water availability and climatic conditions, whereas P. latifolia was more tolerant to drought and experienced increased growth rates in plots where Q. ilex decay rates were high. Conclusions: Our findings reveal that small changes in water availability drive changes in species growth, composition, and distribution, as demonstrated by the continuous and ongoing replacement of the current supradominant Q. ilex by the subdominant P. latifolia, which is better adapted to tolerate hot and dry environments. The consequences of these ecological transformations for ecosystem function and service provision to human society are discussed

Wood vs. canopy allocation of aboveground net primary productivity in a Mediterranean forest during 21 years of experimental rainfall exclusion

A Mediterranean holm oak forest was subjected to experimental partial rainfall exclusion during 21 consecutive years, to study the effects of the expected decrease in water availability for the Mediterranean vegetation in the coming decades. Allocation in woody structures and total aboveground allocation were correlated with annual rainfall, whereas canopy allocation and the ratio wood/canopy allocation were not dependent on rainfall. Fruit productivity was also correlated with annual rainfall, but only in Quercus ilex. In the studied site, there were two types of forest structure: high canopy stand clearly dominated by Quercus ilex, and low canopy stand with more abundance of a tall shrub species, Phillyrea latifolia. In the tall canopy stand, the allocation to woody structures decreased in the experimental rainfall exclusion, but not the allocation to canopy. In the low canopy stand, wood allocation in Quercus ilex was very small in both control and plots with rainfall exclusion, but wood allocation in Phillyrea latifolia was even higher than that obtained in tall canopy plots, especially in the plots receiving the experimental rainfall exclusion. These results highlight likely future changes in the structure and functioning of this ecosystem induced by the decrease in water availability. A serious drop of the capacity to mitigate climate change for this Mediterranean forest can be expected, and the ability of Phillyrea latifolia to take advantage of the limited capacity to cope drought conditions detected in Quercus ilex, makes likely a forthcoming change in species dominance especially in the low canopy stands

L'increment de CO2 no ha fet augmentar el creixement dels arbres

Durant el segle passat la concentració atmosfèrica de CO2 va augmentar en 50 parts per milió, provocant un increment en l'eficiència en l'ús de l'aigua per part de les diverses espècies d'arbres que viuen en els boscos d'arreu del món. Tanmateix, aquesta major eficiència en l'ús de l'aigua no ha comportat, en general, un major creixement dels arbres arreu del món i, per tant, tampoc un augment de la seva capacitat d'embornal de CO2 com el que s'asumeix en els models de canvi climàtic. Altres factors com el canvi climàtic (especialment la sequera), la limitació de nutrients o l'aclimatació a llarg termini a una elevada concentració de CO2 han impedit que els arbres puguin treure profit dels efectes potencials d'aquest augment de CO2 per poder incrementar el seu creixement.Durante el siglo pasado la concentración atmosférica de CO aumentóen 50 partes por millón, provocando un incremento en la eficiencia en eluso del agua de las diversas especies de árboles que viven en losbosques de todo el mundo. No obstante, esta mayor eficiencia en el usodel agua no ha comportado, en general, un mayor crecimiento de los árboles de todo el mundo y por lo tanto tampoco un aumento de su capacidad como sumidero de CO como el que se asume en los modelos de cambio climático. Otros factores como el cambio climático (especialmente la sequía), la limitación de nutrientes o la aclimatación a largo plazo a una elevada concentración de CO han impedido que los árboles, puedan aprovechar los efectos potenciales de este aumento de CO para poder incrementar su crecimient

Acumulació de la biomassa en una brolla del Garraf durant dotze anys en relació amb la precipitació

En una zona experimental situada dins del Parc del Garraf, hem estudiat l'acumulació de biomassa en una brolla des del 1998, quan feia quatre anys del foc del 1994, fins al 2010. En aquest treball, presentem els resultats de l'estudi i explorem la relació entre l'acumulació de biomassa i la precipitació acumulada en dos períodes previs de dotze i sis mesos. Les relacions estan distorsionades per l'ocurrència d'alguns anys anòmals. Quan no es tenen en compte les dades dels anys anòmals, les correlacions mostren el control que la precipitació exerceix sobre l'acumulació de biomassa mentre la brolla es recupera del foc, especialment la precipitació dels sis mesos previs a l'estiu.En una zona experimental situada dentro del Parque de El Garraf, hemos estudiado la acumulación de biomasa en un matorral desde 1998, cuatro años después del incendio de 1994, hasta 2010. Aquí presentamos los resultados y exploramos la relación entre la acumulación de biomasa y la precipitación acumulada en dos períodos previos de doce y seis meses. Las relaciones están distorsionadas por la ocurrencia de algunos años anómalos. Cuando no se tienen en cuenta los datos de los años anómalos, las correlaciones muestran el control que la precipitación ejerce sobre la acumulación de biomasa mientras el matorral se recupera del incendio, especialmente la precipitación de los seis meses previos al verano.We have studied the accumulation of biomass in a shrubland since 1998, four years after the 1994 fire, until 2010, in an experimental area located within the Garraf Natural Park. We explored the relationship between the accumulation of biomass and the precipitation accumulated during the previous twelve and six months. Relationships were distorted by the occurrence of some anomalous years. Once data from anomalous years were discarded, biomass accumulation was positively correlated with the accumulated rainfall, especially with the precipitation falling during the six months preceding summer

Is forest fecundity resistant to drought? Results from an 18-yr rainfall-reduction experiment

Recruitment is a primary determinant of the long-term dynamics of plant populations in changing environments. However, little information is known about the effects of anthropogenic environmental changes on reproductive ecology of trees. We evaluated the impact of experimentally induced 18 yr of drought on reproduction of three contrasting forest trees: Quercus ilex , Phillyrea latifolia and Arbutus unedo. Rainfall reduction did not decrease tree fecundity. Drought, however, affected the allocation of resources in Q. ilex and A. unedo but not the more drought tolerant P. latifolia. Larger crop production by Q. ilex and A. unedo was associated with a stronger decrease in growth in the rainfall-reduction plots compared with the control plots, suggesting that these species were able to maintain their fecundity by shifting their allocation of resources away from growth