Influencia de la espesura de un pinar albar (Sistema Central, España) en la disponibilidad de luz y agua en el sotobosque, y la supervivencia de robles plantados en su interior

Information of tree-nurse shelterwood effects on survival of underplanted seedlings is particularly scant in Mediterranean forest ecosystems. To study light and water resources availability and survival associated to overstory density, two-year-old seedlings of Quercus petraea (Matt.) Liebl. (sessile oak) and Quercus pyrenaica Willd. (pyrenean oak) were planted in the understory of an even-aged Scots pine (Pinus sylvestris L.) plantation located in central Spain, which had been previously assigned to three density treatments: uncut, 33% thinned and 50% thinned of the original density, each replicated four times. Soil moisture was measured with a TDR during the first growing season after planting. Light conditions were estimated by hemispherical photography. Survival was measured at the end of the first growing season in the field and at the beginning of the next two growing seasons. The reduction in density after thinning had a positive effect on light availability and on near-surface soil moisture. Pyrenean oak had higher survival rates than sessile oak, which showed similarly high mortality rates in all three overstory treatments. Thinning had a positive effect on the survival of pyrenean oak, though irrespective of the intensity. Overall, these results point to the necessity to reduce canopy tree density in Mediterranean mountain pinewoods before carrying out enrichment plantations beneath.Con el fin de estudiar el efecto de la espesura de la cubierta forestal en la disponibilidad de luz y agua para las plantas y su supervivencia, se plantaron brinzales de dos savias de Quercus petraea (Matt.) Liebl. (roble albar) y Quercus pyrenaica Willd. (melojo) en el interior de un pinar de Pinus sylvestris L. (pino albar) situado en el NE de la provincia de Madrid sometido previamente a tres tratamientos: clara del 33% de la densidad original, clara del 50% y ausencia de clara. Se midió la humedad volumétrica del suelo con un TDR y la disponibilidad de luz en el sotobosque por medio de fotografias hemisféricas. La supervivencia se midió al final del primer año y al comienzo del verano de los dos años siguientes. La reducción de la densidad del dosel incrementó la luz disponible en el sotobosque, la humedad de los primeros 10 cm de suelo y la supervivencia de las plantas de melojo, aunque no se apreció un efecto diferente segun la intensidad de la clara aplicada. La mortalidad de las plantas de roble albar fue elevada en todos los tratamientos, y superior a la del melojo. Estos resultados apuntan a la necesidad de reducir la espesura del pinar albar en la zona de estudio antes de llevar a cabo plantaciones con melojo en su interior

Impact of successive spring frosts on leaf phenology and radial growth in three deciduous tree species with contrasting climate requirements in central Spain

Rear-edge tree populations forming the equatorward limit of distribution of temperate species are assumed to be more adapted to climate variability than central (core) populations. However, climate is expected to become more variable and the frequency of climate extremes is forecasted to increase. Climatic extreme events such as heat waves, dry spells and spring frosts could become more frequent, and negatively impact and jeopardize rear-edge stands. To evaluate these ideas, we analyzed the growth response of trees to successive spring frosts in a mixed forest, where two temperate deciduous species, Fagus sylvatica L. (European beech) and Quercus petraea (Matt.) Liebl. (sessile oak), both at their southernmost edge, coexist with the Mediterranean Quercus pyrenaica Willd. (Pyrenean oak). Growth reductions in spring-frost years ranked across species as F. sylvatica > Q. petraea > Q. pyrenaica. Leaf flushing occurred earlier in F. sylvatica and later in Q. pyrenaica, suggesting that leaf phenology was a strong determinant of spring frost damage and stem growth reduction. The frost impact depended on prior climate conditions, since warmer days prior to frost occurrence predisposed to frost damage. Autumn Normalized Difference Vegetation Index data showed delayed leaf senescence in spring-frost years and subsequent years as compared with pre-frost years. In the studied forest, the negative impact of spring frosts on Q. petraea and especially on F. sylvatica growth, was considerably higher than the impacts due to drought. The succession of four spring frosts in the last two decades determined a trend of decreasing resistance of radial growth to frosts in F. sylvatica. The increased frequency of spring frosts might prevent the expansion and persistence of F. sylvatica in this rear-edge Mediterranean population

Contrasting responses facing peak drought in seedlings of two co-occurring oak species

To evaluate leaf physiological mechanisms involved in plant tolerance to water deficit between sites of varying overstorey density, we investigated leaf water relations of two broadleaved oaks (Quercus petraea (Matt.) Liebl.;Quercus pyrenaica Willd.) planted within a dense Scots pine stand, in a thinned adjacent area and in a nearby gap at a Mediterranean, mid-mountain field site. Leaf water parameters were estimated in established seedlings at the end of June and August over two consecutive years by measuring pressure-volume relationships with the pressure chamber technique. Plant water status was always similar in both species, and it was lower in August than June due to lower soil moisture at the end of summer. Higher light and diurnal water deficits in the gap were associated to more negative osmotic potentials. Impacts of overstorey density on some leaf water parameters depended on the species. While Q. pyrenaica showed active osmotic adjustment across sites, seedlings of Q. petraea were unable to cope with increased water deficit by osmotic adjustment in the unthinned dense area. While Q. pyrenaica showed consistent increases in the maximum bulk modulus of elasticity (εmax) from June to August across sites, εmax decreased in the unthinned dense area for seedlings of Q. petraea. These results could reflect distinct species strategies to cope with water deficit under the low-light conditions created by a dense overstorey. Higher leaf osmotic adjustment and bulk modulus of elasticity in Q. pyrenaica seedlings at peak summer drought might confer on them a competitive advantage during establishment in dry sub-Mediterranean understories. Opening of moderate canopy gaps in dense Scots pine stands improves some leaf mechanisms involved in drought tolerance in oak seedlings. © 2010 Institute of Chartered Foresters. All rights reserved

Acclimation to light in seedlings of Quercus petraea (Mattuschka) Liebl.;Quercus pyrenaica Willd. planted along a forest-edge gradient

Photosynthetic acclimation of two co-occurring deciduous oaks (Quercus petraea and Quercus pyrenaica) to a natural light gradient was studied during one growing season. In the spring of 2003, 90 seedlings per species were planted along a transect resulting from a dense Pinus sylvestris stand, an adjacent thinned area and a 10-m-wide firebreak (16.5-60.9% Global Site Factor (GSF)). In two dates of the following summer, we measured leaf gas exchange, carboxylation efficiency (CE), chlorophyll and nitrogen content, light-response curves of chlorophyll a fluorescence parameters, and leaf mass per area (LMA). Summer was mild, as evidenced by leaf predawn water potential (Ψpd), which reduced the interactive effect of water stress on the response of seedlings to light. Q. pyrenaica had higher LMA, CE, stomatal conductance (g s max) and photosynthesis per unit area (Amax a) than Q. petraea at all growth irradiances. Amax a, LMA, g s max and electron transport rate (ETR) all increased with light availability (GSF) in a similar fashion in both species. Light had also a clear effect on the organization of Photosystem II (PS II), as deduced by chlorophyll a fluorescence curves. Chlorophyll concentration (Chlm) decreased with increasing light availability in Q. pyrenaica but it did not in Q. petraea. Seedlings of Q. petraea acclimated to higher irradiances showed a greater non-photochemical quenching (NPQ) than those of Q. pyrenaica. This suggests a higher susceptibility to high light in Q. petraea, which would be consistent with a better adaptation to shade, inferred from the lower LMA or the lower rate of photosynthesis. © 2006 Springer-Verlag

Respiratory costs of woody tissues in a Quercus pyrenaica coppice

Long-term coppicing leads to the development of massive root systems. A disproportionate carbon investment in root maintenance has been pointed as a cause of the widespread decline of abandoned coppices. We aimed at assessing how coppicing has influenced root and shoot development and related carbon loss ascribed to maintenance of woody tissues in Quercus pyrenaica. For this goal, results from published studies on root dynamics, woody biomass and respired CO2 fluxes in an abandoned Q. pyrenaica coppice were integrated and extended to quantify overall respiratory expenditures of above- and below-ground woody organs. Internal and external CO2 fluxes together with soil CO2 efflux were monitored in eight stems from one clone across a growing season. Stems and roots were later harvested to quantify the functional biomass and scale up root and stem respiration (RR and RS, respectively) to the clone and stand levels. Below- and above-ground biomass was roughly equal. However, the root-to-shoot ratio of respiration (RR/RS) was generally below one. Relatively higher RS suggests enhanced metabolic activity aboveground during the growing season, and highlights an unexpected but substantial contribution of RS to respiratory carbon losses. Moreover, soil and stem CO2 efflux to the atmosphere in Q. pyrenaica fell in the upper range of reported rates for various forest stands distributed worldwide. We conclude that both RS and RR represent an important carbon sink in this Q. pyrenaica abandoned coppice. Comparatively high energetic costs in maintaining multiple stems per tree and centennial root systems might constrain aboveground performance and contribute to coppice stagnation

Ability to avoid water stress in seedlings of two oak species is lower in a dense forest understory than in a medium canopy gap

The ecophysiological response to summer drought in seedlings of two co-occurring oak species (Quercus petraea and Quercus pyrenaica) outplanted below a dense Pinus sylvestris stand and a medium canopy gap was examined in two experiments carried out in central mountains of the Iberian Peninsula (Spain). Leaf water relations and gas exchange were studied in the first experiment. For both species, lower pre-dawn leaf water potential (Ψpd) evidenced a higher degree of water stress in the understory seedlings, even though soil moisture was similar in both sites. Rates of photosynthesis (Asat) and stomatal conductance (gsat) at saturating light were higher in the gap seedlings in all measuring dates, but the earlier and more pronounced stress imposed in the understory precluded the comparison of tolerance responses between sites. Q. pyrenaica showed always higher Asat and gsat than Q. petraea, independently of Ψpd, which did not differ significantly between species. At the end of summer, values of Asat/gsat and leaf carbon isotopic composition reflected a less efficient water use in Q. pyrenaica. In the second experiment, growth and root development were examined in a different set of seedlings planted in the same sites. Q. petraea allocated less biomass to roots, and attained 20% higher total plant dry mass than Q. pyrenaica at the end of the second growing season in the field. For both species total plant dry mass and coarse plus fine root dry mass reached at this time were approximately three-fold higher in the gap than in the understory. Poor root development could explain the more limited access to water of seedlings outplanted in the understory. This study reveals that seedlings of Q. petraea and Q. pyrenaica planted under a dense, mature pine stand are more susceptible to summer drought and present a delay in growth with respect to a medium canopy gap. © 2007 Elsevier B.V. All rights reserved

Summer field performance of Quercus petraea (Matt.) Liebl and Quercus pyrenaica Willd seedlings, planted in three sites with contrasting canopy cover

In 2000, one-year-old seedlings of pyrenean oak (Quercus pyrenaica Willd.) and sessile oak (Quercus petraea [Matt.] Liebl) were planted in a thinned and an unthinned plot in a pinewood (Pinus sylvestris), and in a nearby clearing. In summer 2002 and 2003, water relations and gas exchange parameters were measured to address the impact of drought on the seedlings. Chlorophyll a fluorescence was also measured to explore leaf photochemistry and a possible non-stomatal limitation to photosynthesis (A). Reduction in stomatal conductance (g) in response to the decrease of predawn water potential (Ψpd) resulted the main cause affecting net carbon uptake. Water potential at midday (Ψmd) was similar in both species but Quercus petraea was more sensitive to soil water deployment occurred along summer, showing slightly lower Ψpd because worse recover of water potential during night. Rate of photosynthesis was higher in Q. pyrenaica probably in relation to its greater leaf mass per area (LMA) and nitrogen content per leaf area (Na). Mortality was highest in the clearing and lowest in the thinned pinewood. Throughout the summer, soil moisture was higher in the thinned area, possibly because of the reduction in tree transpiring surface and interception of rainfall. Accordingly, Ψpd of both species was higher in the thinned site. © 2006 Springer Science+Business Media B.V

Gas exchange and leaf aging in an evergreen oak: causes and consequences for leaf carbon balance and canopy respiration

Leaves of Mediterranean evergreens experience large variations in gas exchange rates over their life span due to aging and seasonally changing environmental conditions. Accounting for the changing respiratory physiology of leaves over time will help improve estimations of leaf and whole-plant carbon balances. Here we examined seasonal variations in light-saturated net CO2 assimilation (Amax), dark respiration (Rd) and the proportional change in Rd per 10 °C change in temperature (Q10 of Rd) in previous-year (PY) and current-year (CY) leaves of the broadleaved evergreen tree Quercus ilex L. Amax and Rd were lower in PY than in CY leaves. Differences in nitrogen between cohorts only partly explained such differences, and rates of Amax and Rd expressed per unit of leaf nitrogen were still significantly different between cohorts. The decline in Amax in PY leaves did not result in the depletion of total non-structural carbohydrates, whose concentration was in fact higher in PY than CY leaves. Leaf-level carbon balance modeled from gas exchange data was positive at all ages. Q10 of Rd did not differ significantly between leaf cohorts; however, failure to account for distinct Rd between cohorts misestimated canopy leaf respiration by 13% across dates when scaling up leaf measurements to the canopy. In conclusion, the decline in Amax in old leaves that are close to or exceed their mean life span does not limit the availability of carbohydrates, which are probably needed to sustain new growth, as well as Rd and nutrient resorption during senescence. Accounting for leaf age as a source of variation of Rd improves the estimation of foliar respiratory carbon release at the stand scale