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We investigated how richness and composition of vascular plant species in the understory of a mixed hardwood forest stand varied with respect to the abundance and composition of the overstory. The stand is in central Spain and represents the southernmost range of distribution of several tree and herbaceous species in Europe. Understory species were identified in 46 quadrats (0.25 m) where variables litter depth and light availability were measured. In addition, we estimated tree density, basal area, and percent basal area by tree species within 6-m-radius areas around each plot. Species richness and composition were studied using path analysis and scale-dependent geostatistical methods, respectively. We found that the relative abundance of certain trees species in the overstory was more important than total overstory abundance in explaining understory species richness. Richness decreased as soil litter depth increased, and soil litter increased as the relative proportion of Fagus sylvatica in the overstory increased, which accounted for a negative, indirect effect of Fagus sylvatica on richness. Regarding understory species composition, we found that some species distributed preferentially below certain tree species. For example, Melica uniflora was most frequent below Fagus sylvatica and Quercus petraea while the increasing proportion of Q. pyrenaica in the overstory favored the presence of Cruciata glabra, Arenaria montana, Prunus avium, Conopodium bourgaei, Holcus mollis, Stellaria media and Galium aparine in the understory. Overall, these results emphasize the importance of individual tree species in controlling the assemblage and richness of understory species in mixed stands. We conclude that soil litter accumulation is one way through which overstory composition shapes the understory community. [ABSTRACT FROM AUTHOR]

The extent to which seedling recruitment is limited by summer drought in Mediterranean-type ecosystems depends on the light microsite, yet the relationship between light availability and water status, functional performance, and survival of seedlings in these systems is still unclear. Over a 3-year period, we studied the pattern of survival and functional performance of seedlings of Quercus petraea (Matt.) Liebl. and Quercus pyrenaica Willd. in a montane forest in central Spain, which is the southern edge of the natural range of Q. petraea. After a mast year of the two species, 72 plots were established in six microhabitats spanning a range of overstorey canopy closure: closed, partial and open canopies dominated by either Q. petraea or Q. pyrenaica adult trees. Seedlings of each species naturally emerged beneath the conspecific-dominated canopies. The second and third years of study were extremely dry. Three years after emergence, the greatest seedling survival occurred beneath the partial canopy of Q. pyrenaica trees (8%) and the lowest (0%) beneath the closed canopies of Q. pyrenaica and Q. petraea. Survival for Q. pyrenaica increased linearly with understorey light across the range of 10-35% Global Site Factor. Plant water deficit (estimated by leaf water potential) was high across microhabitats, and increased with light availability for Q. pyrenaica. Potential for photosynthesis (estimated by the electron transport rate of photosystem II) decreased with canopy closure; and potential for light harvesting (e.g. specific leaf area (SLA) and chlorophyll concentration) increased with closure. Extreme water deficit could be the main contributor to seedling death in the more open microhabitats, whereas light level was insufficient to maintain carbon balance under the water-stressful conditions existing beneath the closed tree canopies. Seedling establishment appears to be a limiting factor for the recruitment of both oaks within this forest in a wide range of microhabitats, especially for the more drought-sensitive Q. petraea. Moderate reductions of tree canopy cover can improve seedling establishment, but extreme summer droughts can prevent the success of any silvicultural practice made. [ABSTRACT FROM AUTHOR]

Abstract: 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 (Ψ pd) evidenced a higher degree of water stress in the understory seedlings, even though soil moisture was similar in both sites. Rates of photosynthesis (A sat) and stomatal conductance (g sat) 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 A sat and g sat than Q. petraea, independently of Ψ pd, which did not differ significantly between species. At the end of summer, values of A sat/g sat 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. [Copyright &y& Elsevier]

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., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Little is known about how environmental factors shape the short- and long-term responses of leaf respiration to temperature under field conditions despite the importance of respiration for plant and stand carbon balances. Impacts of water availability and canopy cover on leaf dark respiration (R) and temperature sensitivity were assessed in beech (Fagus sylvatica L.) seedlings in a sub-Mediterranean population. We studied seedlings established within canopy gaps (39% global site factor; GSF) that were subject to either no watering (unwatered plants; UW) or regular watering (2-10% higher volumetric topsoil water content as summer progressed; W plants) and seedlings established beneath the adjacent understorey (12% GSF). Leaf R rose exponentially with diurnal increases in temperature; the same temperature sensitivity (Q(10): 2.2) was found for understorey and gap plants, irrespective of watering treatment. Respiration estimated at 25 degrees C (R(25)) was lower in the understorey than the gaps and was significantly lower in the unwatered than in the watered gap plants by the end of summer (0.65 versus 0.80 micromol m(-2) s(-1)). R(25) declined with increasing summer temperature in all plants; however, respiration estimated at the prevailing ambient temperature did not change through the summer. There were parallel declines in R(25) and concentrations of starch and soluble sugars with increasing summer temperature for gap plants. We conclude that seasonal shifts in temperature-response curves of beech leaf R occur in both low- and high-light environments; since leaf R decreased with increasing plant water deficit, such shifts are likely to be greater whenever plants experience summer drought compared to scenarios where plants experience high rainfall in summer.