Víctor Resco de Dios

Climate Change Effects on Mediterranean Forests and Preventive Measures

This paper synthesizes and reviews literature concerning climate change effects on Mediterranean forest ecology and management as well as the restorative techniques necessary to maintain forest health, forest yield and biodiversity. Climate change compounded with trends of rural abandonment are likely to diminish forested areas within the Mediterranean basin that will be replaced by fire prone shrub communities. This could be favoured by outbreaks of pathogens, fire and other large-scale disturbances. Landscape fragmentation is expected to impede species migration. Annual increments and subsequent income from forests are expected to decrease. Reafforestations are necessary to ensure the presence of propagules of forest species and their site-specific varieties best adapted to future climatic conditions even though they may be different from the present forest-plant community. Current challenges in biodiversity conservation can only be met by afforestations whose main objective is to maintain ecosystem functioning. A new silviculture must emerge encompassing these habitat displacement and economic concerns while maximizing carbon sequestration.

Gastropod diversity in aspen stands in coastal northern Sweden

Forest management has actively reduced proportion of deciduous trees in Fennoscandian managed forests. Several species of cryptogams and invertebrates depend on deciduous trees, among which aspen (Populus tremula L.) is especially important. The occurrence and abundance of several terrestrial gastropod species are linked to aspen leaf litter. However, the impact of forestry practices on the gastropod communities is largely unknown. We examined difference in species richness, diversity (H 0 ), and species composition in the gastropod fauna in aspen stands in a 400 km 2 managed boreal forest area in northern Sweden by collecting them with masonite boards from 20 selected stands. We contrasted isolated (>500 m from the neighboring stand) and aggregated (<300 m from neighboring stand) stands, stands close to arable land (<50 m) and stands in the forest, and used locations outside the aspen stands in the surrounding forest as controls. Stand sizes ranged from 100 to 3000 m 2 . Gastropod species richness and diversity were high in aspen stands near arable land and in aggregated stands in the forest and low in controls. Both diversity and assembly composition of the gastropods in isolated aspen stands in the forest interior were intermediate between, and did not differ significantly from, controls and other aspen stands. Species diversity and richness increased with stand area up to an area of 700 m 2 . We conclude that aspen stands in this managed boreal forest landscape are important for gastropods, even for species that are not strict habitat specialist connected to aspen. The total amount and the distribution of aspen in the landscape also may be important, since many species can be lost if area and connectivity of aspen stands drop to low levels. The positive species–area relationship suggests that future forestry practice should favor stands at least 500 m 2 (0.05 ha) preferably in connection to other stands. # 2002 Elsevier Science B.V. All rights reserved.

Utilizing intraspecific variation in phenotypic plasticity to bolster agricultural and forest productivity under climate change

Climate change threatens the ability of agriculture and forestry to meet growing global demands for food, fibre and wood products. Information gathered from genotype-by-environment interactions (G × E), which demonstrate intraspecific variation in phenotypic plasticity (the ability of a genotype to alter its phenotype in response to environmental change), may prove important for bolstering agricultural and forest productivity under climate change. Nonetheless, very few studies have explicitly quantified genotype plasticity-productivity relationships in agriculture or forestry. Here, we conceptualize the importance of intraspecific variation in agricultural and forest species plasticity, and discuss the physiological and genetic factors contributing to intraspecific variation in phenotypic plasticity. Our discussion highlights the need for an integrated understanding of the mechanisms of G × E, more extensive assessments of genotypic responses to climate change under field conditions, and explicit testing of genotype plasticity-productivity relationships. Ultimately, further investigation of intraspecific variation in phenotypic plasticity in agriculture and forestry may prove important for identifying genotypes capable of increasing or sustaining productivity under more extreme climatic conditions.

Forests synchronize their growth in contrasting Eurasian regions in response to climate warming

Significance Forests dominate carbon fluxes in terrestrial ecosystems. We demonstrate how an intensified climatic influence on tree growth during the last 120 y has increased spatial synchrony in annual ring-width patterns within contrasting (boreal and Mediterranean) Eurasian biomes and on broad spatial scales. Current trends in tree growth synchrony are related to regional changes in climate factors controlling productivity, overriding local and taxonomic imprints on forest carbon dynamics. Enhanced synchrony is becoming a widespread, although regionally dependent, phenomenon related to warmer springs and increased temperature variability in high latitudes and to warmer winters and drier growing seasons in mid-latitudes. Forests play a key role in the carbon balance of terrestrial ecosystems. One of the main uncertainties in global change predictions lies in how the spatiotemporal dynamics of forest productivity will be affected by climate warming. Here we show an increasing influence of climate on the spatial variability of tree growth during the last 120 y, ultimately leading to unprecedented temporal coherence in ring-width records over wide geographical scales (spatial synchrony). Synchrony in growth patterns across cold-constrained (central Siberia) and drought-constrained (Spain) Eurasian conifer forests have peaked in the early 21st century at subcontinental scales (∼1,000 km). Such enhanced synchrony is similar to that observed in trees co-occurring within a stand. In boreal forests, the combined effects of recent warming and increasing intensity of climate extremes are enhancing synchrony through an earlier start of wood formation and a stronger impact of year-to-year fluctuations of growing-season temperatures on growth. In Mediterranean forests, the impact of warming on synchrony is related mainly to an advanced onset of growth and the strengthening of drought-induced growth limitations. Spatial patterns of enhanced synchrony represent early warning signals of climate change impacts on forest ecosystems at subcontinental scales.

Genetic variation in circadian regulation of nocturnal stomatal conductance enhances carbon assimilation and growth

Circadian resonance, whereby a plants endogenous rhythms are tuned to match environmental cues, has been repeatedly shown to be adaptive, although the underlying mechanisms remain elusive. Concomitantly, the adaptive value of nocturnal transpiration in C3 plants remains unknown because it occurs without carbon assimilation. These seemingly unrelated processes are interconnected because circadian regulation drives temporal patterns in nocturnal stomatal conductance, with maximum values occurring immediately before dawn for many species. We grew individuals of six Eucalyptus camaldulensis genotypes in naturally lit glasshouses and measured sunset, predawn and midday leaf gas exchange and whole-plant biomass production. We tested whether sunrise anticipation by the circadian clock and subsequent increases in genotype predawn stomatal conductance led to rapid stomatal opening upon illumination, ultimately affecting genotype differences in carbon assimilation and growth. We observed faster stomatal responses to light inputs at sunrise in genotypes with higher predawn stomatal conductance. Moreover, early morning and midday stomatal conductance and carbon assimilation, leaf area and total plant biomass were all positively correlated with predawn stomatal conductance across genotypes. Our results lead to the novel hypothesis that genotypic variation in the circadian-regulated capacity to anticipate sunrise could be an important factor underlying intraspecific variation in tree growth.

Woody clockworks: circadian regulation of night‐time water use in Eucalyptus globulus

The role of the circadian clock in controlling the metabolism of entire trees has seldom been considered. We tested whether the clock influences nocturnal whole-tree water use. Whole-tree chambers allowed the control of environmental variables (temperature, relative humidity). Night-time stomatal conductance (gs ) and sap flow (Q) were monitored in 6- to 8-m-tall Eucalyptus globulus trees during nights when environmental variables were kept constant, and also when conditions varied with time. Artificial neural networks were used to quantify the relative importance of circadian regulation of gs and Q. Under a constant environment, gs and Q declined from 0 to 6 h after dusk, but increased from 6 to 12 h after dusk. While the initial decline could be attributed to multiple processes, the subsequent increase is most consistent with circadian regulation of gs and Q. We conclude that endogenous regulation of gs is an important driver of night-time Q under natural environmental variability. The proportion of nocturnal Q variation associated with circadian regulation (23-56%) was comparable to that attributed to vapor pressure deficit variation (25-58%). This study contributes to our understanding of the linkages between molecular and cellular processes related to circadian regulation, and whole-tree processes related to ecosystem gas exchange in the field.

Processes driving nocturnal transpiration and implications for estimating land evapotranspiration

Evapotranspiration is a major component of the water cycle, yet only daytime transpiration is currently considered in Earth system and agricultural sciences. This contrasts with physiological studies where 25% or more of water losses have been reported to occur occurring overnight at leaf and plant scales. This gap probably arose from limitations in techniques to measure nocturnal water fluxes at ecosystem scales, a gap we bridge here by using lysimeters under controlled environmental conditions. The magnitude of the nocturnal water losses (12–23% of daytime water losses) in row-crop monocultures of bean (annual herb) and cotton (woody shrub) would be globally an order of magnitude higher than documented responses of global evapotranspiration to climate change (51–98 vs. 7–8 mm yr−1). Contrary to daytime responses and to conventional wisdom, nocturnal transpiration was not affected by previous radiation loads or carbon uptake, and showed a temporal pattern independent of vapour pressure deficit or temperature, because of endogenous controls on stomatal conductance via circadian regulation. Our results have important implications from large-scale ecosystem modelling to crop production: homeostatic water losses justify simple empirical predictive functions, and circadian controls show a fine-tune control that minimizes water loss while potentially increasing posterior carbon uptake.

Soil phosphorous and endogenous rhythms exert a larger impact than CO2 or temperature on nocturnal stomatal conductance in Eucalyptus tereticornis

High nocturnal transpiration rates (5-15% of total water loss in terrestrial plants) may be adaptive under limited fertility, by increasing nutrient uptake or transport via transpiration-induced mass flow, but the response of stomata in the dark to environmental variables is poorly understood. Here we tested the impact of soil phosphorous (P) concentration, atmospheric CO2 concentration and air temperature on stomatal conductance (gs) during early and late periods in the night, as well as at midday in naturally, sun-lit glasshouse-grown Eucalyptus tereticornis Sm. seedlings. Soil P was the main driver of nocturnal gs, which was consistently higher in low soil P (37.3-79.9 mmol m(-2) s(-1)) than in high soil P (17.7-49.3 mmol m(-2)(-1)). Elevated temperature had only a marginal (P = 0.07) effect on gs early in the night (gs decreased from 34.7 to 25.8 mmol m(-2) s(-1) with an increase in temperature of 4 °C). The effect of CO2 depended on its interaction with temperature. Stomatal conductance responses to soil P were apparently driven by indirect effects of soil P on plant anatomy, since gs was significantly and negatively correlated with wood density. However, the relationship of gs with environmental factors became weaker late in the night, relative to early in the night, likely due to apparent endogenous processes; gs late in the night was two times larger than gs observed early in the night. Time-dependent controls over nocturnal gs suggest that daytime stomatal models may not apply during the night, and that different types of regulation may occur even within a single night. We conclude that the enhancement of nocturnal gs under low soil P availability is unlikely to be adaptive in our species because of the relatively small amount of transpiration-induced mass flow that can be achieved through rates of nocturnal water loss (3-6% of daytime mass flow).

Differences in morpho-physiological leaf traits reflect the response of growth to drought in a seeder but not in a resprouter Mediterranean species

Understanding the mechanisms underlying the response of different plant functional types to current and projected changes in rainfall is particularly important in drought-prone areas like the Mediterranean. Here, we report the responses of two species with contrasting leaf characteristics and post-fire regeneration strategies (Cistus ladanifer L., malacophyllous, seeder; Erica arborea L., sclerophyllous, resprouter) to a manipulative field experiment that simulated a severe drought (45% reduction of historical average rainfall). We measured monthly changes in relative growth rate (RGR), specific leaf area (SLA), bulk leaf carbon isotope composition (δ13C), predawn water potential (Ψpd), photosynthetic gas exchange, bulk modulus of elasticity and osmotic potential at maximum turgor (π). Temporal (monthly) changes in RGR of C. ladanifer were correlated with all measured leaf traits (except π) and followed Ψpd variation. However, the temporal pattern of RGR in E. arborea was largely unrelated to water availability. SLA monthly variation reflected RGR variation reasonably well in C. ladanifer, but not in E. arborea, in which shoot growth and δ13C increased at the time of maximum water stress in late summer. The relationship between water availability, and RGR and carbon assimilation in C. ladanifer, and the lack of any relationship in E. arborea suggest that the former has an enhanced capacity to harness unpredictable rainfall pulses compared with the latter. These contrasting responses to water availability indicate that the projected changes in rainfall with global warming could alter the competitive ability of these two species, and contribute to changes in plant dominance in Mediterranean shrublands.

The stable isotope ecology of terrestrial plant succession

We review the relevance and use of stable isotopes for the study of plant community succession. Stable isotope measurements provide information on the origin of resources acquired by plants, the processes governing resource uptake and transformation, and the physiological and environmental conditions of plant growth. When combined with measurements of the stable isotope ratio values of soil microbial biomass, soil organic matter and plant litter, isotope measurements of plants can indicate effects of successional changes on ecosystem processes. However, their application to questions of plant succession and ecosystem change is limited by the degree to which the underlying assumptions are met in each study, and complementary measures may be required, depending upon the question of interest. First, we discuss the changes that occur in the stable isotope composition of plants and ecosystems with ontogeny and species replacements, as well as their potential evolutionary significance. Second, we discuss the imprints of plant competition and facilitation on leaf and wood tissue, as well as how stable isotopes can provide novel insights on the mechanisms underlying plant interactions. Finally, we discuss the capacity for stable isotope measurements to serve as a proxy record for past disturbances such as fire, logging and cyclones.