Climate control of terrestrial carbon exchange across biomes and continents

Peer reviewe

The effect of vapour pressure deficit on stomatal conductance, sap pH and leaf-specific hydraulic conductance in Eucalyptus globulus clones grown under two watering regimes

Background and Aims Stomatal conductance has long been considered of key interest in the study of plant adaptation to water stress. The expected increase in extreme meteorological events under a climate change scenario may compromise survival in Eucalyptus globulus plantations established in south-western Spain. We investigated to what extent changes in stomatal conductance in response to high vapour pressure deficits and water shortage are mediated by hydraulic and chemical signals in greenhouse-grown E. globulus clones. Methods Rooted cuttings were grown in pots and submitted to two watering regimes. Stomatal conductance, shoot water potential, sap pH and hydraulic conductance were measured consecutively in each plant over 4 weeks under vapour pressure deficits ranging 0 42 to 2 25 kPa. Evapotranspiration, growth in leaf area and shoot biomass were also determined. Key Results There was a significant effect of both clone and watering regime in stomatal conductance and leaf-specific hydraulic conductance, but not in sap pH. Sap pH decreased as water potential and stomatal conductance decreased under increasing vapour pressure deficit. There was no significant relationship between stomatal conductance and leaf-specific hydraulic conductance. Stomata closure precluded shoot water potential from falling below -1 8 MPa. The percentage loss of hydraulic conductance ranged from 40 to 85 %. The highest and lowest leaf-specific hydraulic conductances were measured in clones from the same half-sib families. Water shortage reduced growth and evapotranspiration, decreases in evapotranspiration ranging from 14 to 32 % in the five clones tested. Conclusions Changes in sap pH seemed to be a response to changes in atmospheric conditions rather than soil water in the species. Stomata closed after a considerable amount of hydraulic conductance was lost, although intraspecific differences in leaf-specific hydraulic conductance suggest the possibility of selection for improved productivity under water-limiting conditions combined with high temperatures in the early stages of growth. © The Author 2016. Published by Oxford University Press on behalf of the Annals of Botany Company

Response of pine natural regeneration to small-scale spatial variation in a managed Mediterranean mountain forest

Questions What influence do management practices and previous tree and shrub stand structure have on the occurrence and development of natural regeneration of Pinus sylvestris in Mediterranean mountain forests? How are the fine-scale and environmental patterns of resources affected and what impact does this have on the distribution of the regeneration? Location A Pinus sylvestris Mediterranean mountain forest in central Spain. Methods Upperstory trees and regeneration (seedlings and saplings) were mapped in four 0.5-ha plots located in two types of stand with different management intensities (even-aged and uneven-aged stands). Environmental variables were recorded at the nodes of a grid within the plots. The relationships between the upperstory and regeneration were evaluated by bivariate point pattern analysis; redundancy analysis ordination and variation partitioning were performed to characterize regeneration niches and the importance of the spatial component. Results Seedlings and saplings presented a clumped structure under both types of management and their distribution was found to be related to the spatial distribution of favourable microsites. Regeneration was positively related to conditions of partial cover with high soil water content during the summer. More than half of the explained variance was spatially structured in both types of stand. This percentage was particularly high in the even-aged stands where the pattern of regeneration was highly influenced by the gaps created by harvesting. Conclusions The spatial distribution of the tree and shrub upperstory strongly influences regeneration patterns of P. sylvestris. Current management practices, promoting small gaps, partial canopy cover and moderate shade in even-aged stands, or favouring tree and shrub cover in the case of uneven-aged stands, appears to provide suitable conditions for the natural regeneration of P. sylvestris in a Mediterranean climate. © 2009 International Association for Vegetation Science

Forest structural diversity characterization in Mediterranean pines of central Spain with QuickBird-2 imagery and canonical correlation analysis

Variation in forest structure provides information on vegetation complexity and provides insights on biodiversity. Characterizing forest structural diversity with remotely sensed data supports reporting, monitoring, and policy development. We explored the relationship between forest structural diversity in Mediterranean pines of the Spanish Central Range and variables derived from imagery captured with a commercial high spatial resolution satellite (QuickBird-2; with pixels sided 2.4 m multispectral and 0.68 m panchromatic). To combine multiple aspects of tree conditions at a stand level, “structural diversity” was characterized at the plot level (N= 1022) as a linear combination of the median of absolute differences of individual trees’ bole diameter, height, and crown diameter measured on the field from the local median equivalents. Spectral reflectance variations in the visible and near-infrared, as well as image co-occurrence texture metrics from the panchromatic imagery at various window sizes were generated. All relationships to image-derived values were assessed against circular 0.3 ha areas corresponding with the field measured plots. Canonical correlation analysis aided in identification of combinations of reflectance and texture metrics most highly related with forest structural diversity (R= 0.89). Reflectance diversity was found to be more important than co-occurrence texture features in describing forest structural diversity when forest structure was limited (R= 0.47 vs.R= 0.39), whereas texture was more informative to the model when the forest structural diversity was high (R= 0.88 vs.R= 0.63), relating more complex forest conditions. Our results, although empirically defined by the local conditions and image acquisition characteristics, demonstrated the potential in high spatial resolution imagery for description of forest structural diversity in forests of the Mediterranean environment, especially important for Spain where a national high spatial resolution image data base has been collected. © 2012 CASI

Characterizing 25 years of change in the area, distribution, and carbon stock of Mediterranean pines in Central Spain

Mediterranean pines are subject to continuous change under the influence of natural and human factors. Remotely sensed data provide a means to characterize these changes over large areas. In this study we used a time series of Landsat imagery to capture 25 years (1984-2009) of change in the pine-dominated forests of the Central Range in Spain. Object-based image analysis methods were used to identify landscape-level changes in the area and the distribution of forests. We also propose that in the absence of disturbance, biomass accrual is occurring (or depletion in cases where removal is evident) and may be related to changes to the carbon stock; we describe the detected spectral changes in terms of biomass changes as the carbon stocking process. The primary inputs for the identification of changes in the area and distribution of pine stands were Landsat bands 3, 4 and 5 and the Tasseled Cap Angle (TCA) - a metric derived from the greenness and brightness components of the Tasseled Cap Transformation (TCT). In the identification of carbon stocking processes the temporal derivative of the TCA, the Process Indicator (PI), was used to inform on the rate and directionality of the change present. Our results show that the total area of pine forest has increased by 40%, from 1211 km 2 to 1698 km 2, during this period, with a variable rate of change. The distribution of pine-dominated forest has changed as well there is an area of 765 km 2 permanently covered with pines and 945 km 2 found to be temporarily occupied. Following the logic of carbon stocking processes, our findings show that at the end of the analysis period, 20% of the potential pine area is increasing its carbon stock and 40% of this area is experiencing a decrease. © 2012 Copyright Taylor and Francis Group, LLC

Regeneration dynamics of mixed stands of Pinus pinaster Ait.;Pinus pinea L. in Central Spain

The dynamics of mixed stands are more complex and less studied than those of monospecific stands. The objective of this work was to analyze the variables involved in seedling occurrence and seedling survival in mixed stands of Pinus pinaster and P. pinea in Mediterranean areas. From 2011 to 2016, regeneration of both species was monitored at two sites located in Central Spain. We installed 72 regeneration plots where seedling dynamics were monitored. All the trees in the study areas were measured and mapped. Additionally, we took hemispherical photographs in each regeneration plot. The average density of P. pinea seedlings over the study period was almost 20 times larger than that of P. pinaster. Our results indicate that the seedlings of both species grow under moderate light conditions. In addition, we found that the occurrence of seedlings of both species was related to the structure of the stand. P. pinea seedlings grew where the density and size of P. pinaster trees were low and where P. pinea trees provided moderately sheltered conditions, whereas the number of P. pinaster seedlings was related to under intermediate densities of P. pinaster trees. Furthermore, seedling survival was positively associated with age of the seedlings and negatively with the August average maximum temperature. The temporal continuity of mixed stands of P. pinea and P. pinaster in the study area is compromised by the observed lack of regeneration of P. pinaster

Basal area and diameter distribution estimation using stereoscopic hemispherical images

In recent years, proximal sensing data has increasingly been used to optimize forest inventories. In this paper we present a forest inventory methodology based on stereoscopic hemispherical images. An automated pixel-based approach and a user-guided “region growing” approach have been developed for image matching. To estimate the basal area, number of trees and mean diameter, the sampling probability is determined for each tree. The accuracy and precision of the estimates derived from stereoscopic hemispherical images was analyzed for a set of National Forest Inventory plots. The results revealed that tree matching depends on the species, the distance to the target tree and the diameter. The Pearson correlation coefficient was 0.86 for the mean diameter and 0.89 for the basal area, whereas for the number of trees per hectare it was 0.59. The proposed methods may be used in large scale forest inventories as a cost-efficient way of obtaining data on diameter distribution and basal area from field surveys following a two-stage scheme combined with remote sensing techniques. © 2016 American Society for Photogrammetry and Remote Sensing

Abiotic factors modulate post-drought growth resilience of Scots pine plantations and rear-edge Scots pine and oak forests

The proportion of planted forests in the Mediterranean Basin is one of the largest in the world. These plantations are dominated by pine species and present a series of characteristics such as low elevation, high competition or small tree size that make them more vulnerable to droughts. However, quantitative assessments of their post-drought growth resilience in accordance with species, site factors and tree characteristics are lacking. In this study we sampled 164 trees at four forest sites located in the drought-prone Sierra Nevada, southeastern Spain. We compared growth responsiveness to drought in rear-edge planted vs. relic natural Scots pine (Pinus sylvestris) and coexisting Pyrenean oak (Quercus pyrenaica) stands. Our objective was to characterize and compare the different growth responses to drought between species and sites and the effect of the main physiographic factors (altitude, aspect, and slope) on these responses since the influence of these factors on post-drought resistance and resilience has received little attention to date. Our results reveal that the planted pine sites with the lowest mean growth rates displayed greater resistance during drought, and that higher altitude was associated with improved resistance and/or resilience for all species and sites. Natural pine and Pyrenean oak stands were better adapted to the dry climatic conditions of the Mediterranean region where the study was undertaken, displaying greater resistance and/or resilience and lower influence of drought on growth in comparison to stands of planted pines. These results suggest that promoting the conservation of high-elevation pine plantations and enhancing the regeneration of natural pine and oak may improve the resistance and resilience of these drought-prone forest ecosystems