Paolo Cherubini

Identification, measurement and interpretation of tree rings in woody species from mediterranean climates

We review the literature dealing with mediterranean climate, vegetation, phenology and ecophysiology relevant to the understanding of tree‐ring formation in mediterranean regions. Tree rings have been used extensively in temperate regions to reconstruct responses of forests to past environmental changes. In mediterranean regions, studies of tree rings are scarce, despite their potential for understanding and predicting the effects of global change on important ecological processes such as desertification. In mediterranean regions, due to the great spatio‐temporal variability of mediterranean environmental conditions, tree rings are sometimes not formed. Often, clear seasonality is lacking, and vegetation activity is not always associated with regular dormancy periods. We present examples of tree‐ring morphology of five species (Arbutus unedo, Fraxinus ornus, Quercus cerris, Q. ilex, Q. pubescens) sampled in Tuscany, Italy, focusing on the difficulties we encountered during the dating. We present an interpretation of anomalies found in the wood structure and, more generally, of cambial activity in such environments. Furthermore, we propose a classification of tree‐ring formation in mediterranean environments. Mediterranean tree rings can be dated and used for dendrochronological purposes, but great care should be taken in selecting sampling sites, species and sample trees.

Thresholds for warming‐induced growth decline at elevational tree line in the Yukon Territory, Canada

[1] A few tree ring studies indicate recent growth declines at northern latitudes. The precise causes are not well understood. Here we identify a temperature threshold for decline in a tree ring record from a well-established temperature-sensitive site at elevational tree line in northwestern Canada. The positive ring width/temperature relationship has weakened such that a pre-1965 linear model systematically overpredicts tree ring widths from 1965 to 1999. A nonlinear model shows an inverted U-shaped relationship between this chronology and summer temperatures, with an optimal July– August average temperature of 11.3� C based on a nearby station. This optimal value has been consistently exceeded since the 1960s, and the concurrent decline demonstrates that even at tree line, trees can be negatively affected when temperatures warm beyond a physiological threshold. If warming continues without significant gains in effective precipitation, the large-scale greening of recent decades could be replaced by large-scale browning. Such browning could slow or reverse carbon uptake by northern forests. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1851 Hydrology: Plant ecology; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4221 Oceanography: General: Dendrochronology; KEYWORDS: temperature, threshold, tree rings Citation: D’Arrigo, R. D., R. K. Kaufmann, N. Davi, G. C. Jacoby, C. Laskowski, R. B. Myneni, and P. Cherubini (2004), Thresholds for warming-induced growth decline at elevational tree line in the Yukon Territory, Canada, Global Biogeochem. Cycles, 18, GB3021, doi:10.1029/2004GB002249.

Growth response to climate and drought in Pinus nigra Arn. trees of different crown classes

Tree-ring chronologies were examined to investigate the influence of climate on radial growth of Pinus nigra in southeastern Spain. We addressed whether drought differentially affected the ring-widths of dominant and suppressed trees and if our results supported the hypothesis that, in a Mediterranean climate, suppressed conifer trees suffer greater growth reductions than dominant trees. Climate–growth relationships were analyzed using response and correlation functions, whereas the effect of drought on trees growth was approached by superposed epoch analysis in 10 dry years. A cool, wet autumn and spring, and/or mild winter enhanced radial growth. Latewood was the most sensitive ring section in both kinds of trees and it was primarily influenced by current year precipitations. Earlywood was mostly influenced by climatic conditions previous to the growing season. In general, May was the most influential month. Pinus nigra was shown to be very drought sensitive tree in the study area. Tree-rings in suppressed trees showed lower growth reductions caused by drought than those of dominant trees. However, dominant trees recovered normal growth faster. Dominant trees showed a more plastic response, and suppression appeared to reduce the effect of climate on tree radial growth. Some possible causes for these effects are discussed. Our results support the essential role of the balance between light and moisture limitations for plant development during droughts and show that it is not appropriate to generalize about the way in which suppression affects climate-growth relationship in conifers.

Elevated CO2 increases tree‐level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites

Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances.

Drought tolerance of two black poplar (Populus nigra L.) clones: contribution of carbohydrates and oxidative stress defence

Drought is expected to become an increasingly important factor limiting tree growth caused by climate change. Two divergent clones of Populus nigra (58-861 and Poli) originating from contrasting environments were subjected to water limitation (WL) to elucidate whether they differ in tolerance to drought, which mechanisms to avoid stress they exhibit and whether drought has an impact on the interactions between roots and shoots. Limiting water availability caused photosynthetic rate and total non-structural carbohydrate (TNC) levels to decrease in 58-861. However, starch-degrading enzyme activity and gene expression were induced in roots, and soluble sugar levels were higher than in well-watered (WW) plants. These data suggest that assimilation and partitioning of carbon to the roots are decreased, resulting in mobilization of stored starch. In contrast, the photosynthetic rate of Poli was reduced only late in the treatment, and carbohydrate levels in WL plants were higher than in WW plants. Superoxide dismutase (SOD) activity and gene expression were higher in Poli than in 58-861, even in WW plants, leading to a higher capacity to defend against oxidative stress.

Variations of vessel diameter and δ13C in false rings of Arbutus unedo L. reflect different environmental conditions.

Woody species in Mediterranean ecosystems form intra-annual density fluctuations (IADFs) in tree rings in response to changes in environmental conditions, especially water availability. Dendrochronology, quantitative wood anatomy and high-resolution isotopic analysis (using a laser ablation technique) were used to characterize IADFs in Arbutus unedo shrubs grown on two sites with different water availability on the island of Elba (Italy). Our findings show that IADF characterization can provide information about the relationship between environmental factors and tree growth at the seasonal level. At the more xeric site, IADFs mainly located in the early and middle parts of the annual ring, showed a decrease in vessel size and an increase in δ(13) C as a result of drought deficit. Opposite trends were found at the more mesic site, with IADFs located at the end of the ring and associated with a lower δ(13) C. Moreover, at the first site, IADFs are induced by drought deficit, while at the second site IADFs are linked with the regrowth in the last part of the growing season triggered by favourable wet conditions. This combined approach is a promising way for dating problematic wood samples and interpreting the phenomena that trigger the formation of IADFs in the Mediterranean environment.

Fast response of Scots pine to improved water availability reflected in tree-ring width and δ13C.

Drought-induced forest decline, like the Scots pine mortality in inner-Alpine valleys, will gain in importance as the frequency and severity of drought events are expected to increase. To understand how chronic drought affects tree growth and tree-ring delta(13)C values, we studied mature Scots pine in an irrigation experiment in an inner-Alpine valley. Tree growth and isotope analyses were carried out at the annual and seasonal scale. At the seasonal scale, maximum delta(13)C values were measured after the hottest and driest period of the year, and were associated with decreasing growth rates. Inter-annual delta(13)C values in early- and latewood showed a strong correlation with annual climatic conditions and an immediate decrease as a response to irrigation. This indicates a tight coupling between wood formation and the freshly produced assimilates for trees exposed to chronic drought. This rapid appearance of the isotopic signal is a strong indication for an immediate and direct transfer of newly synthesized assimilates for biomass production. The fast appearance and the distinct isotopic signal suggest a low availability of old stored carbohydrates. If this was a sign for C-storage depletion, an increasing mortality could be expected when stressors increase the need for carbohydrate for defence, repair or regeneration.

An investigation of the common signal in tree ring stable isotope chronologies at temperate sites

We observed a strong common high-frequency (interannual) signal for thed 13 C chronologies, whereas the low-frequency (decadal-scale) signal was more similar among thed 18 O chronologies. For both carbon and oxygen isotopes, we found significant positive relationships with annual and growing season temperatures and negative relationships with precipitation, again of similar magnitude for all species except forFraxinus, which contained only minor climatic information. Averaging of all chronologies resulted in an increase in the climatic signal of the mean chronology. The combinedd 18 O record reflected decadal-scale temperature variations remarkably well (r= 0.72). However, the relationship between climate and carbon isotopes declined over the last 3 decades of the 20th century, probably related to the steep increase in atmospheric CO 2 concentrations, resulting in strongly divergingd 13 C trends of the different chronologies. Our study indicates that combining chronologies from different species enhances the potential of isotope studies for extending climate reconstructions into areas of temperate climate.

Potential sampling bias in long-term forest growth trends reconstructed from tree rings : A case study from the Italian Alps

Abstract Tree-ring studies of long-term growth trends have often produced controversial results. In such studies, the largest-diameter trees in a stand are usually sampled. We assessed the influence of stand dynamics on long-term growth trends by examining the past diameters of all the trees living in two uneven-aged subalpine Norway spruce (Picea abies (L.) Karst.) stands in the Italian eastern Alps, as reconstructed from ring widths. The trees were ordered according to diameter, and groups of 12 trees (the 12 largest, the 12 smallest, etc.) were formed. Different diameter groups have different increment curves. In both stands, the 12 largest trees in 1992 have not had consistently faster growth rates than smaller trees. This indicates that changes in diameter ranking order have occurred in the past and may be expected in the future. During stand development, changes occur in the relative position of individual trees, as ordered by diameter. The largest-diameter trees, at any time, may not always have been the largest trees and may not continue to be so. In a given year, the largest trees on average grow slower than other trees, which will become the largest in the future. The mean chronologies of the trees that were among the largest, prior to the harvest, and which presently (in 1992) are no longer in the top 12, and the mean chronologies of the trees that have moved up into the top 12 show very different growth trends. If analysed out of context, they would be interpreted differently, leading to different conclusions on long-term growth trends. When only the 12 largest-diameter trees are sampled, a bias may be present, as the trees may not have been open grown and free of competition in the past. Consequently, studies of long-term growth may be seriously affected by bias attributable to stand dynamics and sampling strategies. In future studies, the growth patterns of all diameter classes in a stand should be assessed, rather than restricting the sampling to the largest diameters.

Spatial structure along an altitudinal gradient in the Italian central Alps suggests competition and facilitation among coniferous species

Abstract Questions: What is the structure of the anthropogenic upper forest-grassland ecotone and are there differences in the spatial relationships between the tree species involved? Location: Valfurva Valley, Italian central Alps. Methods: We conducted a spatial distribution and structure analysis in three 1-ha permanent plots along an altitudinal gradient, from the treeline to the sub-alpine forest. We reconstructed the age structure from cores from each individual with diameter > 4 cm at 50 cm height. Results: All tree species and age classes examined had a clumped structure. The cluster tendency was more evident at the treeline where the environmental conditions are more severe. In the sub-alpine forest there was a repulsion between Pinus cembra and Pinus mugo but at the treeline P. cembra was frequently found downslope from P. mugo. Conclusions: Although human influence has been the main driving force in shaping the present forest structure, in the last few decades natural dynamics have become the predominant force acting on forest structure and processes, showing a higher magnitude as altitude increases. Our results emphasize the existence of facilitating and interfering mechanisms between different species. P. cembra seems to be favoured compared to the other tree species.