Drought-induced mortality selectively affects Scots pine trees that show limited intrinsic water-use efficiency responsiveness to raising atmospheric CO2

Heres A.-M., Voltas J., Lopez B.C., Martinez-Vilalta J. (2014) Drought-induced mortality selectively affects Scots pine trees that show limited intrinsic water-use efficiency responsiveness to raising atmospheric CO2. Functional Plant Biology. 41: 244-256.
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Doi: 10.1071/FP13067

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Widespread drought-induced tree mortality has been documented around the world, and could increase in frequency and intensity under warmer and drier conditions. Ecophysiological differences between dying and surviving trees might underlie predispositions to mortality, but are poorly documented. Here we report a study of Scots pines (Pinus sylvestris L.) from two sites located in north-eastern Iberian Peninsula where drought-associated mortality episodes were registered during the last few decades. Time trends of discrimination against 13C (Δ13C) and intrinsic water-use efficiency (WUEi) in tree rings at an annual resolution and for a 34 year period were used to compare co-occurring now-dead and surviving pines. Results indicate that both surviving and now-dead pines significantly increased their WUEi over time, although this increase was significantly lower for now-dead individuals. These differential WUEi trends corresponded to different scenarios describing how plant gas exchange responds to increasing atmospheric CO2 (Ca): the estimated intercellular CO2 concentration was nearly constant in surviving pines but tended to increase proportionally to Ca in now-dead trees. Concurrently, the WUEi increase was not paralleled by a growth enhancement, regardless of tree state, suggesting that in water-limited areas like the Mediterranean, it cannot overcome the impact of an increasingly warmer and drier climate on tree growth. © 2014 CSIRO.

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Declining hydraulic performances and low carbon investments in tree rings predate Scots pine drought-induced mortality

Hereş A.-M., Camarero J.J., López B.C., Martínez-Vilalta J. (2014) Declining hydraulic performances and low carbon investments in tree rings predate Scots pine drought-induced mortality. Trees - Structure and Function. 28: 1737-1750.
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Doi: 10.1007/s00468-014-1081-3

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Key message: The retrospective analysis of wood anatomical features evidences how a long-term deterioration of hydraulic performance and carbon use portend drought-induced mortality in Scots pine.Abstract: Widespread episodes of drought-induced tree mortality are predicted to become more frequent as climate becomes warmer and drier. Nevertheless, growth trends and their links to changes in wood anatomy before tree dies are still poorly understood. Wood anatomical features provide valuable information that can be extracted to infer the mechanisms leading to tree death. In this study, we characterize drought-induced mortality affecting two Scots pine (Pinus sylvestris) sites (Prades and Arcalís) located in the North Eastern Iberian Peninsula. Co-occurring now-dead and living Scots pine trees were sampled and their wood anatomical features were measured and compared. We aimed to detect differences in anatomical features between living and dead trees, and to infer past physiological performances that might have determined their subsequent death or survival. Now-dead trees showed lower tracheid and resin duct production, and smaller radial lumen diameters than co-occurring living trees. At the more xeric Prades site, these anatomical differences were larger and chronic, i.e. were observed over the three studied decades, whilst they were less pronounced at the other, more mesic Arcalís site, where tree mortality episodes were more recent. This indicates that dead trees’ hydraulic conductivity was severely affected and that carbon investment in xylem formation and resin duct production was constrained prior to tree death. Our findings show that both hydraulic deterioration and low carbon allocation to xylem formation were associated to drought-induced mortality in Scots pine. Nevertheless, the temporal dynamics of these processes differed between populations as a function of site climatic conditions. © 2014, Springer-Verlag Berlin Heidelberg.

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The age of monumental olive trees (Olea europaea) in northeastern Spain

Arnan X., López B.C., Martínez-Vilalta J., Estorach M., Poyatos R. (2012) The age of monumental olive trees (Olea europaea) in northeastern Spain. Dendrochronologia. 30: 11-14.
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Doi: 10.1016/j.dendro.2011.02.002

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Trees can reach ages that in some cases amount to thousands of years. In the Mediterranean region, olive trees (Olea europaea) have traditionally been considered a particularly long-lived species. The main objective of this study was to assess the age of large olive trees considered to be millenarian and classified as monumental trees in northeastern Spain. We extracted cores of 14 individuals and obtained 8 sections of trees which had already been cut in the area where the largest olive trees in the northeastern Iberian Peninsula are found. The age of the sampled olive trees was assessed by counting the number of annual growth rings. Tree rings did not cross-date well, neither within nor between individuals, but boundaries between likely annual rings were clearly distinct. We found a linear relationship between DBH and tree age (in years) (Age=2.11×diameter(cm)+88.93, R2=0.80), which was used to estimate the age of unsampled olive trees. The maximum estimated age (627±110 years) is among the greatest ages reported for olive trees around the world (700 years) and among the oldest trees in Mediterranean ecosystems. © 2011 Istituto Italiano di Dendrocronologia.

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Growth patterns in relation to drought-induced mortality at two Scots pine (Pinus sylvestris L.) sites in NE Iberian Peninsula

Hereş A.-M., Martínez-Vilalta J., López B.C. (2012) Growth patterns in relation to drought-induced mortality at two Scots pine (Pinus sylvestris L.) sites in NE Iberian Peninsula. Trees - Structure and Function. 26: 621-630.
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Doi: 10.1007/s00468-011-0628-9

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Drought-related tree mortality has become a widespread phenomenon. Scots pine (Pinus sylvestris L.) is a boreal species with high ecological amplitude that reaches its southwestern limit in the Iberian Peninsula. Thus, Iberian Scots pine populations are particularly good models to study the effects of the increase in aridity predicted by climate change models. A total of 78 living and 39 dead Scots pines trees were sampled at two sites located in the NE of the Iberian Peninsula, where recent mortality events have been recorded. Annual tree rings were used to (1) date dead trees; (2) investigate if there was an association between the occurrence of tree death and severe drought periods characterized by exceptionally low ratios of summer precipitation to potential evapotranspiration (P/PET); and (3) to compare the growth patterns of trees that died with those of surviving ones. Mixed models were used to describe the relationships between tree growth (in terms of basal area increment, BAI, and the percentage of latewood, LW%) and climate variables. Our results showed a direct association between Scots pine mortality and severe drought periods characterized by low summer water availability. At the two sites, the growth patterns of dead trees were clearly distinguishable from those of the trees that survived. In particular, the BAI of dead trees was more sensitive to climate dryness (low P/PET summer, high temperatures) and started to decline below the values of surviving neighbors 15-40 years before the time of death, implying a slow process of growth decline preceding mortality. © 2011 Springer-Verlag.

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Stand- and tree-level determinants of the drought response of Scots pine radial growth

Martínez-Vilalta J., López B.C., Loepfe L., Lloret F. (2012) Stand- and tree-level determinants of the drought response of Scots pine radial growth. Oecologia. 168: 877-888.
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Doi: 10.1007/s00442-011-2132-8

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Characterizing the responses of key tree species to extreme climatic events may provide important information for predicting future forest responses to increased climatic variability. Here we aimed at determining which tree- and stand-level attributes were more closely associated with the effect of a severe drought on the radial growth of Scots pine, both in terms of immediate impact and recovery after the drought event. Our dataset included tree-ring series from 393 plots located close to the dry limit of the species range. Time series analysis and mixed-effects models were used to study the growth of each tree and its detailed response to a severe drought event that occurred in 1986. Our results showed that the radial growth responses of Scots pine were determined primarily by tree-level characteristics, such as age and previous growth rate, and secondarily by stand basal area and species richness, whereas local climate had a relatively minor effect. Fast-growing trees were more severely affected by the drought and retained proportionally lower growth rates up to three years after the episode. In absolute terms, however, fast-growing trees performed better both during and after the event. Older trees were found to be less resilient to drought. The effect of stand basal area and species richness indicated that competition for resources worsened the effects of drought, and suggested that the effect of interspecific competition may be particularly detrimental during the drought year. © 2011 Springer-Verlag.

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The age of monumental olive trees (Olea europaea) in northeastern Spain.

Arnan X, López BC, Martínez-Vilalta J, Estorach M, Poyatos R (2011) The age of monumental olive trees (Olea europaea) in northeastern Spain. Dendrochronología doi: 10.1016/j.dendro.2011.02.002.

Twentieth century increase of Scots pine radial growth in NE Spain shows strong climate interactions

Martínez-Vilalta J., López B.C., Adell N., Badiella L., Ninyerola M. (2008) Twentieth century increase of Scots pine radial growth in NE Spain shows strong climate interactions. Global Change Biology. 14: 2868-2881.
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Doi: 10.1111/j.1365-2486.2008.01685.x

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Stem radial growth responds to environmental conditions, and has been widely used as a proxy to study long-term patterns of tree growth and to assess the impact of environmental changes on growth patterns. In this study, we use a tree ring dataset from the Catalan Ecological and Forest Inventory to study the temporal variability of Scots pine (Pinus sylvestris L.) stem growth during the 20th century across a relatively large region (Catalonia, NE Spain) close to the southern limit of the distribution of the species. Basal area increment (BAI) was modelled as a function of tree size and environmental variables by means of mixed effects models. Our results showed an overall increase of 84% in Scots pine BAI during the 20th century, consistent with most previous studies for temperate forests. This trend was associated with increased atmospheric CO2 concentrations and, possibly, with a general increase in nutrient availability, and we interpreted it as a fertilization effect. Over the same time period, there was also a marked increase in temperature across the study region (0.19°C per decade on average). This warming had a negative impact on radial growth, particularly at the drier sites, but its magnitude was not enough to counteract the fertilization effect. In fact, the substantial warming observed during the 20th century in the study area did not result in a clear pattern of increased summer drought stress because of the large variability in precipitation, which did not show any clear time trend. But the situation may change in the future if temperatures continue to rise and/or precipitation becomes scarcer. Such a change could potentially reverse the temporal trend in growth, particularly at the driest sites, and is suggested in our data by the relative constancy of radial growth after ca. 1975, coinciding with the warmer period. If this situation is representative of other relatively dry, temperate forests, the implications for the regional carbon balance would be substantial. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing.

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