Abstract
In the Mediterranean region, the widely predicted rise in temperature, change in the precipitation pattern, and increase in the frequency of extreme climatic events are expected to alter the shape of ecological communities and to affect plant physiological processes that regulate ecosystem functioning. Although change in the mean values are important, there is increasing evidence that plant distribution, survival, and productivity respond to extremes rather than to the average climatic condition. The present study aims to assess the effects of both mean and extreme climatic conditions on radial growth and functional anatomical traits using long-term tree-ring time series of two co-existing Quercus spp. from a drought-prone site in Southern Italy. In particular, this is the first attempt to apply the Generalized Additive Model for Location, Scale, and Shape (GAMLSS) technique and Bayesian modeling procedures to xylem traits data set, with the aim of (i) detecting non-linear long-term responses to climate and (ii) exploring relationships between climate extreme and xylem traits variability in terms of probability of occurrence. This study demonstrates the usefulness of long-term xylem trait chronologies as records of environmental conditions at annual resolution. Statistical analyses revealed that most of the variability in tree-ring width and specific hydraulic conductivity might be explained by cambial age. Additionally, results highlighted appreciable relationships between xylem traits and climate variability more than tree-ring width, supporting also the evidence that the plant hydraulic traits are closely linked to local climate extremes rather than average climatic conditions. We reported that the probability of extreme departure in specific hydraulic conductivity (Ks) rises at extreme values of Standardized Precipitation Index (SPI). Therefore, changing frequency or intensity of extreme events might overcome the adaptive limits of vascular transport, resulting in substantial reduction of hydraulic functionality and, hence increased incidence of xylem dysfunctions.
Highlights
An accepted picture for the Mediterranean region is the ongoing trend toward higher temperature and reduced precipitation, associated to an increase in the frequency and magnitude of climatic extremes, which are expected to have detrimental effects on trees and forest biomes (Easterling et al, 2000; IPCC, 2013)
Relationships between xylem traits and Wr showed that the standard linear regression fit to these data was significantly different from zero for Aav, Dh, and dv (Figure 1, upper panels); in particular, all outcomes appeared to be negatively related to the tree-ring width
Results of the fitted GAMLSS highlighted significant temperature, precipitation, and cambial age effect for Ks, whilst only the age effect was depicted for Wr (Table 1)
Summary
An accepted picture for the Mediterranean region is the ongoing trend toward higher temperature and reduced precipitation, associated to an increase in the frequency and magnitude of climatic extremes, which are expected to have detrimental effects on trees and forest biomes (Easterling et al, 2000; IPCC, 2013). Extreme climatic events are increasingly considered to play a major role in tree mortality, and variation in xylem anatomical traits linked to tree hydraulic properties has received considerable attention in recent decades as an important plant acclimation process (for a review on this issue, see Jentsch et al, 2007). Both observational and experimental studies to date reported that variability and extremes in climate are more important drivers of ecosystem processes than mean conditions (Royer et al, 2011; Smith, 2011; Thompson et al, 2013). Xylem plastic adjustments may not be able to cope with the effect of rapid and extreme climatic events
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