Plasticity in response to environmental drivers can help trees cope with droughts. However, our understanding of the importance of plasticity and physiological adjustments in trees under global change is limited. We examine 20th century growth responses in Gymnosperm trees during (resistance) and following (resilience) years of severe soil and atmospheric droughts occurring in isolation or as compound events. We use high atmospheric vapour pressure deficit (VPD) to select years of atmospheric drought and negative annual values of the Standardised Precipitation-Evapotranspiration Index (SPEI) to select years with a large negative balance between precipitation and evaporation. Sensitivities (i.e., the slopes of the relationships) of resilience to VPD and SPEI changed throughout the 20th century, with the directions of these changes often reversing in the second half of the century. For the 1951-2001 period, variable sensitivities had positive effects on resilience, especially following years of high VPD and compound VPD/SPEI events, avoiding growth losses that would have occurred if sensitivities had remained constant. Despite sensitivity changes, resilience recovered less at the end of the 20th century compared to the beginning of the century. Future adjustments to low SPEI and high VPD are likely to continue to compensate for the trends in climate only partially, leading to further generalised reductions in tree growth of Gymnosperm trees.
Although a substantial body of evidence suggests that large and old trees have reduced metabolic levels, the search for the causes behind this observation has proved elusive. The strong coupling between age and size, commonly encountered in the field, precludes the isolation of the potential causes. We used standard propagation techniques (grafting and air-layering) to decouple the effects of size from those of age in affecting leaf structure, biochemistry and physiology of two broadleaved trees, Acer pseudoplatanus (a diffuse-porous species) and Fraxinus excelsior (a ringporous species). The first year after establishment of the propagated plants, some of the measurements suggested the presence of age-related declines in metabolism, while other measurements either did not show any difference or suggested variability across treatments not associated with either age or size. During the second year after establishment, only one of the measured properties (specific leaf area) continued to show some evidence of an age-mediated decline (although much reduced compared to the field), whereas, for some properties (particularly for F. excelsior), even the opposite trend of age-related increases was apparent. We concluded that (1) our plants suffered from grafting shock during year 1 and they gradually recovered during year 2; (2) the results over 2 years do not support the statement that age directly mediates ageing in either species but instead suggest that size directly mediates ageing processes; and (3) neither shoots nor roots of A. pseudoplatanus showed any evidence of senescence.
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