Abstract
Rising temperatures and aridification, combined with the stressing effect of some hemiparasitic plants such as mistletoes, may contribute to reduce vigour and growth of trees and shrubs leading to dieback and increasing mortality. This has been rarely explored in pioneer shrubs such as junipers, which are assumed to be more drought tolerant than coexisting trees. To test these ideas, we reconstructed radial growth patterns of common junipers (Juniperus communis L.) with different crown cover and infestation degree by dwarf mistletoe (Arceuthobium oxycedri (DC.) M. Bieb.) in two sites with contrasting aspect and water availability located in north-eastern Spain. We used dendrochronology to study the response of junipers’ radial growth to climatic factors (temperature, precipitation, and soil moisture), an index of drought severity, and mistletoe infestation. Juniper growth was constrained by elevated temperatures and low precipitation leading to drought during the growing season. Infestation by dwarf mistletoe contributed to a short-term growth decline in junipers. The interaction between low summer precipitation and high dwarf mistletoe infestation constrained juniper growth, particularly in the north-oriented wetter site, where hosts presented higher growth rates during wet periods. The negative impact of low summer precipitation on juniper growth overrides the effects due to dwarf mistletoe infestation. Aridification and mistletoe infestation could trigger dieback and mortality of shrubs slowing down successional dynamics and delaying shrub encroachment into former croplands and grasslands.
Highlights
Some regions of the Mediterranean Basin have experienced aridification trends after the 1950s due to increasingly warmer conditions [1]
We aim to determine whether there is a relationship between changes in radial growth of common juniper (J. communis) as a function of variations in temperature, precipitation, soil moisture, an index of drought severity, and infestation degree by dwarf mistletoe (A. oxycedri) in two sites of north-eastern Spain, near the southernmost, xeric distribution limit of common juniper [18]
The juniper response to the interaction between summer precipitation and dwarf mistletoe infestation observed in Ribazada suggests that elevated summer precipitation enhanced juniper growth if mistletoe cover was low enough, and the negative influence of mistletoe on radial growth was observed when the infestation degree was very high
Summary
Some regions of the Mediterranean Basin have experienced aridification trends after the 1950s due to increasingly warmer conditions [1]. According to tree-ring reconstructions of droughts in the Mediterranean Basin, the recent dry spells in the late 20th century and early 21st century were among the most severe of the past millennium [2] Such severe and increasingly hotter and drier droughts have been linked to forest dieback episodes characterized by growth and productivity declines, rapid leaf shedding and rises in mortality rates [3,4]. Schleid., Thymus vulgaris L., etc.), where they are encroaching into grasslands as traditional land-use activities (pastures, extensive agriculture, etc.) cease or decrease their intensity [5] This is the case of junipers, which are vulnerable to drought stress because they are anisohydric species, i.e., they show a loose regulation of stomatal conductance rates as soil water decreases, and depend on shallow soil water sources [6,7,8]. Increasingly drier and hotter conditions could lead to more widespread forest dieback but could trigger de-shrubification events by promoting shrub dieback, slowing down successional dynamics [10]
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