Abstract
Crown defoliation is extensively monitored across European forests within the International Co-operative Programme (ICP) as a proxy of forest health. Climate warming and drought are assumed to be the major drivers of tree growth and crown defoliation, particularly in seasonally dry areas such as the Mediterranean Basin. Here we analyse how climate, drought, and atmospheric processes are related to defoliation time series of five oak and five pine species that are dominant across Spanish ICP monitoring forest plots. We found that warmer and drier conditions during April were linked to enhanced defoliation. Warm April conditions were also related to high values of the Atlantic Multi-decadal Oscillation (AMO), thereby indicating large-scale links between atmospheric processes, temperature, and defoliation patterns. The temperature-defoliation association was species-specific since some tree species from wet sites showed a weak association (e.g., Quercus robur L.) whereas others from dry sites (e.g., Quercus ilex L.) presented the strongest associations. The latter tree species could be considered vulnerable to heat stress in terms of leaf shedding. We also explored if defoliation was related to radial growth and found negative associations in relatively dry areas. Warmer and drier conditions linked to increasing AMO values are connected to the post-1990s rise of defoliation in Spanish ICP forest plots. Combined incorporation of defoliation and growth into mortality models can provide insights into assessments of forest vulnerability.
Highlights
The frequency and intensity of droughts and heat waves is expected to increase throughout the21st century as climates warm [1,2]
We confirmed that warmer and drier climate conditions during April are linked to enhanced defoliation in Spanish International Co-operative Programme (ICP)-monitored forest plots
Such climate conditions were linked to elevated values of the Atlantic Multi-decadal Oscillation (AMO), suggesting the existence of large-scale links between atmospheric processes, April temperature, and forest defoliation patterns
Summary
The frequency and intensity of droughts and heat waves is expected to increase throughout the21st century as climates warm [1,2]. Dry spells and heat waves during the early growing season cause rapid morphological responses in affected trees as leaf shedding (defoliation), early senescence, and canopy dieback leading to short- to long-term reductions in leaf area and growth, decreasing forest productivity [3,4]. The negative effects of heat stress on trees are exacerbated by water shortage and can lead to widespread forest dieback, as has been observed in most drought-prone forest biomes [5,6]. Severe canopy defoliation in response to drought or heat stress reflects irreversible tree damage and portends forest dieback [7]. Warm autumn conditions can affect bud dormancy and hardening processes, and delay budburst and leaf-out in the following spring affecting canopy defoliation in the case of boreal tree species [11]
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