Analysing Atmospheric Processes and Climatic Drivers of Tree Defoliation to Determine Forest Vulnerability to Climate Warming

Abstract

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.

Tree diversity mitigates defoliation after a drought‐induced tipping point

“…The fact that, for both beech and sessile oak, the mean defoliation level was highest after dry years than wet years confirms the results of earlier studies (e.g., Carnicer et al., ; Sánchez‐Salguero et al., ; Seidling, ), as well as our hypothesis, that drought can trigger and expedite defoliation in deciduous forests. But perhaps more importantly, our results reveal that the negative impact of drought on the crown defoliation of the three studied species in deciduous temperate forests was mitigated by species mixing through time (i.e., interaction between species richness and year of observation; Figure b).…”

The impact of drought spells on forests depends on site conditions: The case of 2017 summer heat wave in southern Europe

“…However, in north‐western France, vegetation anomalies might be related to local effects on vegetation cover related to increased crown transparency, as indicated by records from European networks on forest health (Michel et al, ). Such enhanced leaf shedding could also be driven by increases in temperature (Sánchez‐Salguero, Camarero, Grau, et al, ), although a consistent link to date has not been established.…”

Site factors and stand conditions associated with Persian oak decline in Zagros mountain forests

“…may have profound implications for carbon storage and ecosystem services (Carnicer et al, 2011). Our results suggest that Persian oak is sensitive in terms of crown defoliation to warm and dry conditions, as well as they have been found for other mediterranean oaks, like Q. faginea, Q. Ilex and Q. suber (Sánchez-Salguero et al, 2017). Warm spring conditions and severe summer drought have been already recognised as triggers of crown defoliation, whereas growth of oaks in Mediterranean environments is linked to late spring-early summer water availability, at both annual and decadal timescales, suggesting oak growth decline was associated with a delayed response to climate (Di Filippo et al, 2010;Natalini et al, 2016).…”

“…The effects of drought are more common visualized at the regional-scale (Keyser & Brown, 2016;Sánchez-Salguero et al, 2017), but its influence on crown dieback and tree death will be modulated at the local-scale, where factors such as topography, soil and stand structure might be as important as regional climate to understand oak forests dynamics (Jenkins & Pallardy, 1995;Chapman et al, 2006;Gea-Izquierdo et al, 2009;Galiano et al, 2010). Topographic characteristics such as slope and aspect also strongly influence soil moisture, and therefore drought-induced tree mortality may vary accordingly (Das et al, 2008).…”

“…As regards the specific case of oak trees, water deficit is the most common stressing factor related to physiological weakness (Jenkins & Pallardy, 1995;Lloret et al, 2004;Ogaya & Peñuelas, 2007;Fan et al, 2008;Haavik et al, 2015), triggering episodic tree declines and mortality (Fan et al, 2012;Keyser & Brown, 2016;Sánchez-Salguero et al, 2017). Furthermore, following drought, other events, such as beetle outbreaks, affect mortality of oak trees or increase its severity Fan et al, 2008;Hosseini, 2012).…”