Drought and heatwave impacts on semi-arid ecosystems' carbon fluxes along a precipitation gradient

Abstract

The inter-annual variability (IAV) of the terrestrial carbon cycle is tightly linked to the variability of semi-arid ecosystems. Thus, it is of utmost importance to understand what the main meteorological drivers for the IAV of such ecosystems are, and how they respond to extreme events such as droughts and heatwaves. To shed light onto these questions, we analyse the IAV of carbon fluxes, its relation with meteorological variables, and the impact of compound drought and heatwave on the carbon cycle of two similar ecosystems, along a precipitation gradient. A four-year long dataset from 2016 to 2019 was used for the FLUXNET sites ES-LMa and ES-Abr, located in central (39°56'25″ N 5°46'28″ W) and southeastern (38°42'6″ N 6°47'9″ W) Spain. We analyse the physiological impact of compound drought and heatwave on the dominant tree species, Quercus ilex. Our results show that the gross primary productivity of the wetter ecosystem was less sensitive to changes in soil water content, compared to the dryer site. Still, the wetter ecosystem was a source of CO 2 each year, owing to large ecosystem respiration during summer; while the dry site turned into a CO 2 sink during wet years. Overall, the impact of the summertime compound event on annual CO 2 fluxes was marginal at both sites, compared to drought events during spring or autumn. This highlights that drought timing is crucial to determine the annual carbon fluxes in these semi-arid ecosystems. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

A historical, geographical and ecological perspective on the 2018 European summer drought

“…However, ecosystem productivity in some regions affected by drought, especially parts of Scandinavia and Eastern Europe, appears to have been close to or even above average, as observed by eddy-flux towers, simulated by process-based vegetation models, and deduced from atmospheric CO 2 measurements and transport modelling in the so-called atmospheric inversion models [33,40,43,44]. In Southern Europe, net CO 2 uptake was generally above average, even though extreme summer temperatures were registered, as wetter than average conditions prevailed in summer 2018 [45]. The asymmetry in the responses of ecosystems to the extreme summer drought and heatwave is the most striking feature of the analyses presented in this issue, but this can be seen as the result of the variety and broad scope of the approaches.…”

“…Rödenbeck et al [40] find a larger role for temperature-correlated NEE anomalies than for those correlated with the SPEI06, and suggest an important contribution of TER in creating the Central European reduced net carbon uptake. Bastos et al [33], Smith et al [42], El-Madany et al [45] and Koebsch et al [53] report covariations of TER reductions with GPP reductions, as also observed during many previous droughts across the world. Atmospheric CO 2 observations suggest the balance of these reductions to have led to reduced net carbon uptake in many ecosystems (though see [38,[40][41][42], for some turning net sinks of carbon into sources).…”

“…But Lindroth et al [44] show that forest ecosystems in Scandinavia were generally able to cope well with the strong drought by accessing deeper water layers, and therefore maintain close to average evaporation rates in spite of drought. Similarly, El-Madany et al [45] find little response of Mediterranean oak trees to the summer drought, while Gharun et al [48] find montane grasslands in Switzerland to have responded with extra carbon uptake. On the other hand, in those sites more strongly affected by soil-moisture deficits, Fu et al [47] reported a doubling of the sensitivity of GPP to soil-water content, indicating a shift to water-limitation conditions compared with the 'normal' year 2016.…”

“…A complication when trying to better define the role of soil moisture is that its effects come on top of the betterunderstood impact of high atmospheric vapour pressure deficits, causing stomatal closure and decreases in latent energy and GPP, as reported widely also for the summer 2018 event in [44][45][46][47][48][49]. Soil moisture stress exacerbated this atmospheric-driven impact, implemented as a reduction of the maximum carboxylation rate (Vcmax) in Gourlez de la Motte et al [46], as reductions in Vcmax, stomatal conductance and mesophyll conductance by Smith et al [42], and through various other mechanisms, sometimes including full plant hydraulics from roots to leaves, in the Dynamic Global Vegetation Models used in Bastos et al [33].…”

Lysimeter based evaporation and condensation dynamics in a Mediterranean ecosystem

“…The rain mainly falls between October and April, with mean annual amounts of ca. 650 mm, with large interannual variations (El-Madany et al, 2020). The ecosystem is a typical Mediterranean semiarid tree-grass ecosystem (Dehesa) with low-density oak tree cover (Quercus Ilex (L.), 20 trees ha −1 ) (Bogdanovich et al, 2021).…”