Warming may extend tree growing seasons and compensate for reduced carbon uptake during dry periods

Abstract

1. Warming and drought alter plant phenology, photosynthesis and growth with important consequences for the global carbon cycle and the earth's climate. Yet, few studies have attempted to tease apart their effects on tree phenology, particularly leaf senescence, and on source and sink activity.2. We experimentally assessed the single and combined effects of warming and reduced soil moisture on the phenology (leaf-out and senescence date, growing season length) and above-ground sink (height and diameter growth, leaf area and Huber values) and source activity (net photosynthesis, photosynthetic efficiency, chlorophyll concentration and total carbon [C] uptake) of two tree species with distinct strategies to deal with drought: European beech and pubescent oak.3. Warming advanced leaf-out, irrespective of soil moisture levels, particularly in oak and to a lower extent in beech, leading to a prolonged growing season in oak but not beech. No impacts of warming on senescence timing were found for both species. Reduced moisture had little impact on the phenology of both species. Warming-induced advances in phenology and higher photosynthetic efficiency increased the annual C uptake for oak and compensated for the reduced photosynthetic activity in the presence of reduced moisture. Conversely, for beech, source activity, including yearly C uptake, was lower in all treatments than the control, indicating no compensation of the C budget by phenological shifts. Synthesis.Our results demonstrate that a warming-driven earlier activity and higher photosynthetic efficiency compensates for reduced photosynthesis during hot and dry periods, but only for pubescent oak, which is a rather droughttolerant species. Current predictions of warming-induced mitigation effects through extended C uptake seem incorrect for beech.

Impact of warmer and drier conditions on tree photosynthetic properties and the role of species interactions

“…( 2021 ), Grossiord et al . ( 2022 )). The OTCs were covered by mobile glass roofs kept closed during the whole experiment to exclude natural precipitation.…”

“…The experimental set-up consisted of 16 hexagonal glass-walled open-top chambers (OTCs) of 3 m height and 6 m 2 area (Fig. 1; Sch€ onbeck et al (2021), Grossiord et al (2022)). The OTCs were covered by mobile glass roofs kept closed during the whole experiment to exclude natural precipitation.…”

INTRAGRO: A machine learning approach to predict future growth of trees under climate change

“…The extension of growing season length might have several positive and negative implications for trees. In boreal forests, longer growing seasons are expected to increase forest productivity due to enhanced carbon sequestration (Devi et al., 2020; Grossiord et al., 2022). However, the growing season extension also means more favourable conditions for the proliferation of pests, potentially leading to outbreaks and increased damage to tree populations.…”

The Rate of Canopy Development Modulates the Link Between the Timing of Spring Leaf Emergence and Summer Moisture

“…Consequently, enhanced springtime ET can lead to reductions in soil moisture which may limit C uptake in late summer and autumn (Buermann et al., 2018; Lian et al., 2020). On the other hand, increased carbon uptake when spring arrives early may dampen the impact of subsequent dry downs on annual C uptake (Grossiord et al., 2022). For example, Roman et al.…”

“…Consequently, enhanced springtime ET can lead to reductions in soil moisture which may limit C uptake in late summer and autumn (Buermann et al, 2018;Lian et al, 2020). On the other hand, increased carbon uptake when spring arrives early may dampen the impact of subsequent dry downs on annual C uptake (Grossiord et al, 2022). For example, Roman et al (2015) showed that an early start to spring substantially mediated the negative impacts of the extreme 2012 flash drought on fluxes in the long-running Morgan-Monroe State Forest AmeriFlux site.…”