Warming and elevated CO2 alter tamarack C fluxes, growth and mortality: evidence for heat stress-related C starvation in the absence of water stress

Abstract

Climate warming is increasing the frequency of climate-induced tree mortality events. While drought combined with heat is considered the primary cause of this mortality, little is known about whether moderately, high temperatures alone can induce mortality, or whether rising CO2 would prevent mortality at high growth temperatures. We grew tamarack (Larix laricina) under ambient (400 ppm) and elevated (750 ppm) CO2 concentrations combined with ambient, ambient +4°C, and ambient +8°C growth temperatures to investigate whether high growth temperatures lead to carbon limitations and mortality. Growth at +8°C led to 40% mortality in the ambient CO2 (8TAC) treatment, but no mortality in the elevated CO2 treatment. Thermal acclimation of respiration led to similar leaf carbon balances across the warming treatments, despite a lack of photosynthetic acclimation. Photosynthesis was stimulated under elevated CO2, increasing seedling growth, but not leaf carbon concentrations. However, growth and foliar carbon concentrations were lowest in the +8°C treatments, even with elevated CO2. Dying 8TAC seedlings had lower needle carbon concentrations and lower ratios of photosynthesis to respiration than healthy 8TAC seedlings, indicating that carbon limitations were likely the cause of seedling mortality under high growth temperatures.

Divergent effects of warming on nonstructural carbohydrates in woody plants: a meta‐analysis

“…Moreover, previous studies have indicated that warming could significantly deplete starch in conifers' leaves (Du et al, 2020; Yu et al, 2019), which echoes our findings of the negative response of leaf starch to warming in all the studied woody plants (Figure 1C), especially in the evergreen species (Figure 3C). Evergreens typically exhibit a conservative survival strategy (Li et al, 2021; Palacio et al, 2018), potentially resulting in the conversion of starch to large amounts of soluble sugars to avoid warming‐induced heat stress (Murphy and Way, 2021; Weber et al, 2019). More importantly, since soluble sugars are substrates for cellular respiration, such a substantial overall decline in leaf starch (Figure 1C) might arise from the acceleration of starch decomposition (Adams et al, 2013; Adams et al, 2009; Tingey et al, 2003; Tjoelker et al, 2008; Yu et al, 2019) in response to the increased warming‐induced respiratory consumption (Geigenberger, 2011).…”

“…However, Xiong et al (2020) and Liu et al (2020) reported that warming led to a marked higher proportion of NSC allocated to roots in evergreen conifers but a prominent lower proportion of NSC allocated to roots in broadleaved deciduous trees. Some studies also provided evidence that warming might result in a decline in whole‐tree C stock, noting that the decreased starch content was prevalent (Dietze et al, 2014; Murphy and Way, 2021; Sevanto et al, 2014; Wiley et al, 2017). Thus, these conflicting findings indicated that warming effects on NSC might vary by plant species and organ types, and might also behave differently in different NSC components.…”

“…Rising temperature raises the probability of ecosystems reaching their ecophysiological thresholds that, once crossed, will trigger self‐amplifying feedbacks (e.g., massive forest dieback), which could in turn exacerbate climate change (Adams et al, 2013; Gampe et al, 2021; Song et al, 2019; Trenberth et al, 2014). Global warming has indisputably posed increasingly irreversible risks to the growth and survival of woody plants (Hughes, 2000; Murphy and Way, 2021; Parmesan and Yohe, 2003). Severe heat stress could even force woody plants to survive longer periods at the expense of depleting their carbon (C) stores, ultimately incurring a complete C imbalance to the whole ecosystem (McDowell et al, 2011).…”

“…Previous studies have found that warming can exacerbate the negative impact of drought by decreasing C assimilation while increasing C consumption, thereby reducing the whole‐plant NSC stock (Duan et al, 2013; Yan et al, 2020). In addition, it has also been suggested that high CO 2 concentration could stimulate photosynthesis, thereby counteracting the negative effects of warming that deplete C stores without replenishing them (Habermann et al, 2019; Murphy and Way, 2021). Indeed, climate warming is predicted to increase the intensity and frequency of other global change drivers, which might strongly influence the C balance of ecosystems and even lead to increasing tree mortality (Adams et al, 2017; Albert et al, 2011; McDowell, 2011; Murphy and Way, 2021; Trenberth et al, 2014).…”

“…In addition, it has also been suggested that high CO 2 concentration could stimulate photosynthesis, thereby counteracting the negative effects of warming that deplete C stores without replenishing them (Habermann et al, 2019; Murphy and Way, 2021). Indeed, climate warming is predicted to increase the intensity and frequency of other global change drivers, which might strongly influence the C balance of ecosystems and even lead to increasing tree mortality (Adams et al, 2017; Albert et al, 2011; McDowell, 2011; Murphy and Way, 2021; Trenberth et al, 2014). Therefore, exploring how woody plants adjust their NSC utilization in response to multiple environmental changes combined with warming could further provide insights into terrestrial ecosystem feedback to global change (Dietze et al, 2014; Hartmann and Trumbore, 2016; Yan et al, 2020).…”