• Energy Research

Thermal sensitivity of tropical trees A key uncertainty in climate change models is the thermal sensitivity of tropical forests and how this value might influence carbon fluxes. Sullivan et al. measured carbon stocks and fluxes in permanent forest plots distributed globally. This synthesis of plot networks across climatic and biogeographic gradients shows that forest thermal sensitivity is dominated by high daytime temperatures. This extreme condition depresses growth rates and shortens the time that carbon resides in the ecosystem by killing trees under hot, dry conditions. The effect of temperature is worse above 32°C, and a greater magnitude of climate change thus risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks are likely to remain higher under moderate climate change if they are protected from direct impacts such as clearance, logging, or fires. Science , this issue p. 869

AbstractTropical forests face increasing climate risk1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, $$\varPsi $$ Ψ 50) and hydraulic safety margins (for example, HSM50) are important predictors of drought-induced mortality risk3–5, little is known about how these vary across Earth’s largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters $$\varPsi $$ Ψ 50 and HSM50 vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both $$\varPsi $$ Ψ 50 and HSM50 influence the biogeographical distribution of Amazon tree species. However, HSM50 was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM50 are gaining more biomass than are low HSM50 forests. We propose that this may be associated with a growth–mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM50 in the Amazon6,7, with strong implications for the Amazon carbon sink.

Living trees constitute one of the major stocks of carbon in tropical forests. A better understanding of variations in the dynamics and structure of tropical forests is necessary for predicting the potential for these ecosystems to lose or store carbon, and for understanding how they recover from disturbance. Amazonian tropical forests occur over a vast area that encompasses differences in topography, climate, and geologic substrate. We observed large differences in forest structure, biomass, and tree growth rates in permanent plots situated in the eastern (near Santarém, Pará), central (near Manaus, Amazonas) and southwestern (near Rio Branco, Acre) Amazon, which differed in dry season length, as well as other factors. Forests at the two sites experiencing longer dry seasons, near Rio Branco and Santarém, had lower stem frequencies (460 and 466 ha(-1) respectively), less biodiversity (Shannon-Wiener diversity index), and smaller aboveground C stocks (140.6 and 122.1 Mg C ha(-1)) than the Manaus site (626 trees ha(-1), 180.1 Mg C ha(-1)), which had less seasonal variation in rainfall. The forests experiencing longer dry seasons also stored a greater proportion of the total biomass in trees with >50 cm diameter (41-45 vs 30% in Manaus). Rates of annual addition of C to living trees calculated from monthly dendrometer band measurements were 1.9 (Manaus), 2.8 (Santarém), and 2.6 (Rio Branco) Mg C ha(-1) year(-1). At all sites, trees in the 10-30 cm diameter class accounted for the highest proportion of annual growth (38, 55 and 56% in Manaus, Rio Branco and Santarém, respectively). Growth showed marked seasonality, with largest stem diameter increment in the wet season and smallest in the dry season, though this may be confounded by seasonal variation in wood water content. Year-to-year variations in C allocated to stem growth ranged from nearly zero in Rio Branco, to 0.8 Mg C ha(-1) year(-1) in Manaus (40% of annual mean) and 0.9 Mg C ha(-1) year(-1) (33% of annual mean) in Santarém, though this variability showed no significant relation with precipitation among years. Initial estimates of the C balance of live wood including recruitment and mortality as well as growth suggests that live wood biomass is at near steady-state in Manaus, but accumulating at about 1.5 Mg C ha(-1) at the other two sites. The causes of C imbalance in living wood pools in Santarém and Rio Branco sites are unknown, but may be related to previous disturbance at these sites. Based on size distribution and growth rate differences in the three sites, we predict that trees in the Manaus forest have greater mean age (approximately 240 years) than those of the other two forests (approximately 140 years).

Résumé Le puits de carbone des forêts tropicales est connu pour être sensible à la sécheresse, mais on ne sait pas quelles forêts sont les plus vulnérables aux événements extrêmes. Les forêts avec des conditions de base plus chaudes et plus sèches peuvent être protégées par une adaptation préalable, ou plus vulnérables parce qu'elles fonctionnent plus près des limites physiologiques. Ici, nous rapportons que les forêts dans les climats sud-américains plus secs ont connu les plus grands impacts du El Niño 2015–2016, indiquant une plus grande vulnérabilité aux températures extrêmes et à la sécheresse. Les réponses à long terme, mesurées au sol, arbre par arbre de 123 parcelles forestières à travers l'Amérique du Sud tropicale montrent que le puits de carbone de la biomasse a cessé pendant l'événement, le bilan carbone devenant indiscernable de zéro (−0,02 ± 0,37 Mg C ha −1 par an). Cependant, les forêts tropicales intactes d'Amérique du Sud dans l'ensemble n'étaient pas plus sensibles à l'extrême El Niño 2015–2016 qu'aux événements précédents moins intenses, restant une défense clé contre le changement climatique tant qu'elles sont protégées. Resumen Se sabe que el sumidero de carbono del bosque tropical es sensible a la sequía, pero no está claro qué bosques son los más vulnerables a los eventos extremos. Los bosques con condiciones de referencia más cálidas y secas pueden estar protegidos por una adaptación previa, o ser más vulnerables porque operan más cerca de los límites fisiológicos. Aquí informamos que los bosques en climas más secos de América del Sur experimentaron los mayores impactos de El Niño 2015–2016, lo que indica una mayor vulnerabilidad a las temperaturas extremas y la sequía. Las respuestas árbol por árbol medidas en el suelo a largo plazo de 123 parcelas forestales en América del Sur tropical muestran que el sumidero de carbono de biomasa cesó durante el evento y el balance de carbono se volvió indistinguible de cero (-0,02 ± 0,37 Mg C ha -1 por año). Sin embargo, los bosques tropicales intactos de América del Sur en general no fueron más sensibles al fenómeno extremo de El Niño 2015–2016 que a eventos anteriores menos intensos, y siguen siendo una defensa clave contra el cambio climático mientras estén protegidos. Abstract The tropical forest carbon sink is known to be drought sensitive, but it is unclear which forests are the most vulnerable to extreme events. Forests with hotter and drier baseline conditions may be protected by prior adaptation, or more vulnerable because they operate closer to physiological limits. Here we report that forests in drier South American climates experienced the greatest impacts of the 2015–2016 El Niño, indicating greater vulnerability to extreme temperatures and drought. The long-term, ground-measured tree-by-tree responses of 123 forest plots across tropical South America show that the biomass carbon sink ceased during the event with carbon balance becoming indistinguishable from zero (−0.02 ± 0.37 Mg C ha −1 per year). However, intact tropical South American forests overall were no more sensitive to the extreme 2015–2016 El Niño than to previous less intense events, remaining a key defence against climate change as long as they are protected. من المعروف أن بالوعة الكربون في الغابات الاستوائية حساسة للجفاف، ولكن من غير الواضح أي الغابات هي الأكثر عرضة للظواهر المتطرفة. قد تكون الغابات ذات الظروف الأساسية الأكثر سخونة وجفافًا محمية عن طريق التكيف المسبق، أو أكثر عرضة للخطر لأنها تعمل بالقرب من الحدود الفسيولوجية. نذكر هنا أن الغابات في المناخات الأكثر جفافًا في أمريكا الجنوبية شهدت أكبر تأثيرات لظاهرة النينيو 2015–2016، مما يشير إلى زيادة التعرض لدرجات الحرارة القصوى والجفاف. تُظهر الاستجابات طويلة الأجل المقاسة بالأرض شجرة بشجرة لـ 123 قطعة أرض حرجية في جميع أنحاء أمريكا الجنوبية الاستوائية أن بالوعة الكربون للكتلة الحيوية توقفت خلال الحدث مع عدم إمكانية تمييز توازن الكربون عن الصفر (−0.02 ± 0.37 Mg C ha −1 في السنة). ومع ذلك، لم تكن غابات أمريكا الجنوبية الاستوائية السليمة بشكل عام أكثر حساسية لظاهرة النينيو الشديدة 2015–2016 من الأحداث السابقة الأقل كثافة، وظلت دفاعًا رئيسيًا ضد تغير المناخ طالما أنها محمية.