Wednesday, September 4, 2013

New paper finds another problem with global carbon-cycle models: plant respiration is 'as different as night and day'

Just a few days following publication of a paper finding global carbon cycle datasets may be significantly biased, a paper published today in Nature finds carbon-cycle models do not take into account that the effects on plant growth of rising night-time temperatures are opposite to those of increasing daytime temperatures.

Biogeochemistry: As different as night and day
Published online

An analysis of northern ecosystems shows that the effects on plant growth of rising night-time temperatures are opposite to those of increasing daytime temperatures — a finding that has implications for carbon-cycle models. See Letter p.88

An under-appreciated aspect of climate  change is the fact that Earth is warming at a higher rate at night than  during the day: over the past 50 years, daily  minimum temperatures have increased about  40% faster than daily maximum temperatures1.  This asymmetric warming may have impor- tant biological consequences, particularly for  fundamental ecosystem metabolic processes  that are strongly sensitive to temperature vari- ations, such as photosynthesis and respiration.  On page 88 of this issue, Peng et al.2 document  regionally significant, and in many cases  opposing, effects of year-to-year (interannual)  variations in daytime and night-time temperatures on plant growth and carbon cycling in  land regions of the Northern Hemisphere. Photosynthesis is driven by light and thus  happens only during the day, whereas plant  and microbial respiration occurs continu-
 ously. Therefore, faster night-time warming  presumably affects respiration more than it  affects photo  synthesis, and this could have far- reaching implications for how ecosystems react  to expected increases in warming in coming  decades. But remarkably little research has been done on how asymmetric warming influences  ecological function, especially at large scales.  To address this issue, Peng and colleagues have  analysed satellite-derived data sets of plant  greenness, which is a proxy for plant growth. The authors found that ecosystems in cool,  wet temperate and boreal regions such as  northwestern North America and Japan, and  those in cold regions such as Siberia and the  Tibetan plateau, seem to have benefited most  from daytime temperature increases over the  period considered (1982–2009). By contrast,  ecosystems in dry temperate regions, such as  central Eurasia and western China, showed the  opposite effect: increasing daytime tempera- tures correlated with decreasing plant green- ness. These contrasting responses broadly  agree with expectations for ecosystems in  which plant growth is limited primarily by  temperature (cool, wet climates) or moisture  (warm, dry climates).  More intriguingly, Peng and colleagues  found that ecosystems in many of the boreal  and wet temperate regions grew less well in  response to increases in night-time minimum  temperatures — the opposite effect to their  response to increasing daytime maximum 
 temperatures (Fig. 1). Conversely, in many  arid and semi-arid regions, such as the grass- lands of China and North America, increasing  night-time minimum temperatures correlated  positively with plant greenness. Peng et al. used a statistical approach to control for other contributing environmental vari- ables, such as solar radiation and precipitation.  This allowed them to isolate the interannual  greenness responses to daytime maximum and  night-time minimum temperature variations.  The authors confirmed the statistical validity  of their findings using other techniques, and  also analysed the sensitivity of the greenness  response to alternative interpolated climate 
 data sets and at individual weather-station  locations. Importantly, the different analyses  all confirmed the same broad conclusions. 

 A strength of this study is that the research- ers explored ecosystem responses to asymmet- ric warming using a variety of other large-scale  data sets, and found similar patterns. One data  set was for the net exchange of carbon between  land and the atmosphere — a quantity that  integrates photosynthesis and respiration, and  which was inferred from a multi-year analysis3.  Peng and co-workers found that this quantity  correlated positively with daytime temperature  variations for cool and wet boreal ecosystems,  but negatively with night-time temperatures  for these ecosystems. They also observed that  the amplitudes of the seasonal cycles of car- bon dioxide levels measured at Point Barrow,  Alaska, and Mauna Loa, Hawaii, vary in the  same way with daytime and night-time tem- perature variations in boreal regions, but not  in temperate areas. Peng et al. focused only on boreal and  temperate  ecosystems.  The response to  asymmetric warming of tropical and subtropi- cal ecosystems, which account for most CO2  exchange between the land and the atmos- phere, is not clear and merits further investiga- tion. Previous work4 at a well-studied tropical 
 forest revealed a negative correlation between  tree growth and annual mean daily minimum  temperatures, a response broadly similar to  Peng and colleagues’ findings for boreal forests. 

 Tropical forests are thought to be vulnerable to  warming5, with some evidence6 suggesting that  they are already near high-temperature thresh- olds above which growth could be restricted.  Future research could help to fill major gaps in  our understanding of thermal tolerance and  acclimation in tropical and subtropical plant  species, and thus their response to warming5,7. So what are the physiological mechanisms  that drive large-scale correlations between  temperature variations and ecosystem metabo- lism? The commonly discussed mechanisms  involve biochemical responses to temperature,  but with some interesting twists. For example,  the positive correlation found between night- time minimum temperatures and greenness in  semi-arid grasslands is puzzling, but might be  related to greater night-time plant respiration  that stimulates increased daytime photosyn- thesis8. Increases in night-time respiration  have also been invoked in a pioneering study9  of nocturnal warming that documented differ- ent plant responses in grassland: the dominant  grass species declined in response to increases  in night-time temperature during spring,  whereas other plant species that use a different  photosynthetic pathway increased in number. A research agenda to investigate these  mechanisms further should include manipu- lative field and mesocosm experiments (in  which small parts of a natural ecosystem are  enclosed and warmed). Experimental warm- ing studies are lacking for many ecosystems.  Even fewer night-time warming experiments  have been conducted so far, with most being  in shrublands10 or grasslands and croplands8;  warming experiments that truly impose asym- metry between day and night warming are  rare11. There is a particularly urgent need for  warming studies in forests, which dominate  the global carbon cycle and climate feedbacks.  However, there are substantial technological  challenges to conducting such experiments in  large-statured ecosystems. Forest mesocosm  experiments would require exceedingly com- plex and expensive facilities. Despite these  limitations, Peng and colleagues’ results argue  strongly for an increased focus on the dif- fering ecological impacts of night-time and  daytime temperatures, to improve our ability  to understand and predict how warming will  affect Earth’s ecosystems. ■

Asymmetric effects of daytime and night-time warming on Northern Hemisphere vegetation

Published online

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