Grassland to shrubland state transitions enhance carbon sequestration in the northern Chihuahuan Desert.

The replacement of native C4 -dominated grassland by C3 -dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m(-2) yr(-1) on average, while nearby native C4 grassland was a net source of 31 g C m(-2) yr(-1) over this same period. Differences in C exchange between these ecosystems were pronounced--grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.

Isoprene Emission from Velvet Bean Leaves (Interactions among Nitrogen Availability, Growth Photon Flux Density, and Leaf Development).

Although isoprene synthesis is closely coupled to photosynthesis, both via ATP requirements and carbon substrate availability, control of isoprene emission is not always closely linked to photosynthetic processes. In this study we grew velvet bean (Mucuna sp.) under different levels of photon flux density (PFD) and nitrogen availability in an effort to understand better the degree to which these two processes are linked. As has been observed in past studies, we found that during early leaf ontogeny the onset of positive rates of net photosynthesis precedes that of isoprene emission by 3 to 4 d. Other studies have shown that this lag is correlated with the induction of isoprene synthase activity, indicating that overall control of the process is under control of that enzyme. During leaf senescence, photosynthesis rate and isoprene emission rate declined in parallel, suggesting similar controls over the two processes. This coordinated decline was accelerated when plants were grown with high PFD and high nitrogen availability. The latter effect included declines in the photon yield of photosynthesis, suggesting that an unexplained stress arose during growth under these conditions, triggering a premature decline in photosynthesis and isoprene emission rate. In mature leaves, growth PFD and nitrogen nutrition affected photosynthesis and isoprene emission in qualitatively similar, but quantitatively different, ways. This resulted in a significant shift in the percentage of fixed carbon that was re-emitted as isoprene. In the case of increasing growth PFD, isoprene emission rate was more strongly affected than photosynthesis rate, and more carbon was lost as isoprene. In the case of increasing nitrogen, photosynthesis rate increased more than isoprene emission rate, and leaves containing high amounts of nitrogen lost a lower percentage of their assimilated carbon as isoprene. Taken together, our results demonstrate that, although the general correlation between isoprene emission rate and photosynthesis rate is consistently expressed, there is evidence that both processes are capable of independent responses to plant growth environment.

Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves.

Isoprene emission from plants represents one of the principal biospheric controls over the oxidative capacity of the continental troposphere. In the study reported here, the seasonal pattern of isoprene emission, and its underlying determinants, were studied for aspen trees growing in the Rocky Mountains of Colorado. The springtime onset of isoprene emission was delayed for up to 4 weeks following leaf emergence, despite the presence of positive net photosynthesis rates. Maximum isoprene emission rates were reached approximately 6 weeks following leaf emergence. During this initial developmental phase, isoprene emission rates were negatively correlated with leaf nitrogen concentrations. During the autumnal decline in isoprene emission, rates were positively correlated with leaf nitrogen concentration. Given past studies that demonstrate a correlation between leaf nitrogen concentration and isoprene emission rate, we conclude that factors other than the amount of leaf nitrogen determine the early-season initiation of isoprene emission. The late-season decline in isoprene emission rate is interpreted as due to the autumnal breakdown of metabolic machinery and loss of leaf nitrogen. In potted aspen trees, leaves that emerged in February and developed under cool, springtime temperatures did not emit isoprene until 23 days after leaf emergence. Leaves that emrged in July and developed in hot, midsummer temperatures emitted isoprene within 6 days. Leaves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene within 2 h of exposure to 32°C. Continued exposure to warm temperatures resulted in a progressive increase in the isoprene emission rate. Thus, temperature appears to be an important determinant of the early season induction of isoprene emission. The seasonal pattern of isoprene emission was examined in trees growing along an elevational gradient in the Colorado Front Range (1829-2896 m). Trees at different elevations exhibited staggered patterns of bud-break and initiation of photosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isoprene emission could be predicted at each of the three sites as the time after bud break required for cumulative temperatures above 0°C to reach approximately 400 degree days. Seasonal temperature acclimation of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 35°C could be accurately predicted during spring and summer using a single algorithm that describes the Arrhenius relationship of enzyme activity. From these results, it is concluded that the early season pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pattern is determined by controls over leaf nitrogen concentration, especially the depletion of leaf nitrogen during senescence. Following early-season induction, isoprene emission rates correlate with photosynthesis rates. During the season there is little acclimation to temperature, so that seasonal modeling simplifies to a single temperature-response algorithm.