Patterns of wood carbon dioxide efflux across a 2,000-m elevation transect in an Andean moist forest

During a 1-year measurement period, we recorded the CO2 efflux from stems (RS) and coarse woody roots (RR) of 13–20 common tree species at three study sites at 1,050, 1,890 and 3,050 m a.s.l. in an Andean moist forest. The objective of this work was to study elevation changes of woody tissue CO2 efflux and the relationship to climate variation, site characteristics and growth. Furthermore, we aim to provide insights into important respiration–productivity relationships of a little studied tropical vegetation type. We expected RS and RR to vary with dry and humid season conditions. We further expected RS to vary more than RR due to a more stable soil than air temperature regime. Seasonal variation in woody tissue CO2 efflux was indeed mainly attributable to stems. At the same time, temperature played only a small role in triggering variations in RS. At stand level, the ratio of C release (g C m−2 ground area year−1) between stems and roots varied from 4:1 at 1,050 m to 1:1 at 3,050 m, indicating the increasing prevalence of root activity at high elevations. The fraction of growth respiration from total respiration varied between 10 (3,050 m) and 14% (1,050 m) for stems and between 5 (1,050 m) and 30% (3,050 m) for roots. Our results show that respiratory activity and hence productivity is not driven by low temperatures towards higher elevations in this tropical montane forest. We suggest that future studies should examine the limitation of carbohydrate supply from leaves as a driver for the changes in respiratory activity with elevation

Modelling diverse root density dynamics and deep nitrogen uptake — a simple approach

We present a 2-D model for simulation of root density and plant nitrogen (N) uptake for crops grown in agricultural systems, based on a modification of the root density equation originally proposed by Gerwitz and Page in J Appl Ecol 11:773–781, (1974). A root system form parameter was introduced to describe the distribution of root length vertically and horizontally in the soil profile. The form parameter can vary from 0 where root density is evenly distributed through the soil profile, to 8 where practically all roots are found near the surface. The root model has other components describing root features, such as specific root length and plant N uptake kinetics. The same approach is used to distribute root length horizontally, allowing simulation of root growth and plant N uptake in row crops. The rooting depth penetration rate and depth distribution of root density were found to be the most important parameters controlling crop N uptake from deeper soil layers. The validity of the root distribution model was tested with field data for white cabbage, red beet, and leek. The model was able to simulate very different root distributions, but it was not able to simulate increasing root density with depth as seen in the experimental results for white cabbage. The model was able to simulate N depletion in different soil layers in two field studies. One included vegetable crops with very different rooting depths and the other compared effects of spring wheat and winter wheat. In both experiments variation in spring soil N availability and depth distribution was varied by the use of cover crops. This shows the model sensitivity to the form parameter value and the ability of the model to reproduce N depletion in soil layers. This work shows that the relatively simple root model developed, driven by degree days and simulated crop growth, can be used to simulate crop soil N uptake and depletion appropriately in low N input crop production systems, with a requirement of few measured parameters

Analysis of gas exchange in seedlings of Acer saccharum : integration of field and laboratory studies

In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 μmol quanta m -2 s -1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 μmol m -2 s -1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO 2 response. At PPFD-saturation CE was 0.018 μmol m -2 s -1 /(μl/l). The apparent quantum efficiency (incident PPFD) at saturating CO 2 was 0.05–0.08 mol/mol. and PPFD-saturated CO 2 exchange was 6–8 μmol m -2 s -1 . The ratio of internal CO 2 concentration to external ( C i / C a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C i / C a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO 2 exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47755/1/442_2004_Article_BF00378907.pd