Effects of climate change and shifts in forest composition on forest net primary production.

Forests are dynamic in both structure and species composition, and these dynamics are strongly influenced by climate. However, the net effects of future tree species composition on net primary production (NPP) are not well understood. The objective of this work was to model the potential range shifts of tree species (DISTRIB Model) and predict their impacts on NPP (PnET-II Model) that will be associated with alterations in species composition. We selected four 200 x 200 km areas in Wisconsin, Maine, Arkansas, and the Ohio-West Virginia area, representing focal areas of potential species range shifts. PnET-II model simulations were carried out assuming that all forests achieved steady state, of which the species compositions were predicted by DISTRIB model with no migration limitation. The total NPP under the current climate ranged from 552 to 908 g C/m(2) per year. The effects of potential species redistributions on NPP were moderate (-12% to +8%) compared with the influence of future climatic changes (-60% to +25%). The direction and magnitude of climate change effects on NPP were largely dependent on the degree of warming and water balance. Thus, the magnitude of future climate change can affect the feedback system between the atmosphere and biosphere.

Within-canopy sampling of global irradiance to describe downwelling light distribution and infer canopy stratification in a broadleaf forest.

A broadleaf mixed forest diversified through partial tree thinning was studied to identify expedient sampling and data analysis procedures to capture the heterogeneous within-canopy downward distribution of instantaneous global photosynthetic photon flux (PPF); to extract foliage structural properties from the acquired light values; and to compute statistics descriptive of the within-canopy light and leaf layer distributions. We sampled PPF at 1-m intervals along vertical gradients using a helium-filled balloon as a platform for a light sensor. A random method was used to identify the forest floor locations for the within-canopy balloon ascents. About 400 PPF measurements were recorded per vertical transect. For each PPF value, we computed, by inversion of the Monsi-Saeki model, the number of leaf strata cumulated along the sunbeam direction from the position where the light was measured. Variability in PPF and leaf layer at different vegetation scales was computed by non-parametric statistics. The methods were evaluated as appropriate for intra-canopy PPF sampling, particularly in an undisturbed canopy. The minimum number of vertical PPF profiles required to capture the within-canopy PPF variability was 9-10 (equivalent to about 4000 measurements). The reliability and sensitivity of the inversion of the Monsi-Saeki method were sufficient to capture the canopy structural differences between undisturbed and partially thinned forests. The proposed PPF canopy sampling and data analysis procedures provide a fast, reliable and inexpensive way to characterize tree crown structure, and to predict plant growth and forest dynamics and could be applied whenever vegetation absorbed radiation is a main driving force for forest canopy processes. The experimental light attenuation data and the extracted canopy leaf layer numbers could serve to corroborate canopy mechanistic models of radiative transfer and net primary production.

Energy investment in leaves of red maple and co-occurring oaks within a forested watershed.

Despite its recent expansion in eastern US forests, red maple (Acer rubrum L.) generally exhibits a low leaf photosynthetic rate, leaf mass per unit area (LMA) and leaf nitrogen concentration ([N]) relative to co-occurring oaks (Quercus spp.). To evaluate these differences from the perspective of leaf energy investment, we compared leaf construction cost (CC) and leaf maintenance cost (MC) with leaf photosynthetic rate at saturating photon flux density and ambient CO2 partial pressure (Amax) in red maple and co-occurring red oak (Quercus rubra L.) and chestnut oak (Quercus prinus L.). We also examined relationships among leaf physiological, biochemical and structural characteristics of upper-canopy leaves of these three species at lower (wetter) and upper (drier) elevation sites of a watershed in the Black Rock Forest, Cornwall, NY, USA. Although A(max), leaf [N], leaf carbon concentration ([C]) and LMA were significantly less in red maple than in either oak species at both sites, CC per unit leaf area of red maple was 28.2 and 35.4% less than that of red oak at the lower and upper site, respectively, and 38.8 and 32% less than that of chestnut oak at the lower and upper site, respectively. Leaf MC per unit leaf area, which was positively associated with leaf CC (r2 = 0.95), was also significantly lower in red maple than in either oak species at both sites. When expressed per unit leaf area, A(max) was positively correlated with both CC (r2 = 0.65) and MC (r2 = 0.59). The cost/benefit ratio of CC/Amax of red maple was significantly less than that of chestnut oak at the lower site, however, CC/A(max) did not exhibit any significant interspecific differences at the upper site. Expressed per unit leaf area, CC was correlated positively with LMA (r2 = 0.90), leaf [N] (r2 = 0.97), and leaf [C] (r2 = 0.89), and negatively correlated with leaf molar carbon to nitrogen ratio (r2 = 0.92). Combined with red maple's general success in many oak-dominated forests, our findings suggest that reduced leaf-level photosynthetic capacity and related leaf characteristics in red maple are partially balanced by lower energy and resource requirements for leaf biomass construction and maintenance, which could enhance the competitive success of this species.

Analysis of the growth of rimu (Dacrydium cupressinum) in South Westland, New Zealand, using process-based simulation models.

Two process-based models were used to identify the environmental variables limiting productivity in a pristine, mature forest dominated by rimu (Dacrydium cupressinum Sol. ex Lamb.) trees in South Westland, New Zealand. A model of canopy net carbon uptake, incorporating routines for radiation interception, photosynthesis and water balance was used to determine a value for quantum efficiency when climate variables were not limiting. The annual net carbon uptake by the canopy was estimated to be 1.1 kg C m(-2) and the quantum efficiency 22.6 mmol mol quanta(-1). This value of quantum efficiency, combined with other parameters obtainable from the literature, was then used in a model of forest productivity (3-PG), to simulate changes in net productivity and the allocation of carbon to tree components. The model was adjusted to match a measured stem increment of 10.6 Mg ha(-1) over a period of 13 years. To achieve this while maintaining a low, but stable value for leaf area index, it was necessary to set the site fertility rating very low and select high values for the parameters describing the proportional allocation of total carbon to roots. This approach highlighted nutrient availability as the principal constraint on productivity for the ecosystem and identified critical measurements that will be necessary for using the model to predict the effects of climate change on carbon sequestration. The low rates of carbon uptake and productivity are consistent with the low nutrient supply available from the highly leached, acid soils, most likely attributable to frequent saturation and a very shallow aerobic zone.

Photosynthetic characteristics in canopies of Quercus rubra, Quercus prinus and Acer rubrum differ in response to soil water availability.

Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (A max) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher A max per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the A max-N relationship. Importantly, the relationships between A max and N area (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (g smax) differed significantly between species, with g smax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, g smax was significantly lower at the upper site than the lower site. There was no significant response of g smax to site in A. rubrum. These stomatal responses were consistent with the C i/C a ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf δ13C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.