ALBERT

Description: Although plants are sessile organisms, they can forage for resources and avoid neighbors by growing towards areas with high resource availability and reduced competition. Apparently because of this morphological flexibility, tree canopies are rarely positioned directly above their stem bases and are often displaced. To determine if contrasts in light availability lead to the development of canopy displacement, we investigated the responses of tree canopies to the heterogeneous light environments at the edges of six experimental gaps. Canopies and trunks of gap edge trees were mapped, and their spatial distributions were analyzed. We found that tree canopies were displaced towards gap centers. The magnitude and precision of canopy displacement were greater for subcanopy trees than for canopy trees. The magnitude and precision of canopy displacement were generally greater for earlier successional trees and hardwoods than for later successional trees and conifers. Canopy depth was significantly greater on gap-facing sides of trees than on forest-facing sides of trees. Thus, trees along gap edges foraged for light by occupying both horizontal and vertical gap space. This morphological flexibility has implications for individual plant success, as well as forest structure and dynamics.

Description: Competitive interactions among plants are largely determined by spatial proximity. However, despite their sessile nature, plants have the ability to avoid neighbors by growing towards areas with high resource availability and reduced competition. Because of this flexibility, tree canopies are rarely centered directly above their stem bases and are often displaced. We sought to determine how a tree's competitive neighborhood influences its canopy position. In a 0.6-ha temperate forest plot, all trees greater than 10 cm DBH (n = 225) were measured for basal area, height, canopy depth, and trunk position. Canopy extent relative to trunk base was determined in eight subcardinal directions, and this information was used to reconstruct canopy size, shape, and position. We found that trees positioned their canopies away from large neighbors, close neighbors, and shade-tolerant neighbors. Neighbor size, expressed as basal area or canopy area, was the best indication of a neighbor's importance in determining target tree canopy position. As neighborhood asymmetry increased, the magnitude of canopy displacement increased, and the precision with which canopies avoided neighbors increased. Flexibility in canopy shape and position appears to reduce competition between neighbors, thereby influencing forest community dynamics.

Description: We used sap flow as a measure of whole-tree function to examine how coniferous and broad-leaved species in mixed temperate forests differ in canopy-level transpiration and photosynthetic rates. We used heat dissipation probes to measure whole-tree sap flow in three species throughout one full year and then combined these measurements with micrometeorological monitoring and leaf-level gas exchange to determine whole-tree carbon gain. Both broad-leaved species (red oak, Quercus rubra L.; red maple, Acer rubrum L.) had two- to four-fold greater annual fluxes of water and carbon on a ground area basis than did the conifer (eastern hemlock, Tsuga canadensis (L.) Carrière), with red oak trees additionally showing 6080% higher fluxes than red maple. Despite fixing one-third of its carbon when broad-leaved species were leafless, hemlock was not able to compensate for its low photosynthetic rates during the growing season. Productivity measures derived from annual growth rings and eddy covariance confirmed that whole-tree sap flow provided a valuable estimate of both the magnitude of current forest fluxes and differences in individual species' fluxes. Our results indicate that the predicted loss of hemlock from mixed temperate forests could potentially increase whole-forest water loss and carbon gain by two- to four-fold, provided sufficient nitrogen and water remain available to support such a change.

Description: We examined how elevated CO2 affected the growth of seven co-occurring tree species: American beech (Fagusgrandifolia Ehrh.), paper birch (Betulapapyrifera Marsh.), black cherry (Prunusserotina Ehrh.), white pine (Pinusstrobus L.), red maple (Acerrubrum L.), sugar maple (Acersaccharum Marsh.), and eastern hemlock (Tsugacanadensis (L.) Carr). We also tested whether the degree of shade tolerance of species and the age of seedlings affected plant responses to enhanced CO2 levels. Seedlings that were at least 1 year old, for all species except beech, were removed while dormant from Harvard Forest, Petersham, Massachusetts. Seeds of red maple and paper birch were obtained from parent trees at Harvard Forest, and seeds of American beech were obtained from a population of beeches in Nova Scotia. Seedlings and transplants were grown in one of four plant growth chambers for 60 d (beech, paper birch, red maple, black cherry) or 100 d (white pine, hemlock, sugar maple) under CO2 levels of 400 or 700 μL•L−1. Plants were then harvested for biomass and growth determinations. The results showed that the biomass of beech, paper birch, black cherry, sugar maple, and hemlock significantly increased in elevated CO2, but the biomass of red maple and white pine only marginally increased in these conditions. Furthermore, there were large differences in the magnitude of growth enhancement by increased levels of CO2 between species, so it seems reasonable to predict that one consequence of rising levels of CO2 may be to increase the competitive ability of some species relative to others. Additionally, the three species exhibiting the largest increase in growth with increased CO2 concentrations were the shade-tolerant species (i.e., beech, sugar maple, and hemlock). Thus, elevated CO2 levels may enhance the growth of relatively shade-tolerant forest trees to a greater extent than growth of shade-intolerant trees, at least under the light and nutrient conditions of this experiment. We found no evidence to suggest that the age of tree seedlings greatly affected their response to elevated CO2 concentrations.

Description: Changes in forest species composition could influence ecosystem carbon uptake rates. To understand how species differed in their contributions to canopy photosynthesis, we investigated how the dominant coniferous (eastern hemlock, Tsuga canadensis (L.) Carr.) and broad-leaved (northern red oak, Quercus rubra L.; red maple, Acer rubrum L.) species in a central Massachusetts forest differed in canopy carbon uptake rates. We considered what factors influenced in situ leaf-level photosynthesis and then used a bottom-up summation approach to estimate species-specific total canopy carbon uptake rates. Variation in canopy light strongly influenced leaf-level photosynthetic rates: sunlit leaves had significantly higher rates than shaded leaves, and photosynthesis increased with canopy height. Species also differed in leaf-level photosynthetic rates, with the broad-leaved species having up to twofold higher rates than hemlock. Within hemlock, needles older than 2 years had lower photosynthesis than younger needles. Variation in leaf-level photosynthesis scaled up to influence canopy carbon uptake rates. Red oak consistently had the highest canopy photosynthetic rates, while through the season, hemlock's relative contribution to carbon flux increased and that of red maple decreased. Thus, in such mixed forests, future changes in species composition could have substantial impacts on forest carbon dynamics, particularly if red oak is the primary broad-leaved species to expand at the expense of hemlock.