F. A. Bazzaz

The fern understory as an ecological filter: emergence and establishment of canopy-tree seedlings

We investigated the role of the fern understory as an ecological filter that influences the organization of the seedling bank in New England deciduous forests. Microenvironmental variables—including light levels, litter depth, soil exposure, soil moisture, and soil organic matter content—were quantified in experimental plots where the fern understory was undisturbed, partially removed or completely removed and were related to natural recruitment and 1st-yr survival of Acer rubrum, Betula lenta, B. alleghaniensis, Fraxinus americana, Pinus strobus, and Quercus rubra. We conducted a series of three field emergence experiments to test hypotheses regarding mechanisms of fern interference with seedling emergence. The fern understory reduced light levels from 3.4% of full sun to 1.1% of full sun beneath its canopy. Soil exposure was lower and litter depth was greater under fern cover, whereas soil moisture and soil organic-matter content were not affected by fern cover. The understory filter differentially infl...

Succession on Abandoned Fields in the Shawnee Hills, Southern Illinois

Much land in the Shawnee Hills of Southern Illinois, originally covered by oak—hickory forests, was cleared for cultivation by the mid 1800s and later abandoned because of severe erosion and low crop production. Plant succession was studied on such abandoned fields of eight different ages ranging from 1 year to 40 years old. First—year fields were occupied principally by annuals. Perennial herbs were prominent in the following few years. Shrubs and trees formed the bulk of vegetation on 40—year fields or more. Seed germination of some of the early invaders was also investigated. The general trend in succession is discussed. See full-text article at JSTOR

The fern understory as an ecological filter: growth and survival of canopy-tree seedlings

We investigated the role of the fern understory of closed-canopy forests as an ecological filter shaping the density, species composition, size structure, and spatial distribution of the seedling bank. In New England deciduous forests we tested the hypothesis that the understory stratum is a selective filter that differentially influences growth and survival of tree-seedling species by comparing performance of Acer rubrum, Betula al- leghaniensis, and Quercus rubra seedlings in plots where the fern understory was undis- turbed, partially removed, or completely removed. We related seedling growth and survival to microenvironmental characteristics of experimental plots in order to further explore mechanisms responsible for the filtering capacity of the fern understory. The presence of a fern understory reduced growth and survival of all seedling species, but the magnitude of the effect differed among species. Mortality resulting from resource limitation in Quercus below the fern understory was balanced by mortality resulting from insect herbivory in fern-free areas. Relative biomass growth rates of all species were neg- atively influenced by the presence of fern cover, whereas relative height growth rates of Acer and Quercus were uniformly low and insensitive to the presence of fern cover. Growth and survival rates indicate that only Quercus seedlings can emerge from the fern stratum in the absence of understory or overstory canopy disturbance. A trade-off between persis- tence in low light and maximum growth in understory light levels was observed among species. The relative growth rate of Betula in terms of biomass and height was more responsive to light levels than were relative growth rates of Acer or Quercus, and the growth rate of Betula was higher than that of Acer and Quercus in all light levels. However, survival of Betula below the fern stratum was lower than survival of Acer and Quercus. The fern understory has the capacity to selectively filter tree seedlings as they grow up through it because seedling species respond differentially to the presence of fern cover. The selective filtering of tree seedlings by the fern understory results in a seedling spatial structure that reflects the spatial heterogeneity of the fern stratum. The seedling pool below the fern stratum has a lower seedling density and different species composition and size structure than the seedling pool in fern-free areas.

Effects of CO_2 and Temperature on Growth and Resource Use of Co‐Occurring C_3 and C_4 Annuals

We examined how CO2 concentrations and temperature interacted to affect growth, resource acquisition, and resource allocation of two annual plants that were supplied with a single pulse of nutrients. Physiological and growth measurements were made on individuals of Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) grown in envi- ronments with atmospheric CO2 levels of 400 or 700 gtL/L and with light/dark temperatures of 28?/22? or 380/3 10C. Elevated CO2 and temperature treatments had significant indepen- dent and interactive effects on plant growth, resource allocation, and resource acquisition (i.e., photosynthesis and nitrogen uptake), and the strength and direction of these effects were often dependent on plant species. For example, final biomass of Amaranthus was enhanced by elevated CO2 at 280 but was depressed at 380. For Abutilon, elevated CO2 increased initial plant relative growth rates at 280 but not at 380, and had no significant effects on final biomass at either temperature. These results are interpreted in light of the interactive effects of CO2 and temperature on the rates of net leaf area production and loss, and on net whole-plant nitrogen retention. At 280C, elevated CO2 stimulated the initial production of leaf area in both species, which led to an initial stimulation of biomass accumulation at the higher CO2 level. However, in elevated CO2 at 280, the rate of net leaf area loss for Abutilon increased while that of Amaranthus decreased. Furthermore, high CO2 apparently enhanced the ability of Amaran- thus to retain nitrogen at this temperature, which may have helped to enhance photosyn- thesis, whereas nitrogen retention was unaffected in Abutilon. Thus, at 280, final biomass of Abutilon was not stimulated in a high CO2 environment whereas the final biomass of Amaranthus was. At 380, Abutilon had slightly reduced peak leaf areas under elevated CO2 in comparison to ambient CO2 grown plants, but increased rates of photosynthesis per unit leaf area early in the experiment apparently compensated for reduced leaf area. For Amaran- thus at 380, peak leaf area production was not affected by CO2 treatment, but the rate of net leaf area loss hastened under elevated CO2 conditions and was accompanied by sub- stantial reductions of whole-plant nitrogen content and leaf photosynthesis. This may have led to the reduced biomass accumulation of high CO2 grown plants that we observed during the last 30 d of growth. Plants of both species grown in elevated CO2 exhibited reduced tissue-specific rates of nitrogen absorption, increased plant photosynthetic rate per unit of conductance, and increased initial allocation of biomass to roots, irrespective of temperature. Plants of both species grown under an elevated temperature regime had substantially decreased repro- ductive allocation, increased allocation to stem biomass, and increased plant water flux at both CO2 treatments. The age of plants also affected our interpretations of plant responses to CO2 and temperature treatments. For example, significant effects of CO2 treatment on the growth of Abutilon were evident early, prior to the initiation of flowering, when nitrogen availability would have been highest and pot space would not have been limited. Never- theless, the opposite was true for Amaranthus, in which significant effects of CO2 treatment on plant growth were not detectable until the final 30 d of the experiment. Elevated CO2 interacted with temperature to affect plant productivity in different ways than would have been predicted from plant responses to elevated CO2 alone. Furthermore, a majority of the interactive effects of CO2 concentration and temperature on plant growth could be interpreted in light of their effects on the rates of net leaf area production and loss, nitrogen retention, and, to a lesser degree, photosynthesis and resource partitioning.

Gap Partitioning Among Maples (Acer) in Central New England: Survival and Growth

We measured survival and growth of three shade-tolerant species of maple (Acer pensylvanicum, A. rubrum, and A. saccharum) in response to understory and exper- imentally created small canopy gaps of two sizes (8 X 12 m, 75 m2; 16 X 24 m, 300 m2) in central New England. Seedlings of the three species (2160 total, 720 per species ) were transplanted into five plot locations (center plus northwest, northeast, southwest, and south- east gap edges) within all gap and understory sites. Measurements of microclimates, ar- chitecture, photosynthetic performance, survival, and growth were made over 1 yr before, and 2 yr following, gap release. Red maple (A. rubrum) survived better overall across the study due to greater persistence in the north and center plots of large gaps. The small gaps and understories showed no differences among the species. Survival rates exceeded 80% in most sites and plots, with low values (30-65%) only in the exposed plots of large gaps. There were no relationships between post-gap survival and previous age, height, or basal diameter. By the end of 2 yr of gap release, both gap sizes induced greater distinctions among the species in all growth variables than the understory. Striped maple (A. pensylvanicum) exhibited greater leader extension, absolute stem height, net height change, absolute basal diameter, and net basal diameter change than red maple and sugar maple (in that order) in nearly all sites and plots. The exception was large-gap center and north plots, where red maple equalled or exceeded striped maple in net basal diameter change, but not net height increase. Sugar maple (A. saccharum) was the least responsive of the species to the gap- understory gradient. As with survival, there were no predictable relationships between pre- gap age or size and post-gap growth. Photosynthetic performance paralleled growth by these species across the gradient, particularly for shoot assimilation. When growth variables were plotted against irradiance and temperature measured at seedling plot positions, there were consistent and clear dis- tinctions among species across the gap-understory gradient, providing limited evidence for gap partitioning in our system. Striped maple appears to be a superior generalist, red maple is a weaker generalist, and sugar maple shows the poorest performance in a manner that is nearly insensitive to the gap-understory gradient in our experimental system.

HOW ENVIRONMENTAL CONDITIONS AFFECT CANOPY LEAF‐LEVEL PHOTOSYNTHESIS IN FOUR DECIDUOUS TREE SPECIES

Species composition of temperate forests vary with successional age (i.e., years after a major disturbance) and seems likely to change in response to significant global climate change. Because photosynthesis rates in co-occurring tree species can differ in their sensitivity to environmental conditions, these changes in species composition are likely to alter the carbon dynamics of temperate forests. To help improve our understanding of such atmosphere-biosphere interactions, we explored changes in leaf-level photosynthesis in a 60-70 yr old temperate mixed-deciduous forest in Petersham, Massachusetts (USA). Diurnally and seasonally varying environmental conditions differentially influenced in situ leaf-level photosynthesis rates in the canopies of four mature temperate deciduous tree species: red oak (Quercus rubra), red maple (Acer rubrum), white birch (Betulapapyrifera), and yellow birch (Betula alleghaniensis). We measured in situ photosynthesis at two heights within the canopies (top of the canopy at -20 m, and in the sub-canopy of the same individual trees at -14-16 m) through a diurnal time course on 7 d over two growing seasons. We simultaneously measured a suite of environmental conditions surrounding the leaf at the time of each measurement. We used path analysis to examine the influence of environmental factors on in situ photosynthesis in the tree canopies. Overall, red oak had the highest photosynthesis rates, followed by white birch, yellow birch, and red maple. There was little evidence for a substantial midday depression in photosynthesis. Instead, photosynthesis declined throughout the day, particularly after 1600. Diurnal patterns of light reaching canopy leaves, leaf and air temperature, and vapor pressure deficit (VPD) contributed to diurnally varying photosynthesis rates. Large differences in these parameters through the growing season partly led to the seasonal differences observed in photosynthesis rates. Path analyses helped to identify the relative contribution of various environmental factors on photosynthesis and further revealed that species-specific sensi- tivities to various environmental conditions shifted through the season. Red oak photo- synthesis was particularly sensitive to air temperatures late in the season when air tem- peratures were low. Further, red maple photosynthesis was particularly sensitive to high VPDs through the growing season. Incorporating data on the physiological differences among tree species into forest carbon models will greatly improve our ability to predict alterations to the forest carbon budgets under various environmental scenarios such as global climate change, or with differing species composition.

CO2 ELEVATION, CANOPY PHOTOSYNTHESIS, AND OPTIMAL LEAF AREA INDEX

We studied the effects of CO2 elevation on leaf and canopy photosynthesis and optimal leaf area index (LAI) for stands of the annual species Abutilon theophrasti and Ambrosia artemisiifolia. Leaf photosynthesis was modeled as a function of photosyn- thetic photon flux density (PPFD) and nitrogen content per unit leaf area (NL). There was a curvilinear relationship between the light-saturated rates of leaf photosynthesis (Pmax) and NL. CO2 elevation significantly increased Pmax as a function of NL in both species. Dark respiration (Rd) was linearly correlated with NL. CO2 elevation slightly but significantly increased Rd in Abutilon, while it had no significant effect on Rd in Ambrosia. The initial slope of a light-response curve was determined from quantum yield (4gabs) multiplied by leaf absorptance and then calibrated against NL. Daily canopy photosynthesis, calculated by integration of leaf photosynthesis with the actual distribution of leaf area, leaf N, and PPFD within a canopy, showed fairly good agreement with the canopy photosynthesis estimated from growth analysis. CO2 elevation increased canopy photosynthesis by 30- 50%. Based on the leaf photosynthesis model for Abutilon, we calculated daily canopy photosynthesis for a given LAI and N availability, in which N was assumed to be distributed optimally within a leaf canopy to maximize daily canopy photosynthesis. An optimal LAI to maximize daily canopy photosynthesis was obtained for each level of N availability and this optimum increased with increasing N availability. Contrary to the often predicted increase in LAI with CO2 elevation, the optimum LAI did not increase at high CO2 when N availability was limited. Two factors were suggested to be involved in counteracting the increase in LAI in a high-CO2 world. One is the higher 4abs of plants grown in elevated C02, which makes leaves in the canopy more N limited, favors higher NL and thus lowers optimal LAI. The other is the higher Rd in elevated C02, which leads to higher light compensation points, and lowers optimal LAI.

Pattern of Leaf Damage Affects Fitness of the Annual Plant Raphanus Sativus (Brassicaceae)

We investigated how the pattern of leaf damage influences reproduction, growth, and allocation in the wild radish, Raphanus sativus (Brassicaceae). We removed an equivalent leaf area from plants with four leaves in five treatments ranging from con- centrated to dispersed damage: one entire mature leaf removed, one entire new leaf re- moved, 50% of two mature leaves removed, 50% of two new leaves removed, and 25% of all four leaves removed. Plants in a control group were undamaged. Reproduction, growth, and allocation were not affected by the age of the damaged leaf. However, the pattern of leaf damage significantly affected our three measures of plant fitness: the number of flowers produced, the reproductive biomass, and the total biomass. Plants in the treatment in which the damage was most dispersed had significantly higher flower number, reproductive biomass, and total biomass than an intermediate damage treatment and significantly more reproductive biomass than the concentrated damage treatment. There were no significant differences between the concentrated and intermediate damage treatments and no differ- ences between the dispersed damage treatment and the undamaged control. Our data indicate that more dispersed damage is less detrimental to the plant than more concentrated damage. Therefore, the pattern of leaf damage must be considered in determining the impact of herbivores on plant performance.

RESOURCE CONGRUENCE AND FOREST REGENERATION FOLLOWING AN EXPERIMENTAL HURRICANE BLOWDOWN

Catastrophic uprooting of forest canopy trees creates mounds, pits, and other microsites that provide opportunities for regeneration of particular species. We measured environmental factors on five types of microsites created by simulated blowdown of a mixed deciduous forest in central New England, United States. We then estimated spatial variation in resource levels and quantified congruence among different resources at each site. Effects of simulated blowdown on light levels and CO2 concentrations were more pronounced after three years than effects on nitrogen availability and other soil resources. Spatial heterogeneity in light levels and net nitrification rates was greater in the blowdown, but heterogeneities of soil organic matter concentration and net mineralization rates were greater in the undisturbed forest. Availability of nitrate, a limiting resource in most New England forests, was low on mounds and in pits, but high on the vertical portion of forest floor resulting from uprooting of canopy trees. At a spatial scale relevant to seedlings, resource congruence was greater in the undisturbed forest than in the experimental blowdown, primarily because of the effects of blowdown on light levels. Congruence in the blowdown increased with an increase in spatial scale, but congruence in the undisturbed forest was similar at both spatial scales. Seedling growth of two birch species was correlated with light levels and with congruence among soil resources. This study shows that immediate disturbance effects on microtopography and light levels determine recruitment patterns of colonizing species, with changes in soil resource levels influencing later community development. Furthermore, some species appear to respond to resource congruence, which may provide an additional dimension to the regeneration niche.

CO2 ELEVATION, CANOPY PHOTOSYNTHESIS, ANDOPTIMAL LEAF AREA INDEX

We studied the effects of CO2 elevation on leaf and canopy photosynthesis and optimal leaf area index (LAI) for stands of the annual species Abutilon theophrasti and Ambrosia artemisiifolia. Leaf photosynthesis was modeled as a function of photosynthetic photon flux density (PPFD) and nitrogen content per unit leaf area (NL). There was a curvilinear relationship between the light-saturated rates of leaf photosynthesis (Pmax) and NL. CO2 elevation significantly increased Pmax as a function of NL in both species. Dark respiration (Rd) was linearly correlated with NL. CO2 elevation slightly but significantly increased Rd in Abutilon, while it had no significant effect on Rd in Ambrosia. The initial slope of a light-response curve was determined from quantum yield (ϕabs) multiplied by leaf absorptance and then calibrated against NL. Daily canopy photosynthesis, calculated by integration of leaf photosynthesis with the actual distribution of leaf area, leaf N, and PPFD within a canopy, showed fairly good agree...