ALBERT

Notizen: Summary The behavior of Polygonum pensylvanicum L., an early successional annual, and Polygonum virginianum L., a later successional perennial were compared on singlefactor gradients of light, nutrients and soil moisture and on a double-factor gradient of light and nutrients. The two species were grown together as well as alone. In most cases, the early successional species was more productive and allocated less of its biomass to belowground parts relative to its later successional congener. The early successional species was also more opportunistic in its utilization of resources as evidenced by its greater responsiveness to varying resource availability. The role of perenniality in the later successional species may be dual. Individuals of P. virginianum started from seed were more responsive to deficiencies in resources than were individuals started from rootstocks in these experiments. In the single-factor experiments where plants were started from seed, there were numerous significant interactions between levels of light, nutrients or moisture with competition in both species. In the double-factor experiment, however, there were no statistically significant interactions between light, nutrients and competition in the perennial started from rootstocks, but all possible interactions were significant in the early successional annual. The result of a large, underground reserve in the perennial was that the effects of resource availabilities of light and nutrients were uncoupled. The competitive superiority of the early successional species over the later successional species was greatly diminished when the later successional species was started from rootstocks instead of from seeds. Regardless of whether individuals of P. virginianum were started from seed or from rootstocks the results of competition between the two species indicate that the early successional species is competitively more competent when resources are abundant and that the later successional species is competitively more competent when resources are scarce.

Notizen: Summary Differences in major selective forces important in early and late successional communities should influence niche breadth and degree of overlap. Early successional species may not experience consistent, strong selection against competition and can be expected to have broader niches with more overlap than later successional species. This paper presents data from 2 early successional winter annuals that show clear niche separation under conditions in which coevolution is unlikely. Lactuca scariola L. and Erigeron annuus (L.) Pers. flower in mid-summer and attract the same insect visitors. Erigeron, the more common species, begins to flower in mid-morning when insect visitors are quite active. Lactuca flowers open earlier in the morning and close just before Erigeron flowers open. The early morning hours are not optimum for Lactuca visitation, as most of the visits occur just before floral closing. Both species can selffertilize without a vector, and Lactuca was introduced from Europe. It is, therefore, not possible that niche separation inflowering time and subsequent time of visit in Erigeron and Lactuca is a result of coevolutionary niche differentiation between the two species. This separation is likely to be a preadaptation resulting from coevolution with other species.

Notizen: Summary Communities, consisting of six co-occurring, disturbed site annuals, were subjected to CO2 unenriched (300 ppm) and to CO2 enriched (450 and 600 ppm) atmospheres at different levels of light and nutrient availability. In general, total community production increased with CO2 enrichment to 450 ppm, but a further increase in CO2 to 600 ppm had little or no effect. The response of community production to CO2 level was not affected by nutrient availability but was affected by light level. Of the six species, four display C3 metabolism. The proportion of total community production contributed by these species increased as a result of CO2 enrichment, and was dependent upon both light and nutrient availability. The relative success of some species, particularly in terms of reproduction (total seed biomass), was significantly altered by CO2 concentration depending on the level of nutrients. There were not only changes in reproductive success (seed biomass) and shoot biomass but also changes in the proportion of biomass allocated to seed. These experiments demonstrate that CO2 enrichment does affect annual plant communities both in terms of productivity and species composition and that the affect of CO2 on such system may depend upon other resources such as light and nutrients.

Notizen: Abstract Plant species differ broadly in their responses to an elevated CO2 atmosphere, particularly in the extent of nitrogen dilution of leaf tissue. Insect herbivores are often limited by the availability of nutrients, such as nitrogen, in their host plant tissue and may therefore respond differentially on different plant species grown in CO2-enriched environments. We reared gyspy moth larvae (Lymantria dispar) in situ on seedlings of yellow birch (Betula allegheniensis) and gray birch (B. populifolia) grown in an ambient (350 ppm) or elevated (700 ppm) CO2 atmosphere to test whether larval responses in the elevated CO2 atmosphere were species-dependent. We report that female gypsy moths (Lymantria dispar) reared on gray birch (Betula populifolia) achieved similar pupal masses on plants grown at an ambient or an elevated CO2 concentration. However, on yellow birch (B. allegheniensis), female pupal mass was 38% smaller on plants in the elevated-CO2 atmosphere. Larval mortality was significantly higher on yellow birch than gray birch, but did not differ between the CO2 treatments. Relative growth rate declined more in the elevated CO2 atmosphere for larvae on yellow birch than for those on gray birch. In preference tests, larvae preferred ambient over elevated CO2-grown leaves of yellow birch, but showed no preference between gray birch leaves from the two CO2 atmospheres. This differential response of gypsy moths to their host species corresponded to a greater decline in leaf nutritional quality in the elevated CO2 atmosphere in yellow birch than in gray birch. Leaf nitrogen content of yellow birch dropped from 2.68% to 1.99% while that of gray birch leaves only declined from 3.23% to 2.63%. Meanwhile, leaf condensed tannin concentration increased from 8.92% to 11.45% in yellow birch leaves while gray birch leaves only increased from 10.72% to 12.34%. Thus the declines in larval performance in a future atmosphere may be substantial and host-species-specific.

Notizen: Abstract We examined leaf dynamics and leaf age gradients of photosynthetic capacity and nitrogen concentration in seedlings of the tropical pioneer tree, Heliocarpus appendiculatus, grown in a factorial design under controlled conditions with two levels each of nutrients, ambient light (light levels incident above the canopy), and self-shading (the gradient of light levels from upper to lower leaves on the shoot). Correlations among these parameters were examined in order to determine the influence of self-shading, and the regulation of standing leaf numbers, on leaf longevity and its association with leaf photosynthetic capacity. Leaf longevity and the number of leaves on the main shoot were both reduced in high light, while in the low light environment, they were reduced in the steeper self-shading gradient. In high nutrients, leaf longevity was reduced whereas leaf number increased. Leaf initiation rates were higher in the high nutrient treatment but were not influenced by either light treatment. Maximum-light saturated photosynthetic rate, on an area basis, was greater in the high light and nutrient treatments, while the decline in photosynthetic capacity in realtion to leaf position on the shoot was more rapid in high light and in low nutrients. Leaf longevity was negatively correlated among treatments with initial photosynthetic capacity. The leaf position at which photosynthetic capacity was predicted to reach zero was positively correlated with the number of leaves on the shoot, supporting the hypothesis that leaf numbers are regulated by patterns of self-shading. The negative association of longevity and initial photosynthetic capacity apparently arises from different associations among gradients of photosynthetic capacity, leaf numbers and leaf initiation rates in relation to light and nutrient availability. The simultaneous consideration of age and position of leaves illuminates the role of self-shading as an important factor influencing leaf senescence and canopy structure and dynamics.

Notizen: Abstract  Within the same forest, photosynthesis can vary greatly among species and within an individual tree. Quantifying the magnitude of variation in leaf-level photosynthesis in a forest canopy will improve our understanding of and ability to model forest carbon cycling. This information requires extensive sampling of photosynthesis in the canopy. We used a 22-m-tall, four-wheel-drive aerial lift to reach five to ten leaves from the tops of numerous individuals of several species of temperate deciduous trees in central Massachusetts. The goals of this study were to measure light-saturated photosynthesis in co-occurring canopy tree species under field conditions, and to identify sampling schemes appropriate for canopy tree studies with challenging logistics. Photosynthesis differed significantly among species. Even though all leaves measured were canopy-top, sun-acclimated foliage, the more shade-tolerant species tended to have lower light-saturated photosynthetic rates (P max) than the shade-intolerant species. Likewise, leaf mass per area (LMA) and nitrogen content (N) varied significantly between species. With only one exception, the shade-tolerant species tended to have lower nitrogen content on an area basis than the intolerant species, although the LMA did not differ systematically between these ecological types. Light-saturated P max rates and nitrogen content, both calculated on either an area or a mass basis, and the leaf mass to area ratio, significantly differed not only among species, but also among individuals within species (P〈0.0001 for both). Differences among species accounted for a greater proportion of variance in the P max rates and the nitrogen content than the differences among individuals within a species (58.5–78.8% of the total variance for the measured parameters was attributed to species-level differences versus 5.5–17.4% of the variance was attributed to differences between individual trees of a given species). Furthermore, more variation is accounted for by differences among leaves in a single individual tree, than by differences among individual trees of a given species (10.7–30.4% versus 5.5–17.4%). This result allows us to compare species-level photosynthesis, even if the sample size of the number of trees is low. This is important because studies of canopy-level photosynthesis are often limited by the difficulty of canopy access. As an alternative to direct canopy access measurements of photosynthesis, it would be useful to find an ”easy-to-measure” proxy for light-saturated photosynthetic rates to facilitate modeling forest carbon cycling. Across all species in this study, the strongest correlation was between nitrogen content expressed on an area basis (mmol m–2, N area) and light-saturated P max rate (μmol m–2 s–1, P maxarea) (r 2=0.511). However, within a given species, leaf nitrogen was not tightly correlated with photosynthesis. Our sampling design minimized intra-specific leaf-level variation (i.e., leaves were taken only from the top of the canopy and at only one point in the season). This implies that easy-to-measure trends in nitrogen content of leaves may be used to predict the species-specific light-saturated P max rates.

Notizen: Abstract Despite predictions that both atmospheric CO2 concentrations and air temperature will rise together, very limited data are currently available to assess the possible interactive effects of these two global change factors on temperate forest tree species. Using yellow birch (Betula alleghaniensis) as a model species, we studied how elevated CO2 (800 vs. 400 μl l−1) influences seedling growth and physiological responses to a 5°C increase in summer air temperatures (31/26 vs. 26/21°C day/night), and how both elevated CO2 and air temperature during the growing season influence seedling ability to survive freezing stress during the winter dormant season. Our results show that while increased temperature decreases seedling growth, temperature-induced growth reductions are significantly lower at elevated CO2 concentrations (43% vs. 73%). The amelioration of high-temperature stress was related to CO2-induced reductions in both whole-shoot dark respiration and transpiration. Our results also show that increased summer air temperature, and to a lesser degree CO2 concentration, make dormant winter buds less susceptible to freezing stress. We show the relevance of these results to models used to predict how climate change will influence future forest species distribution and productivity, without considering the direct or interactive effects of CO2.

Notizen: Abstract  We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes in drought response with age in seedlings, juveniles, and mature trees of Q. rubra. Throughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential of leaves (ΨLeaf) and gas exchange of plants at three sites at the Harvard Forest in Petersham, Massachusetts. In order to determine what fraction of the measured differences in gas exchange between seedlings and mature trees was due to environment versus ontogeny, an experiment was conducted in which seedlings were grown under light and soil moisture regimes simulating the environment of mature trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet year, and six-fold greater during the drought year. The seedling experiment demonstrated that the difference in photosynthetic capacity between seedlings and mature trees is comprised equally of an environmental component (50%) and an ontogenetic component (50%) in the absence of water limitation. Photosynthesis was depressed more severely in seedlings than in mature trees in the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn leaf water potential (ΨPD) of seedlings became increasingly negative (–0.4 to –1.6 MPa), while that of mature trees became only slightly more negative (–0.2 to –0.5 MPa). Again, juveniles showed an intermediate response (–0.25 to –0.8 MPa). During the wet summer of 1996, however, there was no difference in ΨPD between seedlings, juveniles and mature trees. During the dry summer of 1995, seedlings were more responsive to a major rain event than mature trees in terms of ΨLeaf , suggesting that the two age classes depend on different water sources. In all age classes, instantaneous measurements of intrinsic water use efficiency (WUEi), defined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes had higher WUEi during the drought year than in the wet year. Mature trees, however, showed a greater ability to increase their WUEi in response to drought. Integrated measurements of WUE from C isotope discrimination (Δ) of leaves indicated higher WUE in mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to the strong bias in C isotope fractionation at the time of leaf production, which occurred prior to the onset of drought conditions in 1995. From this study, we arrive at two main conclusions:

Notizen: Summary Ulmus alata and Diospyros virginiana are components of the shrubearly tree communities of old-field succession in several areas in the deciduous forests of eastern North America. In these habitats, the plants experience high insolation, high temperatures, and low soil moisture during the summer. They exhibit pronounced daily changes in water potential and usually develop more negative water potentials as the season progresses. The species light saturate at ∼1,150 μE m-2 sec-1 with photosynthetic rates of 15 mg CO2 dm-2 h-1 for U. alata and 17 mg CO2 dm-2 h-1 for D. virginiana. The optimum temperatures for photosynthesis are ∼25°C. Ulmus alata maintains maximum photosynthesis to water potentials of-14 bars and recovers from-20 bars to ∼60% of maximum photosynthesis within 10 hrs after watering. When they are deprived of water, twigs of D. virginiana exhibit faster decline in photosynthesis and leaf conductance than twigs of U. alata. The two species have somewhat different response to the environmental of high insolation and low water supply. Unlike Ulmus, Diospyros virginiana has some adaptations which may explain the persistence of a few individuals in mature forests.

Notizen: Abstract We measured the effect of elevated CO2 on populations of the western flower thrips, Frankliniella occidentalis and on the amount of leaf damage inflicted by the thrips to one of its host plants, the common milkweed, Asclepias syriaca. Plants grown at elevated CO2 had significantly greater aboveground biomass and C:N ratios, and significantly reduced percentage nitrogen. The number of thrips per plant was not affected by CO2 treatment, but the density of thrips (numbers per gram aboveground biomass), was significantly reduced at high CO2. Consumption by thrips, expressed as the amount of damaged leaf area per capita, was significantly greater at high CO2, and the amount of leaf area damaged by thrips was increased by 33%. However overall leaf area at elevated CO2 increased by 62%, more than compensating for the increase in thrips consumption. The net outcome was that plants at elevated CO2 had 3.6 times more undamaged leaf area available for photosynthesis than plants at ambient CO2, even though they had only 1.6 times the overall amount of leaf area. This study highlights the need for measuring the effects of herbivory at the whole-plant level and also the importance of taking herbivory into account when predicting plant responses to elevated CO2.