ALBERT

Notizen: Abstract. Elevated levels of CO2 in the atmosphere are expected to affect plant performance and may alter global temperature patterns. Changes in mean air temperatures that might be induced by rising levels of CO2 and other greenhouse gases could also be accompanied by increased variability in daily temperatures such that acute increases in air temperature may be more likely than at present. Consequently, we investigated whether plants grown in a CO2 enriched atmosphere would be differently affected by a heat shock than plants grown at ambient CO2 levels. Plants of a C3 annual (Abutilon theophrasti), a C3 annual crop (Sinapis alba) and a C4 annual (Amaranthus retroflexus) were grown from seed in growth chambers under either 400 or 700cm3 m−3 CO2, and were fertilized with either a high or low nutrient regime. Young seedlings of S. alba, as well as plants of all species in either the vegetative or reproductive phase of growth were exposed to a 4-h heat shock in which the temperature was raised an additional 14–23°C (depending on plant age). Total biomass and reproductive biomass were examined to determine the effect of CO2, nutrient and heat shock treatments on plant performance. Heat shock, CO2, and nutrient treatments, all had some significant effects on plant performance, but plants from both CO2 treatments responded similarly to heat shocks. We also found, as expected, that plants grown under high CO2 had dramatically decreased tissue N concentrations relative to plants grown under ambient conditions. We predicted that high-CO2-grown plants would be more susceptible to a heat shock than ambient-CO2-grown plants, because the reduced N concentrations of high-CO2 grown plants could result in the reduced synthesis of heat shock proteins and reduced thermotolerance. Although we did not examine heat shock proteins, our results showed little relationship between plant nitrogen status and the ability of a plant to tolerate an acute increase in temperature.

Notizen: This study investigated simultaneous plant and soil feedbacks on growth enhancement with elevated [CO2] within microcosms of yellow birch (Betula alleghaniensis Britt.) in the second year of growth. Understanding the integrated responses of model ecosystems may provide key insight into the potential net nutrient feedbacks on [CO2] growth enhancements in temperate forests. We measured the net biomass production, C:N ratios, root architecture, and mycorrhizal responses of yellow birch, in situ rates gross nitrogen mineralization and the partitioning of available NH4+ between yellow birch and soil microbes. Elevated atmospheric [CO2] resulted in significant alterations in the cycling of N within the microcosms. Plant C/N ratios were significantly increased, gross mineralization and NH4+ consumption rates were decreased, and relative microbial uptake of NH4+ was increased, representing a suite of N cycling negative feedbacks on N availability. However, increased C/N ratios may also be a mechanism which allows plants to maintain higher growth with a constant or reduced N supply. Total plant N content was increased with elevated [CO2], suggesting that yellow birch had successfully increased their ability to acquire nutrients during the first year of growth. However, plant uptake rates of NH4+ had decreased in the second year. This discrepancy implies that, in this study, nitrogen uptake showed a trend through ontogeny of decreasing enhancement under elevated [CO2]. The reduced N mineralization and relatively increased N immobilization are a potential feedback which may drive this ontogenetic trend. This study has demonstrated the importance of using an integrated approach to exploring potential nutrient-cycling feedbacks in elevated [CO2].

Notizen: Increased levels of atmospheric CO2 may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO2 influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L–1) and elevated (700 μL L–1) atmospheric CO2. For both individually and competitively grown paper birch seedlings, there was a greater CO2 growth enhancement for seedlings watered less frequently than for well-watered seedlings. This differential change in CO2 responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO2 only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO2 reduced this sensitivity. Additionally, elevated CO2 improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO2 may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO2 levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.

Notizen: Rising levels of atmospheric CO2 may alter patterns of plant biomass production. These changes will be dependent on the ability of plants to acquire sufficient nutrients to maintain enhanced growth. Species-specific differences in responsiveness to CO2 may lead to changes in plant community composition and biodiversity. Differences in species-level growth responses to CO2 may be, in a large part, driven by differences in the ability to acquire nutrients. To understand the mechanisms of how elevated CO2 leads to changes in community-level productivity, we need to study the growth responses and patterns of nutrient acquisition for each of the species that comprise the community.In this paper, we present a study of how elevated CO2 affects community-level and species-level patterns of nitrogen uptake and biomass production. As an experimental system we use experimental communities of 11 co-occurring annuals common to disturbed seasonal grasslands in south-western U.S.A. We established experimental communities with approximately even numbers of each species in three different atmospheric CO2 concentrations (375, 550, and 700 ppm). We maintained these communities for 1, 1.5, and 2 months at which times we applied a 15N tracer (15NH415NO3) to quantify the nitrogen uptake and then measured plant biomass, nitrogen content, and nitrogen uptake rates for the entire communities as well as for each species.Overall, community-level responses to elevated CO2 were consistent with the majority of other studies of individual- and multispecies assemblages, where elevated CO2 leads to enhanced biomass production early on, but this enhancement declines through time. In contrast, the responses of the individual species within the communities was highly variable, showing the full range of responses from positive to negative. Due to the large variation in size between the different species, community-level responses were generally determined by the responses of only one or a few species. Thus, while several of the smaller species showed trends of increased biomass and nitrogen uptake in elevated CO2 at the end of the experiment, community-level patterns showed a decrease in these parameters due to the significant reduction in biomass and nitrogen content in the single largest species.The relationship between enhancement of nitrogen uptake and biomass production in elevated CO2 was highly significant for both 550 ppm and 700 ppm CO2. This relationship strongly suggests that the ability of plants to increase nitrogen uptake (through changes in physiology, morphology, architecture, or mycorrhizal symbionts) may be an important determinant of which species in a community will be able to respond to increased CO2 levels with increased biomass production. The fact that the most dominant species within the community showed reduced enhancement and the smaller species showed increased enhancement suggest that through time, elevated CO2 may lead to significant changes in community composition.At the community level, nitrogen uptake rates relative to plant nitrogen content were invariable between the three different CO2 levels at each harvest. This was in contrast to significant reductions in total plant nitrogen uptake and nitrogen uptake relative to total plant biomass. These patterns support the hypothesis that plant nitrogen uptake is largely regulated by physiological activity, assuming that physiological activity is controlled by nitrogen content and thus protein and enzyme content.

Notizen: [Auszug] SIR - Genome size, as measured by the content of nuclear DNA per cell, varies over 2,500-fold among species of angiosperm plants1. As genome size is positively correlated with cell size and with the duration of cell cycle2, it may have direct effects on the evolutionary strategy3, life history, ...

Notizen: [Auszug] SIR - There have been several reports suggesting that non-field-based studies of plant responses to elevated atmospheric CO2 may be compromised because small rooting volumes limit a plant's ability to respond to elevated CO2 (refs 1-3). This hypothesis calls into question most of our present ...

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.