Boyd R. Strain

Growth responses of tropical shrubs to treefall gap environments

To investigate the effects of differences in light and nutrient availability on growth, we planted seven species of shrubs in two genera, Miconia (Melastomataceae) and Piper (Piperaceae), into the centers, edges, and adjacent forest understory of four natural treefall gaps (275-335 M2) in the tropical, premontane rain forest of Costa Rica. We used rooted cuttings of species typical of forest understory environments on the one hand and large clearings or disturbed areas on the other. We also compared growth rates of three Miconia species grown in shade houses under 2, 20, and 40% full sunlight. Both light and nutrient availability in newly formed gaps of these sizes were strongly buffered by the canopy and root systems of the surrounding forest. Total incident radiation was higher in gap centers (9-23% full sunlight) than in gap-forest edges (3-11%) or under intact forest canopy (0.4-2%), but varied among similar microhabitats from different sites. Relative stem growth rates (RGRS) of all field-grown plants were significantly greater in gap centers than at edges or beneath forest understories. Fertilization did not significantly affect growth rate in any light environment. Light appears to be the most critical resource limiting growth at these gap sizes. In general, shade-tolerant species were less plastic than light-demanding species, but at these gap sizes grew as fast or faster in the gap centers. In shade-houses, the shade-tolerant species grew fastest at 20% full sunlight and light-demanding species grew fastest at 40% full sunlight. We found no evidence of a trade-off between growth and foliar phenolic concentration in these species.

Direct effects of increasing carbon dioxide on vegetation

CO/sub 2/ is an essential environmental resource. It is required as a raw material of the orderly development of all green plants. As the availability of CO/sub 2/ increases, perhaps reaching two or three times the concentration prevailing in preindustrial times, plants and all other organisms dependent on them for food will be affected. Humans are releasing a gaseous fertilizer into the global atmosphere in quantities sufficient to affect all life. This volume considers the direct effects of global CO/sub 2/ fertilization on plants and thus on all other life. Separate abstracts have been prepared for individual papers. (ACR)

Photosynthetic inhibition after long-term exposure to elevated levels of atmospheric carbon dioxide

The effect of long-term exposure to elevated levels of CO2 on biomass partitioning, net photosynthesis and starch metabolism was examined in cotton. Plants were grown under controlled conditions at 350, 675 and 1000 μl l-1 CO2. Plants grown at 675 and 1000 μl l-1 had 72% and 115% more dry weight respectively than plants grown at 350 μl l-1. Increases in weight were partially due to corresponding increases in leaf starch. CO2 enrichment also caused a decrease in chlorophyll concentration and a change in the chlorophyll a/b ratio. High CO2 grown plants had lower photosynthetic capacity than 350 μl l-1 grown plants when measured at each CO2 concentration. Reduced photosynthetic rates were correlated with high internal (non-stomatal) resistances and higher starch levels. It is suggested that carbohydrate accumulation causes a decline in photosynthesis by feedback inhibition and/or physical damage at the chloroplast level.

Effects of light regime on the growth, leaf morphology, and water relations of seedlings of two species of tropical trees

An experiment was conducted with Heliocarpus appendiculatus, a pioneer or large gap species of tropical moist forest in Costa Rica, and Dipteryx panamensis, a small gap species. Seedlings were grown in full sun, partial (80%) shade, and full (98%) shade. After one month of growth they were switched between environments and grown for two more months.Growth in height of Heliocarpus was greatly affected by irradiance, being increased in response to full shade and decreased in full sun. Height of Dipteryx was unaffected by irradiance level. Survival of Heliocarpus seedlings was only 49% in full shade, whereas Dipteryx had 100% survival. Biomass of Heliocarpus was not significantly greater in full sun than in partial shade whereas it was for Dipteryx. The response of root: shoot ratio was similar for both species. They were lowest in full shade and highest in full sun. Heliocarpus exhibited greater changes in leaf thickness, specific leaf weight, and stomatal density than did Dipteryx. Stomatal conductance of both species was lower in full shade and full sun than in partial shade.The results of the experiment indicate that growth of Heliocarpus is more plastic than that of Dipteryx in response to changes in irradiance. Previous environment did not affect the response to the present environment in either species. Both species responded positively to increases in irradiance.

Carbon gain by plants in natural environments

P hysiological ecologists have long been concerned with photosynthesis, which incorporates carbon to provide plants with all their energy and structural building blocks. This carbon gain is an important aspect of plant performance in natural environments. The acquisition and use of other resources, such as nitrogen and water, are tightly linked to photosynthetic performance. For example, energy from photosynthesis is employed for nitrogen acquisition and reduction, and photosynthetic capacity is strongly coupled to leaf nitrogen content (Chapin et al., p. 49, this issue). Photosynthesis is also closely related to water movement in a plant. To facilitate absorption of carbon dioxide for use in photosynthesis, a leaf must have wet cell surfaces. As a consequence, transpiration uses nearly all of the water taken up

Vegetation effects on microclimate in lowland tropical forest in Costa Rica

The temperature and atmospheric humidity in a tropical lowland rain forest in Costa Rica were measured in order to assess the microclimate in different forest environments. Two disturbed sites, a single tree fall gap (400 m2) and an 0.5 hectare clearing, were compared for periods up to two years after disturbance. Two locations in primary forest, the canopy and understory, were also monitored. Temperatures were highest in the clearing, intermediate in the canopy and gap which were similar, and lowest in the understory. Vapor pressure deficits (VPD) were highest in the clearing, followed by the canopy, the gap and the understory. With regrowth of the vegetation in the gap and clearing sites, the temperatures and vapor pressure deficits significantly decreased. After 1 year, the microclimate at seedling height in the clearing resembled that of the gap, and after two years the microclimate of the gap was very similar to that of the understory. Seasonal differences in temperature and VPD were small compared to differences caused by changes in the stature of the vegetation.

The effect of elevated CO2 and N availability on tissue concentrations and whole plant pools of carbon-based secondary compounds in loblolly pine (Pinus taeda)

Abstract We examined the extent to which carbon investment into secondary compounds in loblolly pine (Pinus taeda L.) is changed by the interactive effect of elevated CO2 and N availability and whether differences among treatments are the result of size-dependent changes. Seedlings were grown for 138 days at two CO2 partial pressures (35 and 70 Pa CO2) and four N solution concentrations (0.5, 1.5, 3.5, and 6.5 mmol l−1 NO3NH4) and concentrations of total phenolics and condensed tannins were determined four times during plant development in primary and fascicular needles, stems and lateral and tap roots. Concentrations of total phenolics in lateral roots and condensed tannins in tap roots were relatively high regardless of treatment. In the smallest seedlings secondary compound concentrations were relatively high and decreased in the initial growth phase. Thereafter condensed tannins accumulated strongly during plant maturation in all plant parts except in lateral roots, where concentrations did not change. Concentrations of total phenolics continued to decrease in lateral roots while they remained constant in all other plant parts. At the final harvest plants grown at elevated CO2 or low N availability showed increased concentrations of condensed tannins in aboveground parts. The CO2 effect, however, disappeared when size differences were adjusted for, indicating that CO2 only indirectly affected concentrations of condensed tannins through accelerating growth. Concentrations of total phenolics increased directly in response to low N availability and elevated CO2 in primary and fascicular needles and in lateral roots, which is consistent with predictions of the carbon-nutrient balance (CNB) hypothesis. The CNB hypothesis is also supported by the strong positive correlations between soluble sugar and total phenolics and between starch and condensed tannins. The results suggest that predictions of the CNB hypothesis could be improved if developmentally induced changes of secondary compounds were included.

Effects of nitrogen supply and elevated carbon dioxide on construction cost in leaves of Pinus taeda (L.) seedlings

Seedlings of loblolly pine (Pinus taeda L.) were grown under varying conditions of soil nitrogen and atmospheric carbon dioxide availability to investigate the interactive effects of these resources on the energetic requirements for leaf growth. Increasing the ambient CO2 partial pressure from 35 to 65 Pa increased seedling growth only when soil nitrogen was high. Biomass increased by 55% and photosynthesis increased by 13% after 100 days of CO2 enrichment. Leaves from seedlings grown in high soil nitrogen were 7.0% more expensive on a g glucose g−1 dry mass basis to produce than those grown in low nitrogen, while elevated CO2 decreased leaf cost by 3.5%. Nitrogen and CO2 availability had an interactive effect on leaf construction cost expressed on an area basis, reflecting source-sink interactions. When both resources were abundant, leaf construction cost on an area basis was relatively high (81.8±3.0 g glucose m−2) compared to leaves from high nitrogen, low CO2 seedlings (56.3±3.0 g glucose m−2) and low nitrogen, low CO2 seedlings (67.1±2.7 g glucose m−2). Leaf construction cost appears to respond to alterations in the utilization of photoassimilates mediated by resource availability.

Photosynthesis and successional status of Costa Rican rain forest trees.

Seven tree species from three different light environments in the wet lowland forests of Costa Rica were grown under controlled environment conditions to assess light related photosynthetic potentials. Light saturated photosynthesis rates were clearly related to light levels of the field environments. Mean saturated, net photosynthetic rates ranged from 6.8 to 11.3 to 27.7 μmol m−2 sec−1 for plants from heavy shade, canopy light gaps and man-made clearings respectively. Light saturation of plants from clearings occurred at photosynthetic photon flux densities greater than 1000 μmol m−2 sec−1 whereas plants from heavy shade environments became light saturated near 500 μmol m−2 sec−1. Plants that normally occur in intermediate light environments were intermediate in light saturation levels. Mean maximum stomatal conductances ranged from 1.0 to 7.3 mm sec−1 and followed a pattern similar to photosynthetic rates.

Effect of elevated CO2 on mycorrhizal colonization of loblolly pine (Pinus taeda L.) seedlings

Interactive effects of elevated atmospheric CO2 and phosphorus supply on mycorrhizal colonization rates were investigated using loblolly pine (Pinus taeda L.) seedlings from Florida and coastal North Carolina. Seedlings from both populations were grown in greenhouses maintained at either 35.5 Pa or 71.0 Pa CO2. In both CO2 treatments, seedlings were grown in a full factorial experiment with or without mycorrhizal inoculum and with an adequate or a limiting supply of phosphorus. Seedlings were harvested 60, 90 and 120 days after emergence and at each harvest root subsamples were examined to determine the percent of fine roots that were mycorrhizal. Additionally, root carbohydrate and nutrient levels were measured at each harvest. Root starch, sugar and total non-structural carbohydrate (TNC) concentrations were increased by growth in elevated CO2 and decreased by mycorrhizal colonization. Phosphorus stress decreased root starch concentrations, increased root sugar concentrations and did not significantly affect TNC concentrations. However, despite significant effects on root carbohydrate levels, there were generally no significant treatment effects on mycorrhizal colonization. Additionally, at all harvests, root starch and sugar concentrations were not correlated with percent of fine roots that were mycorrhizal. These results suggest that although elevated CO2 may significantly increase root carbohydrate levels, the increases may not affect the percent of fine roots that are mycorrhizal.