We studied the effect of removing and adding plant litter in different seasons on biomass, density, and species richness in a Solidago dominated old-field community in New Jersey, USA. We removed all the naturally accumulated plant litter in November (658 g/m2) and in May (856 g/m2) and doubled the amount of litter in November and May in replicated plots (1 m2). An equal number of plots were left as controls. Litter removal and addition had little impact on total plant biomass or individual species biomass in the growing season following the manipulations. Litter removal, however, significantly increased plant densities but this varied depending upon the season of litter removal, species, and life history type. Specifically, the fall litter removal had a much greater impact than the spring litter removal suggesting that litter has its greatest impact after plant senescence in the fall and prior to major periods of early plant growth in spring. Annual species showed the greatest response, especially early in the growing season. Both spring and fall litter removal significantly increased species richness throughout the study. Litter additions in both spring and fall reduced both plant densities and species richness in June, but these differences disappeared near the end of the growing season in September. We concluded than in productive communities where litter accumulation may be substantial, litter may promote low species richness and plant density. This explanation does not invoke resource competition for the decline in species richness. Finally, we hypothesize that there may be broad thresholds of litter accumulation in different community types that may act to either increase or decrease plant yield and diversity.

The role of litter in an old-field community: impact of litter quantity in different seasons on plant species richness and abundance.

Summary
Ambrosia trifida L. is a fast growing annual colonizer of disturbed ground. When present it drastically reduces kinds, numbers and growth of other annuals. Its seedlings emerge before any other annual in early spring. We studied seed germination in relation to temperature, moisture, depth of sowing, and seedling emergence, survivorship and performance of mature plants in relation to time and depth of sowing and seed size.

Seeds germinated under a wide range of temperature (8 to 41°C) with an optimum between 10 to 24 °C. Further germination occurred after restratification suggesting germination polymorphism. The seeds also germinated under a wide range of soil moisture conditions (17 to 55 % dry wt) but highest germination occurred at 20 to 33 % soil moisture. Optimum depth of sowing was 2 cm; very little germination occurred very close to soil surface, and some occurred at the 16 cm depth. In the field seedlings emerged from shallow depths first and then from progressively deeper parts of the soil bank. There was positive correlation between seed size and maximum depth from which seeds of this species and those of five other associated annuals emerge. Time required for seedlings to emerge after germination was directly related to depth. Seedlings that emerged first had the highest probability of survival and those that emerged late suffered progressively higher mortality, irrespective of whether late emergence was caused by late germination or deep germination. Among the survivors that reached maturity, plant weight, height, and number of seeds per plant declined with delayed emergence. However, experimental manipulation showed that delayed emergence per se was not the cause of the mortality but rather competition with individuals which emerged earlier. Furthermore, delayed emergence of cohorts in separate plots did not significantly reduce plant height, weight and number of seeds per plant.

Thus in the field and within a patch there may be strong selection for early germination and emergence. However, this selection may be opposed by the unpredictability of the time of disturbance, the spatial patchiness of disturbance, and the longevity and induced dormancy of Ambrosia seeds. Compared to its associated annuals, A. trifida has the largest seeds and seedlings, the earliest germination and emergence and a very high photosynthetic rates. These properties give A. trifida a decisive advantage over the other species.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1979.tb02313.x

The biology of ambrosia trifida l.

Desert annuals of Death Valley, California have higher average light-saturated photosynthetic capacities and leaf nitrogen contents than do early-successional annuals of Illinois. The leaves of annuals in the light-unlimited Death Valley environment change little in specific weight, nitrogen, or photosynthetic capacity with age. In contrast, these properties decrease markedly with age in the leaves of the Illinois annuals even in leaves not exposed to the usual shading that accompanies canopy development. These results are interpreted in a carbon-gained-per-nitrogen-invested context.

Photosynthetic capacity in relation to leaf position in desert versus old-field annuals

To learn whether niche breadths change during the life of a plant, we tested the responses of three age—classes of six annual plants on four environmental gradients Several ontogenetic niche shifts were observed The differences in response among age—classes of a species were larger than the differences in response among other species of similar ages The species were very similar in their germination responses to the moisture and nutrient gradients, but were quite different from each other on the temperature gradient The biomass response of seedlings was the broadest of the three age—classes only on the nutrient gradient The responses of the species were very similar On the temperature gradient, responses of the species were also similar to each other, but response breadths were low On the light gradient there were more differences among the species as seedlings than on the other gradients The reproductive responses of mature plants were most sensitive to changes in nutrient concentration, resulting in narrower responses than for the other two age—classes Responses of mature plants were relatively broad on the moisture, temperature, and light gradients The co—occurrence of different ages of plants in a given locality makes these ontogenetic shifts in response to resource gradients important in the outcome of interactions between neighbors

Ontogenetic Niche Shifts in Old‐Field Annuals

Ambrosia arteinisiifolia L. dominates abandoned fields after spring plowing in many parts of the eastern United States. Its seeds germinate at or near the soil surface in early spring, and the cotyledons become green shortly after they emerge. They are photosynthetically active, with a net photosynthesis dark respiration ratio (P/R) of 1.9. The primary leaves are photosynthetically more active than the cotyledons with a P/R ratio of 3. Photosynthesis of young A. artemisiifolia seedlings is 21 mg C02 dm-2 hr-1 at 1,076 X 102 lux, 250 C, and 300 ppm CO2. They approach light saturation at 1,076 X 102 lux (1.1 cal cm-2 min-) and exhibit appreciable photosynthesis at temperatures from 5? to 350 C. Their rate of photosynthesis increases with an increase of the ambient C02 concentrations comparable to that measured near the soil surface in the field. Although seedling photosynthesis is quite sensitive to decreasing water potential (*J.) between 0 and -10 bars, there is no further decrease between -10 and -20 bars. Furthermore, at this range of 'P photosynthesis is about 20% of that at 0 bars. In a plowed field these seedlings experience high light intensities, appreciable variation in daily temperature, high CO2 concentrations due to its flux from the soil, and low water potentials since the roots are in the upper part of the soil. Field-grown, mature A. artemisiifolia plants do not light-saturate even at 1,076 X 102 lux. Their photosynthetic rate at optimum light intensity, temperature, ala, and 300 ppm C02 is about 35 mg C02 dm-2 hr-1. This high rate of photosynthesis is accompanied by a high rate of transpiration (3 g H20 dm-2 hr-1). Photosynthesis in these plants is sensitive to temperature and reaches a peak at 200 C. The effect of CO2 enrichment on the rate of photosynthesis is not as pronounced in mature plants as in seedlings. Photosynthetic rates decline sharply between -10 and -15 bars BILL; however, appreciable rates are maintained even at '' of -20 bars for several hours. Photosynthetic recovery from water stress is rapid after watering and is attributed to the very low total plant resistance to water transport (1 X 106 sec cm-1). Mature plants have moderate photorespiration since their CO2 compensation point is reached at 50 ppm C02. These data indicate that A. artemisiifolia can not only efficiently exploit the environmental resources of the field during seedling and mature plant stages of its life cycle but also effectively function under the sometimes extreme conditions encountered in the field. These ecophysio- logical attributes coupled with complex germination behavior and genetic plasticity enable the species to be a successful pioneer in early successional ecosystems where the environment can be severe and unpredictable.

Ecophysiology of Ambrosia Artemisiifolia: A Successional Dominant

Aboveground primary production in a field dominated by Ambrosia artemisiifolia in southeastern Illinois was 630 + 168 g m-2 in 1968 and 595 + 142 g m-2 in 1969. During the 1st year productivity for the entire growing seasorn was 7.8 g m-2 dayand 14.4 g mr2 dayfor a short period during the growing season. Efficiency of aboveground net primary production was 1.3 % between plowing of the field and peak, standing, aboveground biomass of the plant community in mid-August. Efficiency was a high 2.6% during a short period of the growing season when productivity was maximal. Comparisons with other data on primary production of 1-year and 2-year fields in several areas of the eastern deciduous forest biome show that the southeastern Illinois fields are significantly more productive and more efficient. This suggests that generalizations concerning community organization in young successional ecosystems require further investigation.

D. M. Mezga

Papers

Primary Productivity and Microenvironment in an Ambrosia-dominated Old Field