John P. O'Neill

Nitrogen vs. phosphorus limitation across an ecotonal gradient in a mangrove forest

Mangrove forests are characterized by distinctive tree-height gradientsthat reflect complex spatial, within-stand differences in environmentalfactors,including nutrient dynamics, salinity, and tidal inundation, across narrowgradients. To determine patterns of nutrient limitation and the effects ofnutrient availability on plant growth and within-stand nutrient dynamics, weused a factorial experiment with three nutrient treatment levels (control, N,P)and three zones along a tree-height gradient (fringe, transition, dwarf) onoffshore islands in Belize. Transects were laid out perpendicular to theshoreline across a mangrove forest from a fringe stand along the seaward edge,through a stand of intermediate height, into a dwarf stand in the interior ofthe island. At three sites, three trees were fertilized per zone for 2yr. Although there was spatial variability in response, growth byR. mangle was generally nitrogen (N) -limited in thefringe zone;phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in thetransition zone. Phosphorus-resorption efficiency decreased in all three zones,and N-resorption efficiency increased in the dwarf zone in response to Penrichment. The addition of N had no effect on either P or N resorptionefficiencies. Belowground decomposition was increased by P enrichment in allzones, whereas N enrichment had no effect. This study demonstrated thatessential nutrients are not uniformly distributed within mangrove ecosystems;that soil fertility can switch from conditions of N to P limitation acrossnarrow ecotonal gradients; and, that not all ecological processes respondsimilarly to, or are limited by, the same nutrient.

Vertical Profile and Canopy Organization in a Mixed Deciduous Forest

A combination of optical measurements of leaf heights and observations on litterfall provided a vertical and temporal description of the leaf community structure in a tall, Liriodendron forest on the Maryland coastal plain. Leaf area, mass, and number were bimodally distributed with height. Median leaf number occurs far below (7–8 m) and median leaf mass far above (22–23 m) the median leaf area (18–19 m). Tree species exhibited leaf stratification into 3 height levels: understory (0–10 m), mid canopy (10–25 m), and overstory (25–37 m). Species leaf area in litterfall was related to the species basal area, although representation of leaf number in litterfall was not correlated with stem numbers for species in the stand. Species also showed a clear phenological sequence of leaf fall.

Abundance and distribution of Corallorhiza odontorhiza reflect variations in climate and ectomycorrhizae

The abundance and reproductive activity of orchids have been linked to variations in weather conditions, but few investigators have examined the relationships between orchid flowering dynamics and the distribution and abundance of mycorrhizal fungi. We quantified the abundance of flowering individuals of Corallorhiza odontorhiza, a mycoheterotrophic orchid, over a 14-year period and mapped the distribution of individuals in six of the 14 years. For two seasons, we conducted intensive and extensive studies of the mycorrhizal fungi that were associated with C. odontorhiza. The annual abundance of flowering plants was statistically related to growing-season precipitation and winter temperature, and the distribution of individuals within the study plot was related to the abundance and distribution of appropriate host fungi. We used DNA sequencing to identify ectomycorrhizal root tips that hosted Tomentella fungi that could potentially support C. odontorhiza. We found that Tomentella spp. were distributed thro...

Germination ecology of the boreal-alpine terrestrial orchid Dactylorhiza lapponica (Orchidaceae)

In order to reveal some of the germination requirements of The boreal-alpine terrestrial orchid Dactylorhiza lapponica, several experiments were carried out on seeds collected from the Sølendet, nature reserve, central Norway. Seeds were sown in seed packets made of nylon cloth and deployed in situ vertically in the peat in order to study the temporal pattern of seed germination and determine if the seeds became part of the soil seed bank. In vitro germination experiments were carried out varying the growth media, fungal partner and chilling treatment, in order to study the effect of a fungal symbiont on germination and early protocorm development, and the possibility of a physiological seed dormancy. A high rate of germination a short time after sowing in the in vitro experiments together with a very low survival after deployment in the in situ experiments (0.2% after three years) indicate that D. lapponica seeds are not part of a long-lived soil seed bank. In vitro experiments also demonstrated that a fungal symbiont was not required for germination. Seeds sown in situ had very low germination rate (11%–12%) and lack of available nutrients is suggested as a possible explanation. Presence of a fungal symbiont clearly enhanced the early development of protocorms in vitro, and is probably necessary for the seedling to grow beyond the earliest protocorm stages under natural conditions. The results indicate that recruitment is highly variable and very low relative to population size, indicating that survival of established plants is crucial to the fate of a population.