Scott K. Gleeson

Allocation and the Transient Dynamics of Succession on Poor Soils

Biomass and nitrogen allocation to leaf, root, stem, and reproduction was determined in a 35-field chronosequence that spans the first 60 yr of secondary succession on a Minnesota sand plain. Biomass (grams per square metre) in leaf and root increased during succession, but reproductive biomass declined, and that in stem remained constant. Because root biomass increased twice as rapidly as leaf biomass, the proportion of total biomass in root increased during succession, whereas that in leaf, reproduction, and stem declined. In an additional study, biomass allocation was determined on a species-by-species basis for 46 species common at different times during succession. This study showed a similar pattern of increasing proportional root allocation and declining proportional re- productive and stem allocation during succession. These changes were accompanied by an increase in total soil nitrogen and a decrease in light penetration to the soil surface during succession. Increasing root allocation and decreasing reproductive allocation suggest that succession on these nutrient-poor soils is the transient dynamics of colonization and competitive displacement, with later successional species being superior nitrogen competitors because of higher root allocation. Allocation trade-offs between root, stem, leaf, and seed can lead to initial dominance by species with high seed and leaf allocation, presumably because of greater colonization and/or maximal growth rates. Thus, this succession differs markedly from successions on rich soils, for which stem allocation is increasingly important. These results contradict the resource ratio hypothesis as an explanation for the pattern of early succession on impoverished soils.

Plant allocation, growth rate and successional status

Theory predicts a negative correlation between root (and stem) allocation and growth rate in enriched environments (RGR max ), but evidence does not consistently support this. There is one possible supporting example from previous research in which root allocation in the field increases during old-field succession at the Cedar Creek Natural History Area, both community wide and for individual species samples, and RGR max declines for some important species. In this study, we confirm that late successional species have lower RGR max for a larger sample of species (n = 28) as well as higher field root allocation. However, root allocation by seedlings does not increase for the late successional species for those same species in the greenhouse in enriched conditions, indeed it declines

Density Dependence is Better Than Ratio Dependence

Ratio—dependent models make very specific assumptions that prevent them from serving as a foundation for general consumer—resource theory. Traditional density—dependent models can be, and have been, modified to incorporate similar alternative assumptions and many others.

Treefall in a Mixed Oak‐Pine Coastal Plain Forest: Immediate and Historical Causation

In a local convective storm, 85 trees were damaged in an 18.3 ha stand in the Pine Barrens region of southern New Jersey, USA. To determine what factors place individual trees at risk, damaged trees were measured and graded for degree of fire scarring and fungal rotting. Similar measurements were made on 280 undamaged individuals. Ages were determined in a subsample by coring. Only two of the seven species present suffered wind damage (Quercus prinus and Q. velutina). Damaged trees were more severely rotted than undamaged individuals both within and among species. Presence of fungal rot was strongly linked to scarring of the trunk by fire, which in turn was determined by date of recruitment relative to historical episodes of fire. Risk was not influenced by either tree size, proximity to existing gaps, taper of the stem, or interspecific differences in wood strength. Vulnerability to wind damage in this stand was determined by a summation of disturbance effects influencing tree recruitment and health over the previous 90 yr.

Root proliferation and marginal patch value

We describe and test a simple model of optimal root proliferation in a soil consisting of patches of various quality, with quality defined as the rate of supply of limiting soil resource. Assuming that roots deplete resources in the patch, the model predicts that root biomass will be allocated such that the marginal gains from each patch are equilibrated. With the further assumption that marginal returns are correlated with patch quality, the generally assumed positive relation between patch quality and root proliferation is predicted. This secondary prediction is tested with the grass Sorghum vulgare grown in containers consisting of four patches of variable limiting N-supply. Mean fine root biomass showed a strong positive relation between patch quality and, biomass consistent with the foraging hypothesis, especially when expressed on a per primary root basis. However, the high within-individual variation and the insensitivity of primary roots imply that much of the root behavior remains unexplained by the model.

Root growth response to water and nutrients in the New Jersey Pinelands

Resource limitation theory regarding water versus nutrient limitation predicts on the one hand that, because of uptake trade-offs, increases in water will increase limitation by nutrients; on the o...

Short-term impacts of nitrogen fertilization on a montane grassland ecosystem in a South Asian biodiversity hotspot

Background: Elevated anthropogenic nitrogen (N) deposition is an important cause of biodiversity decline and disruption of ecosystem function and services, yet these impacts on many globally important ecosystems remain poorly studied. The Western Ghats of India has a N deposition rate that is highest among the 34 world biodiversity hotspots, and is projected to increase to 3.3 g N m−2 year−1 by 2050. Aims: This study aims to understand N addition effects on the plant community and ecosystem in the montane grasslands of the Nilgiris, Western Ghats. Methods: Plant and soil responses were measured in grassland plots subject to N addition (ambient, 2 and 8 g N m−2year−1) and N immobilisation (500 g sucrose N m−2 year−1) treatments in a region frequently subjected to fire disturbance – the study site was burned 6 months prior to the experiment. Results: Ecosystem responses were consistent with a N-limited system. Soil N availability and leaf tissue N concentration responded positively to N addition. Plant cover and biomass increased with N addition, whereas soil moisture decreased. Unexpectedly, N enrichment had a positive effect on species richness and diversity, although the cover of one important native grass was significantly reduced by the higher N level. Conclusions: These results suggest that there may be a complex interaction of N deposition and the direct and indirect effects of fire in this system, warranting more detailed and longer-term research in this ecosystem.