Changes In Stem Density Research Articles

Mortality and tree‐size distributions in natural mixed‐age forests

Summary Tree‐size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree‐size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests. We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light‐demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even‐aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree‐size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands. Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self‐thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation. Smaller trees were significantly more likely to die than larger trees within the young stands. Tree‐diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands. Large‐scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large‐scale disturbance, but the trend was weak. The diameter‐distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure. The diameter distribution of the forest was not in equilibrium over the 19‐year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree‐diameter distributions away from the long‐term mean, leading us to conclude that modern‐day disequilibrium in natural forests may be the legacy of past disturbance events.

Relationship between stem density and dry-canopy evaporation rates in coniferous forests

Summary Our previous study summarized published data on dry-canopy evaporation in coniferous forests during the growing season and found a decrease in the Priestley–Taylor coefficient α with canopy height. We speculated that the decrease in α was caused by a decrease in stomatal conductance with tree height. However, the decrease in α may also be explained by changes in leaf area index (LAI) and/or stem density with canopy height. We found no clear relationship between LAI and α in our previous study. Here, we examined the relationship between stem density and α and found no clear relationship, indicating that the decrease in α with increasing canopy height is not explained by changes in stem density. Thus, our results support the previous speculation that decrease in α was caused by a decrease in stomatal conductance with tree height.

Tree Growth and Mortality in a Southern Appalachian Deciduous Forest Following Extended Wet and Dry Periods

We inventoried two 1-ha plots on opposing watersheds (WS2-WS-S, WS18-WS-N) three times (1983, 1989, 1998) over a 16-year period to contrast how differing precipitation (P) regimes affect tree mortality. From 1983 to 1989, annual precipitation averaged 16.5% less than the 64-year mean; from 1989 to 1998, it averaged 12.2% above the mean. In 1989 and 1998, standing crop biomass, aboveground net primary productivity, mortality rates and species composition were determined. In 1989, following the dry period, the highest mortality for canopy tree species was in Carya spp. and Quercus velutina in both watersheds. Following the wet period in 1998, mortality was highest in Cornus florida in both watersheds, presumably due to the Anthracnose fungus; however, net change in stem density due to ingrowth was positive for this species in the WS-S watershed (+34%) and negative in the WS-N watershed (-18%). Estimated ANPP for WS-S was 10 and 1,076 kg ha1 yr1 for the periods 1983-89 and 1989-98, respectively. In contrast, ANPP on WS-N was 679 and 93 kg ha1 yr1 for the same periods. Differences in ANPP are due to higher rates of mortality on WS-N compared with WS-S. In this study, species specific rates of mortality varied by watershed, and were likely due to microclimate- related increased susceptibility at the species level to certain proximal causes.

Long‐term growth dynamics of natural forests in Hokkaido, northern Japan

Abstract. The long‐term growth dynamics of natural forest stands on the island of Hokkaido were described on the basis of an analysis of data from 38 permanent plots spanning 15–22 yr. Stand structure was characterized by basal area, stem density and tree size variability. To detect trends in stand structure, regression models for recruitment rate (per ha per yr), mortality rate and the rate of change in stem density and tree size variability were developed by a stepwise method using initial basal area, stem density, tree size variability, species composition summarized by LNMDS ordination, altitude, annual mean temperature, annual precipitation, type of understorey vegetation, topography and slope aspect as candidates for predictor variables. The same analyses were conducted for basal area increment (net growth) and its components: survivor growth = basal area gain by growth of surviving individuals and mortality = basal area loss by death of individuals.Stem density remained generally unchanged; recruitment was relatively low even in very sparse stands. Stand basal area generally increased as survivor growth was approximately double the mortality.Recruitment rate was strongly affected by the presence of dwarf bamboo (Sasa spp.) vegetation on the forest floor which inhibited tree regeneration. Mortality rate was density‐dependent; dense stands had higher mortality than sparse stands. Density change rate (recruitment rate ‐ mortality rate) was, therefore, determined by both the type of understorey vegetation and stem density. Survivor growth was high in stands with high stem density and basal area. Mortality was dependent on basal area and altitude. Net basal area increment (net growth) was dependent only on stem density with other factors that influenced survivor growth and mortality omitted. Tree size variability decreased in stands with high tree size variability whereas it increased in stands with low size variability. Based on the obtained models for density change rate and net basal area increment, trajectories of stands were illustrated on a log‐log diagram of stem density and basal area. The predicted differences in trajectories as affected by the understorey vegetation type indicated the importance of dwarf bamboo vegetation for forest dynamics on Hokkaido.

Does gall diameter affect the parasitism rate of Asphondylia borrichiae (Diptera: Cecidomyiidae)?

Abstract. 1. Fertilized field plots of Borrichia frutescens (L.) de Candolle produced plants with a higher apical‐leaf nitrogen content than control plots.2. Gall frequency of the cecidomyiid Asphondylia borrichiae Rossi & Strong on stems of B.frutescens, increased significantly on fertilized plots after approximately 3 months.3. Fertilizer treatment did not result in changes in stem density but did tend to increase proportion of stems flowering and overall plant size. Galls are not normally found on flowering stems.4. Galls grew at a faster rate and to a larger final size on fertilized plots.5. On three sampling dates, per cent parasitism of galls was the same on fertilized and control plots; therefore, this study does not support the galldiameter hypothesis.

Efficacy of Rotovation in Controlling Eurasian Watermilfoil in the Pend Oreille River, Washington

ABSTRACT Full-scale mechanical rotovation of Eurasian watermilfoil (Myriophyllum spicatum) beds was conducted at selected sites along the Pend Oreille River in eastern Washington during the fall of 1986. Fifteen hectares (36 acres) of milfoil-infested river sediments were mechanically tilled to dislodge rootcrowns from the substrate. Treatment was performed by a prototype float-mounted rotovator equipped with a 2.5 m (8-foot) wide pivoting rototiller head. Treatment effectiveness was quantitatively assessed by comparing changes in stem density of M. spicatum in the plot before and after rotovation. Milfoil stem density reductions of 63 to 90 percent were observed in the rotovated area immediately after treatment. Carryover stem density reduction of 25 to 70 percent measured in the growth season following the 1986 rotovation treatment suggested that control of M. spicatum growth for greater than the season of treatment is possible. Efficiency of milfoil root removal by rotovation was dependent on substrate type, operator skill, and adequately powered machinery. Rotovation appears to be a most promising control technology capable of achieving high rates of milfoil root removal and maintaining long-term reductions in plant growth.