Frederick W. Smith

Relation between size and density in developing stands: A description and possible mechanisms

Abstract A description of stand development is presented which incorporates several ecological concepts, including: full-site occupancy; self-thinning; and the relation between the number and average size of plants in the population. Each of these concepts is integrated by a general size-density model. It is suggested that stands which are undergoing self-thinning are those which have achieved an upper limit in the amount of foliage supported. It is postulated that the predictable size-density relations in self-thinning populations can be explained on the basis of regulation and redistribution of a fixed amount of foliage among a declining number of larger individuals.

Age-related decline in forest growth: an emergent property

Abstract Proposed explanations for the age-related decline in forest production (i.e. ‘culmination of current annual increment’) generally fall into one of two categories: (1) the ecophysiology of individual, generally old, trees; and (2) structural changes at the population level associated with increasing stand age. The decline in production occurs in young forests, is substantial at young stand ages, and timing of decline can be altered simply by changes in stand density. Changes in physiology of old trees do not account for the near-universal decline in production in developing stands. Rather, peak production and its subsequent decline are associated with inevitable changes in the structure of developing forest stands. Peak production almost invariably occurs as peak community leaf area is obtained. Substantial changes in canopy architecture, production efficiency, and tree population structure occur at this point, resulting in declining stand-level production. These changes are emergent properties that must be studied and understood at the population level, and are not derived from individual tree physiological processes.

Volume increment in Pinus contorta var. latifolia: the influence of stand development and crown dynamics

Abstract We examined changes in canopy structure in a Pinus contorta var. latifolia chronosequence to determine if the commonly observed decline in current annual stem increment following canopy closure is associated with declining leaf area efficiency, declining stand-level leaf area, or a combination of both. Leaf area index and stem increment for these P. contorta stands peak at approximately 40–45 years of age. The decline in leaf area index following canopy closure is associated with an increase in crown shyness and a reduction in leaf area efficiency. The characteristic pattern of stand growth, with culmination at about the time of canopy closure, is associated with declines in both the amount and efficiency of leaf area.

Determinants of Stemwood Production in Pinus contorta var. Latifolia Forests: The Influence of Site Quality and Stand Structure

(1) Leaf area index of mature stands of lodgepole pine was independent of stand density. Thus, stand density did not affect total leaf area, but strongly influenced leaf area per tree. (2) Leaf area index and site index were significantly but weakly related in the lodgepole pine stands in this study. (3) While the growth of stem volume was significantly (r2 =0 45) related to leaf area index, residual variation was associated with differences in stand structure. (4) Mean tree growth increased with mean leaf area, but at a decreasing rate; this indicates the relatively low efficiency with which leaf area on large trees produces stemwood volume. (5) Reduced leaf area efficiency of large trees in low density stands was related to increased respiration requirements resulting from greater relative amounts of nonphotosynthetic to photosynthetic tissue. This explanation for differences in leaf area efficiency is supported by the observation that the ratio of foliage to total crown biomass was inversely proportional to mean crown size.

Influence of topography and forest structure on patterns of mixed severity fire in ponderosa pine forests of the South Dakota Black Hills, USA

We examined the influence of topography and stand structure on fire effects within the perimeter of the ~34 000 ha Jasper fire of 2000 in ponderosa pine (Pinus ponderosa Laws.) forests of the South Dakota Black Hills, USA. We used a remotely sensed and field-verified map of post-fire burn severity (accuracy 69%, kappa statistic 0.54), the Digital Elevation Model, and vegetation databases maintained by the Black Hills National Forest to empirically test relationships at 500 randomly located points in each of three severity classes. Burn severity was defined as the relative degree of post-fire change based on fire effects on soil, forest floor, and vegetation. This fire burned rapidly, yet created a patchy mosaic of effects (25, 48, and 27% low, moderate, and high severity). Stands burned by low and moderate severity fire had fewer trees (stand density index 13 cm diameter at breast height ha–1) and were found on less steep sites (slope 470) with larger trees (average stand diameter >24 cm) or many small trees were more likely to burn with high severity effects. Our results suggest that managers should consider topography and stand structure together when making strategic decisions about which stands to thin or otherwise manage to reduce the severity with which forests will burn in wildfires.

Influence of canopy architecture on light penetration in lodgepole pine (Pinus contorta var. latifolia) forests

Abstract We examined the influence of canopy architecture on light penetration in unmanaged lodgepole pine (Pinus contorta var latifolia) forests of the Rocky Mountains. Light extinction coefficients were estimated for 19 stands of varying age and structure from measured light interception using the Beer-Lambert equation. Estimates of G (k corrected for solar angle) varied between 0.28 and 0.70 and were negatively correlated with estimates of leaf area index (LAI) and the percent gap in the canopy. We also simulated the effects of changing canopy architecture on light penetration. Simulation results suggested that the order of importance of the characteristics of canopy architecture on light penetration was: (1) LAI; (2) foliage aggregation; (3) average leaf inclination angle; (4) vertical distribution of foliage. Direct measurement and simulation of light penetration suggested that increased LAI and increased within-crown foliage aggregation result in a reduced light capture efficiency (light capture/unit LAI). Canopy gaps tend to be more pronounced in dense, older lodgepole pine forests. High LAI and aggregated foliage distributions associated with this forest structure would cause increased foliage overlap and foliage self shading, thus reducing light capture efficiency.

Canopy stratification and leaf area efficiency: a conceptualization

Abstract Species differences have been observed in the relationship between leaf area efficiency ( E ) and leaf area ( LA ) of individual trees within stands. In some species, for example, E decreases with greater LA , suggesting that the smallest trees in the stand are the most efficient. In other species E increases with greater LA , at least across a portion of the range of individual tree LA found in a stand. Rather than being distinct physiological behaviors, we propose a single model to explain these observed differences in E-LA relationships. Leaf area efficiency is affected by two countervailing factors. As tree size, relative to neighboring trees, increases with greater LA , E tends to increase, as a result of a generally improved canopy position, and presumably a more favorable light environment. However, carbon allocation patterns within the tree also change with tree size; increased carbon requirements for crown construction and maintenance, as well as root system development and stemwood respiration, result in less fixed carbon available for stem growth and, therefore, lower E . We examined two species with reported differences in E-LA relationships. In Pinus contorta stands, E decreases with increasing LA , whereas, in Abies lasiocarpa stands, E increases with LA up to some intermediate level of LA , and then declines with furtther increases in LA. Pinus contorta stands are characterized by limited canopy stratification, lower leaf area index, and lower canopy coverage, resulting in few trees found in poor light conditions. Decreases in E with greater LA are presumably associated with changing carbon allocation patterns, primarily greater respiratory requirements per unit of leaf area. Abies lasiocarpa stands have greater canopy stratification, higher leaf area index, and greater canopy coverage, and this results in relatively small trees situated in poor light environments. Increases in E with greater LA are associated with improved canopy position, and continue as long as height also increases. A general model is presented that explains production-leaf area relationships in forest-grown trees based on changes in relative tree size and canopy structural characteristics.

The role of near-surface lateral roots in the life cycle of aspen in the central Rocky Mountains

Abstract Above- and below-ground development was studied in 47 aspen (Populus tremuloides Michx.) clones in the central Rocky Mountains of Colorado. Our objective was to determine if characteristics of roots larger than 4 mm and within 0.2 m of the surface could be related to the life cycle of vegetatively regenerating aspen. We found that root diameter increased and end-of-season nonstructural carbohydrate content decreased with average stem diameter. Root volumes were lowest and nonstructural carbohydrate contents highest in clones between 20 and 80 years of age. However, root volumes did not change for several years after vegetative coppice regeneration of clones. Initial root/shoot ratios declined rapidly as young suckers grew, but slowed abruptly 10–25 years after regeneration. Aspen lateral root systems appear to adjust to above-ground conditions in three phases over the life cycle of a clone: (1) establishment of new suckers; (2) efficient growth from youth to mid-age; (3) maintenance at older ages.

Bias in leaf area — sapwood area ratios and its impact on growth analysis in Pinus contorta

SummaryTwo alternative estimators of individual tree leaf area (A1) area are used to derive estimates of leaf-area index (L) for 40 plots in Pinus contorta Dougl. stands. One estimator of A1 is based on the common assumption of a constant ratio between A1 and sapwood cross-sectional area at breast height (As). The second estimator of A1 accounts for tree-to-tree variation in the relation between A1 and As. The apparent relationship between stand growth and leaf-area index is strongly dependent on the way leaf area is estimated. When L is derived from a constant A1∶As ratio, stand growth appears to be strongly correlated with L. However, when L is based on estmates of A1 that account for tree-to-tree variation in the A1 — As relation, stand growth is seen to be only weakly related to L. Stand structure, quantified as percent live-crown, accounts for a great deal of the observed variation in leaf-area efficiency. These contrasting relationships illustrate the importance of unbiased estimates of L in interpreting the link between stand-level processes and leaf area.

Short-term impact of post-fire salvage logging on regeneration, hazardous fuel accumulation, and understorey development in ponderosa pine forests of the Black Hills, SD, USA

We examined the impacts of post-fire salvage logging on regeneration, fuel accumulation, and understorey vegetation and assessed whether the effects of salvage logging differed between stands burned under moderate and high fire severity following the 2000 Jasper Fire in the Black Hills. In unsalvaged sites, fire-related tree mortality created a large standing pool of available fuel, resulting in a rapid increase in surface fuel loads. After 5 years, fine woody debris (FWD) and coarse woody debris (CWD) increased ~1380% and 980% in unsalvaged sites, resulting in FWD and CWD loads of 13 and 25 Mg ha–1, respectively. In contrast, salvage logging limited the rate of accumulation of FWD to ~110% over the same time period and total accumulation of CWD to 16 Mg ha–1. In moderate-severity sites, regeneration was 75% lower in salvaged sites owing to low seed-tree retention, suggesting a re-evaluation of salvage guidelines during future operations in the Black Hills. The likelihood of timely regeneration in high-severity sites, regardless of salvage treatment, is low. We found no discernible effect of salvage logging on understorey development 5 years after fire. Logging caused neither a reduction in total plant cover nor an increase in the abundance of exotic species.