Relative Diameter Growth Rate Research Articles

Vegetation Management for Promoting Ecosystem Resilience in Fire‐Prone Mediterranean Shrublands

Abstract Dense shrublands constitute highly hazardous fuels in Mediterranean countries. The combination of agricultural land abandonment and fire occurrence in many Mediterranean areas has led to a landscape dominated by shrublands where resprouter species are scarce or absent. Major goals in the management of these areas are to reduce both the fuel loads and their continuity and increase the resilience of the ecosystems by introducing resprouter species. We investigated the performance of one‐year‐old seedlings of native resprouters, that is, Pistacia lentiscus, Quercus ilex, and Rhamnus alaternus, in a combination of vegetation clearing and mulching with brush chipping in three sites with high amount of fuel load. The planting holes inside the shrublands showed lower radiation and soil surface temperature than those in the cleared areas, whereas soil water content was higher in the mulched holes than in the unmulched ones, especially when water availability was lowest. Seedling survival of Q. ilex and R. alaternus significantly increased within the shrubland; P. lentiscus showed the opposite effect, but its survival was enhanced by the mulching treatment. The three species grew faster in the cleared plots although, unexpectedly, mulching showed a negative effect on Q. ilex relative growth rate in diameter. Our results suggest that the combination of fuel control and reforestation as techniques for shrubland management is an appropriate option, at least in the short term, to redirect vegetation dynamics toward later successional stages, improving ecosystem resilience by the introduction of woody resprouter species.

Site preparation, stock quality and planting date effect on early establishment of Holm oak ( Quercus ilex L.) seedlings

Nursery researchers tend to study seedling quality instead of other sylvicultural practices such as soil preparation and planting date. The aim of this study was to determine the effects and interactions of site preparation and stock quality on the survival and growth of 1-year-old Quercus ilex L. seedlings planted on different dates. Based on the hypothesis that soil preparation affects out-planting performance more than stock quality does in Mediterranean areas, two different site preparations (subsoiling and manual holing) and three planting dates were studied. Two years after planting, high-quality seedlings planted on an early date over a subsoiling preparation showed the best survival rates (61%), followed by the same quality plant and soil preparation treatments on a mid-season planting date (40%). After two growing seasons, planting date and site preparation affected height growth rate positively, whereas relative diameter growth rate of surviving seedlings was affected by planting date only. A correct selection of the planting date and soil treatment plays an important role in the expression of seedling quality in terms of survival and growth.

Impact of standing vegetation on early establishment of willow cuttings in the flooded area of the Parana River Delta (Argentina)

We assessed the growth and survival of a willow clone (Salix matsudana × Salix alba ‘A 13/44’) growing under different vegetation management in the flooded area of the Parana River Delta (Argentina) during the first 2 years after planting. Treatments consisted in a combination of practices applied in the row and in the inter-row. In the row (1-m wide) vegetation was manually cut with machete (r-M), treated with glyphosate at 3% (r-H) or maintained undisturbed (r-U); in the inter-row vegetation was crushed with a roller (I-R), treated with glyphosate at 3% (I-H) or maintained undisturbed (I-U). Height, diameter and relative growth rate of the dominant sprout were evaluated. Tree survival was high (96%) and not affected by treatments. Growth was modified by the vegetation control at early stages. Height and diameter were higher in r-H plots compared to r-U plots; both were similar in the I-U and I-H plots but greater than in I-R plots. Early differences in diameter relative growth rate among row treatments were found. Possible mechanisms associated with willow growth responses such as plastic responses under resource limited conditions and amelioration of the microenvironment by the native vegetation are discussed.

Growth efficiency–survivorship relationship and effects of spacing on relative diameter growth rate of Japanese cedars

Relative growth rate (RGR) is considered to be one of the most sensitive measures of competitive ability. As such, RGR should correlate positively with survivorship. The proposition, however, has not been fully tested with long-term data. Another question that has not been fully addressed is the long-term effect of initial density on the RGRs of survivors. Based on data from a long-term Japanese cedar ( Cryptomeria japonica) spacing trial (with square spacing of 2, 3, 4, and 5 m), we tested the following two hypotheses: (1) within each spacing, survivors have higher average relative diameter growth rate (RGR DBH) than those trees that did not survive, and (2) the mean RGR DBH of survivors increases with increasing initial spacing. A two-parameter monomolecular model and nonlinear mixed-effects regression were used to model individual tree diameter growth over a 50-year period, with the model's growth-rate parameter as a surrogate measure for RGR DBH. Our results supported both hypotheses conditionally. Survivors were indeed the more efficient ones, but only when competition became intense. Our study suggested that self-thinning likely thins out trees that were inefficient in diameter growth. When young, survivors of the widest spacing did have the highest average RGR DBH. However, after 55 years, all survivors had about the same diameter growth efficiency, regardless of the initial density. In the end, survivors that experienced more intense competition, possibly by making diameter growth a high priority in carbon allocation, achieved the same RGR DBH as did those that experienced less intense competition.

No initial size advantage for Japanese cedars in crowded stands

A long-standing and often contested issue in plant ecology is the “initial size advantage” hypothesis. Resolving the issue will clarify the roles of initial size in several areas of plant ecology. While the hypothesis has been examined several times, yielding supporting evidence for both sides of the argument, the hypothesis has rarely been tested with long-lived perennials under density stress. We tested the hypothesis based on the 2-m and 3-m spacing diameter growth data from a long-term Japanese cedar ( Cryptomeria japonica) spacing trial, with relative diameter growth rate as a measure of overall competitive ability. We also determined the initial size effect duration. A two-parameter monomolecular model and nonlinear mixed-effects regression were employed to model individual tree diameter growth over a 50-year period in a retrospective and sequential manner, with initial diameter size class as the discrete fixed-effect covariate and stand age as the continuous covariate. The model's growth-rate parameter was used as a surrogate measure for relative diameter growth rate. All differences in size-class mortality rates were statistically not significant, except for two early inventories in the 3-m spacing. Survivors’ mean diameter growth efficiency decreased with increasing initial size for the first 35 years for both spacings. Consequently, the differences in mean size-class DBH narrowed quickly for both spacings, and all size classes showed a similar mean DBH after 30 years. Initial size effects in terms of growth efficiency, lasted no more than 45 years for both spacings and with respect to mortality, lasted for no longer than 30 years for the 3-m spacing. Differential growth efficiency eliminated the initial size differences and prevented the development of a “hierarchy of exploitation” in diameter distributions. For Japanese cedars in the two spacings, a larger initial size did not confer a higher competitive ability or survivorship. It was growth efficiency that determined the ultimate fate of individual trees.

Cottonwood growth rate and fine root condensed tannin concentration.

We examined the relationship between trunk diameter and diameter relative growth rate (RGR) and fine root condensed tannin concentration in 12 genotypes of eastern cottonwood (Populus deltoides Bartr. ex Marsh.) planted in three locations across the north central United States. Across genotypes, trunk diameter, diameter RGR and root condensed tannin concentration were negatively correlated at one location (Wisconsin), but showed no significant correlation at the other locations (Iowa and Michigan). The factors responsible for this difference among sites remain unidentified, but may be related to soil fertility.

The relative dominance hypothesis explains interaction dynamics in mixed species Alnus rubra/Pseudotsuga menziesii stands

Summary We used repeated measurements of tree growth and population‐level and neighbourhood conditions from three mixed Alnus rubra/Pseudotsuga menziesii forests in the Pacific Northwest, USA to investigate why previous results regarding the importance of neighbourhood competition as a determinant of plant growth were inconsistent. We propose that relative dominance of a particular species determines the importance of neighbourhood interactions, and tested whether growth performance of both species at various stand ages agrees with this relative dominance hypothesis. Neighbourhood and relative dominance interaction indices were modified to accommodate two‐species mixtures and were incorporated into a growth model predicting relative diameter growth rates. The corrected Akaike Information Criterion (AICc) was used to identify the optimum interaction measures and model forms for each species and measurement period. Interaction indices reflecting the size of a tree relative to the population were the best predictors of growth of the dominant (i.e. taller) species and neighbourhood interaction indices of the subordinate species. Performance of interaction measures as predictors of relative growth rates in our study varied in agreement with the relative dominance hypothesis for both species and on all sites. Results from other studies suggest that the hypothesis may explain growth performances on a species and individual plant level and for a variety of life forms. These findings suggest that the spatial scale of plant interactions is influenced by the size structure of plant populations. The relative dominance hypothesis offers a framework to provide insight into the mechanism of competition, based on the relative performance of competition indices.

Effects of artificial and western spruce budworm (Lepidoptera: Tortricidae) defoliation on growth and biomass allocation of Douglas-fir seedlings.

Artificial defoliation has been used commonly to simulate defoliation by insect herbivores in experiments, in spite of the fact that obvious differences exist between clipping foliage and natural defoliation due to insect feeding. We used a greenhouse experiment to compare the effects of artificial and western spruce budworm (Choristoneura occidentalis Freeman) defoliation on the growth and biomass allocation of 3-yr old half-sib seedlings from mature Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco variety glauca] trees that showed phenotypic resistance versus susceptibility to budworm defoliation in the forest. Artificial clipping of buds mimicked the effects of budworm feeding on total seedling biomass when 50% of the terminal buds were damaged. However, artificial defoliation decreased seedling height, relative growth rate of height, and shoot: root ratio more than budworm defoliation, whereas budworm defoliation decreased stem diameter relative growth rate more than artificial defoliation. Half-sib seedling progeny from resistant maternal tree phenotypes had greater height, diameter, biomass, and shoot: root ratio than seedlings from susceptible phenotypes. We concluded that careful artificial defoliation could generally simulate effects of budworm defoliation on total biomass of Douglas-fir seedlings, but that the two defoliation types did not have equal effects on biomass allocation between shoot and root. Further, an inherently higher growth rate and a greater allocation of biomass to shoot versus root are associated with resistance of Douglas-fir trees to western spruce budworm defoliation.

Heritability of Ozone Sensitivity in Open-Pollinated Families of Black Cherry (Prunus serotina Ehrh.)

Abstract First-year seedlings of black cherry, representing 10 open-pollinated families from wild parents, were exposed in CSTR chambers to replicated treatments of 30, 60, 90, and 120 ppb ambient ozone for a 7 hr day-1, 5 day wk-1. Stem height, occurrence of leaf senescence, and visible foliar injury expressed as adaxial stipple were measured weekly during the 10 wk duration of exposure. Basal stem diameters were measured before and after fumigation. Stipple began to occur in week 1 of the 120 ppb ozone treatment, week 2 of the 90 ppb treatment, and week 4 of the 60 ppb treatment. No stipple occurred on any seedlings exposed to 30 ppb ozone. Premature leaf senescence occurred in all treatments, but sooner and more severely at higher concentrations of ozone. Relative diameter growth rates declined as ozone concentrations increased. Families differed significantly in their response to ozone treatments in severity of adaxial stipple, but not in leaf senescence or height or diameter growth. Family heritability estimates for foliar injury, calculated by treatment and week of measurement, were generally above 0.5 in the 90 and 120 ppb ozone treatments. The relative ozone sensitivity of these 10 families under CSTR conditions corresponded well with sensitivity rankings of their 27-yr-old parents, replicated in a clonal seed orchard, and growing under ambient ozone exposures. The existence of localized, heritable variation in ozone sensitivity in wild populations has obvious implications for the use of bioindicators in forest health monitoring. For. Sci. 48(1):111–117.

Gender, light and water effects in carbon isotope discrimination, and growth rates in the dioecious tree Ilex aquifolium

1. Detailed understanding of the specific physiology of sexes in dioecious species is required to explain patterns in gender dimorphism. Under controlled-environment conditions we tested the hypothesis that sexes of the dioecious tree holly Ilex aquifolium L. (Aquifoliaceae) differed in growth and long-term potential water-use efficiency, as measured by carbon isotope discrimination (Δ13C), and that these differences were dependent on the environmental context. 2. Patterns of response in Δ13C to the various combinations of light and water were gender-specific. Under more xeric conditions, females maintained significantly higher Δ13C than males. 3. Female plants exhibited significantly greater relative diameter growth rates than male plants. 4. As expected, Δ13C significantly increased with decreasing irradiance, and decreased with increasing limitation in water supply. Light and water effects were not independent, with a more pronounced drought effect in decreasing leaf Δ13C under unshaded than under shaded conditions. 5. Our results suggest that between-sex differences in physiology are context-dependent. Future studies attempting to assess gender dimorphism should take more account of gender-specific interactions with the environment. Gender-specific efficiency in water use could play a decisive role in explaining gender differences in growth and ecological interactions.

Gender, light and water effects in carbon isotope discrimination, and growth rates in the dioecious tree Ilex aquifolium

Abstract1. Detailed understanding of the specific physiology of sexes in dioecious species is required to explain patterns in gender dimorphism. Under controlled‐environment conditions we tested the hypothesis that sexes of the dioecious tree holly Ilex aquifolium L. (Aquifoliaceae) differed in growth and long‐term potential water‐use efficiency, as measured by carbon isotope discrimination (Δ13C), and that these differences were dependent on the environmental context.2. Patterns of response in Δ13C to the various combinations of light and water were gender‐specific. Under more xeric conditions, females maintained significantly higher Δ13C than males.3. Female plants exhibited significantly greater relative diameter growth rates than male plants.4. As expected, Δ13C significantly increased with decreasing irradiance, and decreased with increasing limitation in water supply. Light and water effects were not independent, with a more pronounced drought effect in decreasing leaf Δ13C under unshaded than under shaded conditions.5. Our results suggest that between‐sex differences in physiology are context‐dependent. Future studies attempting to assess gender dimorphism should take more account of gender‐specific interactions with the environment. Gender‐specific efficiency in water use could play a decisive role in explaining gender differences in growth and ecological interactions.

Early diameter growth of trees in planted forest stands

A model to describe early diameter growth of trees in planted forest stands is presented. Relative diameter growth rate of free-grown trees is used to estimate potential diameter growth of competitively grown trees of the same species and age. The effect of competition on the ability of competitively grown trees to attain their growth potential is assumed to be a function of site resource availability—growing space. Data from free-grown trees of the correlated curve trend projects in South Africa are used to estimate growth at several spacings at other South African sites and in Hawaii. The model produces typical current annual and mean annual growth curves as well as branched curves of diameter growth over time at different spacings. It is consistent mathematically with relative density concepts. Because individual tree diameter and stand volume are the most important factors in harvesting costs, the model can serve as a decision support tool to assist forest plantation managers choose how many trees to plant per unit area for optimum cost-efficiency in short-rotation intensive-culture operations.

Interference of bull thistle (Cirsiumvulgare) with growth of ponderosa pine (Pinusponderosa) seedlings in a forest plantation

The biennial thistle Cirsiumvulgare (Savi) Tenore significantly reduced ponderosa pine (Pinusponderosa Dougl. ex Laws.) seedling growth during the second year of infestation but had insignificant effects in the first year when all thistles were in the rosette stage. Pine stem diameter relative growth rate was significantly negatively correlated with four different indices of thistle interference and with visual estimates of thistle cover. Total thistle density (adults + rosettes) within 2.0 m of target seedlings best explained differences in stem relative growth rate, but density of adults alone and visual estimates of thistle cover were nearly as good. Simple regressions indicated that soil moisture and pine predawn leaf water potential were significantly negatively correlated with thistle density and significantly positively correlated with stem relative growth rate, but multiple regressions and path analyses indicated that their effects on seedling growth were negligible relative to the other (unexplained) effects of thistle density. Foliar nitrogen, phosphorus, and potassium concentrations were not significantly correlated with thistle density and failed to explain differences in seedling growth. Although it remains unclear how thistles suppressed pine seedling growth, if these results hold true at other sites, plantation managers will have at their disposal relatively easy methods for assessing thistle interference.

Early growth of planted western hemlock in relation to stock type and controlled-release fertilizer application

Four types of western hemlock (Tsugaheterophylla (Raf.) Sarg.) nursery stock, differing in size, morphology, and root growth capacity were planted on a clear-cut forest site on Vancouver Island, British Columbia. The stock types were: (i) 1-year-old, plug-rooted seedlings raised in 36-cm3 containers; (ii) as (i) but transplanted to a bareroot nursery for 1 year; (iii) 1-year-old, bareroot seedlings raised in 1000-cm3 containers; (iv) as (iii) but raised in containers coated on the inside with copper paint to inhibit lateral root elongation upon contact with the container wall. Survival and growth of seedlings with and without fertilization at planting with 50 g of a slow-release (Nutricote®, 12-month release at 25 °C) N–P–K fertilizer were observed for 3 years. Neither stock type nor fertilizer had a statistically significant (P < 0.05) effect on seedling survival. At planting, plug seedlings were significantly (P < 0.01) shorter and smaller in stem volume than the other stock types. These differences were no longer evident by the end of the third growing season because of more rapid growth by the plug stock. The largest differences between the plugs and the other stock types in height and diameter relative growth rates occurred during the first growing season. Fertilization significantly (P < 0.01) increased height and stem volume relative growth rates of all stock types during the first growing season. During the third growing season fertilization had a significant (P < 0.01) negative effect on height, and diameter and stem volume relative growth rates of all stock types, but not on absolute rates. At the end of three growing seasons, fertilized trees remained significantly (P < 0.01) larger in all stock types than in the controls.

Common Bermudagrass (Cynodon dactylon) Interference in Newly Planted Peach (Prunus persica) Trees

Field experiments conducted over a 2-yr period demonstrated that common bermudagrass [Cynodon dactylon (L.) Pers. # CYNDA] inhibited growth of newly planted peach (Prunus persica L. ‘Norman’) trees. Common bermudagrass densities of 100, 75, 50, and 25% ground cover reduced tree fresh weight by 86, 64, 43, and 19%, respectively, the first year (1978) and 87, 62, 44, and 28%, respectively, the second year (1979) after planting. Tree trunk diameter relative growth rate (RGR) was reduced by 75 and 100% common bermudagrass ground cover densities at all measurement dates only in 1978. Tree leaf N and K were reduced in both years by common bermudagrass; however, only at the 100% common bermudagrass density in 1978 was N at a deficient level. Leaf chlorophyll was reduced in trees grown in all densities of common bermudagrass only in 1978. Reduced tree growth cannot be explained entirely by competition for essential nutrients; thus an allelopathic effect of the bermudagrass on young peach roots is suspected.