Litterfall in a secondary forest with special reference to the relationships between leaf-fall rate,basal area and relative growth rate on a species basis

Total basal area (BA), mean relative growth rate of individual basal area (RGR _bam), annual leaf-fall rate (L), and number of trees (N) on a species basis were measured in a secondary forest mixed with evergreen and deciduous trees in the warm-temperate zone of Japan. Every species had a characteristic tree size and composed the tree and the shrub layers. The layer for each species, and the tree size of the species were represented by the species mean basal area per tree (BA/N orba _m). A power form equation was obtained for the relation between the species mean leaf-fall rate per tree (L/N orl _m) andba _m.RGR _bam was correlated with leaf-fall rate per unit basal area (L/BA) rather than withba _m. This suggested thatRGR _bam is associated less with mean tree size (ba _m) and dominance as total basal area (BA), and more with annual leaf production per unit basal area, provided thatL is equal to the annual leaf production per species.Rhus succedanea, the most dominant species in the forest plot, was typical in that it showed lower values ofRGR _bam andL/BA than other tree species. This would suggest a decline ofR. succedanea in a secondary forest.     

Vegetation structure and dynamics of Pinus gerardiana forests in Balouchistan,Pakistan

Abstract. 44 forest stands, including 42 stands with Pinus gerardiana Wall, ex Lamb dominant and two stands with broad-leaved trees, were sampled in the Suleiman Range in Balouchistan. Density oi Pinus gerardiana trees ranged from 24 to 930 trees / ha with a mean of 266 individuals / ha; the average basal area was 25.5 m² ha^-1. Adequate recruitment of Pinus seedlings was observed; higher seedling density is recorded from east-facing slopes, while tree density was higher on west-facing slopes. The average growth rate was estimated as 0.08 cm / yr radial growth. However, trees on high elevations and cooler slopes grow faster. Soil variables showed no correlation with density, basal area or importance values. It is suggested that the present degraded stage of the forests in the study area is of anthropogenic origin.     

Laurel forests in Tenerife,Canary Islands

 Stand structure and leaf area distribution of a laurel forest in the Agua García mountains of Tenerife are described. The site is situated at 820 m a.s.l., faces NNE, and has a humid mediterranean climate. Summer droughts are mitigated by relatively high air humidity and clouds. The natural mixed hardwood forest is composed of six major tree species: Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng, Myrica faya Ait., Erica arborea L. and two species of Ilex (I. platyphylla Webb & Berth. and I. canariensis Poivet.). The experimental stand had a density of 1693 trees ha^– 1, a basal area of 33.7 m²ha^– 1, and a cumulated volume of above-ground parts of trees of 231 m³ ha^– 1 with a corresponding dry mass of 204 ton ha^– 1. Diameters at breast height ranged from 6 to 46 cm. Mean concentration of plant dry mass per volume was 1.17 kg m^– 3. The vertical pattern of leaf area distribution in individual trees for all tree species was characterized by a Gaussian-like curve. Stand leaf area index was 7.8. These evergreen, broad-leaved (laurisilva or lucidophyllous) forests represent a relic forest that was widespread in the Mediterranean region some 20 million years ago. Our data illustrate some of the structural characteristics of this historically widespread forest type. Received: 2 December 1994 / Accepted: 6 November 1995     

Structure of a unique inland mangrove forest assemblage in fossil lagoons on the Caribbean Coast of Mexico

Scrub mangrove wetlands colonize the intertidal zone of fossil lagoons located in carbonate continental margins along the Yucatan Peninsula of Mexico. These unique ecological types were investigated in October, 1994, by locating transects in several mangrove forests along the Caribbean coast of the peninsula. Four species of mangrove occurred at these sites including Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, Conocarpus erecta. This is one of the first examples of a species rich scrub forest. The mangroves fell into three height categories: short scrub less than 1.5 m, tall scrub to 3.0 m, and basin forests between 4.5 and 6 m. Average height, diameter (dbh), basal area, and complexity index generally increased from short scrub to basin forests. Basal area, ranged from 0.16 m² ha^–1 in a short scrub forest intermixed with Cladium jamaicense to 12.9 m² ha^–1 in a basin forest. Density ranged from 1520 trees ha^–1 to over 25,000 trees ha^–1 in a short scrub forest dominated by R. mangle. The complexity index ranged from 0.01 to 8.3. Height, dbh, basal area, and complexity index were positively related. A number of trees were growing as sprouts from larger downed trunks, suggesting that hurricanes, such as Gilbert that occurred in 1988, are important in controlling the structure of these forests. These forests appear isolated from the sea, but are influenced by groundwater exchange occurring at the land-margin zone.     

Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm² m^?2 (28‐y) to 11.5 cm² m^?2 (67‐y) and decreased again to 7 cm² m^?2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m^?2 (28‐y) to 14 kg dry weight m^?2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m^?2 s^?1) and canopy transpiration (1.5 mm d^?1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d^?1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = ? 0.02 + 1.15SA R²) which was determined by stand density and tree sapwood area.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

Tree population studies in low-diversity forests,Guyana. I. Floristic composition and stand structure

Studies were undertaken on the floristic composition and stand structure of four 1 hectare plots in the lowland forests of Kurupukari, Guyana. A total of 3897 trees, covering 153 species and 31 plant families were recorded at greater than 5 cm diameter at breast height (dbh). The number of species per hectare ranged from 61 to 84 (>5.0 cm dbh) and 50–71 (>10.0 cm dbh). The total number of trees per hectare varied two-fold between study plots, with 45–50% of the trees within the 5–10 cm size-class. Mean total basal area varied from 32.39–34.63 m² per 100 m². The four most dominant plant families represented 43.8% of the total number of trees, while representing only 11.2% of the species. No one plant family dominated in more than one of the four study plots, and all four plots held at least one plant family with more than 20% of the total number of trees. Although 14 tree species were common to all four plots, only 26%–35% of the species were represented by a single tree. Between three and seven species represented 50% of the trees within all size-classes, with species dominance occurring within the highest density plot.These tropical forest types of central Guyana may represent some of the lowest diversity forests in the neotropics, whereby the total number of tree species is relatively limited, typically with six dominant canopy species, but the relative abundance of these species is highly variable between the forest types. Mechanisms influencing the competitive interactions associated with species dominance are discussed in relation to the importance of mycorrhizae and the persistence of species dominance.     

Population structure and dynamics oi Juniperus excelsa in Balouchistan,Pakistan

Abstract. 60 monospecific stands of Juniperus excelsa were sampled at four locations in Balouchistan. Density, basal area and height of individuals were recorded. Soils were analysed for selected physical and chemical characteristics and the degree of disturbance due to logging and burning was noted. The density of juniper trees (> 6 cm dbh) ranged from 56 to 332 stems / ha (average 174 stems / ha). Higher densities were recorded for relatively undisturbed stands and on west facing slopes. Density of seedlings and saplings (< 6 cm dbh) was strongly correlated with tree density and tree basal area. Among the edaphic variables CaC0₃ was correlated with juniper density and basal area. Diameter distributions within stands were mostly skewed and unimodal with gaps appearing in large size classes. The male to female ratio was close to 1. Cross-sections of 16 trees were used to determine age and growth rate. Number of rings in trees with 20 to 30 cm dbh ranged from 95 to 221 (x = 160 ± 38). Diameter and age were not related. Mean annual diameter increment ranged from 6 to 16 yr / cm x = 10 ± 3 yr / cm). It is concluded that size class gaps and low seedling / sapling densities are the consequence of anthropogenic disturbance.     

SURFACE AREA RELATIONS OF WOODY PLANTS AND FOREST COMMUNITIES

Surface area of wood and bark is an important dimension of forests, with implications for respiration rate, energy exchange, and water and mineral budgets. Surface area of stem wood and bark can be estimated effectively from linear regressions on conic surface (one-half basal circumference times tree height) or from regressions of the logarithm of area on the logarithm of diameter at breast height. Branch surface can be estimated from a formula using branch basal diameter, length, and number of current twigs, and from logarithmic regressions of branch bark surface on basal diameter of branches and breast-height diameter of trees. In temperate deciduous forests several square meters of plant surface occur above each square meter of ground surface; these plant surfaces include 0.3–0.6 m² of stem bark, 1.2–2.2 m² of branch bark, and 3.0–6.0 m² of leaf blades. Branch bark surface increases more rapidly than leaf surface with increasing size of branches and trees. Growth and aging of trees, and maturation of forests, imply increasing ratios of bark (and wood) surface to the photosynthetic leaf surface which supports its growth and respiration.     

Thinning Jeffrey pine stands to reduce susceptibility to bark beetle infestations in California,U.S.A.

• 1 Bark beetles (Coleoptera: Curculionidae, Scolytinae) are commonly recognized as important tree mortality agents in coniferous forests of the western U.S.A.
• 2 High stand density is consistently associated with bark beetle infestations in western coniferous forests, and therefore thinning has long been advocated as a preventive measure to alleviate or reduce the amount of bark beetle‐caused tree mortality.
• 3 The present study aimed to determine the effectiveness of thinning to reduce stand susceptibility to bark beetle infestations over a 10‐year period in Pinus jeffreyi forests on the Tahoe National Forest, California, U.S.A. Four treatments were replicated three times within 1‐ha square experimental plots. Treatments included thinning from below (i.e. initiating in the smallest diameter classes) to a residual target basal area (cross‐sectional area of trees at 1.37 m in height) of: (i) 18.4 m²/ha (low density thin); (ii) 27.6 m²/ha (medium density thin); (iii) 41.3 m²/ha (high density thin); and (iv) no stand manipulation (untreated control).
• 4 Throughout the present study, 107 trees died as a result of bark beetle attacks. Of these, 71% (75 trees) were Abies concolor killed by Scolytus ventralis; 20.6% (22 trees) were Pinus ponderosa killed by Dendroctonus ponderosae; 4.7% (five trees) were P. jeffreyi killed by Dendroctonus jeffreyi; 1.8% (two trees) were P. jeffreyi killed by Ips pini; 0.9% (one tree) were P. jeffreyi killed by Orthotomicus (= Ips) latidens; 0.9% (one tree) were P. ponderosa killed by both Dendroctonus brevicomis and D. ponderosae; and 0.9% (one tree) were P. jeffreyi killed by unknown causes.
• 5 In the low density thin, no pines were killed by bark beetles during the 10‐year period. Significantly fewer trees (per ha/year) were killed in the low density thin than the high density thin or untreated control. No significant treatment effect was observed for the percentage of trees (per year) killed by bark beetles.

     

Effects of European colonization on indigenous ecosystems: post-settlement changes in tree stand structures in Eucalyptus–Callitris woodlands in central New South Wales, Australia

Aim There has been considerable debate about pre‐settlement stand structures in temperate woodlands in south‐eastern Australia. Traditional histories assumed massive tree losses across the region, whereas a number of recent histories propose that woodlands were originally open and trees regenerated densely after settlement. To reconcile these conflicting models, we gathered quantitative data on pre‐settlement stand structures in Eucalyptus–Callitris woodlands in central New South Wales Australia, including: (1) tree density, composition, basal area and canopy cover at the time of European settlement; and (2) post‐settlement changes in these attributes. Location Woodlands dominated by Eucalyptus species and Callitris glaucophylla, which originally occupied approximately 100,000 km² in central New South Wales, Australia. Methods We recorded all evidence of pre‐settlement trees, including stumps, stags and veteran trees, from 39 relatively undisturbed 1‐ha stands within 16 State Forests evenly distributed across the region. Current trees were recorded in a nested 900 m² quadrat at each site. Allometric relationships were used to estimate girth over bark at breast height, tree basal area, and crown diameter from the girth of cut stumps. A post‐settlement disturbance index was developed to assess correlations between post‐settlement disturbance and attributes of pre‐settlement stands. Results The densities of all large trees (> 60 cm girth over bark at breast height) were significantly greater in current stands than at the time of European settlement (198 vs. 39 trees ha^?1). Pre‐settlement and current stands did not differ in basal area. However, the proportional representation of Eucalyptus and Callitris changed completely. At the time of settlement, stands were dominated by Eucalyptus (78% of basal area), whereas current stands are dominated by Callitris (74%). On average, Eucalyptus afforded 83% of crown cover at the time of settlement. Moreover, the estimated density, basal area and crown cover of Eucalyptus at the time of settlement were significantly negatively correlated with post‐settlement disturbance, which suggests that these results underestimate pre‐settlement Eucalyptus representation in the most disturbed stands. Main conclusions These results incorporate elements of traditional and recent vegetation histories. Since European settlement, State Forests have been transformed from Eucalyptus to Callitris dominance as a result of the widespread clearance of pre‐settlement Eucalyptus and dense post‐settlement recruitment of Callitris. Tree densities did increase greatly after European settlement, but most stands were much denser at the time of settlement than recent histories suggest. The original degree of dominance by Eucalyptus was unexpected, and has been consistently underestimated in the past. This study has greatly refined our understanding of post‐settlement changes in woodland stand structures, and will strengthen the foundation for management policies that incorporate historical benchmarks of landscape vegetation changes.     

Vessel-element dimensions and frequency within the most current growth increment along the length of Eucalyptus globulus stems

The mean number of vessels per unit area in the outer most growth increment at six percentage heights in individual trees, viewed in transverse section, showed an increasing trend from basal to apical regions. The highest frequency of vessels occurred at 90% of total tree height (23.58 mm^-2) while the lowest frequency (9.99 mm^-2) was recorded at 15% of total tree height from the base. Vessel elements were largest at 15% of total tree height and proceeded to decrease in size with increased stem height. Average radial measurements ranged between 111.27 µm and 160.0 µm while average tangential measurements ranged between 76.30 µm and 112.80 µm. There was variation seen at each percentage height between collection dates (i.e. individual trees); however, the trend of increasing vessel frequency from basal to apical regions existed regardless of between-tree variation in dimensions. Vessel-element dimensions showed a similar, but inverse trend to vessel-element frequency with the largest vessel elements located at 15% of total tree height and the smallest vessel elements located at 90% of total tree height.     

Crown profile of foliage area characterized with the Weibull distribution in a hinoki (Chamaecyparis obtusa) stand

Summary Thirten sample trees of various sizes in a 29-year-old hinoki [Chamaecyparis obtusa (Sieb, et Zucc.) Endl.] plantation were felled and subjected to the stratified clip technique. Crown profile of foliage area fitted well with the Weibull distribution. The crown profile tended to be more skewed toward the top of crowns in smaller trees than in larger trees. This tendency was reflected in the value of the shape parameter of the Weibull distribution. The shape parameter ranged from 1.73 to 3.23 and gradually increased up to an asymptotic value with an increase of stem diameter at breast height. The scale parameter of the distribution ranged from 1.0 to 4.2 and tended to increase in proportion to stem diameter at breast height. Foliage area of a tree correlated well with stem diameter at breast height through an ordinary allometric equation. Tree height could be approximated fairly well by a generalized allometric equation as a function of stem diameter at breast height. On the basis of the census of stem diameter at breast height, canopy profile could be constructed synthesizing crown profiles of foliage area for individual trees in the stand. Leaf area index was estimated to be 6.6 ha ha^–1.     

Contrasting leaf characteristics of trees and lianas in secondary and mature forests in southwestern China

We compared variation in sun-canopy leaf anatomy, morphology and photosynthetic rates of coexisting woody species (trees and lianas) in an 8-year-old secondary forest (SF) and mature forest (MF) in the wet season in Xishuangbanna, SW China. Variability of leaf traits of 66 species within growth-form groups in each forest was quantified using coefficients of variation (CV). For the mean values, the woody species in the SF had significantly higher leaf thickness and stomatal density, but lower nonmesophyll/mesophyll ratios than those in the MF. The average leaf area and leaf mass area (LMA) in the studied woody species did not change greatly during the successional process, but differed significantly between the growth forms, with trees having higher values than lianas. The light-saturated photosynthetic rate per unit leaf area (A _a) of the woody species in the SF ranged from 11.2 to 34.5 μmol m⁻² s⁻¹, similarly to pioneer tree species from literature data in southeast Asia. The A _a and photosynthetic nitrogen-use efficiency (PNUE) were significantly higher than those in the MF; whereas A _a in the MF ranged between 9 to 21 μmol m⁻² s⁻¹, with similar values between lianas and trees. For all woody species in both SF and MF, there were no significant differences in the average values of the CV of all measured variables for both lianas and trees. However, considerable variation in leaf anatomy, morphology, and photosynthetic rates within both growth forms and forests existed, as well as substantial variation in leaf size and stomatal density. We concluded that the tropical woody species formed a heterogeneous functional group in terms of leaf morphology and physiology in both secondary and mature forests.     

Succession after stand replacing disturbances by fire,wind throw,and insects in the dark Taiga of Central Siberia

The dark taiga of Siberia is a boreal vegetation dominated by Picea obovata, Abies sibirica, and Pinus sibirica during the late succession. This paper investigates the population and age structure of 18 stands representing different stages after fire, wind throw, and insect damage. To our knowledge, this is the first time that the forest dynamics of the Siberian dark taiga is described quantitatively in terms of succession, and age after disturbance, stand density, and basal area. The basis for the curve–linear age/diameter relation of trees is being analyzed. (1) After a stand-replacing fire Betula dominates (4,000 trees) for about 70 years. Although tree density of Betula decreases rapidly, basal area (BA) reached >30 m²/ha after 40 years. (2) After fire, Abies, Picea, and Pinus establish at the same time as Betula, but grow slower, continue to gain height and eventually replace Betula. Abies has the highest seedling number (about 1,000 trees/ha) and the highest mortality. Picea establishes with 100–400 trees/ha, it has less mortality, but reached the highest age (>350 years, DBH 51 cm). Picea is the most important indicator for successional age after disturbance. Pinus sibirica is an accompanying species. The widely distributed “mixed boreal forest” is a stage about 120 years after fire reaching a BA of >40 m²/ha. (3) Wind throw and insect damage occur in old conifer stands. Betula does not establish. Abies initially dominates (2,000–6,000 trees/ha), but Picea becomes dominant after 150–200 years since Abies is shorter lived. (4) Without disturbance the forest develops into a pure coniferous canopy (BA 40–50 m²/ha) with a self-regenerating density of 1,000 coniferous canopy trees/ha. There is no collapse of old-growth stands. The dark taiga may serve as an example in which a limited set to tree species may gain dominance under certain disturbance conditions without ever getting monotypic.     

Stem water storage and diurnal patterns of water use in tropical forest canopy trees

Stem water storage capacity and diurnal patterns of water use were studied in five canopy trees of a seasonal tropical forest in Panama. Sap flow was measured simultaneously at the top and at the base of each tree using constant energy input thermal probes inserted in the sapwood. The daily stem storage capacity was calculated by comparing the diurnal patterns of basal and crown sap flow. The amount of water withdrawn from storage and subsequently replaced daily ranged from 4 kg d^–1 in a 0·20-m-diameter individual of Cecropia longipes to 54 kg d^–1 in a 1·02-m-diameter individual of Anacardium excelsum, representing 9–15% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and basal sapwood area, suggesting that this relationship was species-independent and size-specific for trees at the study site. According to this relationship there was an increment of 10 kg of diurnal water storage capacity for every 0·1 m² increase in basal sapwood area. The diurnal withdrawal of water from, and refill of, internal stores was a dynamic process, tightly coupled to fluctuations in environmental conditions. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resistance with tree size and thus play an important role in regulating the water status of leaves exposed to the large diurnal variations in evaporative demand that occur in the upper canopy of seasonal lowland tropical forests.     

Tree community structure and stem mortality along a water availability gradient in a Mexican tropical dry forest

We document spatial changes in species diversity, composition, community structure, and mortality of trees across a gradient of water availability in a tropical dry forest in western Mexico. This gradient occurs along the main stream of a small watershed of less than 1 km in length. Four 30 × 80 m plots were established systematically to include the driest (ridge top of the watershed) to the wettest sites (watershed bottom) within this watershed. All stems larger than 5 cm were identified, and measured for diameter and height. Dead stems larger than 5 cm were measured and classified as: a) found on live or dead trees, and b) standing (“snags”) or lying (“downlogs”) on the ground. The number of recorded species per plot declined from 73 to 44 species as water availability decreased. A decline in estimated total richness, and in Shannon-Wiener and Simpson diversity indices was also observed in the drier plots. Species composition strongly changed along the gradient, with the two ends of the gradient sharing only 11% of the species. Stem density and percentage of dead stems and trees increased in abundance and basal area from the wetter to the drier sites. Tree and stem size (basal area, height and stem diameter) showed the opposite trend. Nonetheless, total basal area of live trees was largest at the two end gradient locations and oscillated between 12.22 m² ha⁻¹ and 7.93 m² ha⁻¹. Proportion of snags increased towards the driest site (from 46 to 72%), while that of down logs decreased. Overall, our results suggest that small-scale gradients of water availability play a paramount role in the spatial organization of tree communities in seasonal tropical environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

STABILITY AND INSTABILITY IN PLANT COMMUNITIES FOLLOWING FIRE

Tree species composition and density were monitored in closed oak-hickory forest and forest-prairie edge for a period of five years after a prescribed burn. In the closed forest, tree stem density declined markedly following the burn. Tree basal area and density decreased from 17.5 m²/ha and 630 trees/ha in the preburn sample to 12.0 m²/ha and 310 trees/ha five years later. In contrast, on the forest-prairie edge, tree basal area and density increased slightly during the same time period from 3.0 m²/ha and 117 trees/ha to 5.2 m²/ha and 172 trees/ha. Our data suggest that closed canopy forests in fire susceptible areas accumulate fuels to levels that encourage fires of sufficient intensity to destabilize forest systems and convert them to open forests or savannahs. Conversely, on the forest-prairie edge, amounts and patterns of fuel accumulation, and species response to burning, are such that fire can be considered to be a factor promoting stability.     

Dependence of the aboveground CO2 exchange rate on tree size in field-grown hinoki cypress (Chamaecyparis obtusa)

The CO₂ exchange of the aboveground parts for five different-sized 17-year-old (as of 1991) hinoki cypress (Chamaecyparis obtusa) trees growing in the field was non-destructively measured over one year, using an open CO₂ exchange system. The CO₂ exchange of individual trees decreased with decreasing tree sizes, such as aboveground phytomass, leaf mass and leaf area. However, the CO₂ exchange abruptly decreased near the smallest-suppressed sample tree. The size dependence was well described by a generalized power function. The annual gross photosynthesis of individual trees was proportional to the square root of leaf mass or leaf area. The dependence of CO₂ exchange on annual phytomass increment was described by a simple power function with an exponent value less than unity, suggesting that CO₂ exchange per unit of phytomass increment was lower in larger-sized trees than in smaller-sized trees. The mean photosynthetic activity of a tree, i.e., gross photosynthesis per unit of leaf area, slightly increased to its highest value with decreasing leaf area and then decreased abruptly near the smallest sample tree. The maximum value of mean photosynthetic activity was estimated to be 2.85 kg CO₂ m⁻² year⁻¹ for a leaf area of 1.56 m² tree⁻¹. The ratio of mean photosynthetic activity to the maximum photosynthetic activity was the highest in an intermediate tree and decreased gradually toward larger-sized trees by ca. 60% and also decreased toward the smallest suppressed tree by ca. 35%.     

Biomass and litter dynamics in a Melaleuca forest on a seasonally inundated floodplain in tropical,northern Australia

Litterfall from a Melaleuca forest was investigated as part of chemical cycling studies on the Magela Creek floodplain in tropical, northern Australia. The forest contained two species of tree, Melaleuca cajaputi and Melaleuca viridiflora, with a combined average density of 294 trees ha^–1. The M. viridiflora trees had diameter breast height measurements ranging from 11.8 to 62.0 cm, median class 25.1–30.0cm and a mean value of 29.2±1.0 cm, compared to 13.0 to 66.3 cm, 30.1–35.0cm and 33.5±1.0cm for M. cajaputi trees. A regression model between tree height, diameter breast height and fresh weight was determined and used to calculate average tree weights of 775±1.6kg for M. viridiflora and 1009±1.6kg for M. cajaputi, and a total above-ground fresh weight of 263±0.3t ha^–1. The weight of litter recorded each month on the ground beneath the tree canopy ranged from 582±103 to 2176±376 g m^–2 with a monthly mean value of 1105±51 g m^–2. The coefficient of variation of 52% on this mean indicates the large spatial and temporal variability in litter distribution over the study site. This variability was greatly affected by the pattern of water flow and litter transport during the Wet season. Litterfall from the trees was evaluated using two techniques - nets and trays. The results from these techniques were not significantly different with annual litterfall collected in the nets being 705 ± 25 g m^–2 and in the trays 716±49 g m^–2. The maximum monthly amount of litterfall, 108 ±55g m^–2, occurred during the Dry season months of June–July. Leaf material comprised 70% of the total annual weight of litter, 480±29 g m^–2 in the nets and 495 ± 21 g m^–2 in the trays. The tree density and weight of litter suggest that the Melaleuca forests are highly productive and contribute a large amount of material to the detrital/debris turnover cycle on the floodplain.