Litterfall Mass Research Articles

INVASION BY A N2-FIXING TREE ALTERS FUNCTION AND STRUCTURE IN WET LOWLAND FORESTS OF HAWAII

Invasive species pose major threats to the integrity and functioning of ecosystems. When such species alter ecosystem processes, they have the potential to change the environmental context in which other species survive and reproduce and may also facilitate the invasion of additional species. We describe impacts of an invasive N2-fixing tree, Falcataria moluccana, on some of the last intact remnants of native wet lowland forest undergoing primary succession on 48-, 213-, and 300-yr-old lava flows of Kilauea Volcano on the island of Hawai‘i. We measured litterfall, soil nitrogen (N) and phosphorus (P) availability, light availability, species composition, and forest structure in native-dominated stands and in stands invaded by Falcataria. Litter inputs increased 1.3–8.6 times, N mass of litterfall increased 4–55 times, and P mass of litterfall increased 2–28 times in invaded stands relative to native stands. C:N and C:P ratios of litterfall were lower, and N:P ratios higher, in invaded stands relative to native stands. Resin-captured soil N and P values were 17–121 and 2–24 times greater, respectively, in invaded stands relative to native stands on each of the three lava flows. Native species accounted for nearly 100% of total basal area and stem density in native stands, while alien species accounted for 68– 99% of total basal area, and 82–91% of total stem density, in invaded stands. Compositional changes following Falcataria invasion were due both to increases in alien species, particularly Psidium cattleianum, and decreases in native species, particularly Metrosideros polymorpha. Results provide a clear example of how invasive tree species, by modifying the function and structure of the ecosystems that they invade, can facilitate invasion by additional nonnative species and eliminate dominant native species. Given the rarity and limited extent of remaining native-dominated wet lowland forests in Hawaii, and the degree to which Falcataria invasion alters them, we expect that the continued existence of these unique ecosystems will be determined, in large part, by the spread of this invasive species.

Litter production, seasonal pattern and nutrient return in seven natural forests compared with a plantation in southern China

Summary The amount and pattern of litterfall and its nutrient return were studied in seven natural forests of Schima superba Gardn. and Champ. (SCS), Castanopsis fabri Hance (CAF), Tsoongiodendron odorum Chun (TSO), Cinnamomum chekiangense Nakai (CIC), Altingia gracilipes Hemsl. (ALG), Castanopsis carlesii (Hemsl.) Hayata (CAC) and Pinus massoniana D. Don (PIM), and compared with that of an adjacent 29-year-old plantation of Chinese fi r ( Cunninghamia lanceolata Lamb.) (CUL) in Jianou, Fujian, China. Mean annual total litterfall over 3 years of observations varied from 4.63 Mg ha � � in the CUL to 8.85 Mg ha � � in the PIM; of this litterfall, the leaf contribution ranged from 62 to 73 per cent. Litterfall in the CAF, ALG and CAC showed an unimodal distribution pattern, while for the fi ve other forests, the litterfall pattern was multi-peak. The rank order of the eight forests, according to nutrient return mass with the exception of P, was different from the order when rank was according to total mass of litterfall. The highest annual N, K and Ca returns from total litterfall were noticed in the TSO, the CAF and the CUL, respectively. The amounts of P and Mg potentially returned to the soil were the highest in the PIM. The leaf fraction provided greater potential returns of N, P, K, Ca and Mg to the soil than other litter fractions. The results of this study demonstrate that natural forests have a greater capability for maintaining site productivity than the monoculture coniferous plantation, due to higher amount of above-ground litter coupled with greater nutrient returns; therefore conservation of natural forests is recommended as a practical measure in forest management to realize sustainable development of forestry in mountainous areas of southern China.

Effect of typhoon disturbance on fine litterfall and related nutrient input in a subtropical forest on Okinawa Island, Japan

Abstract Typhoons are frequent on Okinawa Island, southwestern Japan. The effects of typhoon disturbance on the patterns of fine litterfall and related nutrient inputs in a subtropical evergreen broad-leaved forest were studied over 5 years from May 1996 to April 2001. Annual fine litterfall averaged 7558 kg ha −1 (range from 6188 to 9439 kg ha −1 ) for six sampling plots over 5 years, which differed significantly among years ( p p =0.122). A seasonal maximum was most evident for leaf litter component. Woody litter fell more irregularly through the year, and peak fall varied with typhoon and windstorm. The mean ratio of annual litterfall mass of sexual organs to leaves was 0.06, much lower than that in other tropical and subtropical rain forests. Nutrient concentrations varied in litterfall components, but were not significantly different among plots. The lowest concentrations of N and P in leaf litter were observed in March, which is also the month with the greatest leaf fall. However, the highest concentrations were recorded in typhoon season. Nitrogen and P concentrations were 34% and 106% greater in the green leaves that fell during typhoons than in senescent leaves. Mean nutrient inputs by litterfall were: N 83, P 3.2, K 25, Ca 71, Mg 19, Al 12, Na 10, Fe 0.86 and Mn 3.9 kg ha −1 yr −1 , and differed significantly among years for all elements ( p p p

Impacts of the exotic, nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen-cycling in a pine–oak ecosystem

We investigated the influence of the exotic nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen cycling in a pitch pine (Pinus rigida) −scrub oak (Quercus ilicifolia, Q. prinoides) ecosystem. Within paired pine-oak and adjacent black locust stands that were the result of a 20-35 year-old invasion, we evaluated soil nutrient contents, soil nitrogen transformation rates, and annual litterfall biomass and nitrogen concentrations. In the A horizon, black locust soils had 1.3-3.2 times greater nitrogen concentration relative to soils within pine-oak stands. Black locust soils also had elevated levels of P and Ca, net nitrification rates and total net N-mineralization rates. Net nitrification rates were 25-120 times greater in black locust than in pine-oak stands. Elevated net N-mineralization rates in black locust stands were associated with an abundance of high nitrogen, low lignin leaf litter, with 86 kg N ha–1 yr–1 in leaf litter returned compared with 19 kg N ha–1 yr–1 in pine-oak stands. This difference resulted from a two-fold greater litterfall mass combined with increased litter nitrogen concentration in black locust stands (1.1% and 2.6% N for scrub oak and black locust litter, respectively). Thus, black locust supplements soil nitrogen pools, increases nitrogen return in litterfall, and enhances soil nitrogen mineralization rates when it invades nutrient poor, pine-oak ecosystems.

The Effects of Exotic Grasses on Litter Decomposition in a Hawaiian Woodland: The Importance of Indirect Effects

Exotic grasses and grass-fueled fires have altered plant species composition in the seasonal submontane woodlands of Hawaii Volcanoes National Park. These changes have altered both structural and functional aspects of the plant community, which could, in turn, have consequences for litter decomposition and nitrogen (N) dynamics. In grass-invaded unburned woodland, grass removal plots within the woodland, and woodland converted to grassland by fire, we compared whole-system fluxes and the contributions of individual species to annual aboveground fine litterfall and litterfall N, and litter mass and net N loss. We assessed the direct contribution of grass biomass to decomposition and N dynamics, and we determined how grasses affected decomposition processes indirectly via effects on native species and alteration of the litter layer microenvironment. Grasses contributed 35% of the total annual aboveground fine litterfall in the invaded woodland. However, total litterfall mass and N were not different between the invaded woodland and the grass removal treatment because of compensation by the native tree Metrosideros polymorpha, which increased litter production by 37% ± 5% when grasses were removed. The 0.3 g N m−2/y−1 contained in this production increase was equal to the N contained in grass litter. Litter production and litterfall N was lowest in the grassland due to the loss of native litter inputs. Decomposition of litterfall on an area basis was highest in the grass-invaded woodland. We attributed this effect to increased inherent decomposability of native litter in the presence of grasses because (a) the microenvironment of the three vegetation treatments had little effect on decomposition of common litter types and (b) M. polymorpha litter produced in the invaded woodland decomposed faster than that produced in the grass removal plots due to higher lignin concentrations in the latter than in the former. Area-weighted decomposition was lowest in the grassland due to the absence of native litter inputs. Across all treatments, most litter types immobilized N throughout the incubation, and litter net N loss on an area basis was not different among treatments. Our results support the idea that the effects of a plant species or growth form on decomposition cannot be determined in isolation from the rest of the community or from the direct effects of litter quality and quantity alone. In this dry woodland, exotic grasses significantly altered decomposition processes through indirect effects on the quantity and quality of litter produced by native species.

A Method to Determine the Minimum Number of Litter Traps in Litterfall Studies1

ABSTRACTLitterfall was collected in a 0.64 ha area for three months, using 25 litter traps. Resampling methods were used to calculate the total litterfall mass grand mean and coefficient ofvariation (CV) for each month using different numbers of litter traps (2–25). Based on the variation of those two statistics, it was possible to determine the minimum number of litter traps necessary to achieve accuracy (variation in grand mean) and precision (magnitude of CV)

Belowground biomass, production, and carbon cycling in mature Norway spruce, Maine, U.S.A.

Two 55-year-old stands of Norway spruce (Picea abies (L.) Karst.) were sampled to determine belowground biomass, production and decomposition of fine and small roots, litterfall and litter decay rates, soil respiration rates, and carbon cycling patterns. Mean biomass of live fine (<1 mm) Norway spruce roots to a soil depth of 40 cm was 359 ash-free dry g·m–2, whereas biomass of live fine + small roots ([Formula: see text]3 mm) amounted to 561 g·m–2. Mean root production averaged 297 g·m–2·year–1for roots <1 mm and 418 g·m–2·year–1for roots [Formula: see text]3 mm. By comparison, needle litter production was 205 g·m–2·year–1. Norway spruce roots [Formula: see text]3 mm decomposed at a rate of 9.5% of total root mass per month during the growing season, compared with a needle litter annual decay rate of 22.3%·year–1. A partial carbon budget for Norway spruce indicated that annual needle litterfall mass was 98 g C·m–2·year–1, and annual carbon release from fresh litter decay was approximately 22 g·C·m–2·year–1. During the growing season, monthly CO2flux from soil respiration was 69 g C·m–2·month–1, belowground C allocation to net production of roots [Formula: see text]3 mm was 33.5 g C·m–2·month–1, and C release from decomposition of roots [Formula: see text]3 mm was 31 g C·m–2·month–1. These values were also extrapolated to annual estimates of C fluxes.

The relationship between N, P returned via litter production and nutrient use efficiency of heath and peat swamp forests in Central Kalimantan

The study of nutrient use efficiency in a heath and a peat swamp forest was carried out in Palangkaraya, Central Kalimantan. There was a stronger correlation in the heath forest between nitrogen and phosphorus in above ground litterfall than in the peat swamp forest, and higher positive correlation between litterfall mass and nitrogen return via litterfall in the heath than in the peat swamp forest, compared to phosphorus. The heath forest used nutrient more efficiently than the peat swamp forest and thus was more efficient in retranslocating nutrients. This phenomenon is important for plants to adapt to the heath forest conditions. Both the heath and peat swamp forests in this study were more efficient in their nutrient use than the mixed dipterocarp forests. There was a specific pattern of NUEP (phosphorus used efficiency): the NUEP was higher when P in litterfall was low, suggesting a seasonal effect. Thus in the heath and peat swamp forests, plants use interesting and important nutrient use strategy to adapt to seasonal change.

A Method to Determine the Minimum Number of Litter Traps in Litterfall Studies1

Litterfall was collected in a 0.64 ha area for three months, using 25 litter traps. Resampling methods were used to calculate the total litterfall mass grand mean and coefficient of variation (CV) for each month using different numbers of litter traps (2–25). Based on the variation of those two statistics, it was possible to determine the minimum number of litter traps necessary to achieve accuracy (variation in grand mean) and precision (magnitude of CV).

Forest floor mass and litterfall in Pinus luchuensis plantations with and without broad-leaved trees

Dynamics of forest floor organic matter and litterfall were studied for three years in Luchu pine ( Pinus luchuensis Mayr.) plantations with and without broad-leaved tree species generated naturally in the northern part of Okinawa Island. The mixed stand was dominated by Luchu pine and Schima wallichii Korthals. Annual fine litterfall for the mixed stand averaged 11 Mg ha −1 yr −1 compared with 8 Mg ha −1 yr −1 for the pure pine stand. The present study revealed that there was a strong relationship between the mean litterfall per individual and the mean basal area per individual for dominant and sub-dominant species (r=0.94; p=0.017) . Typhoons have a large influence on litterfall. The mean amount of premature litter were 1.8 and 1.5 Mg ha −1 yr −1 or 17 and 19% of the annual litterfall mass for the mixed and the pure pine stands, respectively. The mean forest floor mass (of material ≤2.0 cm in diameter) was 22% greater in the pure pine stand compared with the mixed stand. A mass balance approach for fine litter decomposition revealed that the residence times of forest floor organic matter were 6 and 8 years for the mixed and pure pine stands, respectively.

Ecosystem structure and productivity of tropical rain forests along altitudinal gradients with contrasting soil phosphorus pools on Mount Kinabalu, Borneo

Summary We measured above‐ground net primary productivity (ANPP) and ecosystem structure and processes in eight rain forest stands at four elevations (700, 1700, 2700 and 3100 m) and on two geological substrates (sedimentary vs. ultrabasic rock) on Mount Kinabalu, Borneo. All ultrabasic sites had smaller pools of total soil phosphorus (P) and of labile inorganic P than did the sedimentary sites at the same altitudes. We predicted that the magnitude of altitudinal changes in ANPP would be less on ultrabasic than on sedimentary substrates, reflecting lower temperature dependency of ANPP under stronger P limitation. Although ANPP declined with increasing altitude on both substrates, the slopes of the two regression lines were similar. The intercept was, however, marginally greater on sedimentary than on ultrabasic substrate. Stand‐level nutrient‐use efficiencies (the ratio of litterfall mass to nutrient return) for N and P were only affected by altitude on ultrabasic substrate where they increased exponentially. Mean foliar N and P contents per unit leaf area of the canopy species increased with altitude on both substrates, but differed between substrates at the same altitude only for P (lower on ultrabasic). Leaf area index (LAI) decreased upslope on both substrates. We assumed that half of primary production was allocated below‐ground in order to evaluate stand level net assimilation rate (NAR). This was nearly constant on sedimentary substrate, but declined linearly with increasing altitude on ultrabasic substrate, where it may have to be added to LAI to explain ANPP patterns. We suggest that on sedimentary substrate trees may be able to maintain NAR under colder environments by increasing foliar N and P per unit leaf area, but P deficiency prevents them from adjusting on ultrabasic substrate.

Edge effects on litterfall mass and nutrient concentrations in forest fragments in central Amazonia

Forest edges bordering on pasture were created by cutting and burning the surrounding Amazonian lowland rain forest in the dry season (June) of 1990. Litterfall was measured for 3.5 y along transects 10, 50, 100 and 250-m into the forest from the forest edge. Litterfall along the 10-m transects increased by up to 2.5 times that on spatial controls (250-m transects) in the dry season in which the edge was created. In the second dry season after edge creation litterfall at 10-m was lower than on controls, after which it returned to control rates in the second wet season, 1.5 y after edge creation. Litterfall 50-m into the forest was less affected; there was a smaller rise in the dry season in which the edge was cut, and no significant effects after that. At 100-m there was no effect of edge creation on litterfall. Phosphorus concentrations in litterfall were elevated along 10-m transects, 10–12 wk after edge creation, possibly because of reduced retranslocation prior to abscission. The changes in litterfall described here are inextricably linked to the biomass collapse, which occurs near forest-fragment edges in the same area.

Biodegradable dissolved organic carbon in forest soil solution and effects of chronic nitrogen deposition

Using a flow-through bioreactor, biodegradable dissolved organic carbon (BDOC) in forest floor solution was determined for a red pine ( Pinus resinosa) plantation and a naturally regenerated mixed hardwood forest (dominant species: Quercus velutina, Q. rubra, Acer rubrum, Fagus grandifolia, Prunus serotina). The forests have received chronic-N fertilization at three different rates (0, 50, 150 kg N ha −1 year −1) since 1988. Both BDOC concentration and %BDOC [% of dissolved organic carbon (DOC)] were significantly higher in the summer (18–20 mg C l −1, ≈30%) than spring or fall for both forest types. The BDOC depletion that was hypothesized to occur with chronic-N application and N saturation was not found in either stand. Instead, for the hardwood stand, BDOC concentration increased with N application, probably due to increased BDOC production in the forest floor with increases in available N. The strong negative relationship between litterfall mass and both BDOC concentration ( r 2=0.36) and %BDOC ( r 2=0.60) for the hardwood stand suggests that the primary source of BDOC in the hardwood forest floor is recent photosynthate allocated to fine roots for their growth, and organic compounds released from them (e.g., exudates and mucilage) rather than the leachate of freshly fallen autumn litter. A significant but weaker positive relationship between BDOC and ambient temperature for both stands suggests that the decomposition of forest floor organic matter is an additional source of BDOC in the forest floor. The production of non-biodegradable DOC (NBDOC) in the forest floor was not sensitive to ambient temperature or the chronic-N treatment. This suggests that an abiotic process, such as chemical equilibration between forest floor and forest floor solution, is responsible for the concentration of non-labile DOC. High %BDOC in throughfall (50–75%) for both stands suggests that throughfall is possibly an additional source of BDOC in the forest floor.

EFFECTS OF FREE-AIR CO2ENRICHMENT (FACE) ON BELOWGROUND PROCESSES IN APINUS TAEDAFOREST

Terrestrial vegetation and soils may act as important carbon sinks if rising atmospheric CO2 stimulates plant production. We used free-air CO2 enrichment (FACE) technology to expose three 30 m diameter plots of a loblolly pine (Pinus taeda) forest to elevated CO2 at 200 μL/L above ambient levels, while three control plots were outfitted with FACE apparatus but were fumigated with ambient air. We quantified litterfall mass and chemistry, fine root biomass increment and turnover, CO2 efflux from soils, δ13C in soil CO2, soil CO2, soil microbial biomass C and N, and potential net N mineralization. After two growing seasons, elevated CO2 caused significant increases in loblolly pine litterfall mass and fine root increment. Within the first year of FACE treatment, the concentration of CO2 in soil had increased, and soil surface CO2 efflux was generally higher at elevated CO2, but this difference was not statistically significant. Loblolly pine litter C:N ratio, fine root turnover, microbial biomass C and N, and potential net N mineralization were not significantly affected by elevated CO2. Our results suggest that elevated atmospheric CO2 may accelerate inputs of organic matter to soil C pools in loblolly pine forests, but it may also accelerate losses of C from belowground by stimulating soil respiration.

Detritus Production and Soil N Transformations in Old-Growth Eastern Hemlock and Sugar Maple Stands

To examine the linkage between forest cover type, litter inputs, and patterns of net N mineralization versus the turnover of N among soil microbes, we measured both the net and gross rates of N mineralization in replicated, adjacent old-growth eastern hemlock [Tsuga canadensis(L.) Carr.] or sugar maple (Acer saccharum Marsh.) stands in upper Michigan. Mean aboveground net primary production and annual litterfall mass were significantly higher (P 0.10) between the two forest types. In situ gross rates of N mineralization were higher (P < 0.06) in the maple forests than in the hemlock forests (7.5 and 6.1 mg N·kg soil-1·d-1 respectively), but in situ net N mineralization varied independently of forest type and stand-level litterfall N concentration. Cover type–dependent differences in detritus production and detritus C quality appear to result in different N turnover rates, but the balance between gross mineralization and immobilization of N is very sensitive to within stand variability and varies at a scale smaller than cover type alone can predict.

Effects of nitrogen and phosphorus fertilization in a lowland evergreen rainforest.

A nutrient addition experiment was set up in August 1993 in a species-rich primary lowland dipterocarp forest in Barito Ulu, Central Kalimantan, Indonesia. The following treatments were applied: control, +N, +P and +NP. There were five blocks of four 50 m x 50 m plots with a separate treatment for each plot. Fine litterfall was measured on all the plots from 1 May 1994 for 12 months. Litterfall mass and phosphorus concentrations were significantly higher in all the fertilizer treatments compared with the controls. All trees (> or = 10 cm dbh) were measured in August 1993 and in August 1998, and there was no significant girth increment response to fertilization in dipterocarps or non-dipterocarps. Dipterocarps of the red meranti group showed a doubling of girth increment in the +NP treatment, however, the difference from the control fell short of significance.

Productivity, nutrient cycling, and succession in single- and mixed-species plantations of Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala in Puerto Rico

Tree growth, biomass productivity, litterfall mass and nutrient content, changes in soil chemical properties and understory forest succession were evaluated over a 8.5-year period in single- and mixed-species (50 : 50) plantations of two N 2-fixing species, Casuarina equisetifolia and Leucaena leucocephala, and a non-fixing species, Eucalyptus robusta. At the optimal harvest age for maximum biomass production (4 years), total aboveground biomass ranged from 63 Mg ha −1 in the Eucalyptus monoculture to 124 Mg ha −1 in the Casuarina/ Leucaena mixture, and was generally greater in the mixed-species than in single-species treatments due to increased productivity of the N-fixing species in the mixed stands. Total litterfall varied from 5.3 to 10.0 Mg ha −1 year −1 among treatments, or between 5.9% and 13.2% of net primary production. Litterfall production and rates of nutrient return for N, P, K, Ca and Mg were generally highest for Leucaena, intermediate for Casuarina and lowest for Eucalyptus. These rates were usually higher in the mixed-species than in monospecific stands due to differences in biomass productivity, but varied considerably depending on their species composition. Total system carbon and nutrient pools (in biomass plus soils to 40-cm depth) for N, P, K, Ca, Mg, Mn at four years were consistently greater in the plantation treatments than in the unplanted control plots. Relative to the single-species plantations, these system pools were generally larger in the mixed-species plantations for C (−10% to +10%), N (+17% to +50%), P (−1% to +63%), K (−19% to +46%), Ca (−10% to +48%), Mg (+5% to +57%) and Mn (+19% to +86%). Whole-tree harvests at four years would result in substantial system carbon and nutrient losses, although these estimated losses would not exceed the estimated gains realized during the four-year period of tree growth at this site. At 7.5 years, soil organic matter and effective cation exchange capacity were reduced in all plantation treatments relative to the control. Changes in soil nutrient content from 0 to 7.5 years were highly variable and not significantly different among treatments, although stands containing Leucaena generally showed higher rates of nitrogen and phosphorus accretion in soils than those with Eucalyptus and/or Casuarina. Natural regeneration of secondary forest tree and shrub species increased over time in all plantation treatments. A total of 24 native or naturalized forest species were recorded in the plantations at 8.5 years. Woody species abundance at this age was significantly greater beneath Casuarina than either Eucalyptus or the Eucalyptus/ Leucaena mixed stands. Species richness and diversity, however, were greatest beneath stands containing Eucalyptus and/or Leucaena than in stands with Casuarina.

Nutrient Sinks in a Constructed Melaleuca Wetland Receiving Secondary Treated Effluent

This study examined N, P and K partitioning in the sinks of a 130 m2 constructed Melaleuca wetland after receiving secondary treated sewage effluent for 21 months. The sinks examined were: 1) biomass, which was further partitioned into the harvestable above ground portion and the roots; 2) sediment; 3) gravel and; 4) the clay base. Gravel was the major nutrient storage sink, however this was a function of the high gravel particle mass (525 kg m−2) rather than high nutrient concentrations. M. alternifolia trees had the highest biomass due to high growth rates, higher planting densities, and low litterfall. M. quinquenervia trees were severely attacked by a sap sucking Heteropteron (Eucerocoris. suspectus) which stunted growth, but resulted in a cumulative litterfall mass three times that of the M. alternifolia trees. The sediment sink was strongly influenced by litterfall, with the sediment sink in the M. quinquenervia terraces storing more than twice the nutrients in the sediment sink of the M. alternifolia terraces. Because of their higher grwoth rates and above ground biomass fraction, and their potential to produce tea tree oil, M. alternifolia is most suitable for constructed wetlands incorporating a harvesting regime. Because of their high rate of transfer from the biomass to sediment sink via litterfall, M. quinquenervia is more suited to non-harvested constructed wetlands.

Litterfall Mass, Chemistry, and Nutrient Retranslocation in a Monodominant Forest on Maracá Island, Roraima, Brazil1

ABSTRACTMaracá Island is on the boundary of the Amazon rain forest and the Rio Branco‐Rupunini savanna. The annual rainfall is ca. 1800 mm, with a dry season from October‐March and a wet season from April‐September. A forest type dominated by Peltogyne gracilipes (Caesalpiniaceae) occurs on parts of the island and the adjacent mainland, the least species‐rich (for trees &gt; 10 cm DBH) of any recorded in Brazilian Amazonia. The goal of this study was to test the hypothesis that the dominance by Peltogyne is associated with an unusual litterfall mass, nutrient concentrations, or retranslocation. Three plots (50 × 50 m) were set up in each of three forest types: (1) Peltogyne‐rich forest (PRFa), dominated by Peltogyne gracilipes; (2) Peltogyne‐poor forest (PPF), with a sparse occurrence of Peltogyne; and (3) forest without Peltogyne (FWPa). Litterfall was collected for 360 d at ca 15‐d intervals in 11 randomly positioned traps (0.32 m2) per plot. Mature Peltogyne leaves were collected at random from three randomly selected understory Peltogyne trees in each of the three PRFa plots during the early dry and early wet season. Additionally, young Peltogyne leaves were collected during the early wet season. Chemical analyses were made on bulked (on a 2‐mo basis) litterfall samples and young and mature Peltogyne leaves. The estimated annual litterfall was similar among PRFa (7.9 t/ha), PPF (9.1 t/ha) and FWPa (8.6 t/ha); however, PRFa was distinguished by its litterfall seasonality, because of the deciduous Peltogyne, and its higher concentrations of Ca (9.3 compared to 6.5 mg/g in FWPa) and Mg (3.2 compared to 1.9mg/g in FWPa). About 40–50 percent of N and P, 25–41 percenr of K, and 2–13 percent of Mg were rerranslocated from Peltogyne leaves.

N:P balance in wetland forests: Productivity across a biogeochemical continuum

The nature of and driving forces behind variation among wetland forests in terms of biogeochemistry and vegetation production are not well understood. We suggest that insight into biogeochemical and productivity differences may be gained by examining the degree to which nitrogen and phosphorus are balanced within wetland vegetation. On the basis of examinations of data related to N:P balance and nutrient use efficiencies, vegetation productivity in both depressional and riverine forests appears to be primarily N limited. In contrast to some current theories of wetland biogeochemistry, these data suggest that when P deficiency occurs at all, it represents a secondary productivity constraint in comparison to N. Similarly, a biogeochemical continuum is suggested for wetland forests based on the relationship between N:P ratios in senesced foliage vs. annual litterfall mass. We theorize that the position of a particular wetland forest on this continuum reflects the integration of its geomorphic position and biogeochemical history. In addition, the position of a particular system on the continuum may have predictive value with regard to net primary productivity and nutrient transformation capabilities.