Rebecca Ostertag

Neotropical secondary forest succession: changes in structural and functional characteristics

Abstract In this review, we highlight the main biotic and abiotic factors that influence the patterns of Neotropical secondary forest successions, referred as the woody vegetation that regrows after complete forest clearance due to human activities. We focus on both patterns of species replacement and various processes that occur during succession, and suggest that the sequence of processes may be predictable even if species composition is not. We describe forest recovery with respect to old-growth conditions, which we define as the structure, function, and composition of the original forest before conversion, and we examine this recovery process within the context of type and intensity of past land use. The various phases of the recovery process are described in detail: from factors affecting early colonization, changes in light and soil properties, soil–vegetation feedbacks at initial and later successional stages, biomass accumulation, forest productivity, rates of species accumulation, and species composition. The consensus of these analyses is that the regenerative power of Neotropical forest vegetation is high, if propagule sources are close by and land use intensity before abandonment has not been severe. Nevertheless, the recovery of biophysical properties and vegetation is heavily dependent on the interactions between site-specific factors and land use, which makes it extremely difficult to predict successional trajectories in anthropogenic settings. We attempt, throughout this review, to integrate the structural and functional characteristics of secondary succession as a way to enhance our ability both to predict and manage successional forest ecosystems due to their increasing importance as timber sources, providers of environmental services, and templates for restoration purposes.

Early stages of root and leaf decomposition in Hawaiian forests : effects of nutrient availability

Abstract We examined the effects of soil nutrient availability and tissue chemistry on decomposition of both fine roots (<2 mm diameter) and leaves in three sites along a forest chronosequence in the Hawaiian Islands. These sites form a natural fertility gradient, with the youngest and oldest sites having lower nutrient availability than the intermediate-aged site. Nitrogen (N) limits aboveground net primary productivity (ANPP) in the youngest site, while phosphorus (P) limits ANPP in the oldest site. Both root and leaf litter decomposed most slowly in the 4.1-Myear-old site. We also investigated root decomposition in fertilized plots at the youngest and oldest sites; when roots were produced and decomposed in fertilized plots, root decomposition rates increased with N and P additions at the 4.1-Myear-old site. At the 300-year-old site, however, root decomposition rates did not respond to N or P additions. Roots decomposed faster than leaves at the more infertile sites, in part because of lower lignin-to-nitrogen ratios in roots than in leaf litter. Decomposing roots immobilized more nutrients than did decomposing leaves, and may serve an important role in retaining nutrients in these forests.

CARBON SEQUESTRATION AND PLANT COMMUNITY DYNAMICS FOLLOWING REFORESTATION OF TROPICAL PASTURE

Conversion of abandoned cattle pastures to secondary forests and plantations in the tropics has been proposed as a means to increase rates of carbon (C) sequestration from the atmosphere and enhance local biodiversity. We used a long-term tropical refor- estation project (55-61 yr) to estimate rates of above- and belowground C sequestration and to investigate the impact of planted species on overall plant community structure. Thirteen tree species (nine native and four nonnative species) were planted as part of the reforestation effort in the mid to late 1930s. In 1992, there were 75 tree species ( .9.1 cm dbh) in the forest. Overall, planted species accounted for 40% of the importance value of the forest; planted nonnative species contributed only 5% of the importance value. In the reforested ecosystem, the total soil C pool (0-60 cm depth) was larger than the aboveground C pool, and there was more soil C in the forest (102 6 10 Mg/ha (mean 6 1 SE)) than in an adjacent pasture of similar age (69 6 16 Mg/ha). Forest soil C (C3-C) increased at a rate of ;0.9 Mg·ha 21 ·yr 21 , but residual pasture C (C4-C) was lost at a rate of 0.4 Mg·ha 21 ·yr 21 , yielding a net gain of 33 Mg/ha as a result of 61 years of forest regrowth. Aboveground C accumulated at a rate of 1.4 6 0.05 Mg·ha 21 ·yr 21 , to a total of 80 6 3 Mg/ha. A survey of 426 merchantable trees in 1959 and 1992 showed that they grew faster in the second 33 years of forest development than in the first 22 years, indicating that later stages of forest development can play an important role in C sequestration. Few indices of C cycling were correlated with plant community composition or structure. Our results indicate that significant soil C can accumulate with reforestation and that there are strong legacies of pasture use and reforestation in plant community structure and rates of plant C sequestration.

Forest Floor Decomposition Following Hurricane Litter Inputs in Several Puerto Rican Forests

Hurricanes affect ecosystem processes by altering resource availability and heterogeneity, but the spatial and temporal signatures of these events on biomass and nutrient cycling processes are not well understood. We examined mass and nutrient inputs of hurricane-derived litter in six tropical forests spanning three life zones in northeastern Puerto Rico after the passage of Hurricane Georges. We then followed the decomposition of forest floor mass and nutrient dynamics over 1 year in the three forests that experienced the greatest litter inputs (moist, tabonuco, and palm forests) to assess the length of time for which litter inputs influence regeneration and nutrient cycling processes. The 36-h disturbance event had litterfall rates that ranged from 0.55 to 0.93 times annual rates among the six forests; forest floor ranged between 1.2 and 2.5 times prehurricane standing stocks. The upperelevation forest sites had the lowest nonhurricane litterfall rates and experienced the lowest hurricane litterfall and the smallest relative increase in forest floor standing stocks. In the three intensively studied forests, the forest floor returned to prehurricane values very quickly, within 2‐10 months. The palm forest had the slowest rate of decay (k 0.74 0.16 y ‐1 ), whereas the tabonuco forest and the moist forest had similar decay rates (1.04 0.12 and 1.09 0.14, respectively). In the moist forest, there were short-term increases in the concentrations of nitrogen (N), phosphorus (P), calcium (Ca), and magnesium (Mg) in litter, but in the other two forests nutrient concentrations generally decreased. The rapid disappearance of the hurricane inputs suggests that such pulses are quickly incorporated into nutrient cycles and may be one reason for the extraordinary resilience of these forests to wind disturbances.

BELOWGROUND EFFECTS OF CANOPY GAPS IN A TROPICAL WET FOREST

Canopy gaps are a fairly well-studied phenomenon in tropical forests, but less well known are their belowground effects on patterns of root length and biomass and the consequences of such changes on regeneration and community structure. I examined canopy gaps and adjacent understory sites on two soil types of contrasting fertility in a lowland rain forest in Costa Rica to determine (1) whether canopy gaps create root gaps beneath the canopy opening, and (2) whether the effects of such root gaps on root ingrowth rates, root competition, and root responses to nutrient heterogeneity are related to soil type. Based on soil coring, canopy gaps had less fine (<2 mm) root length and biomass than comparable closed canopy sites. Reductions in root length and biomass were greater on the infertile (residual) soil type than the fertile (alluvial) type but were unrelated to gap size, age, or percentage canopy openness. Additionally, I measured root competition in a trenching experiment using the pioneer tree Hampea appendiculata as a bioassay species. Light appeared to be the only factor limiting relative growth rate, as seedlings grew faster in the canopy gaps than in the understory, regardless of soil type or trenching treatment (trench lined with root restriction cloth, trench open to colonization by neighboring roots, or no trench). The accumulation rate of live fine roots was unaffected by soil or light conditions. Finally, I altered nutrient heterogeneity of the gap and understory sites through the creation of fertilized microsites. Root proliferation into these microsites was enhanced only in the canopy gaps on the infertile soil. The differences between the two soil types in the amount of root length and biomass and in root proliferation responses suggest that the consequences of canopy gap formation may be dependent on background levels of soil fertility.

LONG-TERM PATTERNS IN TROPICAL REFORESTATION: PLANT COMMUNITY COMPOSITION AND ABOVEGROUND BIOMASS ACCUMULATION

Primary tropical forests are renowned for their high biodiversity and carbon storage, and considerable research has documented both species and carbon losses with deforestation and agricultural land uses. Economic drivers are now leading to the abandonment of agricultural lands, and the area in secondary forests is increasing. We know little about how long it takes for these ecosystems to achieve the structural and compositional characteristics of primary forests. In this study, we examine changes in plant species composition and aboveground biomass during eight decades of tropical secondary succession in Puerto Rico, and compare these patterns with primary forests. Using a well-replicated chronosequence approach, we sampled primary forests and secondary forests established 10, 20, 30, 60, and 80 years ago on abandoned pastures. Tree species composition in all secondary forests was different from that of primary forests and could be divided into early (10-, 20-, and 30-year) vs. late (60- and 80-year) successional phases. The highest rates of aboveground biomass accumulation occurred in the first 20 years, with rates of C sequestration peaking at 6.7 +/- 0.5 Mg C x ha(-1) x yr(-1). Reforestation of pastures resulted in an accumulation of 125 Mg C/ha in aboveground standing live biomass over 80 years. The 80 year-old secondary forests had greater biomass than the primary forests, due to the replacement of woody species by palms in the primary forests. Our results show that these new ecosystems have different species composition, but similar species richness, and significant potential for carbon sequestration, compared to remnant primary forests.

Foliar nitrogen and phosphorus accumulation responses after fertilization: an example from nutrient-limited Hawaiian forests

How plants respond to long-term nutrient enrichment can provide insights into physiological and evolutionary constraints in various ecosystems. The present study examined foliar concentrations after fertilization—to determine if nutrient accumulation responses of the most abundant species in a plant community reflect differences in N and P uptake and storage. Using a chronosequence in the Hawaiian Islands that differs in N and P availability, it was shown that after fertilization, plants increase foliar P to a much greater degree than foliar N, as indicated by response ratios. In addition, foliar P responses after fertilization were more variable and largely driving the observed changes in N:P values. Across species, both inorganic and organic P increased but neither form of N increased significantly. This pattern of P accumulation was consistent across 13 species of varying life forms and occurred at both the N-limited and P-limited site, although its magnitude was larger at the P-limited site. Foliar P accumulation after nutrient enrichment may indicate nutrient storage and may have evolved to be a general strategy to deal with uncertainties in P availability. Storage of P complicates interpretations of N:P values and the determination of nutrient limitation.

Litterfall and Decomposition in Relation to Soil Carbon Pools Along a Secondary Forest Chronosequence in Puerto Rico

Secondary forests are becoming increasingly widespread in the tropics, but our understanding of how secondary succession affects carbon (C) cycling and C sequestration in these ecosystems is limited. We used a well-replicated 80-year pasture to forest successional chronosequence and primary forest in Puerto Rico to explore the relationships among litterfall, litter quality, decomposition, and soil C pools. Litterfall rates recovered rapidly during early secondary succession and averaged 10.5 (± 0.1 SE) Mg/ha/y among all sites over a 2-year period. Although forest plant community composition and plant life form dominance changed during succession, litter chemistry as evaluated by sequential C fractions and by 13C-nuclear magnetic resonance spectroscopy did not change significantly with forest age, nor did leaf decomposition rates. Root decomposition was slower than leaves and was fastest in the 60-year-old sites and slowest in the 10- and 30-year-old sites. Common litter and common site experiments suggested that site conditions were more important controls than litter quality in this chronosequence. Bulk soil C content was positively correlated with hydrophobic leaf compounds, suggesting that there is greater soil C accumulation if leaf litter contains more tannins and waxy compounds relative to more labile compounds. Our results suggest that most key C fluxes associated with litter production and decomposition re-establish rapidly—within a decade or two—during tropical secondary succession. Therefore, recovery of leaf litter C cycling processes after pasture use are faster than aboveground woody biomass and species accumulation, indicating that these young secondary forests have the potential to recover litter cycling functions and provide some of the same ecosystem services of primary forests.

Fertilization with nitrogen and phosphorus increases abundance of non-native species in Hawaiian montane forests

Weexamined the effects of fertilization on the diversity, abundance, and cover ofthe understory plant community of two montane wet forests in Hawaii. One siteoccupies a young substrate, where aboveground tree growth is limited bynitrogen(N), while the other site is on an older substrate, where aboveground treegrowth is limited by phosphorus (P). Both sites contained an on-going,long-termfactorial fertilization experiment in which plots were fertilized semi-annuallywith N, P, or N and P in combination. In each fertilization treatment, wemeasured density of species ≥ 0.5 m tall and percent cover ofspecies < 0.5 m tall. Fertilization with N reducedspeciesrichness at the young, N-limited site, but none of the nutrient additionsaltered species richness at the older, P-limited site. Species diversity andevenness were not affected by fertilization at either site. At the site withlowN availability, plots fertilized with NP had higher densities of the non-nativeginger Hedychium gardnerianum, and at the site with lowP-availability, densities of the exotic shrub Rubusargutuswere higher in P- and NP-fertilized plots. Other effects included declines inmoss cover with fertilization at both sites, and reduced abundance of nativeseedlings in response to N and NP addition at the N-limited site. Continuedlong-term fertilization could lead to greater dominance of non-native speciesbyencouraging their growth at the expense of native species, which may sufferdecreased recruitment as fertilization and increased abundance of thenon-nativespecies may reduce suitable substrates for seedling establishment.

Trends in Above and Belowground Carbon with Forest Regrowth After Agricultural Abandonment in the Neotropics

Increasing forest cover on lands which were recently forested (reforestation), as well as on lands which have not supported forest growth in recent times (afforestation), has been proposed by the Intergovernmental Panel on Climate Change (IPCC) to help mitigate anthropogenic C emissions from land-use change and fossil fuel use (Brown et al. 1995a, Watson et al. 2000, Metz et al. 2001). Recent research has suggested that these strategies would be most effective in the tropical latitudes (Gibbard et al. 2005). Tropical forests have higher potential carbon (C) uptake rates than forests in temperate or boreal biomes (Brown et al. 1995a, Watson et al. 2000). Globally, 40% of terrestrial biomass C is in tropical forests (Dixon et al. 1994), and 40% of this is in secondary forests (Brown and Lugo 1990). In addition to providing opportunities for C sequestration, reforestation can lead to the recovery of important forest ecosystem goods and services. These include, but are not limited to, watershed protection, erosion control, regional climate stabilization, wood and nontimber products, and habitat for biodiversity (Brown and Lugo 1990, Guariguata and Ostertag 2001, Naughton-Treves and Chapman 2002, De Walt et al. 2003). Although deforestation is still a dominant trend across the tropics, secondary forests resulting from human disturbance are becoming an increasingly important forest cover type (Brown and Lugo 1990). Just as rates of deforestation are difficult to establish with certainty (see Achard et al. 2002, DeFries et al. 2002, Eva et al. 2003, Fearnside and Laurance 2003), estimates for rates of secondary forest growth also differ, although most agree that the trend is positive. In the 1980s, tropical secondary forests were estimated to cover more than 600 million hectares (ha) globally with an annual rate of formation of 9 million hectares per year (ha/yr), and growing (Brown and Lugo 1990). In 1993, the estimated area of tropical America covered by secondary forests was 165 million ha (Weaver 1995 in Kammesheidt 2002). The United Nations Food and Agriculture Organization in 1990 classified 33 million ha of previously agricultural or pasture lands in Latin America as fallow (cited in Kammesheidt 2002). These fallow lands are key components of forest regrowth