Francis E. Putz

The Natural History of Lianas on Barro Colorado Island, Panama

Liana (woody vine) abundance, height, diameter, and climbing mode were studied in the mature tropical moist forest on Barro Colorado Island, Panama (BCI). Because lianas are capable of extended horizontal as well as vertical growth, sample plots were 100—m2 cylinders extending from the ground up to the treetops. The plots were randomly located in areas representing different stages of regeneration following treefalls. In order to examine canopy lianas closely, trees in or adjacent to the sample plots were climbed freehand or with the aid of mechanical rope ascenders. Lianas are abundant on BCI and play important roles in forest dynamics. A hectare of old—growth forest had 1597 climbing lianas distributed among 43% of the canopy trees. Trees with a least one liana has higher than random probability of having more than one liana, and individual lianas connected an average of 1.56 canopy trees. In the understory, 22% of the upright plants <2 m tall were lianas, and, depending on the species, between 15 and 90% of these plants were vegetative offshoots (ramets) and not true seedlings (genets). Lianas were most abundant in recent treefall gaps and decreased in abundance with time since last disturbance. Trellis availability was found to be a major factor limiting liana access to the forest canopy. Experimental manipulation of supports and experimental planting of Dioclea reflexa seedlings revealed that trellises consisting of small diameter, closely spaced supports are most abundant on the edges of treefall gaps. Tree and liana stems on the edges of treefall gaps provided a major pathway to the canopy for climbing plants. Trees carrying lianas suffered higher mortality rates and upon falling caused more other trees to fall than did liana—free trees. Few lianas died when their host tree fell, and many grew back to the canopy using the abundant trellises that occur on the edges of treefall gaps. Tree sapling growth rates in treefall gaps and Luehea seemannii growth rates in the canopy were slower where lianas were abundant.

Treefall Pits and Mounds, Buried Seeds, and the Importance of Soil Disturbance to Pioneer Trees on Barro Colorado Island, Panama

In mature forest on Barro Colorado Island (BCI), Republic of Panama, 0.09% of the ground surface is covered by pits and mounds created by uprooted trees as compared to 14-60% of the ground surface in temperate forests. Newly formed treefall pits on BCI rapidly filled with soil at an average rate of 8 cm/yr. Although pits and mounds rapidly disappeared on BCI, pioneer (gap phase or forest nomad) trees were more concentrated on mineral soil associated with freshly uprooted trees than elsewhere in treefall gaps. There was a large population (742 seeds/M2) of viable buried seeds, primarily of common pioneer tree species, in the top 10 cm of soil in closed forest; these seeds may give rise to many of the pioneers colonizing the disturbed soil. Large treefall gaps are more frequently colonized by pioneer trees than are small treefall gaps, partially because large gaps are more often caused by uprooted, thus soil-disturbing, trees.

The dynamics of tree populations in tropical forest: a review

Published work on the dynamics of forest tree recruitment, growth and mortality in natural tropical forest is reviewed. In most forests studied, annual mortality is between 1% and 2% and is independent of size class in trees >10 cm dbh; mortality is negatively correlated with growth rate and crown illumination; growth rate is highly variable between individual trees, but shows strong autocorrelation between successive measurements on the same tree. Differences in the rate of dynamic processes can be detected between some species at a site, but data are presently insufficient to determine whether these differences are preserved at other sites where the species occur. None of the studies discussed are of sufficient duration to permit us to draw any conclusions about the equilibrium or non-equilibrium of floristic composition.

Abrupt increases in Amazonian tree mortality due to drought-fire interactions

Significance Climate change alone is unlikely to drive severe tropical forest degradation in the next few decades, but an alternative process associated with severe weather and forest fires is already operating in southeastern Amazonia. Recent droughts caused greatly elevated fire-induced tree mortality in a fire experiment and widespread regional forest fires that burned 5–12% of southeastern Amazon forests. These results suggest that feedbacks between fires and extreme climatic conditions could increase the likelihood of an Amazon forest “dieback” in the near-term. To secure the integrity of seasonally dry Amazon forests, efforts to end deforestation must be accompanied by initiatives that reduce the accidental spread of land management fires into neighboring forest reserves and effectively suppress forest fires when they start. Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW⋅m−1). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change.

Sustainable forestry in the tropics: panacea or folly?

The profitability of uncontrolled logging can be a significant obstacle to sustainable forest management, especially in the tropics. Rice et al. (1997) have argued that not only does traditional selective logging provide higher returns but also incurs less damage to forests than sustainable forest management systems that involve harvesting of many species and the creation of large gaps in the forest canopy to foster regeneration of light-demanding species. They claimed that protected areas were the only viable way to conserve forest ecosystems and proposed that loggers be allowed to log forests selectively once, after which the forests should become parks. Here we respond to the challenge posed by Rice et al. by exhaustively reviewing the evidence regarding the viability and desirability of sustainable forest management in the tropics. Following Rice et al., we use the term conventional timber harvesting to refer to existing practice, which typically pays little attention to maintaining long-term timber supply. Sustainable timber management implies taking steps to ensure forests continue to produce timber in the longer term, while maintaining the full complement of environmental services and non-timber products of the forest. Empirical studies tend to confirm the conclusion of Rice et al. (1997) that although sustainable timber management sometimes provides reasonable rates of return, conventional timber harvesting is generally more profitable. This implies that without additional incentives, one cannot expect companies to adopt sustainable management. The shortsightedness of many loggers, the slow rise in international timber prices, political uncertainty, and tenure insecurity simply reinforce this tendency. However, we reject the claim that sustainable timber management generally damages forests more than conventional logging. Rice et al. base their conclusion largely on the particular case of mahogany extraction in Bolivia, and even there it may not hold. In many cases, sustainable timber management performs better in terms of carbon storage and biodiversity conservation than conventional logging approaches, as well as producing more timber. If new carbon markets emerge, sustainable forest management might compete effectively with conventional timber harvesting. Timber certification systems may also provide a sufficient incentive for sustainable forest management in certain circumstances.

Tree growth, dynamics, and productivity in a mature mangrove forest in Malaysia

Abstract Growth of selected Rhizophora apiculata (Rhizophoraceae) trees was monitored from 1920 through 1981 in a 0.16 ha plot of protected forest in the Matang Mangroves. Starting in 1950, the sample was increased to include monitoring the growth of all the trees more than 10 cm dbh (diameter at 1.3 m or above prop roots). All seedlings were censused by species and removed in 1920 and recensused in 1926, 1927, and 1981. Total above-ground dry weight (biomass) of the forest was estimated using stand tables and a regression equation of biomass on dbh calculated for destructively sampled R. apiculata trees from elsewhere in the Matang Mangroves. Net primary productivity (1950–1981) was calculated from estimated biomass increments and published litter-fall rates. Rhizophora apiculata has maintained its dominance of the plot since 1920 but Bruguiera gymnorrhiza (Rhizophoraceae) and several other more shade-tolerant species have steadily increased in abundance. Between the 1920s and 1981, R. apiculata declined in relative abundance in the seedling layer while B. parviflora and B. cylindrica increased. Mean mortality rate (1950–1981) for trees more than 10 cm dbh was 3.0% per year with a range of 1.3–5.4% per year. When trees fell over and hit other trees, the damaged trees usually died within 10 years. A major cause of mortality appeared to be sapwood-eating termites. Net primary productivity averaged 17.7 t/ha/year over the 1950–1981 observation period. Biomass ranged from 270 to 460 t/ha with a mean of 409 t/ha. It is suggested that Rhizophora spp. trees greater than 50 cm dbh and mangrove forests with total above-ground biomass exceeding 300 t/ha would develop in other areas outside of the region affected by hurricanes if the forest was protected from human disturbance.

A place for alien species in ecosystem restoration

Blanket condemnation of alien species in restoration efforts is counterproductive. Where their presence does not unduly threaten surrounding ecosystems, alien species can be tolerated or even used to good advantage, if they provide essential ecological or socioeconomic services. By speeding restoration or making it more effective, non-native species can provide economic and ecological payoffs. Risk is always an issue when alien species are involved, but greater risk taking is warranted where environmental conditions have been severely modified through human activity than where reassembly of a biological community is the sole goal of restoration.

Effect of disturbance intensity on regeneration mechanisms in a tropical dry forest

We examined the effect of disturbances of varying intensity on the dominant modes of regeneration among woody plants in tropical dry forest in lowland Bolivia. Seed survival and density, mortality, height, crown area, and basal diameters of seedlings and sprouts were compared among four treatments of varying disturbance intensity (high-intensity burn, low-intensity burn, plant removal, and harvesting gap) over a period of 18 months following treatments. High- and low-intensity burns reduced densities of viable seed by an average of 94 and 50%, respectively. Tree seedlings were more abundant than tree sprouts in all treatments. There were few differences in seedling density among treatments. Sprouts were most common in the plant removal and low-intensity burn treatments than in harvesting gap and high-intensity burn treatments. Seedling mortality was higher than sprout mortality during the first year following treatments. Sprouts were taller, had more stems per individual, larger crown areas, and larger basal diameters than seedlings. Origin of sprout differed among treatments. Eighteen months following treatments, 85% of individuals >2.5 m tall were sprouts. Most seedlings >2.5 m tall after 18 months had established in high-intensity burn treatments. Sprouting individuals dominated regeneration after all treatments, however, in high-intensity burn treatments, sprouts were relatively less dominant due to smaller sprouts and larger seedlings after high-intensity burns.

Sprouting of Broken Trees on Barro Colorado Island, Panama

The perception of mature forests as homogeneous stands of large trees has given way to the more dynamic idea that, due to natural disturbances, forests consist of mosaics of areas in different stages of regeneration (for recent reviews see Brokaw 1985a, Denslow 1987). Much attention has focused on canopy gaps formed by the death of large trees and the response of previously suppressed plants and buried seeds to canopy opening. Although several researchers have recognized the importance of vegetative recovery of seedlings damaged when gaps form (e.g., Spurr 1956, Nicholson 1965, Platt and Hermann 1986), sprouting of large broken trees in natural forests has not been well studied. The goal of the research reported here was to assess the ecological importance of sprouting of naturally broken trees > 10 cm dbh (stem diameter at 1.4 m) in gapphase regeneration of a moist tropical forest in Panama. Sprouting is a well-recognized mode of regeneration where trees are cut down and where their above-ground portions are killed by fire (for a review see Blake 1983). Stump sprouts contribute greatly to vegetative regrowth in logged temperate (e.g., Zahner et al. 1985) and tropical forests (e.g., Webb et al. 1972, Knight 1975, Uhl et al. 1981). Management of coppice stands for firewood and small dimension timber probably represents the most ancient form of silviculture (Smith 1986). Stems of sprout origin make use of at least part of the root system of the original tree and thus are capable of extremely rapid growth rates (e.g., Wilson 1968). Due to frequent incidence of heart rots (e.g., Leffelman and Hawley 1925, Roth and Hepting 1943, but see Zahner et al. 1985), coppiced stands are generally managed on a short rotation basis for poles, firewood, and charcoal (e.g., Troup 1928, Smith 1986). In a study of treefalls on Barro Colorado Island, Panama, Putz et al. (1983) reported that of trees suffering complete stem breakage between 1976 and 1980, approximately half sprouted. Trees smaller than the median size (dbh = 22 cm) had a higher propensity to sprout than larger trees. They also observed that the height of stem breakage was fairly evenly distributed

SEA‐LEVEL RISE AND COASTAL FOREST RETREAT ON THE WEST COAST OF FLORIDA, USA

We investigated patterns, rates, and mechanisms of forest replacement by salt marsh in relation to sea-level rise on the west coast of Florida, USA. The geomorphology of this region typifies that of low-lying, limestone coastlines considered highly susceptible to sea-level rise (e.g., much of the eastern Gulf of Mexico, the Yucatan Peninsula, and low-lying limestone islands throughout the world). This coast is microtidal, shallowly sloping, and has a rate of relative sea-level rise similar to that of eustatic rise. To determine patterns of forest change in relation to sea-level rise, we examined patterns of tree species zonation, tree recruitment, and tree mortality in relation to site elevation and tidal-flooding frequency. To reconstruct histories of forest change in relation to sea-level rise, we estimated age distributions of Sabal palmetto, the most widely distributed tree species at our site, relating age structures of stands to reconstructed histories of tidal flooding in the stands. Finally, to assess the relative roles of flooding stress (hypoxia), salt exposure, and competition from encroaching salt-marsh vegetation in the decline of forest stands, we examined patterns of soil redox potential, groundwater salinity, and density of halophytic vegetation among stands in different stages of decline. Zonation among tree species was related to tidal-flooding frequency. For most trees, seedlings were absent from the most frequently flooded stands in which the species occurred. Reconstructed flooding histories of stands and age estimates for S. palmetto suggest that many decades elapse between cessation of regeneration and local elimination of a tree species. Even during the relatively short duration of the study (4 yr), however, composition of some stands changed in the direction predicted from species zonation and sea-level rise. Forest understory replacement by halophytic vegetation appeared to follow, rather than cause, failure of tree regeneration. Tidal flooding rarely produced severe reducing conditions in soil, but groundwater salinity was correlated with tidal-flooding frequency. Forest retreat in this system, therefore, involves the development of relict (non-regenerating) stands of different tree species at different flooding frequencies. Exposure to salt appears to be the major cause of tree regeneration failure, with flooding stress and interference from marsh vegetation playing minor or negligible roles. These interactions differ somewhat from those on deltaic coasts or coasts with high freshwater outflows, where flooding stress may play a larger role in regeneration failure, and from sandy coasts, where erosion may play a larger role in forest retreat. Regardless of the cause of tree regeneration failure, the development of relict stands may be a general forest response to sea-level rise.