Thermophilization – changes in community composition towards greater relative abundances of species associated with warmer environments – has been described for plants and animals in many locations ...around the world. Disturbances of various kinds have increased rates of thermophilization in temperate sites, and this has been proposed, but not demonstrated, for some tropical environments. In this study, we tested whether disturbance by a category 4 hurricane in 1988 (Hurricane Gilbert) increased thermophilization in a Jamaican montane forest by using pre‐ and post‐hurricane data collected over four decades (1974–2014). We analysed tree species composition in permanent plots at ca 1580 m above sea level in Jamaica's Blue Mountains. There were 66 tree species with stem diameters ≥ 3 cm at breast height. We used published data on the altitudinal distribution of 62 species (94% of genetic individuals (genets)) to calculate the mean community altitude scores (MCAS) of the trees recorded in each census, as well as the MCAS of the survivors, recruits and dead trees after each decade. We found that thermophilization did occur (i.e. MCAS decreased significantly over time), and that this was due both to a decreasing MCAS of recruits through the four decades (significantly lower than expected in the last three decades) as well as a high MCAS of trees that died. Thermophilization was fastest in the post‐hurricane decade, during which time there was marked and significant increase in the MCAS of dead trees; this change was above and beyond expectations of long‐term successional dynamics. The rate of compositional change equates to an overall decrease in MCAS of 1.6 m yr−1 over the forty‐year study period. We conclude that this Jamaican montane forest is undergoing thermophilization (likely due to rising temperature) and that the hurricane‐caused disturbance accelerated thermophilization through differential mortality.
Conservation of tropical forest biodiversity increasingly depends on its recovery following severe human disturbance. Our ability to measure recovery using current similarity indices suffers from two ...limitations: different sized individuals are treated as equal, and the indices are proportionate (a community with twice the individuals of every species as compared with the reference community would be assessed as identical). We define an alternative recovery index for trees - the Tanner index, as the mean of the quantitative Bray-Curtis similarity indices of species composition for stem density and for basal area. We used the new index to compare the original (pre-gap) and post-gap composition of five experimental gap plots (each 90-100 m(2)) and four control plots over 24-35 years in the Blue Mountains of Jamaica. After 24-35 years, these small gaps surrounded by undisturbed forest had recovered 68% of the sum of per species stem density and 29% of the sum of per species basal area, a recovery index of 47%. Four endemic species were especially reduced in density and basal area. With the incorporation of basal area and stem density, our index reduces over-estimations of forest recovery obtained using existing similarity indices (by 24%-41%), and thus yields more accurate estimates of forest conservation status. Finally, our study indicates that the two kinds of comparisons: 1) over time between pre-gap and post-gap composition and 2) over space between gap plots and spatial controls (space-for-time substitution) yield broadly similar results, which supports the value of using space-for-time substitutions in studying forest recovery, at least in this tropical montane forest.
The majority of above‐ground carbon in tropical forests is stored in wood, which is returned to the atmosphere during decomposition of coarse woody debris. However, the factors controlling wood ...decomposition have not been experimentally manipulated over time scales comparable to the length of this process.
We hypothesized that wood decomposition is limited by nutrient availability and tested this hypothesis in a long‐term litter addition and removal experiment in a lowland tropical forest in Panama. Specifically, we quantified decomposition using a 15‐year chronosequence of decaying boles, and measured respiration rates and nutrient limitation of wood decomposer communities.
The long‐term probability that a dead tree completely decomposed was decreased in plots where litter was removed, but did not differ between litter addition and control treatments. Similarly, respiration rates of wood decomposer communities were greater in control treatments relative to litter removal plots; litter addition treatments did not differ from either of the other treatments. Respiration rates increased in response to nutrient addition (nitrogen, phosphorus, and potassium) in the litter removal and addition treatments, but not in the controls.
Established decreases in concentrations of soil nutrients in litter removal plots and increased respiration rates in response to nutrient addition suggest that reduced rates of wood decomposition after litter removal were caused by decreased nutrient availability. The effects of litter manipulations differed directionally from a previous short‐term decomposition study in the same plots, and reduced rates of bole decomposition in litter removal plots did not emerge until after more than 6 years of decomposition. These differences suggest that litter‐mediated effects on nutrient dynamics have complex interactions with decomposition over time.
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Arbuscular mycorrhizal fungi (AMF) are widespread in tropical forests and represent a major sink of photosynthate, yet their contribution to soil respiration in such ecosystems remains unknown. Using ...in-growth mesocosms we measured AMF mycelial respiration in two separate experiments: (1) an experiment in a semi-evergreen moist tropical forest, and (2) an experiment with 6-m-tall Pseudobombax septenatum in 4.5-m³ containers, for which we also determined the dependence of AMF mycelial respiration on the supply of carbon from the plant using girdling and root-cutting treatments. In the forest, AMF mycelia respired carbon at a rate of 1.4 t ha⁻¹ yr⁻¹, which accounted for 14 ± 6% of total soil respiration and 26 ± 12% of root-derived respiration. For P. septenatum, 40 ± 6% of root-derived respiration originated from AMF mycelia and carbon was respired < 4 h after its supply from roots. We conclude that arbuscular mycorrhizal mycelial respiration can be substantial in lowland tropical forests. As it is highly dependent on the recent supply of carbon from roots, a function of aboveground fixation, AMF mycelial respiration is therefore an important pathway of carbon flux from tropical forest trees to the atmosphere.
Tropical forest floor characteristics such as depth and nutrient concentrations are highly heterogeneous even over small spatial scales and it is unclear how these differences contribute to ...patchiness in forest floor arthropod abundance and diversity. In a lowland tropical forest in Panama we experimentally increased litter standing crop by removing litter from five plots (L-) and adding it to five other plots (L+); we had five control plots. After 32 mo of treatments we investigated how arthropod abundance and diversity were related to differences in forest floor physical (mass, depth, water content) and chemical properties (pH, nutrient concentrations). Forest floor mass and total arthropod abundance were greater in L+ plots compared with controls. There were no treatment differences in nutrient concentrations, pH or water content of the organic horizons. Over all plots, the mass of the fermentation horizon (Oe) was greater than the litter horizon (Oi); arthropod diversity and biomass were also greater in the Oe horizon but nutrient concentrations tended to be higher in the Oi horizon. Arthropod abundance was best explained by forest floor mass, while arthropod diversity was best explained by phosphorus, calcium and sodium concentrations in the Oi horizon and by phosphorus concentrations in the Oe horizon. Differences in arthropod community composition between treatments and horizons correlated with phosphorus concentration and dry mass of the forest floor. We conclude that at a local scale, arthropod abundance is related to forest floor mass (habitat space), while arthropod diversity is related to forest floor nutrient concentrations (habitat quality). Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp
Globally, anthropogenic disturbances are occurring at unprecedented rates and over extensive spatial and temporal scales. Human activities also affect natural disturbances, prompting shifts in their ...timing and intensities. Thus, there is an urgent need to understand and predict the response of ecosystems to disturbance. In this study, we investigated whether there are general determinants of community response to disturbance across different community types, locations, and disturbance events. We compiled 14 case studies of community response to disturbance from four continents, twelve aquatic and terrestrial ecosystem types, and eight different types of disturbance. We used community compositional differences and species richness to indicate community response. We used mixed‐effects modeling to test the relationship between each of these response metrics and four potential explanatory factors: regional species pool size, isolation, number of generations passed, and relative disturbance intensity. We found that compositional similarity was higher between pre‐ and post‐disturbance communities when the disturbed community was connected to adjacent undisturbed habitat. The number of generations that had passed since the disturbance event was a significant, but weak, predictor of community compositional change; two communities were responsible for the observed relationship. We found no significant relationships between the factors we tested and changes in species richness. To our knowledge, this is the first attempt to search for general drivers of community resilience from a diverse set of case studies. The strength of the relationship between compositional change and isolation suggests that it may be informative in resilience research and biodiversity management.
Leaf litter is an important source of nutrients to tropical forest trees, but its importance for understorey seedling growth is not well understood. Seedlings of Licania platypus (n = 190) and ...Coussarea curvigemmia (n = 304) were transplanted into deeply shaded forest plots in Panama having received 2 y of litter addition or removal and 7 y of fertilization with nitrogen, phosphorus and potassium combined, and their growth and foliar nutrients measured after 13 and 6 mo respectively. Licania platypus growing in litter addition and removal plots had faster height growth and slower leaf growth respectively than in control plots; C. curvigemmia showed no significant effects apart from lower survival in litter addition plots. These effects may be driven by soil nutrients, as suggested by differences in foliar nitrogen and potassium (but not phosphorus) concentrations, and by a pot experiment in a shadehouse using Ochroma pyramidale seedlings, which showed higher leaf area in soils from litter-addition plots, although seedling dry weight was higher only in fertilized soils. Overall, these results show that for one of two species, understorey seedling growth was increased by 2 y of doubled litterfall, and thus that they were probably nutrient limited even in the relatively fertile soils of this semi-deciduous tropical forest.
Although competition between plants is nearly universal in vegetation, we know relatively little about belowground competition and how it interacts with aboveground competition in tropical forests, ...and almost nothing about such interactions on soils of intermediate fertility in sites with a moderate dry season, despite the fact that such forests are extensive. We investigated this over one year in a Panamanian tropical semi-evergreen rain forest, using tree seedlings (Simarouba amara, Gustavia superba, Tachigali versicolor, and Aspidosperma cruenta; least to most shade tolerant), experimental gaps, and trenching. Gaps increased growth and decreased mortality; growth increases were: Simarouba 684% (increase in height relative growth rate), Gustavia 411%, Aspidosperma 364%, and Tachigali 324%. Trenching in gaps increased growth in three species (Simarouba 49%, Gustavia 63%, and Aspidosperma 38%) but had very small effects in the understory; trenching did not affect mortality. We infer that trenching caused increased growth due to increased nutrients in the wet season, and increased water and/or nutrients in the dry season. Thus, across the tropics, in all but the wettest sites with fertile soils, seedlings of many species will be limited by belowground competition, at least in gaps. This is similar to the pervasive importance of belowground competition in temperate forests.
Because reducing aboveground competition for photosynthetically active radiation (PAR) increases growth and survival rates of tropical rain forest seedlings, belowground competition for nutrients is ...often assumed to be of little or no importance. We tested this assumption. We measured the growth, over one year, of seedlings of Aspidosperma carapanauba (shade tolerator) and Dinizia excelsa (light demander) transplanted into understory (high aboveground competition; 1% incoming photosynthetic photon flux density PPFD) and single treefall gap areas (lower aboveground competition; 6% PPFD in centers of gaps) without and with trenches cut around plots (high and low belowground competition). Over the wetter six months, location in gaps significantly increased relative height growth rates by 320% and 570%, and relative leaf production rates by 190% and 280%, while trenching significantly increased height by 330% and 520%, and leaf production by 170% and 260%, for Aspidosperma and Dinizia, respectively, with significant interactions between treatments. Trenching approximately doubled height growth in the understory. Leaf loss rates were unaffected by either treatment but were 3.7 times higher in Dinizia compared with Aspidosperma. Aspidosperma could persist in the understory, as leaf production and loss were balanced at 0.60% PPFD, whereas Dinizia could not, as it required 2.4% PPFD. Reducing either aboveground competition or belowground competition allowed Dinizia to persist as leaf production was increased above unchanging leaf loss rates. Gap and trenching treatments both significantly reduced mortality rates in Dinizia. Location in gaps significantly reduced mortality rates in Aspidosperma. Both species significantly increased allocation to leaves and significantly decreased allocation to roots in response to trenching. The increased growth in the wet season caused by trenching, and increased foliar nitrogen and phosphorus amount, showed that both species were probably competing with adult trees for nutrients as well as PAR. We deduced that phosphorus did not limit growth; however, we could not deduce which nutrient did limit growth. Given that 60-70% of the world's tropical rain forests occur on low-fertility oxisols, intense adult-juvenile competition both aboveground and belowground may be the norm, not the exception.
Climate change and rising atmospheric carbon dioxide (CO2) concentrations are likely to alter tropical forest net primary productivity (NPP), potentially affecting soil C storage. We examined ...biochemical and physical changes in soil C fractions in a humid tropical forest where experimental litter manipulation changed total soil C stocks. We hypothesized that: (1.) low-density soil organic C (SOC) fractions are more responsive to altered litter inputs than mineral-associated SOC, because they cycle relatively rapidly. (2.) Any accumulation of mineral-associated SOC with litter addition is relatively stable (i.e. low leaching potential). (3.) Certain biomolecules, such as waxes (alkyl) and proteins (N-alkyl), form more stable mineral-associations than other biomolecules in strongly weathered soils. A decade of litter addition and removal affected bulk soil C content in the upper 5 cm by +32% and −31%, respectively. Most notably, C concentration in the mineral-associated SOC fraction was greater in litter addition plots relative to controls by 18% and 28% in the dry and wet seasons, respectively, accounting for the majority of greater bulk soil C stock. Radiocarbon and leaching analyses demonstrated that the greater mineral-associated SOC in litter addition plots consisted of new and relatively stable C, with only 3% of mineral-associated SOC leachable in salt solution. Solid-state 13C NMR spectroscopy indicated that waxes (alkyl C) and microbial biomass compounds (O-alkyl and N-alkyl C) in mineral-associated SOC are relatively stable, whereas plant-derived compounds (aromatic and phenolic C) are lost from mineral associations on decadal timescales. We conclude that changes in tropical forest NPP will alter the quantity, biochemistry, and stability of C stored in strongly weathered tropical soils.
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•Tropical forest soil carbon chemistry was sensitive to changed litter biomass.•Soil carbon in stable organo-mineral associations increased with litter addition.•Waxes and proteins were the most stable component of organo-mineral associations.•Phenolic and aromatic carbon was lost from mineral associations with litter removal.
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