Tropical montane cloud forest: Environmental drivers of vegetation structure and ecosystem function

Abstract:Tropical montane cloud forests (TMCF) are characterized by short trees, often twisted with multiple stems, with many stems per ground area, a large stem diameter to height ratio, and small, often thick leaves. These forests exhibit high root to shoot ratio, with a moderate leaf area index, low above-ground production, low leaf nutrient concentrations and often with luxuriant epiphytic growth. These traits of TMCF are caused by climatic conditions not geological substrate, and are particularly associated with frequent or persistent fog and low cloud. There are several reasons why fog might result in these features. Firstly, the fog and clouds reduce the amount of light received per unit area of ground and as closed-canopy forests absorb most of the light that reaches them the reduction in the total amount of light reduces growth. Secondly, the rate of photosynthesis per leaf area declines in comparison with that in the lowlands, which leads to less carbon fixation. Nitrogen supply limits growth in several of the few TMCFs where it has been investigated experimentally. High root : shoot biomass and production ratios are common in TMCF, and soils are often wet which may contribute to N limitation. Further study is needed to clarify the causes of several key features of TMCF ecosystems including high tree diameter : height ratio.This is the author accepted manuscript. The final version is available from Cambridge University Press via http://dx.doi.org/10.1017/S026646741500017

Decadal-scale litter manipulation alters the biochemical and physical character of tropical forest soil carbon

© 2018 Elsevier Ltd 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-state13C 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

Data from: Litter removal in a tropical rain forest reduces fine root biomass and production but litter addition has few effects

Many old-growth lowland tropical rain forests are potentially nutrient limited, and it has long been thought that many such forests maintain growth by recycling nutrients from decomposing litter. We investigated this by continuously removing (for ten years) freshly fallen litter from five (45 m x 45 m) plots, adding it to five other plots, there were five controls. From monthly measures over one year we show that litter removal caused lower: fine root (≤2 mm diameter) standing mass, fine root standing length, fine root length production and fine root length survivorship. Litter addition did not significantly change fine root mass or length or production. Nutrient concentrations in fine roots in litter removal plots were lower than those in controls for nitrogen (N), calcium (Ca) and magnesium (Mg), concentrations in fine roots in litter addition plots were higher for N and Ca. Chronic litter removal has resulted in reduced forest growth due to lack of nutrients, probably nitrogen. Conversely, long-term litter addition has had fewer effects

Nitrogen and phosphorus constrain labile and stable carbon turnover in lowland tropical forest soils

Tropical forests contain a large stock of soil carbon, but the factors that constrain its mineralization remain poorly understood. Microorganisms, when stimulated by the presence of new inputs of labile organic carbon, can mineralize (‘prime’) soil organic matter to acquire nutrients. We used stable carbon isotopes to assess how nutrient demand and soil properties constrain mineralization of added labile (sucrose) carbon and pre-existing (primed) soil carbon in tropical forest soils. In a series of lowland tropical forest soils from Panama, we found that the mineralization of fresh labile carbon was accelerated foremost by phosphorus addition, whereas the mineralization of pre-existing soil carbon was constrained foremost by nitrogen addition. However, there was variation in the relative importance of these nutrients in different soils and the largest effects on the acceleration of sucrose metabolism and constraint of priming occurred following the addition of nitrogen and phosphorus together. The respiration responses due to sucrose or primed soil carbon mineralization were reduced at pH below 4.8 and above 6.0. We conclude that in these tropical forest soils, phosphorus availability is more important in promoting microbial mineralization of sucrose carbon, whereas nitrogen availability is more important in constraining the priming of pre-existing soil organic carbon. This response likely arises because nitrogen is more closely coupled to organic matter cycling, whereas phosphorus is abundant in both organic and inorganic forms. These results suggest that the greatest impact of priming on soil carbon stocks will occur in moderately acidic tropical forest soils of low nitrogen availability. Given long-term changes in both atmospheric carbon dioxide and nitrogen deposition, the impact of priming effects on soil carbon in tropical forest soils may be partially constrained by the abundance of nitrogen

Ecology and distribution of neotropical Podocarpaceae

odocarps are a frequent, but rarely a dominant, component of neotropical wet forests extending from South America into central Mexico and the Greater Antilles. Although podocarps are often considered to be predominantly montane taxa, several species occur in lowland forest and are locally abundant on some Pacific and Atlantic coastal islands in Central America. Here we review literature on the origins and distribution of neotropical podocarps and highlight their apparent association with resourcepoor environments. As a consequence of forest conversion and logging, many podocarps that were already habitat specialists are now further restricted to small and increasingly fragmented populations. Unfortunately, there is little information on the regeneration ecology of podocarps with which to assess the recruitment potential of these populations. An exception is the long-term studies of the dynamics of Podocarpus urbanii, a common species in montane forest in Jamaica. Podocarpus urbanii is moderately shade tolerant and successfully regenerates beneath undisturbed forest. The low juvenile mortality rate of P. urbanii, coupled with relatively high diameter growth, suggests that this species and possibly other podocarps may have greater utility for reforestation than is currently recognized

Soil carbon release enhanced by increased tropical forest litterfall

Tropical forests are a critical component of the global carbon cycle and their response to environmental change will play a key role in determining future concentrations of atmospheric carbon dioxide (CO2). Increasing primary productivity in tropical forests over recent decades has been attributed to CO2 fertilization, and greater biomass in tropical forests could represent a substantial sink for carbon in the future. However, the carbon sequestration capacity of tropical forest soils is uncertain and feedbacks between increased plant productivity and soil carbon dynamics remain unexplored. Here, we show that experimentally increasing litterfall in a lowland tropical forest enhanced carbon release from the soil. Using a large-scale litter manipulation experiment combined with carbon isotope measurements, we found that the efflux of CO2 derived from soil organic carbon was significantly increased by litter addition. Furthermore, this effect was sustained over several years. We predict that a future increase in litterfall of 30% with an increase in atmospheric CO2 concentrations of 150 ppm could release about 0.6 t C ha-1 yr-1 from the soil, partially offsetting predicted net gains in carbon storage. Thus, it is essential that plant–soil feedbacks are taken into account in predictions of the carbon sequestration potential of tropical forests

Litter manipulation and the soil arthropod community in a lowland tropical rainforest

Tropical soil arthropod communities are highly diverse and provide a number of important ecosystem services, including the maintenance of soil structure, regulation of hydrological processes, nutrient cycling and decomposition. Experiments in temperate regions suggest that litter dynamics are important in determining the abundance, richness and community composition of soil fauna, but there is little information for lowland tropical forests. We used a long-term litter manipulation experiment (removing, doubling and control) in a neotropical forest to investigate the consequences of changing litter dynamics on the soil arthropod community. The abundance and biomass of arthropods were reduced significantly by the removal of litter, but not affected by litter addition. Litter manipulation had no effect on simple measures of taxonomic richness or diversity, but multivariate ordination techniques revealed a significant shift in arthropod community composition with the removal, but not addition, of litter. This suggests the overall importance of top-down controls on the arthropod community in this ecosystem, with bottom-up influences only important following the removal of large quantities of litter. Of the parameters measured, the faunal composition of experimental plots was best predicted by litter depth and the concentrations of total carbon and readily-exchangeable phosphorus (in order of importance), highlighting the influential role of soil chemical properties, in addition to the physical properties of litter, in shaping soil arthropod communities. Comparison with the results of a previous study of litter-dwelling fauna in the same litter manipulation experiment suggested that the soil and litter arthropod communities are influenced by different parameters: total carbon and litter depth for the soil community, but sodium and calcium for the litter community, although phosphorus was important in both environments. We conclude that arthropod community composition is controlled by different factors in the soil than in the litter and is affected by decreasing, but not increasing, depth of litter

Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 × 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1–10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles