R. Flint Hughes

High-resolution forest carbon stocks and emissions in the Amazon

Efforts to mitigate climate change through the Reduced Emissions from Deforestation and Degradation (REDD) depend on mapping and monitoring of tropical forest carbon stocks and emissions over large geographic areas. With a new integrated use of satellite imaging, airborne light detection and ranging, and field plots, we mapped aboveground carbon stocks and emissions at 0.1-ha resolution over 4.3 million ha of the Peruvian Amazon, an area twice that of all forests in Costa Rica, to reveal the determinants of forest carbon density and to demonstrate the feasibility of mapping carbon emissions for REDD. We discovered previously unknown variation in carbon storage at multiple scales based on geologic substrate and forest type. From 1999 to 2009, emissions from land use totaled 1.1% of the standing carbon throughout the region. Forest degradation, such as from selective logging, increased regional carbon emissions by 47% over deforestation alone, and secondary regrowth provided an 18% offset against total gross emissions. Very high-resolution monitoring reduces uncertainty in carbon emissions for REDD programs while uncovering fundamental environmental controls on forest carbon storage and their interactions with land-use change.

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.

Invasive plants transform the three-dimensional structure of rain forests

Biological invasions contribute to global environmental change, but the dynamics and consequences of most invasions are difficult to assess at regional scales. We deployed an airborne remote sensing system that mapped the location and impacts of five highly invasive plant species across 221,875 ha of Hawaiian ecosystems, identifying four distinct ways that these species transform the three-dimensional (3D) structure of native rain forests. In lowland to montane forests, three invasive tree species replace native midcanopy and understory plants, whereas one understory invader excludes native species at the ground level. A fifth invasive nitrogen-fixing tree, in combination with a midcanopy alien tree, replaces native plants at all canopy levels in lowland forests. We conclude that this diverse array of alien plant species, each representing a different growth form or functional type, is changing the fundamental 3D structure of native Hawaiian rain forests. Our work also demonstrates how an airborne mapping strategy can identify and track the spread of certain invasive plant species, determine ecological consequences of their proliferation, and provide detailed geographic information to conservation and management efforts.

The response of native species to removal of invasive exotic grasses in a seasonally dry Hawaiian woodland

. Non-native perennial grasses form 30% of the live understory biomass in seasonally dry, submontane forests in Hawaii Volcanoes National Park, yet their effects on native species are unknown. We removed these grasses from plots of 20 m × 20 m in 1991 and maintained removal and control areas over the next three years. Two fast growing shrub species, Dodonaea viscosa and Osteomeles anthylidifolia, increased in size significantly more in removal areas than in controls. Individuals of the most abundant shrub species, Styphelia tameiameia showed no net growth response to grass removal. They did, however, change their architecture: many branches along the mid and upper sections of the main trunk died and a proliferation of new leaves and shoots occurred in the lower 40 cm of trunk. Basal diameter increase was very small in Metrosideros polymorpha, the dominant tree species in these sites. All species except Styphelia had significantly increased leaf tissue nitrogen in removal plots by 18 months after removal when compared to shrubs in control areas suggesting that removal plot shrubs had greater access to soil nitrogen. Available soil-N pools, which were generally higher in the removal plots, support this interpretation. Light levels near the soil surface were also higher where grasses were removed than where they were present which may have contributed to increased shrub growth. By contrast, soil moisture was consistently lower where grasses were removed than where they were still present. Shrub tissue carbon isotope values were consistent with the interpretation that shrubs in removal plots had less rather than more water available to them. Hence, the increased growth observed in removal plot shrubs could not be due to release from moisture competition. Lastly, our results showed that seedlings of all woody species except Metrosideros were significantly more abundant in removal plots at both one and three years after removal and initially high sapling mortality was balanced by high recruitment into the sapling class. We believe that over time this will result in increased densities of native shrubs if grasses are kept out. With the presence of grasses, shrub growth in these woodlands is reduced and biomass is shifting towards grasses.

CARBON DYNAMICS AND LAND-USE CHOICES: BUILDING A REGIONAL-SCALE MULTIDISCIPLINARY MODEL

Policy enabling tropical forests to approach their potential contribution to global-climate-change mitigation requires forecasts of land use and carbon storage on a large scale over long periods. In this paper, we present an integrated modeling methodology that addresses these needs. We model the dynamics of the human land-use system and of C pools contained in each ecosystem, as well as their interactions. The model is national scale, and is currently applied in a preliminary way to Costa Rica using data spanning a period of over 50 years. It combines an ecological process model, parameterized using field and other data, with an economic model, estimated using historical data to ensure a close link to actual behavior. These two models are linked so that ecological conditions affect land-use choices and vice versa. The integrated model predicts land use and its consequences for C storage for policy scenarios. These predictions can be used to create baselines, reward sequestration, and estimate the value in both environmental and economic terms of including C sequestration in tropical forests as part of the efforts to mitigate global climate change. The model can also be used to assess the benefits from costly activities to increase accuracy and thus reduce errors and their societal costs.

FACTORS INFLUENCING DYNAMICS OF TWO INVASIVE C4 GRASSES IN SEASONALLY DRY HAWAIIAN WOODLANDS

The introduced C4 bunchgrass, Schizachyrium condensatum, is abundant in unburned, seasonally dry woodlands on the island of Hawaii, where it promotes the spread of fire. After fire, it is partially replaced by Melinis minutiflora, another invasive C4 grass. Seed bank surveys in unburned woodland showed that Melinis seed is present in locations without adult plants. Using a combination of germination tests and seedling outplant ex- periments, we tested the hypothesis that Melinis was unable to invade the unburned wood- land because of nutrient and/or light limitation. We found that Melinis germination and seedling growth are depressed by the low light levels common under Schizachyrium in unburned woodland. Outplanted Melinis seedlings grew rapidly to flowering and persisted for several years in unburned woodland without nutrient additions, but only if Schizachyrium individuals were removed. Nutrients alone did not facilitate Melinis establishment. Competition between Melinis and Schizachyrium naturally occurs when individuals of both species emerge from the seed bank simultaneously, or when seedlings of one species emerge in sites already dominated by individuals of the other species. When both species are grown from seed, we found that Melinis consistently outcompetes Schizachyrium, re- gardless of light or nutrient treatments. When seeds of Melinis were added to pots with well-established Schizachyrium (and vice versa), Melinis eventually invaded and overgrew adult Schizachyrium under high, but not low, nutrients. By contrast, Schizachyrium could not invade established Melinis pots regardless of nutrient level. A field experiment dem- onstrated that Schizachyrium individuals are suppressed by Melinis in burned sites through competition for both light and nutrients. Overall, Melinis is a dominant competitor over Schizachyrium once it becomes estab- lished, whether in a pot or in the field. We believe that the dominance of Schizachyrium, rather than Melinis, in the unburned woodland is the result of asymmetric competition due to the prior establishment of Schizachyrium in these sites. If Schizachyrium were not present, the unburned woodland could support dense stands of Melinis. Fire disrupts the priority effect of Schizachyrium and allows the dominant competitor (Melinis) to enter the system where it eventually replaces Schizachyrium through resource competition.

Carbon pool and biomass dynamics associated with deforestation, land use, and agricultural abandonment in the neotropics

Current rates of deforestation and the resulting C emissions in the tropics exceed those of secondary forest regrowth and C sequestration. Changing land-use strategies that would maintain standing forests may be among the least expensive of climate change mitigation options. Further, secondary tropical forests have been suggested to have great value for their potential to sequester atmospheric C. These options require an understanding of and capability to quantify C dynamics at landscape scales. Because of the diversity of physical and biotic features of tropical forests as well as approaches and intensities of land uses within the neotropics, there are tremendous differences in the capacity of different landscapes to store and sequester C. Major gaps in our current knowledge include quantification of C pools, rates and patterns of biomass loss following land-cover change, and quantification of the C storage potential of secondary forests following abandonment. In this paper we present a synthesis and further analyses from recent studies that describe C pools, patterns of C decline associated with land use, and rates of C accumulation following secondary-forest establishment--all information necessary for climate-change mitigation options. Ecosystem C pools of Neotropical primary forests minimally range from approximately 141 to 571 Mg/ha, demonstrating tremendous differences in the capacity of different forests to store C. Most of the losses in C and nutrient pools associated with conversion occur when fires are set to remove the slashed forest to prepare sites for crop or pasture establishment. Fires burning slashed primary forests have been found to result in C losses of 62-80% of prefire aboveground pools in dry (deciduous) forest landscapes and 29-57% in wet (evergreen) forest landscapes. Carbon emissions equivalent to the aboveground primary-forest pool arise from repeated fires occurring in the first 4 to 10 years following conversion. Feedbacks of climate change, land-cover change, and increasing habitat fragmentation may result in increases of both the area burned and the total quantity of biomass consumed per unit area by fire. These effects may well limit the capacity for future tropical forests to sequester C and nutrients.

The Kyoto protocol and payments for tropical forest: an interdisciplinary method for estimating carbon-offset supply and increasing the feasibility of a carbon market under the CDM.

Abstract Protecting tropical forests under the Clean Development Mechanism (CDM) could reduce the cost of emissions limitations set in Kyoto. However, while society must soon decide whether or not to use tropical forest-based offsets, evidence regarding tropical carbon sinks is sparse. This paper presents a general method for constructing an integrated model (based on detailed historical, remote sensing and field data) that can produce land-use and carbon baselines, predict carbon sequestration supply to a carbon-offsets market and also help to evaluate optimal market rules. Creating such integrated models requires close collaboration between social and natural scientists. Our project combines varied disciplinary expertise (in economics, ecology and geography) with local knowledge in order to create high-quality, empirically grounded, integrated models for Costa Rica.

Novel forests maintain ecosystem processes after the decline of native tree species

The positive relationship between species diversity (richness and evenness) and critical ecosystem functions, such as productivity, carbon storage, and nutrient cycling, is often used to predict the consequences of extinction. At regional scales, however, plant species richness is mostly increasing rather than decreasing because successful plant species introductions far outnumber extinctions. If these regional increases in richness lead to local increases in diversity, a reasonable prediction is that productivity, carbon storage, and nutrient cycling will increase following invasion, yet this prediction has rarely been tested empirically. We tested this prediction in novel forest communities dominated by introduced species (;90% basal area) in lowland Hawaiian rain forests by comparing their functionality to that of native forests. We conducted our comparison along a natural gradient of increasing nitrogen availability, allowing for a more detailed examination of the role of plant functional trait differences (specifically, N2 fixation) in driving possible changes to ecosystem function. Hawaii is emblematic of regional patterns of species change; it has much higher regional plant richness than it did historically, due to .1000 plant species introductions and only ;71 known plant extinctions, resulting in an ;100% increase in richness. At local scales, we found that novel forests had significantly higher tree species richness and higher diversity of dominant tree species. We further found that aboveground biomass, productivity, nutrient turnover (as measured by soil-available and litter-cycled nitrogen and phosphorus), and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests. We found that the addition of introduced N2-fixing tree species on N-limited substrates had the strongest effect on ecosystem function, a pattern found by previous empirical tests. Our results support empirical predictions of the functional effects of diversity, but they also suggest basic ecosystem processes will continue even after dramatic losses of native species diversity if simple functional roles are provided by introduced species. Because large portions of the Earths surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.

Ecosystem development on Hawaiian lava flows: biomass and species composition

. The strong environmental gradients and ‘natural experimental design’ of Mauna Loa volcano, Hawaii, provide an outstanding opportunity to study controls on ecosystem development. We measured above-ground vascular plant biomass and species composition on 42 sites on which precipitation, temperature, substrate texture, and substrate age varied substantially and largely independently. Biomass and species richness of live plants were strongly correlated with precipitation and lava flow age, but not with temperature or lava flow texture. Species composition, as measured by correspondence analysis, was likewise correlated with precipitation and flow age, but composition was also strongly influenced by temperature. Lava texture had a complex effect on vegetation, with ‘a’ a lava favoring vegetation development on wet sites and pāhoehoe favoring development on dry sites. Many locations remain virtually free of invasion by alien species; aliens appear where disturbance has facilitated invasion, either from stand-level dieback in rainforest or a grass-fire cycle on the dry, leeward side of the mountain. All four of the environmental factors studied here (precipitation, temperature, substrate texture, and substrate age) exert significant and independent control over vegetation biomass and/or species composition on Mauna Loa.