Ayron M. Strauch

Climate change and land use drivers of fecal bacteria in tropical Hawaiian rivers

Potential shifts in rainfall driven by climate change are anticipated to affect watershed processes (e.g., soil moisture, runoff, stream flow), yet few model systems exist in the tropics to test hypotheses about how these processes may respond to these shifts. We used a sequence of nine watersheds on Hawaii Island spanning 3000 mm (7500-4500 mm) of mean annual rainfall (MAR) to investigate the effects of short-term (24-h) and long-term (MAR) rainfall on three fecal indicator bacteria (FIB) (enterococci, total coliforms, and ). All sample sites were in native Ohia dominated forest above 600 m in elevation. Additional samples were collected just above sea level where the predominant land cover is pasture and agriculture, permitting the additional study of interactions between land use across the MAR gradient. We found that declines in MAR significantly amplified concentrations of all three FIB and that FIB yield increased more rapidly with 24-h rainfall in low-MAR watersheds than in high-MAR watersheds. Because storm frequency decreases with declining MAR, the rate of change in water potential affects microbial growth, whereas increased rainfall intensity dislodges more soil and bacteria as runoff compared with water-logged soils of high-MAR watersheds. As expected, declines in % forest cover and increased urbanization increased FIB. Taken together, shifts in rainfall may alter bacterial inputs to tropical streams, with land use change also affecting water quality in streams and near-shore environments.

Leaf-litter inputs from an invasive nitrogen-fixing tree influence organic-matter dynamics and nitrogen inputs in a Hawaiian river.

Abstract.  We examined how invasion of tropical riparian forests by an exotic N-fixing tree (Falcataria moluccana) affects organic-matter dynamics in a Hawaiian river by comparing early stages of leaf-litter breakdown between the exotic F. moluccana and native Metrosideros polymorpha trees. We examined early decomposition stages because of low leaf-litter retention rates (<20 d) that result from the flashy nature of tropical Pacific Island streams. Leaf breakdown rates, fungal biomass, and invertebrate abundances were 40, 120, and 30% greater, respectively, for F. moluccana than M. polymorpha leaves. Leaf-litter breakdown was largely a result of stream flow and to a lesser extent fungal colonization. Invertebrates were not an important factor in leaf-litter breakdown. Initial tannin content, leaf C∶N, and toughness were important intrinsic factors inhibiting leaf breakdown and fungal colonization. Regression analyses between remaining N content (%) and ash-free dry mass of leaf litter revealed that the early stages of F. moluccana leaf-litter breakdown are a source of N to streams invaded by F. moluccana and contribute a conservatively estimated 2.1 to 5.7% to the available total dissolved N pool. Direct input of F. moluccana leaf litter influences early stages of leaf-litter breakdown in tropical streams with low leaf-litter retention rates. Direct input of leaf litter also contributes somewhat to N inputs, but subsurface flows through N-rich soils of F. moluccana-invaded riparian forests probably are a greater source.

Modeled Effects of Climate Change and Plant Invasion on Watershed Function Across a Steep Tropical Rainfall Gradient

ABSTRACTClimate change is anticipated to affect freshwater resources, but baseline data on the functioning of tropical watersheds is lacking, limiting efforts that seek to predict how watershed processes, water supply, and streamflow respond to anticipated changes in climate and vegetation change, and to management. To address this data gap, we applied the distributed hydrology soil vegetation model (DHSVM) across 88 watersheds spanning a highly constrained, 4500 mm mean annual rainfall (MAR) gradient on Hawai‘i Island to quantify stream flow at 3-h time-steps for eight years in response to the independent and interactive effects of (1) large observed decrease in MAR; (2) projected warming and altered precipitation; and (3) four scenarios of forest invasion by the high water-demanding non-native tree species Psidium cattleianum. The model captured 62% of variability in measured flow at daily time scales, 95% at monthly time scales, and 98% at annual time scales. We found that low DHSVM modeled flow (Q90) and storm flow (Q10) responses to observed declines in rainfall dwarfed those of projected temperature increase or invasion, with flow decline positively correlated with MAR. As a percentage of streamflow, temperature and invasion reductions were negatively correlated with MAR. By comparison, warming alone had little effect on Q90 or Q10, but both decreased with increasing P. cattleianum cover, and projected effects of declining MAR were accentuated when combined with P. cattleianum and warming. Restoration mitigated some effects of climate warming by increasing stream base flows, with the relative effects of restoration being larger in drier versus wetter watersheds. We conclude that potential changes in climate in tropical environments are likely to exert significant effects on streamflow, but managing vegetation can provide mitigating benefits.

Leaf Litter Breakdown of Native and Exotic Tree Species in Two Hawaiian Streams that Differ in Flow

Abstract: Riparian leaf litter is a major source of allochthonous organic material to temperate and tropical streams, promoting primary and secondary productivity in lotic and nearshore habitats. In tropical island streams, where native leaf-shredding macroinvertebrates are absent, physical fragmentation from stream flow is an important factor affecting leaf litter breakdown and, thus, organic matter dynamics. Additionally, the invasion of exotic plants into riparian areas is expected to affect litter composition and, consequently, its degradation. We compared the interactions of stream flow and inputs of leaf litter from native and exotic plants on leaf litter breakdown in two streams of varying flows on Hawai‘i Island. Decay rates were greater in the high flow stream than in the low flow one for exotic Spathodea campanulata (0.037 vs. 0.023 day-1), but not significantly different for exotic Psidium cattleianum (0.003 vs. 0.003 day-1), and native Metrosideros polymorpha (0.005 vs. 0.002 days-1). In contrast, the exotic Falcataria moluccana (a nitrogen fixer) decomposed more rapidly in the low flow stream (0.017 day-1) than in the high flow stream (0.010 day-1). Breakdown rates also varied among species, with S. campanulata > F. moluccana > M. polymorpha > P. cattleianum. Breakdown rates were generally positively correlated to leaf nitrogen content and negatively correlated with leaf structure characteristics (toughness, organic carbon content, percentage lignin). Our findings indicate that stream flow regimes altered by climate change are likely to influence leaf litter decomposition, and S. campanulata and F. moluccana will likely impact organic matter dynamics in Hawaiian and other Pacific Island streams. However, predicted changes depend on the species composition of riparian leaf litter.

Stakeholder-Driven Research in Phragmites Management in New England Salt Marshes

Overview Methods of resource management and conservation originated in economic theory during the late 19th century. Historically, communities controlled their environments to enhance development and maximize the harvest of key species. For generations tidal marshes were drained and tidal flooding eliminated in New England to utilize coastal property. These methods improved harvests and development but left coastal ecosystems vulnerable and invasive plants have begun to out-compete native plants. More recently, new theories of conservation biology have focused on the role of humans in the management of local ecosystems. We now recognize the benefits of ecosystem services such as water filtration, breeding habitat, and aesthetic value to society that do not necessarily have direct financial rewards. However, management strategies are still frequently imposed in a top-down fashion. This type of management is largely done by small groups of experts imposing strategies over large regions. In the process, local community needs and values are often ignored resulting in dissatisfaction and even hostility towards managers. Effective invasive species management must first focus on the environmental changes that have occurred as a consequence of the invasion and then develop a strategy to restore the system back to the original state (Schroeder and Keller 1990). Without accurate environmental monitoring, these changes are best understood through the experiences of local Stakeholder-Driven Research in Phragmites Management in New England Salt Marshes fiEld notEs