Fred G.R. Watson

Factors determining relations between stand age and catchment water balance in mountain ash forests

Abstract There is a well-documented empirical relationship between stand age and water yield for mountain ash ( Eucalyptus regnans ) forested catchments in the Central Highlands of Victoria, Australia. Catchments covered with old-growth stands of mountain ash yield almost twice the amount of water annually as those covered with re-growth stands aged 25 years. In this paper, we provide a mechanistic hydro-ecologic explanation for this phenomenon. We measured leaf area index (LAI), sapwood area index (SAI) and various water balance components in several mountain ash stands, ranging in age between 5 and 240 years. Sap flow measurements show that sap velocity does not vary appreciably amongst stands of different ages, but a systematic decline in SAI with age produces a concomitant decrease in stand transpiration. The decline in overstory SAI is accompanied by a decline in overstory LAI. Understory LAI increases as the overstory LAI decreases, but this layer transpires at only about 63% of the mountain ash rate on a per unit leaf area basis. Hence, while total stand LAI decreases are modest over time, the trend is for a significant decline in total stand transpiration as the forest ages. Rainfall interception also declined over time and there was some indication that interception per unit leaf area also declined. Such reductions can be explained by lesser turbulent mixing and elevated humidity around the bulk of the leaf area in the mature forest. There were small decreases in forest floor evaporation through time, though this only accounted for about 5–8% of the site water balance. Our water balance measurements agree qualitatively with empirical water yield relationships developed for mountain ash forests, though the magnitudes of change differ.

Large-scale modelling of forest hydrological processes and their long-term effect on water yield

A water balance model was used to simulate the long-term increases in water yield with forest age which are observed in the mountain ash (Eucalyptus regnans) forests of Victoria, Australia. Specifically, the hypothesis was tested that water yield changes could be explained by changes in evapotranspiration resulting from changes in leaf area index (LAI). A curve predicting changes in the total LAI of mountain ash forest was constructed from ground-based observations and their correlation with Landsat Thematic Mapper measurements of the transformed normalized difference vegetation index (TNDVI). A further curve for mountain ash canopy LAI was constructed from destructive LAI measurements and stem diameter measurements. The curves were incorporated within Macaque, a large-scale, physically based water balance model which was applied to three forested catchments (total area 145 km2). The model was used to evaluate the effect of changes in LAI on predicted stream flow over an 82-year period spanning the 1939 wildfires which burnt most of the area. The use of the LAI curves induced improvement in the predicted hydrographs relative to the case for constant LAI, but the change was not large enough to account for all of the difference in water yield between old-growth and regrowth forests. Of a number of possibilities, concomitant changes in leaf conductance with age were suggested as an additional control on stream flow. These were estimated using data on stand sapwood area per unit leaf area and coded into Macaque. The hydrograph predicted using both the LAI curves and a new leaf conductance versus age curve accurately predicted the observed long-term changes in water yield. We conclude that LAI is a partial control on long-term yield changes, but that another ‘water use efficiency per unit LAI’ control is also operative. Copyright

Assessment Of Prey Vulnerability Through Analysis Of Wolf Movements And Kill Sites

Within predator-prey systems behavior can heavily influence spatial dynamics, and accordingly, the theoretical study of how spatial dynamics relate to stability within these systems has a rich history. However, our understanding of these behaviors in large mammalian systems is poorly developed. To address the relationship between predator selection patterns, prey density, and prey vulnerability, we quantified selection patterns for two fine-scale behaviors of a recovering wolf (Canis lupus) population in Yellowstone National Park, Wyoming, USA. Wolf spatial data were collected between November and May from 1998-1999 until 2001-2002. Over four winters, 244 aerial locations, 522 ground-based telemetry locations, 1287 km of movement data from snow tracking, and the locations of 279 wolf kill sites were recorded. There was evidence that elk (Cervus elaphus) and bison (Bison bison) densities had a weak effect on the sites where wolves traveled and made kills. Wolf movements showed a strong selection for geothermal areas, meadows, and areas near various types of habitat edges. Proximity to edge and habitat class also had a strong influence on the locations where elk were most vulnerable to predation. There was little evidence that wolf kill sites differed from the places where wolves traveled, indicating that elk vulnerability influenced where wolves selected to travel. Our results indicate that elk are more vulnerable to wolves under certain conditions and that wolves are capable of selecting for these conditions. As such, vulnerability plays a central role in predator-prey behavioral games and can potentially impact the systems to which they relate.

Identification and explanation of continental differences in the variability of annual runoff

Continental differences in the variability of annual runoff were investigated using an expanded and improved database to that used in previous work. A statistical analysis of the data, divided by continent and Koppen climate type, revealed that continental differences exist in the variability of annual runoff. The variability of annual runoff for temperate Australia, arid southern Africa and possibly temperate southern Africa were noted to be generally higher than that of other continents with data in the same climate type. A statistical analysis of annual precipitation by continent and Koppen climate type revealed that differences in the variability of annual precipitation could account for some but not all the observed differences in the variability of annual runoff. A literature review of potential causes of continental differences in evapotranspiration resulted in the hypothesis that the significantly higher variability of annual runoff in temperate Australia and possibly temperate southern Africa may be due to the distribution of evergreen and deciduous vegetation. The process model Macaque was used to test this hypothesis. The model results indicate that the variability of annual runoff may be between 1 and 99% higher for catchments covered in evergreen vegetation as opposed to deciduous vegetation, depending on mean annual precipitation and the seasonality of precipitation. It is suggested that the observed continental differences in the variability of annual runoff are largely caused by continental differences in the variability of annual precipitation and in temperate regions the distribution of evergreen and deciduous vegetation in conjunction with the distribution of mean annual precipitation and precipitation seasonality.

Improved methods to assess water yield changes from paired-catchment studies: application to the Maroondah catchments

The statistical methods underpinning analysis of streamflow data from paired-catchment studies have not changed much since the 1960s. Whilst such analyses are widespread in hydrologic practice and research, attention is rarely given to the problems of heteroscedacity, seasonality, serial correlation, and non-normally distributed variates. Each of these problems can potentially invalidate the basic assumptions upon which traditional statistical methods are based. We describe methods to contend with some of these problems and apply them to mountain ash (Eucalyptus regnans) forested catchments in the Maroondah Basin, south-eastern Australia. A seasonal regression model with lag-one auto-regressive (AR1) error was developed to predict monthly streamflow at treated catchments based on streamflow data from a control catchment. It is particularly well-suited to situations where little pre-treatment data is available. Differences between observed and predicted streamflow were used to quantify the effect of forest treatment on streamflow. Results from two catchment groups broadly matched the trend predicted by a previous regional model, with 2–3 year increases in streamflow, followed by decreases over the following one or two decades. A third group of catchments also showed initial increases, but expected subsequent decreases in streamflow were offset by the flow-increasing effects of insect infestations.

Covariates affecting spatial variability in bison travel behavior in Yellowstone National Park

Understanding mechanisms influencing the movement paths of animals is essential for comprehending behavior and accurately predicting use of travel corridors. In Yellowstone National Park (USA), the effects of roads and winter road grooming on bison (Bison bison) travel routes and spatial dynamics have been debated for more than a decade. However, no rigorous studies have been conducted on bison spatial movement patterns. We collected 121 380 locations from 14 female bison with GPS collars in central Yellowstone to examine how topography, habitat type, roads, and elevation affected the probability of bison travel year-round. We also conducted daily winter bison road use surveys (2003-2005) to quantify how topography and habitat type influenced spatial variability in the amount of bison road travel. Using model comparison techniques, we found the probability of bison travel and spatial distribution of travel locations were affected by multiple topographic and habitat type attributes including slope, landscape roughness, habitat type, elevation, and distances to streams, foraging areas, forested habitats, and roads. Streams were the most influential natural landscape feature affecting bison travel, and results suggest the bison travel network throughout central Yellowstone is spatially defined largely by the presence of streams that connect foraging areas. Also, the probability of bison travel was higher in regions of variable topography that constrain movements, such as in canyons. Pronounced travel corridors existed both in close association with roads and distant from any roads, and results indicate that roads may facilitate bison travel in certain areas. However, our findings suggest that many road segments used as travel corridors are overlaid upon natural travel pathways because road segments receiving high amounts of bison travel had similar landscape features as natural travel corridors. We suggest that most spatial patterns in bison road travel are a manifestation of general spatial travel trends. Our research offers novel insights into bison spatial dynamics and provides conceptual and analytical frameworks for examining movement patterns of other species.

Climate change intensification of herbivore impacts on tree recruitment

Altered species interactions are difficult to predict and yet may drive the response of ecological communities to climate change. We show that declining snowpack strengthens the impacts of a generalist herbivore, elk (Cervus elaphus), on a common tree species. Thick snowpack substantially reduces elk visitation to sites; aspen (Populus tremuloides) shoots in these areas experience lower browsing rates, higher survival and enhanced recruitment. Aspen inside herbivore exclosures have greatly increased recruitment, particularly at sites with thick snowpack. We suggest that long-term decreases in snowpack could help explain a widespread decline of aspen through previously unconsidered relationships. More generally, reduced snowpack across the Rocky Mountains, combined with rising elk populations, may remove the conditions needed for recruitment of this ecologically important tree species. These results highlight that herbivore behavioural responses to altered abiotic conditions are critical determinants of plant persistence. Predictions of climate change impacts must not overlook the crucial importance of species interactions.

Tarsier: a practical software framework for model development, testing and deployment

Abstract A new software framework for environmental simulation modeling is described. The framework facilitates fast, powerful model development by providing a system for implementing separate model elements as autonomous modules, which may then be tightly and flexibility integrated. The system is object-oriented, with integration of modules achieved through the sharing of common objects. Module autonomy is achieved by the use of generic messages passed between modules through shared objects. Shared objects represent concepts such as data, time, or sub-models. This facilitates effective model integration with no loss of speed as a result of the integration process. A large existing collection of modules, including IO, data representation and visualization, allows new models to be developed quickly with minimal attention to repetitive tasks. To date, the applications of the framework have been in the domains of hydrological, wildlife and ecosystem modeling. Experience has shown that the persistent merits of the framework include the data sharing/message passing approach, the generic conceptualization of modeling objects, and the body of modules assembled to date. The disadvantages are proving to be the dependence on a specific language and compiler. This hinders the utility of the framework for the broader model development community.

Large-scale distribution modelling and the utility of detailed ground data

A large-scale distribution function model was used to investigate the effect of differing parameter mapping schemes on the quality of hydrological predictions. Precipitation was mapped over a large forested catchment area (163 km 2 ) using both one-dimensional linear and three-dimensional non-linear interpolation schemes. Lumped stream flow predictions were found to be particularly sensitive to the different precipitation maps, with the three-dimensional map predicting 12% higher mean annual precipitation, resulting in 36% higher modelled stream flow over a three-year period. However, spatial predictions of stream flow appeared worse when derived from the three-dimensional map, which is considered the better of the two precipitation maps. This implies uncertainty in either the models response to precipitation or the precipitation mapping process (the 12% precipitation difference was strongly determined by a single, short term gauge). Leaf area index (LAI) was mapped using both remote sensing and species based methods. The two LAI maps had similar lumped mean values but exhibited significant spatial differences. The resulting lumped predictions of stream flow did not vary. This suggests a linear response of water balance to LAI in the non-water-limited conditions of the study area, and de-emphasizes the importance of quantifying relative spatial variations in LAI. Topographic maps were created for a small experimental subcatchment (15 ha) using both air photographic interpretation and ground survey. The two maps differed markedly and lead to significantly different spatial predictions of runoff generation, but nearly identical predicted hydrographs. Thus, at scales of small basins, accurate topographic mapping is suggested to be of little importance in distribution function modelling because models are unable to make use of complex spatial data. Predictions of water yield can be very sensitive (in the case of precipitation) or insensitive (in the case of small-scale topography) to changes in spatial parameterization. In either case, increased complexity in spatial parameterization does not necessarily result in better, or more certain prediction of hydrological response.

Predicting Bison Migration out of Yellowstone National Park Using Bayesian Models

Long distance migrations by ungulate species often surpass the boundaries of preservation areas where conflicts with various publics lead to management actions that can threaten populations. We chose the partially migratory bison (Bison bison) population in Yellowstone National Park as an example of integrating science into management policies to better conserve migratory ungulates. Approximately 60% of these bison have been exposed to bovine brucellosis and thousands of migrants exiting the park boundary have been culled during the past two decades to reduce the risk of disease transmission to cattle. Data were assimilated using models representing competing hypotheses of bison migration during 1990–2009 in a hierarchal Bayesian framework. Migration differed at the scale of herds, but a single unifying logistic model was useful for predicting migrations by both herds. Migration beyond the northern park boundary was affected by herd size, accumulated snow water equivalent, and aboveground dried biomass. Migration beyond the western park boundary was less influenced by these predictors and process model performance suggested an important control on recent migrations was excluded. Simulations of migrations over the next decade suggest that allowing increased numbers of bison beyond park boundaries during severe climate conditions may be the only means of avoiding episodic, large-scale reductions to the Yellowstone bison population in the foreseeable future. This research is an example of how long distance migration dynamics can be incorporated into improved management policies.