Impact of forest disturbance on jarrah (Eucalyptus marginata) forest hydrology

Globally, forests cover 31% of the Earth’s land mass and are critical areas for water supply. In Australia, forested catchments provide 77% of urban water supplies to capital cities. However, recent studies have reported worldwide examples of forest damage resulting from drought or heat related events. The jarrah (Eucalyptus marginata) forests of south-west Western Australia (SWWA) have experienced both sudden and unprecedented forest collapse and profound reductions in streamflows. Projected further declines with climate change, reinforce the need to understand the hydrologic impact of forest disturbance and what management responses are needed to enhance forest resilience and productive capacity. The aim of this thesis was to understand the impact of disturbance and climate on the hydrology of the forests of SWWA, with objectives to: 1. Examine the characteristics of forest hydrology; 2. Evaluate the hydrologic response to forest disturbance and climate variability; and 3. Evaluate forest water management options in the context of forest disturbance and climate change. This thesis thus develops an understanding of the impact of disturbance and climate on the hydrology of the forests of SWWA. Using hillslope and paired catchment studies (Chapter 3), it develops an understanding of the process of infiltration and soil water dynamics and examines the hydrologic impact of forest disturbance. The studies demonstrate the important roles of infiltration, soil water dynamics, and groundwater on the forest water balance, and identify the major factors that impact forest disturbance and forest hydrology. These studies have improved understanding of factors contributing to catchment water balance, and streamflow generation processes. Four catchments underwent land use change and the impact on catchment hydrology was studied by comparing with a control catchment (Chapter 4). These paired catchment studies evaluated the impact of converting forest to agriculture and of timber harvesting. They explored the streamflow generation mechanisms for forested and cleared catchments, the streamflow generation and salinity export changes due to clearing for agriculture, and the hydrologic impact of intense timber harvesting for increased water production. The impacts of deforestation, forest thinning, bauxite mining, bushfires, dieback disease, and reforestation were evaluated using several paired catchment studies across SWWA (Chapter 5). The long-term implications for management of water yield, the impact of a range of disturbances at a catchment scale, and the impact of forest disturbances on stream salinity are also examined. The relationship between the drying climate observed in SWWA over the last 40 years and observed changes in rainfall, groundwater levels, streamflow volumes and flow duration were studied in Chapter 6. The changing relationship between rainfall and streamflow and the likely implications of recent climate change scenarios are also studied. The major forest water issues that have been identified in this thesis are the declining water values in forested areas, such as less water volumes, shorter flow periods, and declining groundwater levels. The adaptive strategies for forest ecosystems are identified to include resistance (protect highly valued areas), resilience (improve capacity to return to pre-disturbance conditions) and response (assist transition to new condition) are discussed in Chapter 7. Drivers identified (Chapter 7) by this thesis include (a) a drying climate with direct and indirect impacts on both the forest itself and on the overall water balance, (b) responses to historical forest management including forest harvest, deforestation and reforestation, (c) long-term impacts of bauxite mining and subsequent rehabilitation, and (d) the interaction of these forest disturbances at a catchment scale. The major findings from this study include: - The high saturated hydraulic conductivity of the sandy gravel topsoil overlies lateritic durcirust with a much lower saturated hydraulic conductivity; - The presence of large infilled “holes” within the lateritic duricrust; - Saturation above the lateritic duricrust was observed confirming subsurface flow concepts; - Presence of vertical preferential flow observed confirming soil water concepts; - The critical importance of the groundwater discharge area in streamflow generation; - Increase in stream salinity directly linked to groundwater levels approaching the surface; - The time to leaching of the salt from the catchment estimated at 200 years; - Forest disturbances such as clearing, timber harvesting and forest thinning led to increased streamflow but with significant delays related to the presence or lack of a groundwater discharge area; and - The extensive reduction in streamflow across the south west has ranged from 36 to 52% (1975 to 2000 compared to 2001 to 2012) seen as a delayed response to rainfall reductions from 1930 to 2000. The challenge for the future is for forest hydrology research to influence current and future forest management to improve environmental and water supply outcomes for the forests of not only SWWA, but globally. Understanding the impact of land-use change on hydrology, water quality and on water resources, and separating this from climate variability and change, is a recurring problem globally. Further understanding is thus needed of the causes of changing forest hydrology and of management options to ultimately improve forest outcomes