Keith Smettem

The altitudinal changes of plant phenology in response to climate change remain poorly understood in subtropical mountain areas. Using the satellite phenology and climate dataset (temperature, precipitation and solar radiation) from 2001 to 2016 in southwest China, we analyzed the spatiotemporal changes of climate and phenological characteristics of the growing season length (LOS), start of the growing season (SOS) and end of the growing season (EOS) for the deciduous broadleaf forest (DBF). Results show that LOS was shortened by 25 and 15.2 days/km rise in elevation, respectively, using two regression methods based on “Hopkins' bioclimatic law” (expressing LOS as a function of altitude, latitude and longitude) and altitudinal mean annual LOS. The majority of the shortened LOS towards high elevations was attributed to the postponed SOS and the advanced EOS as the elevation is higher and lower than 2.2–2.3 km, respectively. The recent climate warming has significantly prolonged LOS in the entire DBF area. This increase in LOS differs with altitude due to altitudinal heterogeneity of climate change. In the cold high mountain environment, changes of phenological parameters are more sensitive to climate warming, characterized by a significantly advanced SOS, postponed EOS and prolonged LOS driven by spring and autumn warming. In the warm environment of the low elevation areas, changes of phenological parameters are relatively smaller even though the temperature rise is greater than that in the cold high mountains. Furthermore, winter wetting can significantly weaken the advanced SOS and prolonged LOS at lower elevations in the warm south, but winter drying and declining solar radiation in spring can enhance the advanced SOS and prolonged LOS at the extremely high elevations in the cold north. These results highlight the critical need to include altitudinal heterogeneity when assessing phenological changes from remote sensing platforms.

Karst plant habitats are generally harsh and vulnerable to climate and human interferences. Restoration of karst vegetation and its effect on water resources are affected not only by climate and anthropological interferences but also by different bedrock lithologies. In this study, changes to climatic factors, vegetation indices of the normalized difference vegetation index (NDVI), the leaf area index (LAI) and runoff during 1982–2015 were statistically identified over the Yangchang river basin (YRB) with two contrasting bedrock lithologies (carbonate rock and detrital rock) in southwest China. The Budyko equation and the hydrological model in LPJ (the Lund-Potsdam-Jena Dynamic Global Vegetation Model) were calibrated and improved to separate runoff change that could be attributed to climate and vegetation changes during 1982 ~ 2015. LPJ was used to predict the potential vegetation for natural recovery without human interference and its effect on the hydrological budget in YRB. Results reveal that the execution of the major ecological restoration project (the Grain for Green Project) in YRB since the early 2000 s has significantly increased NDVI and LAI even in the drier period of 2004 ~ 2015. However, the human effort of vegetation restoration only reaches about 25% of the potential capacity in the whole study area. The reforestation in YRB could decrease runoff by about 7.0 to 7.6% in the period from 2004 to 2015, and 11.2% in the future when vegetation shifts from artificial to natural vegetation recovery. Meanwhile, thick soil in the non-carbonate area can store more water to support a large proportion of forest, and raise drought resistance. However, the limited water holding capacity of weathered bedrock in the carbonate area restricts large tree growth, and thus reduces drought resistance. These results point a need for consideration of how bedrock geology influences available water limitations when designing suitable reforestation strategies in southwest China.

Soil water repellency (SWR) is a widespread phenomenon that influences patterns of soil wetting, runoff, evapotranspiration and availability of water for plants. In natural ecosystems there is emerging evidence that some plants can take advantage of non-uniform wetting patterns, leading to the emergence of co-evolutionary behaviour. In this review, SWR is considered in terms of five spheres of influence. Given the presence of hydrophobic organic material in the biosphere, the strength, severity and persistence of SWR is influenced by properties at the surface of the lithosphere and prevailing conditions in the atmosphere and hydrosphere. These in turn, can be modified by activities in the anthroposphere. This review thus examines the strength, severity and persistence of non-wetting behaviour with reference to these five spheres of influence and also the interactions between the spheres. It is focused on (i) how SWR is characterised to provide insight into how different measurement techniques have specific operational ranges, (ii) how SWR has developed as an indirect consequence of evolution in natural ecosystems and (iii) how feedbacks across the different spheres have emerged. It demonstrates that management and restoration of natural ecosystems with water repellent soils is very different from management of productive crops in monocultural agricultural systems, controlled in the anthroposphere.

Salinization of land is a form of desertification; salinization of rivers is a global threat to biodiversity and compromises the ecosystem goods and services of rivers, wetlands, and lakes. Salinization is caused by flooding or inundation with saline waters, breaching of dykes, storm surges, tsunamis, or the drying of large inland water bodies. Salinization can result where irrigation waters are compromised by salinity. Salinity intersects with major global concerns, including food security, desertification, and biodiversity protection. Soil salinity results from an excess of salts in the soil that reduces plant growth and crop productivity and affects soil biological activity. Salinized soils impose an osmotic stress on plants, reducing water uptake and concentrating toxic level of sodium and chloride. Different plant species exhibit different degrees of salinity tolerance. Salinization removes arable land from production, causing abandonment globally of 0.3–1.5 million hectare year−1. With adequate drainage, salts can be leached and the soil recovered but where the water table remains near the surface, the salinity problem will remain. It may be possible to reverse the effects of salinization. A crucial consideration is whether the desired end point is stabilizing the soils against further change, or reversing the process and restoring soils to another state. Approaches include prevention, stabilization, active management, or land retirement or abandonment. Successful restoration of salinity at the landscape-scale relies on broadscale land-use change. This is problematic where the most profitable land-use is agriculture, thus there has therefore been considerable investigation of land-use systems that at least replicate the profitability of the current agricultural system. Recent approaches have explored how to make the higher water using farming systems acceptable by making the replacement plants profitable in their own right.

Understanding the spatiotemporal patterns of vegetative cover in relation to climate and land uses is essential for effective management of ecology and the environment. In this study, spatial and temporal changes of the normalized difference vegetation index (NDVI) and potential influencing factors were analyzed in different elevations and land uses across southwest China. Results showed: (1) there was a critical elevation of 3400 m, with different NDVI responses to climate and human interventions above and below 3400 m. Below 3400 m, mean NDVI in each land use area and the whole region did not change with elevation due to compensative effects of decreasing cultivated land and increasing woodland and grassland towards high elevations. Above 3400 m, cultivation effectively ceases. NDVI decreased with elevation as alpine plant species shifted from woody trees to alpine grass, primarily related to declining temperature towards high altitudes. (2) NDVI responses to climate change and human activities are also different above and below 3400 m. NDVI below 3400 m increased significantly after 1980s, primarily as a result of reforestation on hillslopes and improved agricultural productivity. Above 3400 m, under climate warming since the 1980s, NDVI did not increase significantly in 1990s and even decreased in 2000s as the consecutive rise of temperature is higher towards higher altitudes in the 2000s. (3) The area-weighted NDVIs illustrated that from 1980s to 2000s, the increased mean NDVI in the whole region arose from contributions of 20.93, 60.66 and 18.41% changes in NDVIs in cultivated land, woodland and grassland, respectively. In 2000s, the proportion of the woody trees contribution to NDVI increased due to reforestation in the low elevation area (<3400 m), but decreased due to shift of the woody trees to alpine grass under the consecutive climate warming in the high elevation area (>3400 m). The decease of NDVI in the high elevations did not alter increasing trend of NDVI across the whole region during 1982–2015. However, in future, the greening could diminish or even cease as climate warming continues and effects of artificially managed ecological restoration reduce.

Substrate conditions for denitrifying bacteria were enhanced by adding carbon sources to a laboratory-scale sand filter system. Temperature, oxidation–reduction potential, and hydrogen ion concentration were measured through the recirculation of nitrogen-dosed wastewater and carbon sources that were mixed to encourage microbial growth, with denitrifying bacteria identified by standard plate counts. Two different external carbon sources (sucrose and ethanol) were added, with and without activated sludge amendments. Nitrate, nitrite, and chemical oxygen demand (COD) concentrations were monitored relative to an untreated control and a treatment with activated sludge under an initial hydraulic loading rate of 0.508 m3 /m2 d and a hydraulic retention time of 2.5 h. Nitrate decay rates were only significantly enhanced for the ethanol treatment without addition of activated sludge. Nitrite initially accumulated when carbon sources were added, but no accumulation was evident by the end of the experiment after 150 min. COD declined when carbon sources were added, but activated sludge had no effect on the rate at which the COD declined. The increased rate of nitrate removal with the addition of ethanol is of technical interest, as the volume of waste-water treated in a unit volume of filter medium for denitrification doubled with ethanol compared with sucrose at the same concentration.

Increasing atmospheric CO2 is both leading to climate change and providing a potential fertilisation effect on plant growth. However, southern Australia has also experienced a significant decline in rainfall over the last 30 years, resulting in increased vegetative water stress. To better understand the dynamics and responses of Australian forest ecosystems to drought and elevated CO2, the magnitude and trend in water use efficiency (WUE) of forests, and their response to drought and elevated CO2 from 1982 to 2014 were analysed, using the best available model estimates constrained by observed fluxes from simulations with fixed and time-varying CO2. The ratio of gross primary productivity (GPP) to evapotranspiration (ET) (WUEe) was used to identify the ecosystem scale WUE, while the ratio of GPP to transpiration (Tr) (WUEc) was used as a measure of canopy scale WUE. WUE increased significantly in northern Australia (p < 0.001) for woody savannas (WSA), whereas there was a slight decline in the WUE of evergreen broadleaf forests (EBF) in the southeast and southwest of Australia. The lag of WUEc to drought was consistent and relatively short and stable between biomes (≤3 months), but notably varied for WUEe, with a long time-lag (mean of 10 months). The dissimilar responses of WUEe and WUEc to climate change for different geographical areas result from the different proportion of Tr in ET. CO2 fertilization and a wetter climate enhanced WUE in northern Australia, whereas drought offset the CO2 fertilization effect in southern Australia.

The addition of biodegradable carbon sources to sand filters can enhance microbial activity but may lead to substrate clogging, a major operational problem. In laboratory-scale soil columns emulating vertical up-flow filters, the clogging effect of two readily biodegradable organic substrates—sucrose as a sugar source and ethanol as an alcohol source—were examined with coarse sand as the substrate medium. Wastewater without the addition of supplemental organics and a ‘control’ treated with tap water were monitored as references. Changes in saturated hydraulic conductivity were measured for all treatments over time. Other parameters that can influence the clogging rate, including temperature, dissolved oxygen, chemical oxygen demand, protein, and polysaccharides, were measured in the influent and effluent wastewater on a weekly basis. At the end of the clogging experiment, the main layer of each filter bed was separated into three sections and saturated hydraulic conductivity, organic matter content, and protein and polysaccharide concentrations were measured in each section. The rate of clogging development in the columns depended on treatment, with ethanol-treated cores clogging more quickly than sucrose-treated cores. Wastewater-treated cores took far longer to clog and the tap water control did not clog, but the saturated hydraulic conductivity declined by 60% over a year. Saturated hydraulic conductivity within the treated cores declined far less than the calculated decline in saturated hydraulic conductivities for the entire cores at the end of the experiment, indicating that clogging in the vicinity of the inlet plate by microbial mats was a major factor influencing the reduction in flow through the columns. To reduce bio-clogging in inlet filters, it may be advantageous to inject organic amendments directly into the bed, rather than pass them through the filters, as is usually the case.

Southwestern Australia has experienced recent climate change, with an increase in air temperature of 0.6°C and a reduction in mean annual precipitation of -15% since 1970. Along with the warming and drying trends, dramatic declines of streamflow have occurred across the region. However, both forest mortality and an increase in leaf area index have been observed in the southwestern forest, suggesting varied responses of vegetation to climate change. In this study, 30 catchments were analyzed using the Mann-Kendall trend test, Pettitt’s change point test and the theoretical framework of the Budyko curve to study the rainfall-runoff relationship change, and effects of climate and land cover change on streamflow. A declining trend and relatively consistent change point (2000) of streamflow were found in most catchments, with 14 catchments showing significant declines (p < 0.05, -68.1% to -35.6%) over 1970-2000 and 2001-2015. Most of the catchments have been shifting towards a more water-limited climate condition since 2000. For the period of 1970 to 2015, the dynamic of vegetation attributes (land cover/use change and growth of vegetation) dominated the decrease of streamflow in about half the study catchments. In general, a coequal role of climate and vegetation on the decline in streamflow was found in the study, suggesting the importance of vegetation management on future water management and production. Precipitation is predicted to decline in the future; therefore, some forest management intervention is required to maintain forest growth and water supply in the southwest of Australia.

Key message In a major Australian city, water supply has been decoupled from forests as a result of management and climate change. Water yield and quality are closely related to forest cover and have been manipulated through broad-scale intervention. The forests remain important for biodiversity protection and considering water as a forest product will fund interventions that maintain the forest’s environmental values. Context Perth, an Australian city of 2 million people and a potable water demand of 300 GL/year, occurs in a region that has experienced a decline in rainfall and a major reduction in surface runoff to water supply reservoirs over the last 40 years. This has led to a major impact on water policies, with the collapse of surface water supply from forested watersheds resulting in the almost complete substitution of Perth’s water supply with groundwater and desalinated water. Thus, water supply has been decoupled from forests and forest management processes. Aims In this paper, we review the interactions between forest cover and water supply in the drying environment of south-western Australia, exploring studies on the hydrological effects of extensive deforestation for agricultural development, widespread reforestation, forest management, and reduced annual rainfall. We draw conclusions applicable to other regions that are experiencing the combined impacts of climate change and pressures from land-use intensification. Results We find that streamflow and water quality are clearly linked to forest cover and this is affected by both climate and forest management. Streamflow increases with a reduction of forest cover (through deforestation or thinning) and decreases with reforestation and reduced rainfall. Stream salinity increases with deforestation and decreases with reforestation. Hydrological responses occur where forest cover treatments have been applied and maintained at watershed-scales. Surprisingly, where water yield or quality has been improved, this has not been rewarded financially and there is a need to develop methods of financing treatments to maintain streamflow. Conclusion Whereas forests were initially maintained for water and timber supply, with biodiversity protection as a co-benefit without a defined value, the decoupling of forests from water supply has substantially reduced the financial resources for any form of direct forest management. As the forests remain important for biodiversity protection, a key recommendation is to consider water as a forest product and thus provide funds for watershed-scale treatments, such as forest thinning, that maintain the forest’s environmental values in a drying climate.