Mature trees scattered throughout agricultural landscapes are critical habitat for some biota and provide a range of ecosystem services. These trees are declining in intensively managed agricultural landscapes globally. We developed a simulation model to predict the rates at which these trees are declining, identified the key variables that can be manipulated to mitigate this decline, and compared alternative management proposals. We used the initial numbers of trees in the stand, the predicted ages of these trees, their rate of growth, the number of recruits established, the frequency of recruitment, and the rate of tree mortality to simulate the dynamics of scattered trees in agricultural landscapes. We applied this simulation model to case studies from Spain, United States, Australia, and Costa Rica. We predicted that mature trees would be lost from these landscapes in 90-180 years under current management. Existing management recommendations for these landscapes--which focus on increasing recruitment--would not reverse this trend. The loss of scattered mature trees was most sensitive to tree mortality, stand age, number of recruits, and frequency of recruitment. We predicted that perpetuating mature trees in agricultural landscapes at or above existing densities requires a strategy that keeps mortality among established trees below around 0.5% per year, recruits new trees at a rate that is higher than the number of existing trees, and recruits new trees at a frequency in years equivalent to around 15% of the maximum life expectancy of trees. Numbers of mature trees in landscapes represented by the case studies will decline before they increase, even if strategies of this type are implemented immediately. This decline will be greater if a management response is delayed.

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Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrubBaccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (H1) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distribution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of H1). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

This study investigated relationships between birds and characteristics of woodland remnants in the central wheat/sheep belt of New South Wales, in order to provide information for conservation management of woodland birds and their habitats in this region. Birds were surveyed in standard area plots in 36 woodland remnants, which ranged from 1 to 1 376 hectares in size, in the central Lachlan catchment in the wheat/sheep belt of New South Wales. Habitat (e.g., shrub cover) and landscape (measures of isolation) characteristics of the remnants were recorded also. Presence/absence occurrences of 20 or more bird species, mostly woodland birds, were positively related to remnant area, shrub cover, number of shrub species, pine cover, fallen logs and branches, and number of remnants within two and five kilometres of the survey remnant. Species of birds that are habitat generalists showed negative or no relationships with these habitat and landscape variables. Richness of all bird species was positively related to remnant area and shrub cover. Number of woodland bird species was positively related to remnant area, shrub cover and tree hollows. Lower species diversities in smaller remnants were most likely caused by lower chances of survival or of successful breeding in small remnants, effects of isolation on the ability of birds to recolonize smaller remnants, small remnants being of insufficient area for some species to form territories, and generally poor habitat quality in small remnants. Retention, management and enhancement of woodland remnants 10 to 20 hectares or larger in size with good understorey cover are necessary for the future viability of woodland birds in the wheat/sheep belt of New South Wales.