The leaf surfaces of the salt-excreting tree Tamarix aphylla harbor a wide diversity of halophilic microorganisms, including Halomonas sp., but little is known of the factors that shape community composition in this extreme habitat. We isolated a strain of Halomonas variabilis from the leaf surface of T. aphylla and used it to determine the heterogeneity of salt concentrations experienced by bacteria in this environment. This halophilic strain was transformed with a proU::gfp reporter gene fusion, the fluorescence of which was responsive to NaCl concentrations up to 200 g liter ؊1 . These bioreporting cells were applied to T. aphylla leaves and were subsequently recovered from dew droplets adhering to the leaf surface. Although cells from within a given dew droplet exhibited similar green fluorescent protein fluorescence, the fluorescence intensity varied between droplets and was correlated with the salt concentration measured in each drop. Growth of H. variabilis was observed in all droplets, regardless of the salt concentration. However, cells found in desiccated microniches between dew drops were low in abundance and generally dead. Other bacteria recovered from T. aphylla displayed higher desiccation tolerance than H. variabilis, both in culture and on inoculated plants, despite having lower osmotic tolerance. Thus, the Tamarix leaf surface can be described as a salty desert with occasional oases where water droplets form under humid conditions. While halotolerant bacteria such as Halomonas grow in high concentrations of salt in such wet microniches, other organisms are better suited to survive desiccation in sites that are not wetted.T he leaf surfaces of Tamarix aphylla (hereafter referred to as Tamarix), a resilient tree capable of growth in soils having a wide variety of water availabilities, are colonized by a diverse microbial community (1, 2). Tamarix is successful in saline soils due to an adaptation mechanism that allows it to excrete excess salt onto its scale-like leaf surface. Some of the salt is subsequently shed from the leaf and salinizes the topsoil, thereby inhibiting the growth of other plants and thus enabling some members of this genus to be aggressive invasive species (3). The high levels of salt (NaCl and other salts) and other exudates on the leaves are hygroscopic, and liquid water readily forms droplets on the leaves when the ambient humidity is sufficiently high (4). Chemical characterization of dew droplets and leaf washes from Tamarix reveal that the leaf surface has a high pH (pH Ͼ 9.0) and dissolved salt concentrations that are at least five times higher than those of seawater (2). Thus, in addition to dealing with the normally harsh conditions encountered on all leaves such as high UV irradiation flux and fluctuating temperatures (5), Tamarix epiphytes must also contend with high pH and salinity.Prior research found that the leaves of Tamarix are mainly colonized by halophilic microorganisms adapted to this highly saline environment (2). Although trees at all sites sampled harb...