Flood disturbance affects morphology and reproduction of woody riparian plants

Riparian forests are structured and maintained by their hydrology. Woody riparian plants typically adapt to the local flood regime to maximise their likelihood of survival and reproductive success. Understanding how extant trees form and reproduce in response to flood disturbance is crucial for predicting vegetation changes and informing restoration. Working in a temperate evergreen riparian forest, we aimed to determine whether disturbance-based responses of plants found in other ecosystems also typify woody plants in riparian forests where disturbances are often mild or chronic, non-lethal, annual events. Using plant surveys and 20-year modelled hydrological data, we examined whether (1) the morphology (main stem diameter, height, crown width, crown extent, stem leaning) and (2) reproduction type (sexual and asexual reproduction) and extent of three dominant woody species (Eucalyptus camphora, Leptospermum lanigerum and Melaleuca squarrosa) vary with flood regime (flood frequency and flood duration); and (3) whether different morphology is associated with different reproductive strategies. Increased flooding generally resulted in increased stem numbers and greater stem leaning—morphologies associated with asexual reproduction—of our study species. More frequent flooding also reduced plant size and sexual reproduction in E. camphora. Sexual reproduction in the studied species was more common in taller plants with single, more upright stems in good condition. Flexible morphology and plastic reproductive strategy may constitute an adaptation of trees to mild or chronic disturbance in floodplains. Our findings suggest that flood regime (i.e. variable frequency and duration of flooding events) is critical to the structural integrity and self-maintenance of species-diverse riparian forests.

Equids engineer desert water availability

Megafauna play important roles in the biosphere, yet little is known about how they shape dryland ecosystems. We report on an overlooked form of ecosystem engineering by donkeys and horses. In the deserts of North America, digging of ≤2-meter wells to groundwater by feral equids increased the density of water features, reduced distances between waters, and, at times, provided the only water present. Vertebrate richness and activity were higher at equid wells than at adjacent dry sites, and, by mimicking flood disturbance, equid wells became nurseries for riparian trees. Our results suggest that equids, even those that are introduced or feral, are able to buffer water availability, which may increase resilience to ongoing human-caused aridification.

Flood disturbance affects morphology and reproduction of woody riparian plants

Riparian forests are structured and maintained by their hydrology. Woody riparian plants typically adapt to the local flood regime to maximise their likelihood of survival and reproductive success. Understanding how extant trees form and reproduce in response to flood disturbance is crucial for predicting vegetation changes and informing restoration. Working in a temperate evergreen riparian forest, we aim to determine whether disturbance-based responses of plants found in other ecosystems also typify woody plants in riparian forests where disturbances are non-lethal, annual events. Using plant surveys and 20-year modelled hydrological data, we examined whether i) the morphology (main stem diameter, height, crown width, crown extent, stem leaning) and ii) reproduction type (sexual and asexual reproduction) and extent of three dominant woody species (Eucalyptus camphora, Leptospermum lanigerum and Melaleuca squarrosa) vary with flood regime (flood frequency and flood duration); and iii) whether different morphology is associated with different reproductive strategies. Increased flooding generally resulted in increased stem numbers and greater stem leaning – morphologies associated with asexual reproduction – of our study species. More frequent flooding also reduced plant size and sexual reproduction in E. camphora. Sexual reproduction in the studied species was more common in taller plants with single, more upright stems in good condition. Flexible morphology and plastic reproductive strategy may constitute an adaptation to mild or chronic disturbance in floodplains. Our findings suggest that woody plants respond to physical disturbance in consistent ways regardless of the nature of the disturbance – be it fires, hurricanes or floods.

Mind the gap: extending the conceptual model of river island development to different environmental conditions and tree species

<p>We present recent results of field observations on an island braided reach of the middle Tagliamento River, Italy, where riparian vegetation survival and establishment depends on an unstable balance between vegetation growth and flood disturbance. We combined field observations and information extracted from aerial images, airborne lidar data, and river flow time series for the period 1986-2017 to investigate the changing spatial distribution of woody vegetation and the associated changes in river topography. We also explored the role of Alnus incana (a member of the Betulaceae family), in an environment dominated by the Salicaceae family (e.g. Populus nigra).</p><p>We observed that gaps between established islands and/or floodplain offer shelter to vegetation, supporting higher colonisation success and different vegetation-landform evolution pathways.</p><p>In particular, A. incana predominantly grows in lines along channel, island and floodplain edges, bordering wooded areas dominated by P. nigra. Given their association with floodplain and island edges and the relationship of taller (older) trees with more elevated surfaces, A. incana in the study reach appears to complement the physical engineering of the dominant species, P. nigra. This suggests that P. nigra may facilitate colonisation by alder but then both species trap sediments to aggrade channel edges and bar surfaces and build island and floodplain landforms.</p><p>Time sequences of aerial images in combination with the flood disturbance time series allowed us to interpret vegetation dynamics and to identify the fate of sexual and asexual reproduction strategies by observing vegetation expansion from lines of young plants and shrubs and from uprooted deposited trees and pioneer islands, respectively. Field observations are then generalized to extend a conceptual model of island development.</p><p>Growing conditions, disturbance energy, and time (window of opportunity) between major floods are the main controls on vegetation colonization. These vary among rivers, among reaches along the same river and locally, as in the investigated gaps, allowing different tree species with different life history traits (e.g. Populus nigra, Alnus incana) to engineer local river landforms in different and complementary ways.</p><p>Although the conceptual model is inspired by observations on the Tagliamento River, consideration of species life history traits and the joint influences of growing conditions, disturbance energy and windows of opportunity provide a framework that may be applied to other temperate rivers where trees drive landform development.</p>