Author(s): Diego Garcia De Jalon; Vanesa Martinez Fernandez; Marta Gonzalez Del Tanago
Linked Author(s): Diego García de Jalón
Keywords: No Keywords
Abstract: The encroachment of riparian vegetation (ERV) is a widespread phenomenon in most of Mediterranean rivers. The causes of this process have been linked to changes in land use, to decrease of extensive livestock, to a great demand of water for irrigation, and currently, climate change is reinforcing it. These drivers have led to the stabilization of river channels; reducing their dynamism and favoring the growth of woody vegetation in the river banks, and reciprocally, the development of woody vegetation promotes this stabilization. Moreover, the process associated with ERV is a hysteresis phenomenon that requires greater inputs of resources to reverse the process, representing a challenge to river managers. Specifically, flow regulation pressure is frequently associated with irrigation in Mediterranean regions. This type of regulation changes the flow regime of the river, reducing natural discharge during the wet season when the reservoir is filling and increasing natural discharge during the dry season when irrigation occurs. As a consequence, growth of riparian vegetation is promoted as soil moisture levels are increased during summer as opposed to the lower discharge that would otherwise limit growth. Additionally, the reduction of floods promotes the ageing of riparian vegetation and hinders the recruitment of pioneer species. The aim of this work is to assess the response of woody riparian vegetation to changes in catchment land use and flow regulation in Spanish rivers. To that purpose, we have quantified the annual vegetation encroachment ratio and developed a model to understand its controlling factors, such as: land use, floodplain and channel traits; flow regulation intensity; type of regulation; current vegetation cover; distance to the dam; and time since dam completion. The study was conducted in twenty river reaches downstream large dams. A diachronic analysis using aerial photographs from 1956 to 2014 was performed to evaluate the morphological evolution of land use and river reaches. Floodplain dimensions, and channel and riparian vegetation changes were assessed by comparing pre-dam with different post-dam conditions. Recent colored aerial photographs had a resolution 0.25 and 0.5 m in all cases, but older ones were black-and-white at 1:33 000 resolution scales. Similar visual scales were used to cope with different air photographs resolution. Our results show a generalized natural vegetation encroachment process. Only two exceptions were found associated with farming and poplar plantation that occupied riparian soils. Average Annual ERV ratio is 12 Ha per km of river length, but fluctuates from 1 to 55 Ha/km. highest values are found in lowest reaches far from the dam. Also, in these highest values were reach in the years following to beginning dams to operate. However, in other reaches there was a delay of several years in the encroaching process associated to scarcity or absence of initial woody vegetation at the time when dam started working. The process associated with ERV reduces native biodiversity and is a hysteresis phenomenon, and thus represents a problem difficult to be solved. Few Counter-measures may be suggested. Preventing RVE by directly avoiding new recruitment or eliminating saplings with controlling flows may be a simple task. However, once the vegetation has been stablished and grown, especially if it is in dense spots, the flushing flows or peaks flows needed to uproot the plants are often unattainable. Biological control of vegetation encroachment may be an option but it has also secondary effects. Increasing grazing by ungulates may effectively limit the vegetation recruitment and prevent height growth.