A method for dendrochronological assessment of medium-term gully erosion rates

A method based on dendrochronology to estimate gully erosion rates was deve loped as an alternative of traditional methods for assessing medium-term gu lly retreat rates, such as field monitoring of headcuts or aerial-photo int erpretation of gully retreat. The method makes use of trees or parts of a t ree affected by gully erosion revealing information on the history of the e rosion process by datable deviations of their normal growth pattern, hence defined as 'datable objects'. These include roots exposed by erosion; brows ing scars made by ungulates on exposed roots or on above-ground parts of fa llen trees; exposed and dead root ends; root suckers; stems, branches and l eading shoots of fallen trees; and a sequence of trees within a gully. The method is based on the differentiation between three main conditions depend ing on the relation between the dynamics of the datable object (part of the tree) and the development of the gully. The first condition implies that t he datable object was created before erosion of the gully volume to be date d, e.g. exposed tree roots. According to the second condition, the datable object developed as an immediate consequence of the erosion event, e.g. gro wth reactions of a fallen tree. The third condition implies that the databl e object was created some time after the erosion event took place, e.g. tre es colonising the gully bed. Each principle has consequences for the accura cy and the correct interpretation of the estimated erosion rate, i.e. wheth er the true erosion rate is underestimated, exact or overestimated. In spit e of methodological limitations and dendrochronological dating problems, th e method was successfully applied in southeast Spain. Conservative estimati ons of gully-head retreat rate resulted in an average medium-term (3-46 yea rs) value of 6 m(3) year(-1) (n = 9). For gully sidewall processes, the ave rage minimum erosion rate per unit sidewall length amounted 0.1 m(3) year(- 1) m(-1) (n = 9). A strong correlation was found between the headcut retrea t rate (upsilon (m(ortho)) m(3) year(-1)) and the drainage-basin area (A, m (2)) of the gullies, expressed by upsilon (m(ortho)) = 0.02 A(0.57) (R-2 = 0.93, n = 9). Comparing the findings from this study with those obtained by short-term headcut retreat monitoring suggests a high reliability of the e stimated retreat rates, supporting the applicability of the developed dendr ochronological method. (C) 2001 Elsevier Science B.V. All rights reserved.