Alteration of the hydrologic cycle due to forest clearing and its consequences for rainforest succession

Abstract

Since the 19th century, 41% of the land on Chiloé Island (41°50′S, 73°40′W) in Chile was cleared. Following clearing and burning, much of the converted land remains in sparse shrub cover. We hypothesized that the arrested conversion back to forests may reflect a nearly permanent condition associated with a rise in the water table. To evaluate this possibility we acquired data from a 60-year old evergreen forest and an area in shrub cover to parameterize two hydrologic models; one that accounts for hourly interception losses and predicts net precipitation (Gash model), the other that calculates hourly transpiration from both overstory and understory components as well as evaporation from the soil (a modified Penman–Monteith model). In addition, standpipes were installed to record water table levels over 18 months. The fraction of a total annual precipitation (∼2100 mm) transpired by shrub and forest cover differed (8% versus 22%) roughly in proportion to differences in the leaf area index (2.2 versus 5.0). Although whole canopy (stomatal) conductances were similar, the aerodynamic conductance was more than three-fold higher for forests compared with shrub cover (∼12 mol m −2 s −1 versus 3 mol m −2 s −1). The frequent wetting of tree canopies, combined with an average wind speed of 0.74 m s −1, resulted in ∼30% interception losses from forests compared with 1% of annual precipitation lost through this pathway from shrub cover. As a result of these differences, only about half of the precipitation enters the ground under forest cover compared to 90% under shrub cover. This difference in canopy interception losses accounts for a rise in the water table from an average of 45–10 cm. The high water table prevents normal tree regeneration. This condition is stable unless an effort is made to provide an elevated substrate for tree seedlings to become established.