FOREST ECOSYSTEM PROCESSES AT THE WATERSHED SCALE - DYNAMIC COUPLING OF DISTRIBUTED HYDROLOGY AND CANOPY GROWTH

The hydrological recovery of watersheds from disturbances such as fire and harvest can change the magnitude and distribution of flow paths a s the canopy regenerates. The spatial distribution of net water input to the soil-topography system is mediated by vegetation patterns throu gh the processes of interception, evapotranspiration and snowmelt. We have previously described RHESSys, a distributed model of water and ca rbon flux with a prescribed canopy cover. Although the canopy structur e varied spatially it did not change through time. We present an expan ded model in which carbon and nitrogen are dynamically coupled with di stributed hydrology. The model fixes and allocates canopy carbon annua lly to reflect changes in climate forcing. We test the interactions of the forest ecosystem to distributed hydrology through controlled expe riments. In the first experiment, we prescribe canopy cover and examin e the sensitivity of the hydrological outputs to the distribution of v egetation. The canopy distribution is found to have significant effect s on simulated hydrological outputs where evaporative demand exceeds a vailable water. In a second experiment we simulate the canopy leaf are a index (LAI) across the topography and through time. The model is exe cuted over 100 years using repeated 10-year meteorological records to investigate spatial and temporal patterns of LAI. Annual precipitation and temperature differences result in temporally fluctuating LAI abou t a reasonably stable long-term mean. The topographical position has a strong effect on local forest canopy characteristics. As expected, si mulated ecosystem processes are found to be sensitive to rooting depth in more water limited environments. (C) 1997 by John Wiley & Sons, Lt d.