Simulating historical variability in the amount of old forests in the Oregon Coast Range.

We developed the landscape age-class demographics simulator (LADS to model historical variabil- ity in the amount of old-growth and late-successional forest in the Oregon Coast Range over the past 3,000 years. The model simulated temporal and spatialpatterns offorestfires along with the resultingfluctuations In the distribution of forest age classes across the landscape. Parameters describing historical fire regimes were derived from datafrom a number of existing dendroecological and paleoecological studies. Our results indicated that the historical age-class distribution was highly variable and that variability increased with de- creasing landscape size. Simulated old-growth percentages were generally between 25% and 75% at the prov- ince scale (2,250,000 ha) and neverfell below 5%. In comparison, old-growth percentages varied from 0 to 100% i at the late-successional reserve scale (40,000 ha). Province-scale estimates of current old-growath (5%) and late-successionalforest ( 1%) in the Oregon Coast Range were lower than expected tinder the simulated historical fire regime, even when potential errors in our parameter estimates were considered. These uncer- tainties do, however, limit our ability to precisely define ranges of historical variability. Our results suggest that in areas where historical disturbance regimes were characterized by large, infrequent fires, manage- ment of fiorest age classes based on a range of historical variability may be feasible only at relatively large spatial scales. Comprehensive landscape management strategies will need to consider otherfactors besides thepercentage of oldforests on the landscape, Including the spatialpattern of stands and the rates andpath- ways of landscape change. Simulaci6n de Variabilidad Hist6rica en la Cantidad de Bosque Maduro en la Cordillera Costera de Oregon Resumen: Desarrollamos un simulador de demograffas de clases de edadles en paisajes (LADS) para mode- los la variabilidad hist6rica de la cantidad de bosque maduro y bosque sucesional tardfo en la cordillera costera tle Oregon, USA para los ziltimos 3,000 aitos. El modelo simul6patrones espacialesy temporales lte in- cendlos junto con fluctuaciones en la distribuciln de clases de edades del bosque ttilizando datos te un nmimero de estbdios dendroec6logicos y paleoecol6gicos existentes Nuestros resultados indican que la dis- tribuci6n histdrica de las clases de edades fue altamente variable y qtue la variabilidad incrementaha con uLna disminuci6n en el tamanio delpaisaje. Procentajes de bosque maduro simuladosfueron generalmente entre 25%y 75% a escala deprovincia (2,250,000 ha)y nuncaflee menor a 5%. En comparaci6n, losporcen- tajes de bosque maduro vailaron de 0% a 100% a escala de reserva sucesional tardfa (40,000 ha). Las esti- maciones actuales a escala de provincia para bosque maduro (5%) y bosque stcesional tardio (11%) en la cordillera costera de Oregon fueron mds bajas que las esperadas bajo el rdgimen de incendios hist6ricos sim- ulado, ad n caando se consider6 un error potencial en nuestrosparametros estimados. Estas incertiflumbres limitan, sin embargo, nuestra habilidad para definir con precisl6n los rangos hist6ricos de variabilidad. Nuestros resultados sugleren que en dreas donde los regimenes deperturbaci6n hist6rica estuvleron caracter-