An Investigation of Forest-Grassland Dynamics in Southwest Yukon, Canada
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Forest encroachment has been documented across North America, from British Columbia to New Mexico, and is a growing concern due to loss of essential grassland habitat. Climate change, fire suppression, changes in grazing regimes, and differences in microclimate between topographic gradients are the main factors associated with forest encroachment into grasslands. Small-scale factors, such as competition and facilitation also play an important role in forest-grassland dynamics. I examined forest-grassland dynamics in southwest Yukon through dendroecological techniques and repeat image analysis. Dendroecological techniques were used to identify periods of tree encroachment, changes in age structure, pulses of tree establishment and possible correlations with climatic variables. Dendroecological results indicated that over the last 60-80 years, trees have invaded an average of 30 meters into grasslands on south-facing slopes and flat terrain in southwest Yukon. Ecotones on north-facing slopes appear stable with little advance into grasslands over the last 60 years. Results indicate forest encroachment varies across topography and between tree species. Repeat image analysis was also used to examine landscape changes over a 60-year time period near Kluane Lake. Forest encroachment was investigated using landscape metrics which characterized changes in grassland configuration across three time periods (1947, 1979, 2007) in a 10km2 area. Total grassland area decreased from 214.4 hectares in 1947 to 137 hectares in 2007 and coincided with extensive grassland fragmentation. One hundred and seventy grassland patches were identified in 1947 which increased to 270 patches in 2007. Although tree invasion was found across all topographic gradients, results complement dendroecological analyses with flat terrain and south-facing slopes experiencing the greatest loss of grassland. It is possible that the increase in trembling aspen (Populus tremuloides) on south-facing slopes and flat terrain is due to warmer temperatures in the area. Aspen establishment coincided with warmer temperatures however further work is needed to identify the influence of changes in fire and grazing regimes. Although southwest Yukon grasslands are limited in distribution they are ecologically significant and provide habitat for unique assemblages of flora and fauna.