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ABSTRACT:Cold air drainage and pooling occur in many mountain valleys, especially at night and during winter. Local climate regimes associated with frequent cold air pooling have substantial impacts on species phenology, distribution and diversity. However, little is known about how the degree and frequency of cold air drainage and pooling will respond to a changing climate. Evidence suggests that, because cold pools are decoupled from the free atmosphere, these local climates may not respond in the same way as regional-scale climates estimated from coarse-grid general circulation models. Indeed, recent studies have demonstrated that historical changes in the frequency of synoptic conditions have produced complex spatial variations in the resulting climatic changes on the ground. In the mountainous terrain of the Oregon Cascades, we show that, at relatively exposed hill slope and ridge top locations, air temperatures are highly coupled to changes in synoptic circulation patterns at the 700-hPa level, whereas in sheltered valley bottoms, cold air pooling at night and during winter causes temperatures to be largely decoupled from, and relatively insensitive to, 700-hPa flow variations. The result is a complex temperature landscape composed of steep gradients in temporal variation, controlled largely by gradients in elevation and topographic position. When a projected climate warming of 2.5°C was combined with likely changes in the frequency distribution of synoptic circulation, modelled temperature changes at closely spaced locations diverged widely (by up to 6°C), with differences equalling or exceeding that of the imposed regional temperature change. Because cold air pooling and consequent atmospheric decoupling occur in many mountain valleys, especially at high latitudes, this phenomenon is likely to be an important consideration in understanding the impacts of climate change in mountainous regions.

Net canopy interception (I net ) during rainfall in an old-growth Douglas-fir-western hemlock ecosystem was 22.8 and 25.0% of the gross rainfall (P G ) for 1999 and 2000, respectively. The average direct throughfall proportion (p) and canopy storage capacity (S) derived from high-temporal resolution throughfall measurements were 0.36 and 3.3 mm, respectively. Derived values of S were very sensitive to the estimated evaporation during canopy wetting (I w ). Evaporation during wetting was typically small due to low vapor pressure deficits that usually occur at the start of an event, therefore I w is best estimated using the Penman method during canopy wetting, rather than assuming a constant evaporation rate over an entire event. S varied seasonally, from an average of 3.0 mm in the spring and fall, to 4.1 mm in the summer, coincident with canopy phenology changes. Interception losses during large storms that saturated the canopy accounted for 81% of I net . Canopy drying after events comprised 47% of I net , evaporation during rainfall comprised 33%, and evaporation during wetting accounted for 1%. Interception associated with small storms insufficient to saturate the canopy accounted for 19% of I net . The Gash analytical model accurately estimated both I net and the individual components of I net in this system when applied on an event basis, and when the Penman method was used to compute evaporation during rainfall. The Gash model performed poorly when applied on a daily basis, due to a rainfall regime characterized by long-duration events, which violated the assumption of one rain event per day.

Drought stress plays an important role in determining both the structure and function of forest ecosystems, because of the close association between the carbon (C) and hydrological cycles. We used a detailed model of the soil-plant-atmosphere continuum to investigate the links between carbon uptake and the hydrological cycle in a mature, open stand of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at the Metolius river in eastern Oregon over a 2-year period (1996-1997). The model was parameterized from local measurements of vegetation structure, soil properties and meteorology, and tested against independent measurements of ecosystem latent energy (LE) and carbon fluxes and soil water content. Although the 2 years had very different precipitation regimes, annual uptake of C and total transpiration were similar in both years, according to both direct observation and simulations. There were important differences in ratios of evaporation to transpiration, and in the patterns of water abstraction from the soil profile, depending on the frequency of summer storms. Simulations showed that, during periods of maximum water limitation in late summer, plants maintained a remarkably constant evapotranspirative flux because of deep rooting, whereas changes in rates of C accumulation were determined by interactions between atmospheric vapor pressure deficit and stomatal conductance. Sensitivity analyses with the model suggest a highly conservative allocation strategy in the vegetation, focused belowground on accessing a soil volume large enough to buffer summer droughts, and optimized to account for interannual variability in precipitation. The model suggests that increased allocation to leaf area would greatly increase productivity, but with the associated risk of greater soil water depletion and drought stress in some years. By constructing sparse canopies and deep rooting systems, these stands balance reduced productivity in the short term with risk avoidance over the long term.

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