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The relative greenness and wetness of Giant Sequoia (Sequoiadendron giganteum, SEGI) groves and the surrounding Sierra Nevada, California forests were investigated using patterns in vegetation indices from Landsat imagery for the period 1985-2015. Vegetation greenness (normalized difference vegetation index) and thus forest biomass in groves increased by about 6% over that 30-year period, suggesting a 10% increase in evapotranspiration. No significant change in the surrounding non-grove forest was observed. In this period, local temperature measurements showed an increase of about 2.2 °C. The wetness of groves (normalized difference wetness index) showed no overall long-term trend, but responded to changes in annual water-year precipitation and temperature. The long-term trends of grove greenness and wetness were elevation dependent, with the lower rain-snow transition elevation zone (1700 – 2100 m) marking a change from an increasing trend at lower elevations to a decreasing trend at higher elevations. The 2011-2015 drought brought an unprecedented drop in grove wetness, over five times the 1985-2010 standard deviation; and wetness in SEGI groves dropped 50% more than in non-grove areas. Overall, the wetness and greenness of SEGI groves showed a larger response to the warming climate and drought than non-grove areas. The influence of droughts on the wetness of SEGI groves reflected effects of both the multi-decadal increase in forest biomass and the effects of warmer drought-year temperatures on the evaporative demand of current grove vegetation, plus sufficient regolith water storage of rain and snowmelt to sustain that vegetation through seasonal and multi-year dry periods.

Lakes are dynamic processors of nutrients and may hold an active role in modifying the water chemistry of stream systems. In this study, we examine the influence of lakes and an important terrestrial component—forest cover—on the nutrient levels of stream water in 11 Rocky Mountain (Idaho) watersheds. Water samples were collected from the inflow and outflow of lakes with varying amounts of upstream lakes and forested area during spring runoff and summer base flow. During base flow, mean total nitrogen concentrations at the inflow of final lakes were significantly related to relative upstream lake area, increasing from 34 to 103 μg N L-1 as upstream lake cover in the watersheds increased from 0% to 4%. Forest cover was not significantly correlated with total nitrogen concentrations exported from the watersheds. However, similar to other studies, inflow nitrate concentrations were negatively correlated with relative forest cover. At the scale of individual lakes, dissolved organic carbon and dissolved organic nitrogen increased, and nitrate was significantly reduced from inflows to outflows. The results indicate a potentially critical role of mountain lakes in governing nitrogen flux downstream, and suggest the role of lakes as nitrogen sinks or sources may reverse between spring runoff and summer base flow.

In glaciated mountains, lakes are interspersed through watersheds and connected by streams. Although lakes or streams are frequently studied as individual water bodies, studying them as integrated functional units provides considerable insight on temperature patterns, nutrient transport and other functions. In the Sawtooth Mountains (Idaho), inter-lake distance averages 2.8 km. In summer, lakes are solar collectors, and warm outflow streams as much as 10 C, thus increasing metabolic rates. These streams seldom cool to equilibrium temperatures before encountering another lake, where the waters tend to overflow and mix into the epilimnion. With overflow, N-15 tracer experiments demonstrated that water and nutrients pass quickly through the epilimnion to the outflow stream where they can be taken up by periphyton. In contrast, in watersheds without upstream lakes, cold stream inflows plunge into the metalimnia and bring nutrients to the deep chlorophyll layer where they are retained, thus reducing downstream transport to other lakes and stream reaches as much as 5-fold. A landscape approach is consequently necessary to understand how the lakes and streams in these watersheds function.