Vegetation Trends In Whitehorse Yukon, 1984 To 2009

To assess temporal greenness trends at the landscape scale for Whitehorse, Yukon (417 km2), this study derived a Landsat normalized difference vegetation index (NDVI) time-series from 1984 to 2009. Using a greenest annual pixel approach, NDVI trend analysis revealed that 37% of studied area had significant greening (p<0.05) and that only 1% of the studied land area had significant browning. Yearly mean NDVI values declined in drought years and increased in years with greater precipitation. Greening pixels were most prevalent in white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa) dominant forests, suggesting that increased amounts of precipitation and rising temperatures have benefited both species and associated shrub communities. Forests where trembling aspen (Populas tremuloides) are dominant displayed the least greening, which may be explained by the proliferation of aspen serpentine leaf miner (Phyllocnistis populiella), and drought related die-back on south-facing slopes that have become warmer across the study period.

Vegetation Trends In Whitehorse Yukon, 1984 To 2009

To assess temporal greenness trends at the landscape scale for Whitehorse, Yukon (417 km2), this study derived a Landsat normalized difference vegetation index (NDVI) time-series from 1984 to 2009. Using a greenest annual pixel approach, NDVI trend analysis revealed that 37% of studied area had significant greening (p<0.05) and that only 1% of the studied land area had significant browning. Yearly mean NDVI values declined in drought years and increased in years with greater precipitation. Greening pixels were most prevalent in white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa) dominant forests, suggesting that increased amounts of precipitation and rising temperatures have benefited both species and associated shrub communities. Forests where trembling aspen (Populas tremuloides) are dominant displayed the least greening, which may be explained by the proliferation of aspen serpentine leaf miner (Phyllocnistis populiella), and drought related die-back on south-facing slopes that have become warmer across the study period.

Aspen Soils Retain More Dissolved Organic Carbon Than Conifer Soils in a Sorption Experiment

The effect tree species have on soil organic carbon (SOC) has been hotly debated but, so far, few clear patterns have emerged. One example of a differing tree species effect on SOC are aspen forests in North America, which have been found to have more stable SOC than adjacent conifer forest stands. An important source for the formation of stable organo-mineral complexes in soil is dissolved organic carbon (DOC). DOC concentrations in mineral soil are often higher under the thick O-horizons of conifer forests than under aspen forests, but this does not correspond to more stable mineral SOC. This suggests that, instead of DOC concentration, DOC quality could be driving the observed differences in SOC. Therefore, we quantified the retention of contrasting forest detritus DOC in soils. Using a batch sorption experiment approach, we compared the retention of detritus leachates from four sources – aspen leaves (AL), aspen roots (AR), conifer (subalpine fir) needles (CN), and conifer (subalpine fir) roots (CR) – on soils sampled under aspen and conifer (subalpine fir and Douglas fir) overstories. The calculated sorption isotherms showed a higher retention of AL DOC than AR DOC, as indicated by all four sorption parameters – k and n (curve-fitting parameters), null point concentration (NPC; net sorption = net desorption), and endpoint (EP, retention at the highest initial DOC concentration). Leachates from CN and CR showed very similar retention behavior, and between the two species the retention of root leachates was more similar than the retention of foliage leachates. Soils sampled from aspen forests showed higher affinity for new DOC than conifer soils [higher sorption rate (n), lower NPC, and higher EP] regardless of the DOC source. The findings suggest that the higher DOC sorption on aspen soils might be a major driver for more stable SOC under aspen stands in North America.

Subalpine Fir (Abies laciocarpa) and Norway Spruce (Picea abies) Seedlings Show Different Growth Responses to Blue Light

Blue light (BL) affects different growth parameters, but information about the physiological effects of BL on conifer seedlings is limited. In northern areas, conifer seedlings are commonly produced in heated nursery greenhouses. Compared with Norway spruce, subalpine fir seedlings commonly show poor growth in nurseries due to early growth cessation. This study aimed to examine the effect of the BL proportion on the growth and development of such conifer seedlings in growth chambers, using similar photosynthetic active radiation, with 5% or 30% BL (400–500 nm) from high pressure sodium (HPS) lamps (300 μmol m−2 s−1) or a combination of HPS (225 μmol m−2 s−1) and BL-emitting diodes (75 μmol m−2 s−1), respectively. Additional BL increased transpiration and improved the growth of the Norway spruce seedlings, which developed thicker stems, more branches, and a higher dry matter (DM) of roots and needles, with an increased DM percentage in the roots compared with the shoots. In contrast, under additional BL, subalpine fir showed reduced transpiration and an increased terminal bud formation and lower DM in the stems and needles but no change in the DM distribution. Since these conifers respond differently to BL, the proportion of BL during the day should be considered when designing light spectra for tree seedling production.

Stable or seral? Fire-driven alternative states in aspen forests of western North America

As important centres for biological diversity, aspen forests are essential to the function and aesthetics of montane ecosystems in western North America. Aspen stands are maintained by a nuanced relationship with wildfire, although in recent decades aspen mortality has increased. The need to understand the baseline environmental conditions that favour aspen is clear; however, long-term fire history reconstructions are rare due to the scarcity of natural archives in dry montane settings. Here, we analyse a high-resolution lake sediment record from southwestern, Utah, USA to quantify the compositional and burning conditions that promote stable (or seral) aspen forests. Our results show that aspen presence is negatively correlated with subalpine fir and that severe fires tend to promote persistent and diverse aspen ecosystems over centennial timescales. This information improves our understanding of aspen disturbance ecology and identifies the circumstances where critical transitions in montane forests may occur.