Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi-arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad-scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors-the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)-are stronger drivers of drought sensitivity than soil and stand characteristics. Drought-induced reductions in tree growth were greatest when the droughts occurred during early-season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ ) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early-season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.

Decreasing trends in acidic deposition levels over the past several decades have led to partial chemical recovery of surface waters. However, depletion of soil Ca from acidic deposition has slowed surface water recovery and led to the impairment of both aquatic and terrestrial ecosystems. Nevertheless, documentation of acidic deposition effects on soils has been limited, and little is known regarding soil responses to ongoing acidic deposition decreases. In this study, resampling of soils in eastern Canada and the northeastern U.S. was done at 27 sites exposed to reductions in wet SO4(2-) deposition of 5.7-76%, over intervals of 8-24 y. Decreases of exchangeable Al in the O horizon and increases in pH in the O and B horizons were seen at most sites. Among all sites, reductions in SO4(2-) deposition were positively correlated with ratios (final sampling/initial sampling) of base saturation (P < 0.01) and negatively correlated with exchangeable Al ratios (P < 0.05) in the O horizon. However, base saturation in the B horizon decreased at one-third of the sites, with no increases. These results are unique in showing that the effects of acidic deposition on North American soils have begun to reverse.

Previous studies have shown in noncalcareous soils that acid deposition may have increased soil leaching of basic cations above the input rate from soil weathering and atmospheric depositions. This phenomenon may have increased soil acidity levels, and, as a consequence, may have reduced the availability of these essential nutrients for forest growth. Fourteen plots of the Forest Ecosystem Research and Monitoring Network in Québec were used to examine the relation between post-industrial growth trends of sugar maple (Acer saccharum Marsh.) and acid deposition (N and S), stand decline rate, and soil exchangeable nutrient concentrations. Atmospheric N and S deposition and soil exchangeable acidity were positively associated with stand decline rate, and negatively with the average tree basal area increment trend. The growth rate reduction reached on average 17% in declining stands compared with healthy ones. The results showed a significant sugar maple growth rate reduction since 1960 on acid soils. The appearance of the forest decline phenomenon in Québec can be attributed, at least partially, to soil acidification and acid deposition levels.

Pe´rie´, C. and Ouimet, R. 2008. Organic carbon, organic matter and bulk density relationships in boreal forest soils. Can. J. Soil Sci. 88: 315Á325. Relationships between soil organic carbon (SOC), organic matter (SOM), and bulk density (BD) were established in acidic loamy to sandy loam fine fractions of forest soils in Quebec (Canada). The interest of such relationships rests with the possibility of using simple and rapid techniques to estimate SOC and BD. It is also a crucial step in establishing the correspondence among several data bases when SOC data are obtained using different measurement techniques. In this study, SOC was measured by dry combustion (SOC DC ) and wet digestion (SOC WD ) methods, and organic matter by loss-on-ignition (LOI). Our results suggest that, in these soils: (1) LOI can be used for estimating SOC (r 2 00.95, RMSEP 016%) and SOC DC /SOM significantly decreased with increasing depth from 0.49 to 0.27; (2) SOC DC and SOC WD were highly correlated. Even if SOC WD provided near complete recovery of SOC DC , dry combustion remains the preferred method for SOC analysis since SOC WD recovery decreased with increasing depth from 100 to 83%. (3) BD was also strongly related to SOM (r 2 00.81). We recommend using the organic density approach to estimate BD from SOM because it allows BD to be predicted without significant bias and with a degree of accuracy of 14%.Key words: Forest soils, soil organic carbon, soil organic matter, soil bulk density Pe´rie´, C. et Ouimet, R. 2008. Liens entre le carbone organique, la matie`re organique et la masse volumique apparente dans le sol des foreˆts bore´ales. Can. J. Soil Sci. 88: 315Á325. Les auteurs ont pre´cise´les liens entre le carbone organique (COS), la matie`re organique (MOS) et la masse volumique apparente (MVA) du sol dans les fractions fines de loam et de loam sablonneux des sols forestiers acides du Que´bec. De tels liens pre´sentent un certain inte´reˆt, car on pourrait y recourir comme technique simple et rapide pour e´valuer la concentration de COS et la MVA. Il s'agit aussi d'une e´tape cruciale quand on souhaite e´tablir une correspondance entre des bases de donne´es ou`la concentration de COS e´mane de diverses techniques. Dans le cadre de la pre´sente e´tude, la quantite´de COS a e´te´de´termine´e par combustion se`che (COS CS ) et digestion par voie humide (COS DH ) tandis qu'on a mesure´la concentration de matie`re organique au moyen des pertes par calcination (PPC). Les auteurs ont obtenu les re´sultats que voici pour les sols concerne´s. (1) La me´thode PPC permet d'estimer le COS (r 2 0 0,95, RMSEP 0 16 %) et le ratio COS CS /MOS diminue de manie`re significative, de 0,49 a`0,27, avec la profondeur. (2) Il existe une importante corre´lation entre le COS CD et le COS DH . Bien que la technique COS DH permette de re´cupe´rer la quasi totalite´du COS CS , la combustion se`che demeure la me´thode privile´gie´e pour analyser le COS, la quantite´de carbone re´cupe´re´e par COS DH passant de 100 a`83 % a`mesure qu'augmente la profond...

The first tree health decline symptoms usually observed are foliar deficiency symptoms, foliage loss, and dieback. To improve the subjective nature and unspecificity of these assessments, we examined sugar maple (Acer saccharum Marsh.) radial growth and health to develop an indicator of sugar maple tree health status based on radial growth pattern. We used the basal area increment (BAI) of 328 tree-ring collections from 16 sites located in southern Quebec, throughout the sugarbush natural range, that were categorized by defoliation class. BAI of trees with decline symptoms was significantly lower than that of healthy trees in 9 of the 16 stands. BAI trends since 1955 showed an inverse relationship with tree decline class measured in 1989, irrespective of tree age. The results indicate that declining trees in these stands have not recovered based on BAI. They also suggest that the decrease in slope of BAI predated the observed symptoms of sugar maple decline by at least one decade. Results suggest that sugar maple vigor and health can be assessed by measuring tree's BAI trend, an indicator that may be useful for the diagnosis of sugar maple health and status years before the appearance of visible canopy symptoms.