Experimental approach and initial forest response to a simulated ice storm experiment in a northern hardwood forest.

Ice storms are a type of extreme winter weather event common to north temperate and boreal forests worldwide. Recent climate modelling studies suggest that these storms may become more frequent and severe under a changing climate. Compared to other types of storm events, relatively little is known about the direct and indirect impacts of these storms on forests, as naturally occurring ice storms are inherently difficult to study. Here we describe a novel experimental approach used to create a suite of ice storms in a mature hardwood forest in New Hampshire, USA. The experiment included five ice storm intensities (0, 6.4, 12.7 and 19.1 mm radial ice accretion) applied in a single year, and one ice storm intensity (12.7 mm) applied in two consecutive years. Results demonstrate the feasibility of this approach for creating experimental ice storms, quantify the increase in fine and coarse woody debris mass and nutrients transferred from the forest canopy to the soil under the different icing conditions, and show an increase in the damage to the forest canopy with increasing icing that evolves over time. In this forest, little damage occurred below 6.4 mm radial ice accretion, moderate damage occurred with up to 12.7 mm of accretion, and significant branch breakage and canopy damage occurred with 19.1 mm of ice. The icing in consecutive years demonstrated an interactive effect of ice storm frequency and severity such that some branches damaged in the first year of icing appeared to remain in the canopy and then fall to the ground in the second year of icing. These results have implications for National Weather Service ice storm warning levels, as they provide a quantitative assessment of ice-load related inputs of forest debris that will be useful to municipalities creating response plans for current and future ice storms.

Simulating Impacts of Ice Storms on Forest Ecosystems.

Ice storms can have profound and lasting effects on the structure and function of forest ecosystems in regions that experience freezing conditions. Current models suggest that the frequency and intensity of ice storms could increase over the coming decades in response to changes in climate, heightening interest in understanding their impacts. Because of the stochastic nature of ice storms and difficulties in predicting when and where they will occur, most past investigations of the ecological effects of ice storms have been based on case studies following major storms. Since intense ice storms are exceedingly rare events it is impractical to study them by waiting for their natural occurrence. Here we present a novel alternative experimental approach, involving the simulation of glaze ice events on forest plots under field conditions. With this method, water is pumped from a stream or lake and sprayed above the forest canopy when air temperatures are below freezing. The water rains down and freezes upon contact with cold surfaces. As the ice accumulates on trees, the boles and branches bend and break; damage that can be quantified through comparisons with untreated reference stands. The experimental approach described is advantageous because it enables control over the timing and amount of ice applied. Creating ice storms of different frequency and intensity makes it possible to identify critical ecological thresholds necessary for predicting and preparing for ice storm impacts.

Calcium addition at the Hubbard Brook Experimental Forest increases sugar storage, antioxidant activity and cold tolerance in native red spruce (Picea rubens).

In fall (November 2005) and winter (February 2006), we collected current-year foliage of native red spruce (Picea rubens Sarg.) growing in a reference watershed and in a watershed treated in 1999 with wollastonite (CaSiO(3), a slow-release calcium source) to simulate preindustrial soil calcium concentrations (Ca-addition watershed) at the Hubbard Brook Experimental Forest (Thornton, NH). We analyzed nutrition, soluble sugar concentrations, ascorbate peroxidase (APX) activity and cold tolerance, to evaluate the basis of recent (2003) differences between watersheds in red spruce foliar winter injury. Foliar Ca and total sugar concentrations were significantly higher in trees in the Ca-addition watershed than in trees in the reference watershed during both fall (P=0.037 and 0.035, respectively) and winter (P=0.055 and 0.036, respectively). The Ca-addition treatment significantly increased foliar fructose and glucose concentrations in November (P=0.013 and 0.007, respectively) and foliar sucrose concentrations in winter (P=0.040). Foliar APX activity was similar in trees in both watersheds during fall (P=0.28), but higher in trees in the Ca-addition watershed during winter (P=0.063). Cold tolerance of foliage was significantly greater in trees in the Ca-addition watershed than in trees in the reference watershed (P<0.001). Our results suggest that low foliar sugar concentrations and APX activity, and reduced cold tolerance in trees in the reference watershed contributed to their high vulnerability to winter injury in 2003. Because the reference watershed reflects forest conditions in the region, the consequences of impaired physiological function caused by soil Ca depletion may have widespread implications for forest health.

The surprising recovery of red spruce growth shows links to decreased acid deposition and elevated temperature.

Following growth declines and increased mortality linked to acid deposition-induced calcium depletion, red spruce (Picea rubens Sarg.) in the northeastern United States are experiencing a recovery. We found that more than 75% of red spruce trees and 90% of the plots examined in this study exhibited increasing growth since 2001. To understand this change, we assessed the relationship between red spruce radial growth and factors that may influence growth: tree age and diameter, stand dynamics, plot characteristics (elevation, slope, aspect, geographical position), and a suite of environmental variables (temperature, precipitation, climate and precipitation indices (degree days, SPEI [standardized precipitation evapotranspiration index], and acid deposition [SO42-, NO3-, pH of rainfall, cation:anion ratio of rainfall]) for 52 plots (658 trees) from five states (spanning 2.5°N × 5°W). Examining the growth relationships from 1925 to 2012, we found that while there was variability in response to climate and acid deposition (limited to 1980-2012) by elevation and location, plot and tree factors did not adequately explain growth. Higher temperatures outside the traditional growing season (e.g., fall, winter, and spring) were related to increased growth. Nitrogen deposition (1980-2012) was associated with lower growth, but the strength of this relationship has lessened over time. Overall, we predict sustained favorable conditions for red spruce in the near term as acid deposition continues to decline and non-traditional growing season (fall through spring) temperatures moderate, provided that overall temperatures and precipitation remain adequate for growth.

Accretion, partitioning and sequestration of calcium and aluminum in red spruce foliage: implications for tree health.

Calcium (Ca) is an essential macronutrient in plants and is an important component of many cellular structures and physiological processes as well as overall forest function. Aluminum (Al) in soil solution can inhibit Ca uptake by plants and disrupt many Ca-dependent metabolic and physiological processes of plants. The ratio of Ca to Al in soil solution can be an important indicator of forest health, especially on acid soils. We used sequential chemical extractions (water, acetic acid and hydrochloric acid) to assess the chemical availability of Ca and Al in foliage from mature red spruce (Picea rubens Sarg.) trees growing under ambient environmental conditions. In plants deficient in Ca and with intermediate total foliar Ca concentration ([Ca]), Ca preferentially accrued in labile and physiologically available forms (water- and acetic acid-extractable). In plants with total foliar [Ca] above a "sufficiency" threshold, Ca also accrued in a chemically sequestered form with low solubility (HCl-extractable), suggesting that Ca sequestration is an inducible process in response to excess foliar Ca. Because it has low solubility, it is likely that sequestered Ca is unavailable for Ca-dependent physiological processes. Immobilization of Al in foliage was related to Ca sequestration, suggesting that Ca sequestration may provide a passive mechanism for Al tolerance in the foliage of these trees. Aluminum immobilization was evident based on the ratio of HCl-extractable Al to the more labile (water- and acetic acid-extractable) forms of Al. Sufficient labile Ca combined with Al sequestration was associated with plant health, including enhanced foliar accretion of Mg and Mn, greater tree growth, enhanced foliar cold hardiness and reduced winter injury. These findings demonstrate that not all chemical forms of foliar Ca and Al are of equal physiological significance and underscore the importance of assessing the biologically significant element forms in biogeochemical research.

Calcium and aluminum impacts on sugar maple physiology in a northern hardwood forest.

Forests of northeastern North America have been exposed to anthropogenic acidic inputs for decades, resulting in altered cation relations and disruptions to associated physiological processes in multiple tree species, including sugar maple (Acer saccharum Marsh.). In the current study, the impacts of calcium (Ca) and aluminum (Al) additions on mature sugar maple physiology were evaluated at the Hubbard Brook Experimental Forest (Thornton, NH, USA) to assess remediation (Ca addition) or exacerbation (Al addition) of current acidified conditions. Fine root cation concentrations and membrane integrity, carbon (C) allocation, foliar cation concentrations and antioxidant activity, foliar response to a spring freezing event and reproductive ability (flowering, seed quantity, filled seed and seed germination) were evaluated for dominant sugar maple trees in a replicated plot study. Root damage and foliar antioxidant activity were highest in Al-treated trees, while growth-associated C, foliar re-flush following a spring frost and reproductive ability were highest in Ca-treated trees. In general, we found that trees on Ca-treated plots preferentially used C resources for growth and reproductive processes, whereas Al-treated trees devoted C to defense-based processes. Similarities between Al-treated and control trees were observed for foliar cation concentrations, C partitioning and seed production, suggesting that sugar maples growing in native forests may be more stressed than previously perceived. Our experiment suggests that disruption of the balance of Ca and Al in sugar maples by acid deposition continues to be an important driver of tree health.