White Ash (Fraxinus Americana) Decline and Mortality: the Role of Site Nutrition and Stress History ⇑ Alejandro A
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Forest Ecology and Management 286 (2012) 8–15 Contents lists available at SciVerse ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco White ash (Fraxinus americana) decline and mortality: The role of site nutrition and stress history ⇑ Alejandro A. Royo a, , Kathleen S. Knight b a USDA Forest Service, Northern Research Station, Irvine, PA 16365, USA b USDA Forest Service, Northern Research Station, Delaware, OH, USA article info abstract Article history: Over the past century, white ash (Fraxinus americana) populations throughout its range have deteriorated Received 3 August 2012 as a result of declining tree health and increased mortality rates. Although co-occurring factors including Received in revised form 30 August 2012 site nutritional deficiencies and punctuated stress events (e.g., defoliations, drought) are hypothesized to Accepted 31 August 2012 trigger white ash decline, there are no empirical assessments of these factors at regional scales. In this study, we evaluated ash crown dieback, crown health condition, and mortality on 190 plots paired along a topographic gradient known to differ in site nutrition across a 3000 km2 area of northwestern Pennsyl- Keywords: vania, USA. Additionally, we assessed white ash foliar nutrient content and additional factors including Dieback defoliation history as potential explanatory variables at all sites. White ash populations on upper slopes Health Magnesium consistently had significantly greater dieback, poorer crown condition, and greater mortality than popu- Calcium lations in paired plots on lower slopes. Despite nearly two decades since the last major elm spanworm Defoliation defoliation, this stressor further amplified the differences in health and mortality seen between slope positions. On the relatively cation deficient upper slope positions, crown dieback and crown condition improved with increasing foliar cation (Ca2+,Mg2+) concentration. Our results indicate that white ash health is strongly influenced by site nutrition, and defoliation can trigger declines in cation-deficient sites. Knowledge of how landscape position and nutrition influence white ash health may alter manage- ment responses to insect outbreak events. Published by Elsevier B.V. 1. Introduction factors (e.g., pathogens; Castello et al., 1995), more often the exact cause is unknown (Hibben and Silverborg, 1978; Mueller-Dombois, Sudden, and often unexplained, episodes of dieback and mortal- 1987; Woodcock et al., 1993). Stand age, drought, landscape posi- ity in forest tree species remain one of the most vexing and vital tion, pests, pathogens, and phytoplasmas are hypothesized to act forest health issues facing managers and conservationists world- in concert to incite ash decline, but, evidence for any of these factors wide (Blank et al., 1988). In North America, ash species (Fraxinus remains equivocal (Tobiessen and Buchsbaum, 1976; Hibben and spp.), and white ash (F. americana) in particular, have exhibited Silverborg, 1978; Castello et al., 1985; Woodcock et al., 1993; Sin- extensive dieback and increased mortality throughout their range clair and Griffiths, 1994; Ward, 1997; Feeley et al., 2001). Indeed, since at least the 1920s (Hibben and Silverborg, 1978; Sinclair our current knowledge of the relative importance and potential et al., 1990; Han et al., 1991). These episodes often decimate dom- linkages among any of these factors in influencing ash decline re- inant or co-dominant tree populations with reported mortality mains woefully inadequate, thereby limiting our ability to predict reaching 48% (Castello et al., 1985) and reductions in live basal area dieback patterns across the landscape. as great as 60% (Morin et al., 2006). The collapse of these popula- Growing evidence suggests site nutrient deficiencies, putatively tions is critical as ash is an important component of eastern decid- linked to anthropogenic changes in soil chemistry, may underlie uous forests and a valued timber species with an estimated 1.25 the tree declines observed in North America and elsewhere (Blank billion trees (P5 in. DBH) growing throughout the upper midwest- et al., 1988; DeHayes et al., 1999; Horsley et al., 1999; Demchik ern and northeastern United States (USDA Forest Service, 2010). and Sharpe, 2000). Although empirical assessments of the influ- Although episodic and extensive Fraxinus spp. mortality is occa- ence of site nutrition on white ash health remain lacking, several sionally linked to individual and readily identifiable endogenous lines of evidence suggest this species may be sensitive to nutri- (e.g., synchronous cohort senescence; Ward, 1982) or exogenous tional imbalances and these, in turn, may increase risk of decline. First, studies indicate white ash has high requirements for base 2+ 2+ ⇑ Corresponding author. Tel.: +1 814 563 1040; fax: +1 814 563 1048. cations (e.g., Ca ,Mg ) and is consistently associated with soils E-mail address: [email protected] (A.A. Royo). with higher pH and greater base cation availability (Boerner and 0378-1127/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.foreco.2012.08.049 A.A. Royo, K.S. Knight / Forest Ecology and Management 286 (2012) 8–15 9 Koslowsky, 1989; van Breemen et al., 1997; Finzi et al., 1998; Bige- white ash mortality from unknown causes was responsible for an low and Canham, 2002). Second, anthropogenic changes in precip- observed 60% decrease in ash live basal area in the 1990s. itation chemistry and acidity over the latter half of the 20th century are associated with declines in soil pH and pronounced 2.2. Sampling reductions in base cation concentrations ranging from 50% up to as high as order of magnitude (e.g., Likens et al., 1996; Drohan We superimposed a grid over the ANF ownership and, using and Sharpe, 1997; Driscoll et al., 2001; Bailey et al., 2005). Finally, existing stand information on ash basal area and landform classifi- researchers have found tree declines are often associated with cat- cations, we searched each 700 ha block to identify pairs of plots ion availability and stress: for sugar maple (Acer saccharum), an- containing a range of 0.23–4.59 m2 haÀ1 of white ash, where one other tree species with high cation requirements, interactions plot in the pair was established on a lower slope position (foot or between nutrient imbalances and stress lead to regional declines bottom) and the other on an upper slope position (shoulder or pla- (reviewed by Cronan and Grigal (1995)). Thus, tree species with teau). At each plot, a focal ash tree was defined as plot center and high cation requirements, like sugar maple and white ash, may the surrounding tree community was inventoried in a variable ra- be bellwethers of regional nutrient imbalances and point to wide- dius plot using a 10 factor prism. Within each pair of upper and spread changes in overall forest health. lower slope positions, care was taken to choose the healthiest Although nutrient imbalances alone may predispose trees to dominant or co-dominant focal trees in similar size and crown health decline symptoms, punctuated stress events appear to be classes. Diameter at breast height (DBH) was measured for each important in triggering these weakened, yet otherwise healthy tree and trees with two trunks were counted as separate trees if trees, into decline and mortality episodes (Manion, 1991; Horsley they divided below breast height (140 cm). We established 105 et al., 2000). Of the myriad short-term stressors potentially affect- blocks with 20 being incomplete (e.g., they were missing a lower- ing tree health, drought and insect defoliations are the most com- or upper-site or site was harvested following establishment) and monly suspected stress agents inciting decline in white ash as well the median distance between plots in a pair was (805 m). Thus, as other species (reviewed by Houston (1987), Millers et al. (1989), we present data from 190 mature, second-growth plots. We noted and Auclair et al. (1997)). Both drought and defoliation stress alter the presence of seeps at each site as within the unglaciated portion tree carbohydrate budgets by reducing carbohydrate storage of the Allegheny plateau weatherable minerals, including Ca and through tissue loss or prolonged stomatal closure and accelerating Mg, are confined to lower portions of the bedrock and are delivered carbohydrate use via increased assimilation and respiration (Tobi- to lower slope positions via water flow paths (Bailey et al., 2004). essen and Buchsbaum, 1976; Wargo, 1999). The net effect is a In summer 2010, we assessed the health status of all ash trees greatly reduced root starch and sugar content, which ultimately using a variety of well-established canopy rating metrics. We rated exerts strong control over tree vigor and predisposes individuals crowns using the Forest Inventory and Analysis (FIA) metric of to infection by opportunistic pathogens (Wargo, 1981; Gregory crown dieback defined as the percentage of the live crown area and Wargo, 1986; Han et al., 1991; Kosola et al., 2001). that exhibits signs of recent fine twig dieback, excluding snag In this study, we examined the relationship among various bio- branches and gaps in the canopy (USDA Forest Service, 2007). tic and abiotic factors linked to variability in white ash health. Our We also rated crowns using a categorical ash crown health condi- specific objectives were to: (i) identify the factors that contribute tion rating developed specifically to assess ash decline and mod- to white