The succession of droughts in unmanaged overcrowded stands is affecting many physiological processes associated with water transport and carbon metabolism and making trees more susceptible to further abiotic and biotic stresses. A meta-analysis of physiological data compiled from recent data bases indicates that, comparatively to other species, oaks are mostly tolerant to drought and anisohydric, although also relatively vulnerable to severe water stress.
fire and coppicing suppression) changes that have been occurring through the last century. The increasing duration and intensity of drought is precipitating widespread episodes of forest decline around the world, possibly in combination with other climatic (e.g. During the most recent inventory period (2006-2010), over 60%, 7% and 5% of red oak, white oak and non-oak groups, respectively, had relative mortality rates of >12%. In contrast, sites with elevated white oak and non-oak mortality occurred sporadically, mainly in the southern portion (Arkansas) of the Ozark Highlands. Spatially, high red oak mortality sites (hot spots with proportional basal area mortality >0.12) initially occurred in the central Ozarks and spread gradually over most of the Ozark Highlands as regional droughts continued. These drought events triggered the escalation of mortality starting around year 2000. The Ozark Highlands experienced a severe drought extending from 1998 to 2000 and another milder drought from 2005 to 2006. Moreover, based on the past 17 years PDSI data, it appears that the cumulative impacts of drought may last up to 10 years. Cross-correlation analyses indicate that mortality in the red oak group was significantly correlated with the growing season Palmer drought severity index (PDSI) and usually lagged two to three years following single drought events. Oak decline and associated escalating mortality have occurred primarily in red oak species while the white oak group has maintained a relatively stable mortality rate that is comparable to non-oak species. These values are three to five times higher than for white oak group and non-oak species. Oak decline has elevated cumulative mortality of red oak species to between 11 and 15 percent in terms of relative density and basal area of standing dead oak trees, respectively. In contrast, fewer than 18% of declining white oaks improved during the study period.Īt the forest landscape/region level, based on annual Forest Inventory and Analysis plot data from 1999 to 2010, oak decline and mortality trends for major oak species (groups) were examined in the Ozark Highlands of Arkansas and Missouri. We also observed that more than 50% of the red oaks that initially exhibited moderate to severe decline symptoms appeared to improve during the study, regardless of harvest treatment. Even in the absence of the improvement harvests, more than 70% of red oaks that initially exhibited little or no crown dieback remained in the same crown dieback classes during the 14-year study period. Although we found that improvement harvests did not mitigate oak decline, they did not make decline worse and had the benefits of increasing the diameter growth of trees in the residual stand. We conducted a replicated, 14-year study in southeastern Missouri to determine if improvement harvests would reduce the severity of oak decline symptoms and increase forest growth compared with untreated stands. However, few studies have experimentally evaluated the effectiveness of cutting treatments for mitigating oak decline, and some scientists have cautioned that cutting in declining stands exacerbates the problem. Consequently, some scientists and managers have advocated thinning and intermediate harvesting to maintain or improve tree vigor and growth, thereby leaving stands less susceptible to pathogens and pests that are frequently the ultimate cause of mortality of declining trees. Prior research indicates that environmental stress, particularly drought, leads to the onset of oak decline. Since the 1970s, oak decline has been a chronic problem throughout the oak-dominated forests of the Missouri Ozarks.
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