Seeds capture the diversity of genetic resource collections of <Emphasis Type="Italic">Malus sieversii</Emphasis> maintained in an orchard

Many species in genebank collections are maintained vegetatively either as in vitro cultures or as plants in the field or greenhouse. In these cases, individual genotypes (cultivars) are the focus of conservation. In crop wild relatives, where the value of an accession is in the heterogeneity of segregating genotypes, seeds may be the most representative form of conservation. We used two sets of field-planted Malus sieversii apple trees that originated from seeds collected from two wild populations in Kazakhstan as a model system to determine if seeds from hand-pollinated crosses could be used to represent the allelic diversity of the parent trees in a genebank orchard. A crossing design was developed that used each of the selected trees from the two Kazakhstan collection sites as mothers and also as fathers in the form of bulked pollen pools. The sets of offspring from these crosses were genotyped using microsatellite markers. Paternity analyses revealed the percentage of offspring that was derived from each of the parent trees. We also determined the contribution of each parent to the fertilization events from the pollen pools. Although there were some unintended pollination events (fertilization by trees in unexpected pollen pools) and unequal contribution of pollen parents within pollen pools, we demonstrated that the seedlings effectively captured the allelic frequency and diversity of the parents. A seed collection strategy is the most efficient way to conserve the diversity of crop wild relatives even in collections typically conserved as individual clones.     

Probabilistic viability calculations for cryopreserving vegetatively propagated collections in genebanks

Vegetatively propagated plant collections within genebanks are vulnerable to threats when they are maintained in field or greenhouse conditions. International genebanking standards recommend that these collections be securely backed-up at either secondary locations or by using in vitro culture or cryopreservation. A series of tools and tables were developed to assist genebank managers in determining the predicted number of viable explants after cryopreservation procedures have been employed based on the number of samples processed and the viability for a range of confidence levels (0.5, 0.75, 0.90, 0.95, and 0.99). In addition, critical viabilities were estimated which represent the minimum viability required to have at least one viable explant for a given number of explants stored per cryovial or tube. The plant cryobanking literature was surveyed to summarize the sample processing practices that have been implemented within genebanks. The minimum number of explants predicted to be viable for accessions within these cryobanked collections was calculated based on the published minimum viability levels and numbers of explants processed. Implementation of the proposed tools and tables will allow genebank managers to determine if shifts in their current methods could result in more efficient back-up procedures.     

Influence of tree shelters on seedling success in an afforested riparian zone

The restoration of a natural riparian ecosystem is a key component to improving water quality and restoring stream health in a disturbed watershed. The rate and degree of riparian restoration, and hence stream restoration, depends in part upon afforestation practices. Successful afforestation is determined largely by the rates of survivorship and growth of the tree species planted or recruited in a riparian zone. This study was part of a project involving the restoration of a channelized section of Wilson Creek located in the Bernheim Arboretum and Research Forest, Nelson County, Kentucky. Riparian restoration activities focused on reestablishing a native riparian corridor using American sycamore (Platanus occidentalis), green ash (Fraxinus pennsylvanica var. subintegerrima), and pin oak (Quercus palustris). This study evaluated techniques for improving the growth and survivorship of planted seedlings along Wilson Creek. Specifically, two tree shelter types (Tubex^® vs. continental mesh), with or without herbicide treatments, were compared. Additionally, the influence of these techniques on debris retention within the riparian zone was also examined. Results showed that use of tree shelters significantly increased the growth of seedlings (but not survivorship), provided physical protection especially during heavy flooding events, and accelerated woody debris retention. Both Tubex^® and continental mesh tree shelters were effective in enhancing seedling growth, with Tubex^® shelters yielding significantly better growth when combined with herbicide to control competing vegetation. Thus, the most cost effective choice may depend on the environmental setting and ability to combine the shelters with other growth enhancing treatments.     

Estimating the quadratic mean diameters of fine woody debris in forests of the United States

Most fine woody debris (FWD) line-intersect sampling protocols and associated estimators require an approximation of the quadratic mean diameter (QMD) of each individual FWD size class. There is a lack of empirically derived QMDs by FWD size class and species/forest type across the U.S. The objective of this study is to evaluate a technique known as the graphical estimation (GE) method for estimating FWD QMDs across forests of the U.S. Results indicate tremendous inter- and intra-specific variation in small FWD diameters. In addition, GE model fitting results demonstrated a lack of substantial difference in FWD QMDs between common forest types. It is postulated that the mixing, fracturing, and decay of both tree and shrub downed woody debris in diverse forest types across the U.S. can homogenate FWD QMDs at state/national levels. In the absence of site-specific empirical measurement of FWD QMDs, it is suggested that a general national set of FWD QMDs derived from the GE method be adopted for large-scale FWD monitoring efforts.     

Thinning method and intensity influence long-term mortality trends in a red pine forest

Tree mortality shapes forest development, but rising mortality can represent lost production or an adverse response to changing environmental conditions. Thinning represents a strategy for reducing mortality rates, but different thinning techniques and intensities could have varying impacts depending on how they alter stand structure. We analyzed trends in stand structure, relative density, stand-scale mortality, climate, and correlations between mortality and climate over 46 years of thinning treatments in a red pine forest in Northern Minnesota, USA to examine how thinning techniques that remove trees of different crown classes interact with growing stock manipulation to impact patterns of tree mortality. Relative density in unharvested plots increased during the first 25 years of the study to around 80%, then began to plateau, but was lower (12–62%) in thinned stands. Mortality in unharvested plots claimed 2.5 times more stems yr⁻¹ and 8.6 times as large a proportion of annual biomass increment during the last 21 years of the study compared to the first 25 years, but showed few temporal trends in thinned stands. Mortality in thinning treatments was generally lower than in controls, particularly during the last 21 years of the study when mortality averaged about 0.1% of stems yr⁻¹ and 4% of biomass increment across thinning treatments, but 0.8% of stems yr⁻¹ and 49% of biomass increment in unharvested plots. Treatments that combined thinning from above with low growing stock levels represented an exception, where mortality exceeded biomass production after initial thinning. Mortality averaged less than 0.1% of stems yr⁻¹ and less than 1% of annual biomass production in stands thinned from below. These trends suggest thinning from below minimizes mortality across a wide range of growing stock levels while thinning from above to low growing stock levels can result in dramatic short-term increases in mortality. Moderate to high growing stock levels (21–34 m² ha⁻¹) may offer greater flexibility for limiting mortality across a range of thinning methods. Mean and maximum annual and growing season temperatures rose by 0.6–1.8 °C during the study, and temperature variables were positively correlated with mortality in unharvested plots. Mortality increases in unharvested plots, however, were consistent with self-thinning principles and probably not driven by rising temperatures. These results suggest interactions between thinning method and intensity influence mortality reductions associated with thinning, and demonstrate the need for broader consideration of developmental processes as potential explanations for increased tree mortality rates in recent decades.     

Regeneration dynamics in remnant Tsuga canadensis stands in the northern Lake States: Potential direct and indirect effects of herbivory

We examined the effects of white-tailed deer (Odocoileus virginianus) herbivory and microsite limitation on Tsuga canadensis regeneration in 39 randomly selected remnant T. canadensis stands in Michigan's Upper Peninsula. Deer of the region migrate to and congregate in T. canadensis stands in winter resulting in strong seasonal habitat use patterns. In each study stand, we quantified vegetation, microsite availability, and deer use (via pellet counts). While some stands contained high densities of T. canadensis regeneration (stems < 4.0 cm dbh), we found complete T. canadensis regeneration failures in 6 out of 39 stands. Additionally, 17 and 22 stands respectively, had complete failures in the small and large sapling categories. General linear models suggested that deer use was the primary limiting factor in the small sapling size class, even at relatively low levels of deer use. T. canadensis seedling density was positively associated with the availability of high-decay coarse woody debris and negatively associated with basal area of Acer saccharum in the overstory. This latter association may be due, at least in part, to negative effects of broadleaf litter on T. canadensis establishment and a general trend toward increasing Acer abundance in the regeneration layer. Our results suggest that differential tolerance to browsing (Tsuga vs. Acer) in conjunction with reduced germination substrate availability may set up a scenario where successful T. canadensis establishments is more limited by legacy and indirect than direct effects given contemporary levels of deer use.