Oak Ecosystems Research Articles

Effects of forest management on the amount of deadwood in Mediterranean oak ecosystems

Deadwood is an important habitat for vertebrate and invertebrate species, a key factor in the nutrient cycle and a valuable carbon pool. Difference in the deadwood C stock depends on many variables such as forest type, microclimate and forest management. We investigated the relationship between C stock in deadwood and forest management in oak stands of southern Italy. Deadwood C stock was quantified in 74 forest stands under intensive, extensive or multifunctional management. The relationship between management and C stock in deadwood was assessed taking into account the decay class and the type of deadwood (snags, stumps and logs). Intensively managed forests are characterized by significantly lower mean levels of deadwood C than the other forms of management. Multifunctional management resulted in the highest level of C, mainly in logs and snags, while extensive management resulted in high levels of C storage concentrated in snags. Results of the research highlight that forest management practices greatly influence the quantitative and qualitative presence of deadwood in forests.

Environmental change in Garry oak (Quercus garryana) ecosystems: the evolution of an eco-cultural landscape

Globally, colonialism resulted in the suppression of aboriginal land manage- ment practices, abetted by the concept of terra nullius, ''belonging to no one''; the belief that aboriginal people had little influence on or ownership of the land. Until recently, this ideology was entrenched in resource management and policy. Traditional ecological knowledge, historical ecology, archaeology, and palaeoecological research have shown these assumptions to be wrong. In this paper we take a multidisciplinary approach (bio- geography, paleoecology, dendrochronology, and bioclimatic envelope modeling) to better understand the role of climate and fire in the formation of eco-cultural landscapes. We synthesize results from pollen and charcoal analysis in Garry oak ecosystems that indicate there were continuous and frequent prescribed burning events, with more severe fires occurring every 26-41 years in southwest British Columbia throughout the Anthropocene (*last 250 years) that substantially altered forest structure and composition. These results are consistent with stand age reconstructions in BC and Washington with Garry oak establishment beginning *1850 AD, corresponding with modern fire exclusion, aboriginal population decline, and end of the Little Ice Age. Douglas-fir recruitment has been con- tinuous since *1900, with succession of oak woodland to closed conifer forest at most sites. These findings indicate that the structure of many Garry oak ecosystems have been profoundly influenced by eco-cultural practices. Overwhelming evidence indicates that in many cases these ecosystems are dependent on prescribed fire for their open structure. In

Erratum to: Comparison of methane, nitrous oxide fluxes and CO2 respiration rates from a Mediterranean cork oak ecosystem and improved pasture

Erratum to: Comparison of methane, nitrous oxide fluxes and CO2 respiration rates from a Mediterranean cork oak ecosystem and improved pasture

Study on the mobility and bioavailability of PTEs in soils from Urban Forest Parks in Sofia, Bulgaria

Forest parks in many urbanized areas are affected by the anthropogenic pollution as demonstrated by the poor health status of many trees. The concentrations of harmful agents like Potentially Toxic Elements (PTEs) can be very high in urban soils and their availability is an important issue for biological components in these environments. This study focuses on the mobile forms of PTEs extracted by 1М NH4OAc at pH=7 in relation to determining their bioavailability for forest trees in oak ecosystems in urban areas. The area studied is along an urban–rural gradient from the Sofia region, Bulgaria. The NH4OAc extracts mainly “biologically active” fraction of elements in soils comprising primarily the soluble and exchangeable forms and in some cases showed good positive correlation with PTE accumulation in trees. Four experimental forest park sites (U1, U2, U3 and U4) created on Hortic/Technic Anthrosols were selected along the urban gradient in the Sofia region. The control sites located in rural zones with natural soils Cambisols and Luvisols (R1, R2, R3 and R4) were used as reference. Strong correlations were found between mobile forms of Cu, Zn, Pb, Cd in urban soils and a number of soil parameters (pH, CEC and texture). The mobility is related to soil pH and with more acidic soils a higher mobility is shown, which is statistically significant for Pb. The element Cu is less mobile in the soil and is localized in the top layer, where it is mainly associated with soil organic matter. The mobile (NH4OAc extractable) portions of Cu, Zn, Pb, and Cd (mg/kg soils) in Urban Soils ranged between: 0.74 and 2.61; 2.94 and 3.29; 1.34 and 3.28; and 0.02 and 0.16, respectively and those in Natural Soils ranged between: 0.73 and 1.65; 1.20 and 5.29; 1.10 and 6.80; and 0.02 and 0.07. The significance of the mobile proportion was greatest for Cd in urban soils where it reached 33.3% of the total content. Despite the determined correlation between mobile forms of Pb and Cd with their total content in leaves of oak trees there is no clear visible effect on the tree health characteristics using parameters as defoliation and decoloration of crowns.

The history of widespread decrease in oak dominance exemplified in a grassland–forest landscape

Regionally-distinctive open oak forest ecosystems have been replaced either by intensive agriculture and grazing fields or by denser forests throughout eastern North America and Europe. To quantify changes in tree communities and density in the Missouri Plains, a grassland–forest landscape, we used historical surveys from 1815 to 1864 and current surveys from 2004 to 2008. To estimate density for historical communities, we used the Morisita plotless density estimator and applied corrections for surveyor bias. To estimate density for current forests, we used Random Forests, an ensemble regression tree method, to predict densities from known values at plots using terrain and soil predictors. Oak species decreased from 62% of historical composition to 30% of current composition and black and white oaks historically were dominant species across 93% of the landscape and currently were dominant species across 42% of the landscape. Current forest density was approximately two times greater than historical densities, demonstrating loss of savanna and woodlands and transition to dense forest structure. Average tree diameters were smaller than in the past, but mean basal area and stocking remained similar over time because of the increase in density in current forests. Nevertheless, there were spatial differences; basal area and stocking decreased along rivers and increased away from rivers. Oak species are being replaced by other species in the Missouri Plains, similar to replacement throughout the range of Quercus. Long-term commitment to combinations of prescribed burning and silvicultural prescriptions in more xeric sites may be necessary for oak recruitment. Restoration of open oak ecosystems is a time-sensitive issue because restoration will become increasingly costly as oaks are lost from the overstory and the surrounding matrix becomes dominated by non-oak species.

Physiological strategies of co-occurring oaks in a water- and nutrient-limited ecosystem

Oak species are well suited to water-limited conditions by either avoiding water stress through deep rooting or tolerating water stress through tight stomatal control. In co-occurring species where resources are limited, species may either partition resources in space and/or time or exhibit differing efficiencies in the use of limited resources. Therefore, this study seeks to determine whether two co-occurring oak species (Quercus prinus L. and Quercus velutina Lam.) differ in physiological parameters including photosynthesis, stomatal conductance, water-use (WUE) and nitrogen-use efficiency (NUE), as well as to characterize transpiration and average canopy stomatal responses to climatic variables in a sandy, well-drained and nutrient-limited ecosystem. The study was conducted in the New Jersey Pinelands and we measured sap flux over a 3-year period, as well as leaf gas exchange, leaf nitrogen and carbon isotope concentrations. Both oak species showed relatively steep increases in leaf-specific transpiration at low vapor pressure deficit (VPD) values before maximum transpiration rates were achieved, which were sustained over a broad range in VPD. This suggests tight stomatal control over transpiration in both species, although Q. velutina showed significantly higher leaf-level and canopy-level stomatal conductance than Q. prinus. Average daytime stomatal conductance was positively correlated with soil moisture and both oak species maintained at least 75% of their maximum canopy stomatal conductance at soil moistures in the upper soil layer (0-0.3 m) as low as 0.03 m(3) m(3)(-3). Quercus velutina had significantly higher photosynthetic rates, maximum Rubisco-limited and electron-transport-limited carboxylation rates, dark respiration rates and nitrogen concentration per unit leaf area than Q. prinus. However, both species exhibited similar WUEs and NUEs. Therefore, Q. prinus has a more conservative resource-use strategy, while Q. velutina may need to exploit niches that are locally higher in nutrients and water. Likewise, both species appear to tap deep, stable water sources, highlighting the importance of rooting depth in modeling transpiration and stomatal conductance in many oak ecosystems.

Soil Properties and Plant Community Changes along a Goat Grazing Intensity Gradient in an Open Canopy Oak Forest

Understanding how the management practices of silvopastoral systems affect the long-term sustainability of oak ecosystems and what their influence is on nutrient cycling and plant community, is of great interest. The aim of this study was to examine the effects of relative grazing intensity on soil properties and on vegetation characteristics in an open canopy oak forest dominated by Quercus frainetto. The research was conducted in the area of Pentalofos, which is located in Evros region, north-eastern Greece and is grazed by goats. The distance from a goat corral was used to represent relative grazing intensity. In June 2011, soil and vegetation samples were collected along transects placed at 50, 150, 300, 600 and 1200 m from the goat corral, running perpendicular to three replicates. Soil measurements included pH, phosphorous (P) and nitrogen (N) concentrations while vegetation measurements included plant cover, species composition and diversity. Plant cover was not significantly different among grazing intensities. Species diversity, especially of the woody vegetation layer, was significantly higher in the light grazing intensity in comparison to both the heavy and the very light grazing. Heavy grazing reduced soil organic matter while it increased total nitrogen. Grazing intensity did not affect available P and soil pH. Light to moderate goat grazing could ameliorate floristic diversity and increase sustainability of oak forests in the Mediterranean region.

Diplodia corticola and Phytophthora cinnamomi: the main pathogens involved in holm oak decline on Caprera Island (Italy)

SummarySince 2008, severe and widespread tree decline and mortality has been observed at the main growing Quercus ilex L. (holm oak) forest on Caprera Island, Italy. To clarify the symptomatology and aetiology of this phenomenon, field surveys and isolations from symptomatic trees were carried out in summer 2010. Affected trees exhibited crown thinning, branch dieback, sunken cankers, epicormic shoots, exudates on branches and trunk, root losses and sudden death symptoms. Four fungal species belonging to Botryosphaeriaceae family, namely Botryosphaeria dothidea, Diplodia corticola, D. seriata and Neofusicoccum parvum, were isolated from cankers on trunk and branches, whereas three species of Phytophthora, namely P. cinnamomi, P. cryptogea and P. gonapodyides, were isolated from fine roots and rhizosphere soil samples. Isolates were identified using both morphological analysis and DNA‐based techniques. Pathogenicity trials on holm oak seedlings showed that all the isolated species are pathogenic. D. corticola proved to be the most aggressive species. Our results provide the first evidence for a combined involvement of D. corticola and P. cinnamomi in the aetiology of holm oak decline in Italy and suggest that these pathogens are not only important contributing factors in the onset of long‐term tree decline, but also may cause the rapid devastation of extensive oak ecosystems.

Long-term variability and environmental control of the carbon cycle in an oak-dominated temperate forest

Our understanding of the long-term carbon (C) cycle of temperate deciduous forests and its sensitivity to climate variability is limited due to the large temporal dynamics of C fluxes. The goal of the study was to quantify the effects of environmental variables on the C balance in a 70-year-old mixed-oak woodland forest over a 7-year period in northwest Ohio, USA. The net ecosystem exchanges (NEE) of C were measured using the eddy-covariance technique. Long-term mean NEE, ecosystem respiration (ER), and gross ecosystem productivity (GEP) were −339±34, 1213±84, and 1552±82gCm−2year−1, respectively. Warming increased ER more than GEP when the available water was not limited, but decreased GEP more than ER when the available water was limited, resulting in decreasing net C fluxes under both conditions. The decreasing net C sink in summer was associated with increasing air temperature (Ta) in spring. The leaf area index (LAI), photosynthetically-active radiation (PAR), and Ta were the most important determinants of NEE for spring, summer, and winter, respectively; however, these variables failed to explain NEE for autumn. The most important determinants of ER and GEP were soil temperature (Ts) in spring, Ta and PAR in summer, and Ta in autumn. Ta was the only control of ER in winter. The annual variation in NEE was larger than that of GEP or ER. The controls of GEP on NEE were more pronounced seasonally and annually than those of ER. The annual GEP was consistently more variable than the annual ER. GEP was also seasonally and annually correlated with ER. Practical models derived from different combinations of independent variables effectively predicted 87%, 80%, and 93% of the monthly variability in NEE, ER, and GEP, respectively. We concluded that the variability in C fluxes was more responsive to increasing Ta and Ts than to variations in seasonal and annual precipitation. The study implies that a warmer climate is likely to reduce the forest productivity and C-sink capacity of oak ecosystems in the future, especially in instances when water inputs become limiting.

Comparison of methane, nitrous oxide fluxes and CO2 respiration rates from a Mediterranean cork oak ecosystem and improved pasture

Background and aims During the recent decades, cork oak (Q. suber) mortality has been increasing in Mediterranean oak woodland endangering the economical and environmental sustainability of the “montado” ecosystem. This fact in combination with climate change and conversion of forestland to pasture may significantly affect the soil-atmosphere greenhouse gases (GHGs) exchange. Our study evaluates the impact of oak trees as compared to pasture on net ecosystem GHG (CH4, N2O, and CO2) exchange as well as the main environmental factors influencing this exchange.

Novel ecosystems in the restoration of cultural landscapes of Tl’chés, West Chatham Island, British Columbia, Canada

The small archipelago of Tl’ches, in coastal British Columbia, has gone through drastic social-ecological change culminating in the decline of traditional management practices, invasion of exotic plant species and, in the early 1960s, land abandonment. This is a common trend in cultural landscapes around the world. Cultural landscapes have great social-ecological significance, including cultural safeguarding and renewal, as well as maintenance of biodiversity, ecosystem functions and services, making them objects of special attention in conservation and restoration efforts. This study investigates the relationship between the Lekwungen people and heavily altered ecosystems in the context of ecological restoration for the cultural landscapes of Tl’ches. In order to thoroughly explore the ecological, social and cultural aspects and processes involved in the restoration of Tl’ches, this study combined different environmental sciences methodologies, including historical and archival research, semi-structured interviews and participatory observation with Lekwungen participants, as well as ecological field assessments and an innovative mapping approach termed Terrestrial Cultural Ecosystem Mapping (TCEM). This study generated historical and spatial references for ecosystem and land use change in West Chatham Island, as well as cultural and ecological understandings to support restoration. Although important native species such as Camassia spp. are still thriving, present-day ecosystems on this island are heavily altered from the natural Garry oak ecosystem reference sites and are largely dominated by exotic invasive shrubs and graminoid species. This investigation indicates that the Lekwungen value both the historical ecological conditions of the island and particular types of the present-day novel or hybrid ecological states, such as the overgrown heritage orchard and nonnative berries found on the islands. Present-day ecosystems of Tl’ches are a result of a hybrid management system (traditional and conventional), coupled with land abandonment for many decades. Restoration of Tl’ches must involve both ecological and cultural components. Therefore, intervention strategies should consider the incorporation of novel and hybrid ecosystems (i.e. naturalized exotic species) into a restoration plan for ecological, cultural, historical, and subsistence values.

Translocation of rare plant species to restore Garry oak ecosystems in western Canada: challenges and opportunities

Covering just 2000 ha, Garry oak ecosystems (GOEs) in western Canada contain about 10% of the Species at Risk Act listed species at risk in Canada, including 30 plants listed by Committee on the Status of Endangered Wildlife in Canada as endangered. Since European settlement ca. 1840, GOE sites have been largely degraded by human disturbance, habitat fragmentation, invasive species, overgrazing, and fire suppression. A key strategy to mitigate this loss of biodiversity is to translocate rare plants to GOE restoration sites. The Garry Oak Ecosystem Recovery Team provides advice on proposed translocations but strongly encourages restoration practitioners to focus on plant populations already present on a site. There is a need for a closer look at challenges and opportunities afforded by translocation. If the approach taken is too precautionary, some rare species in this highly threatened ecosystem may be jeopardized. Current translocation efforts are being spearheaded by Parks Canada for golden paintbrush (Castilleja levisecta Greenm.), seaside birds-foot lotus (Lotus formossimus Greene), and white-top aster (Aster curtus Cronq.). Translocations like these together with further research on the genetics and ecology of rare plant species are critical to species recovery efforts within GOE and other similarly compromised ecosystems.

Ecosystem-level dynamics of soil-vegetation features, with implications for conserving a narrowly endemic reptile

Narrow endemism presents challenges to species occurrence modeling particularly when the distribution of key local habitat features changes across space and time as a function of processes operating at larger scales. One need facing conservation in such settings is a better understanding of the biogeographic dynamics of the larger features that govern occurrence of critical local habitat. The Mescalero–Monahans shinnery sands region of western North America is a dynamic landscape where sand shinnery oak interacts with wind-driven sand to establish dune habitat. This ecosystem supports several narrowly endemic dune-dwelling species including the dunes sagebrush lizard. Using near-anniversary satellite and aerial imagery from 1986, 1998, and 2011, we integrated object-based image classification and statistical analysis to develop and validate a spatially explicit estimate of the sand shinnery oak ecosystem, including dynamics associated with its attrition and emergence, at high resolution throughout an 89,849-km2 study area encompassing the range of the dunes sagebrush lizard. The spatial estimate of the distribution and extent of the sand shinnery oak soil-vegetation association validated reasonably well (overall accuracy = 0.79; sensitivity = 0.49; specificity = 0.91) and showed that the association declined 10.3 % in extent during the 25-year assessment window. The presence of sand shinnery oak, patch size, and patch isolation were dynamic across space and time; a regression model showed that smaller, isolated patches on the periphery of the system were more likely to be lost over time whereas larger, less isolated, and centrally distributed patches were more likely to persist or expand. This study details broadly applicable methods to accurately delineate landforms throughout large extents, and offers mapping tools specific to issues surrounding Mescalero–Monahans shinnery sands endemics that are readily amenable to testing, refinement, and application in efforts to focus sustainable landscape management including conservation of endemic species.

Evolution of soil organic matter after prescribed fire: A 20-year chronosequence

Vegetation fires influence the properties and turnover of soil organic matter in numerous ecosystems. Thermal alteration tends to increase the stability of soil organic matter. Fire induces the destruction of above‐ground biomass, which is associated with a large production of new partially-charred litter. Plant regeneration is directly affected by the fire itself, and by the ensuing modifications to the C, N and nutrient cycles associated with soil organic matter transformations. The objectives of the present study were to determine 1) changes in soil organic matter composition induced by fire, and 2) the dynamics of soil organic matter recovery in the first two decades following a fire event.. In a Florida scrub oak ecosystem, a chronosequence of soils protected from vegetation fire for 1 to 20years was studied. The bulk organic matter and oxidation resistant elemental carbon were quantified in depths 0–5cm, 5–15cm, and 15–25cm. The soil organic matter was characterized by solid-state 13C NMR spectroscopy and Curie point pyrolysis.The combination of these techniques allowed us to identify three steps in soil organic matter evolution: first the un-charred litter brought by the fire degraded between 1 and 4years after fire; second the contribution of aryl carbon, and most probably the pyrogenic carbon, significantly decreased between 4 and 11years after fire; Thirdly, after 11years, the soil organic matter quality appeared driven again by the fresh litter input from regenerating plant ecosystem. In this study, the soil organic matter did not appear strongly modified by the fire. The pyrogenic carbon did not dominate the soil organic matter composition and underwent significant degradation at the decadal timescale. Our results also highlight a potential underestimated effect of dead root input to soil organic matter after the fire.

Cork oak vulnerability to fire: the role of bark harvesting, tree characteristics and abiotic factors.

Forest ecosystems where periodical tree bark harvesting is a major economic activity may be particularly vulnerable to disturbances such as fire, since debarking usually reduces tree vigour and protection against external agents. In this paper we asked how cork oak Quercus suber trees respond after wildfires and, in particular, how bark harvesting affects post-fire tree survival and resprouting. We gathered data from 22 wildfires (4585 trees) that occurred in three southern European countries (Portugal, Spain and France), covering a wide range of conditions characteristic of Q. suber ecosystems. Post-fire tree responses (tree mortality, stem mortality and crown resprouting) were examined in relation to management and ecological factors using generalized linear mixed-effects models. Results showed that bark thickness and bark harvesting are major factors affecting resistance of Q. suber to fire. Fire vulnerability was higher for trees with thin bark (young or recently debarked individuals) and decreased with increasing bark thickness until cork was 3–4 cm thick. This bark thickness corresponds to the moment when exploited trees are debarked again, meaning that exploited trees are vulnerable to fire during a longer period. Exploited trees were also more likely to be top-killed than unexploited trees, even for the same bark thickness. Additionally, vulnerability to fire increased with burn severity and with tree diameter, and was higher in trees burned in early summer or located in drier south-facing aspects. We provided tree response models useful to help estimating the impact of fire and to support management decisions. The results suggested that an appropriate management of surface fuels and changes in the bark harvesting regime (e.g. debarking coexisting trees in different years or increasing the harvesting cycle) would decrease vulnerability to fire and contribute to the conservation of cork oak ecosystems.

Acorn viability following prescribed fire in upland hardwood forests

Restoration of structure and function of mixed-oak (Quercus spp.) forests is a focal issue of forest land managers in the eastern United States due to widespread regeneration failure and poor overstory recruitment of oaks, particularly on productive sites. Prescribed fire is increasingly used as a tool in oak ecosystem restoration, with the goal of reducing competition, and creating light and seedbed conditions conducive to germination and growth of oak seedlings. Yet, oak seedling establishment is dependent on the presence of viable acorns, which may be vulnerable to prescribed fire. We assessed the effect of prescribed burning and fire temperature on the viability of white oak and northern red oak acorns placed on the leaf litter surface, in the duff, or in the mineral soil during five winter prescribed burns in southern Appalachian upland hardwood forests. Fire temperatures varied among acorn plots, ranging from <79 to <371°C. After the burns, acorns were planted in trays with water-saturated vermiculite, exposed to 14 continuous hours of light daily, and maintained at 27°C on germination beds in a greenhouse. After three weeks we recorded the proportion of acorns germinating and the proportion of germinants with shoots. Our study indicated that patchy, low-intensity dormant season prescribed fire in upland hardwood forests reduced viability of white oak and northern red oak acorns located on the leaf litter surface, but did not generally affect acorns in the duff or mineral soil. Germination rates of both northern red oak and white oak acorns on the leaf litter surface decreased with increasing fire temperature. Shoot production by northern red oak germinants from acorns on the leaf litter surface (and less so in the duff), also decreased with increasing fire temperature. Acorns of both species on the leaf litter surface burned at temperatures ⩾204°C showed high mortality levels, with mortality virtually 100% at temperatures ⩾260°C. Fall burns, especially after a heavy acorn crop, could result in high acorn mortality, potentially impacting oak regeneration from seedlings for many years given erratic acorn production patterns among years and species. Frequent burning that reduces litter and duff depth could compromise availability of ‘safe sites’ where acorns are insulated from high fire temperatures. When oak ecosystem restoration is a goal, land managers should consider the timing and size of acorn crops, as well as the forest floor condition when determining the timing and frequency of prescribed burning.

Using a Down-Scaled Bioclimate Envelope Model to Determine Long-Term Temporal Connectivity of Garry oak (Quercus garryana) Habitat in Western North America: Implications for Protected Area Planning

Under the Canadian Species at Risk Act (SARA), Garry oak (Quercus garryana) ecosystems are listed as "at-risk" and act as an umbrella for over one hundred species that are endangered to some degree. Understanding Garry oak responses to future climate scenarios at scales relevant to protected area managers is essential to effectively manage existing protected area networks and to guide the selection of temporally connected migration corridors, additional protected areas, and to maintain Garry oak populations over the next century. We present Garry oak distribution scenarios using two random forest models calibrated with down-scaled bioclimatic data for British Columbia, Washington, and Oregon based on 1961-1990 climate normals. The suitability models are calibrated using either both precipitation and temperature variables or using only temperature variables. We compare suitability predictions from four General Circulation Models (GCMs) and present CGCM2 model results under two emissions scenarios. For each GCM and emissions scenario we apply the two Garry oak suitability models and use the suitability models to determine the extent and temporal connectivity of climatically suitable Garry oak habitat within protected areas from 2010 to 2099. The suitability models indicate that while 164 km(2) of the total protected area network in the region (47,990 km(2)) contains recorded Garry oak presence, 1635 and 1680 km(2) of climatically suitable Garry oak habitat is currently under some form of protection. Of this suitable protected area, only between 6.6 and 7.3% will be "temporally connected" between 2010 and 2099 based on the CGCM2 model. These results highlight the need for public and private protected area organizations to work cooperatively in the development of corridors to maintain temporal connectivity in climatically suitable areas for the future of Garry oak ecosystems.

A review of the development of Mediterranean pine–oak ecosystems after land abandonment and afforestation: are they novel ecosystems?

Context Mediterranean landscapes are composed of different interacting vegetation patches. Pine and oak ecosystems form contiguous patches within these landscapes, in pure stands, or as mixed pine–oak ecosystems. During the nineteenth century, pine forest distribution in the Mediterranean Basin increased dramatically as a result of large-scale re-forestation and spontaneous forest regeneration. At the same time, secondary succession of abandoned agricultural land allowed development of pine and oak ecosystems. Consequently, a pine–oak mosaic has developed, which created opportunities for cross-colonization, i.e. species colonization from one ecosystem in the reciprocal system. Pines shed their wind-dispersed seeds and colonize Mediterranean oak vegetation. Oaks regenerate in different ecosystems, including pine forest understories.

Dendrochronological Potential of Quercus garryana, Saltspring Island, British Columbia

Garry oak (Quercus garryana Dougl.) is the cornerstone tree species of one of the most biologically diverse yet threatened ecosystems in Canada. The Canadian Garry oak ecosystem is located only in southern coastal British Columbia and is home to numerous endangered species. This study tests the dendrochronological potential of Garry oak trees near their northern range limit by successfully crossdating a sample from the Crow's Nest Ecological Research Area (CNERA) on Saltspring Island, British Columbia. Forty-five samples were collected from 39 trees to determine accurate age estimates and develop a crossdated tree-ring chronology that spans 180 years (A.D. 1825–2005). Wood anatomy and periodic growth suppression made crossdating challenging. However, distinct marker rings were readily identified on nearly all cores showing a synchronous response among trees to a common radial growth signal. The majority of trees sampled were established during 1860–1880 and 1947–1967. Correlation analyses show that radial growth of Garry oak at CNERA is influenced by both temperature and moisture availability. Our study demonstrates that extraction of high-quality dendrochronological data from Garry oak trees near their northern range limit is possible, so this species should be recognized as having good dendrochronological potential.

Techniques to Promote Garry Oak Seedling Growth and Survival in Areas with High Levels of Herbivory and Competition

Though regeneration of Garry oak (Quercus garryana) seedlings within Garry oak ecosystems is important to maintain this threatened habitat, seedling regeneration is often poor. Two independent studies were initiated in the early 2000s to examine questions surrounding Garry oak recruitment: one at the Crow's Nest Ecological Research Area (CNERA) on Salt Spring Island, BC, and another at the Pacific Rim Institute for Environmental Stewardship (PRI) on Whidbey Island, WA. In both areas, Garry oak seedlings were caged and monitored for growth and relative health. Over 8 years at CNERA, growth of caged plants outpaced growth of controls, which was indicative of high browsing pressure and relatively high palatability of Garry oak seedlings to black-tailed deer and feral sheep. The PRI oaks grew more slowly and at four PRI sites survival was 30–50% over 5–6 years as compared to 90% survival at CNERA over 8 years. The mortality at PRI was likely due to a combination of dry summer conditions and large numbers of voles, which took advantage of the cages for protection from predators; furthermore, lack of mycorrhizal hosts could have inhibited seedling establishment. Weed block inside exclosure cages provided ideal nesting conditions for voles. This study demonstrated that the caging of Garry oak seedlings, although labor-intensive and requiring frequent maintenance, provides valuable protection from large ungulate browsers and can be maintained as a relatively long-term measure, but vigilance is required to protect young seedlings from other threats such as voles or competing vegetation.