Spatial and temporal patterns of recent forest encroachment in montane grasslands of the Valles Caldera, New Mexico, USA

Aim Recent forest encroachment into montane and subalpine grasslands has occurred in the Rocky Mountains and many other mountain ranges globally. The timing, rate, and extent of tree invasion can depend on interactions among topography, positive spatial feedbacks, and temporally variable factors (especially climate, grazing, and fire). Here we examine spatial and temporal patterns of tree invasion in the Valles Caldera of the Jemez Mountains. Location This study was conducted in the Valles Caldera (35°50′–36°00′ N; 106°24′–106°37′ W), a 24‐km‐wide volcanic basin in northern New Mexico, USA. Grasslands in this otherwise forested region occur in broad valley bottoms of the caldera floor between 2575 and 2700 m, and on south‐facing slopes and mountain tops up to 3300 m. Methods We used a GIS analysis of orthorectified aerial photos taken in 1935 and 1996, covering a 40,000‐ha study area, to quantify the extent of tree invasion and to assess its relationship to spatial factors. We obtained dates of establishment from 299 increment cores and basal disks from 50 sites in the Valles Caldera National Preserve (VCNP) to reconstruct temporal patterns of tree invasion. Results The area of grasslands in our study area declined from 11,747 to 9336 ha (nearly 18%) between 1935 and 1996. Tree invasion increased with slope, elevation, and proximity to the previous tree line, but showed no relationship to aspect. Tree invasion was more rapid and continuous on upper mountain slopes, while the invasion of valley‐bottom grasslands below reversed tree lines was more episodic, and appeared to track mean summer minimum temperatures. Main conclusions The rapid and continuous invasion of steep, high‐elevation slopes suggests that frequent fire was the single most important factor in maintaining grassy communities in these sites. The slower, episodic invasion of valley‐bottom grasslands, and the apparent relationship between increased invasion and years of higher summer minimum temperatures are consistent with the hypothesis that these grasslands have been maintained by low temperatures or frosts damaging to tree seedlings. We encourage prescribed fire to restore and maintain grasslands in the VCNP, especially small patches on steep, high‐elevation slopes.     

A world-wide study of high altitude treeline temperatures

Aim At a coarse scale, the treelines of the world's mountains seem to follow a common isotherm, but the evidence for this has been indirect so far. Here we aim at underpinning this with facts. Location We present the results of a data‐logging campaign at 46 treeline sites between 68° N and 42° S. Methods We measured root‐zone temperatures with an hourly resolution over 1–3 years per site between 1996 and 2003. Results Disregarding taxon‐, landuse‐ or fire‐driven tree limits, high altitude climatic treelines are associated with a seasonal mean ground temperature of 6.7 °C (±0.8 SD; 2.2 K amplitude of means for different climatic zones), a surprisingly narrow range. Temperatures are higher (7–8 °C) in the temperate and Mediterranean zone treelines, and are lower in equatorial treelines (5–6 °C) and in the subarctic and boreal zone (6–7 °C). While air temperatures are higher than soil temperatures in warm periods, and are lower than soil temperatures in cold periods, daily means of air and soil temperature are almost the same at 6–7 °C, a physics driven coincidence with the global mean temperature at treeline. The length of the growing season, thermal extremes or thermal sums have no predictive value for treeline altitude on a global scale. Some Mediterranean (Fagus spp.) and temperate South Hemisphere treelines (Nothofagus spp.) and the native treeline in Hawaii (Metrosideros) are located at substantially higher isotherms and represent genus‐specific boundaries rather than boundaries of the life‐form tree. In seasonal climates, ground temperatures in winter (absolute minima) reflect local snow pack and seem uncritical. Main conclusions The data support the hypothesis of a common thermal threshold for forest growth at high elevation, but also reflect a moderate region and substantial taxonomic influence.     

Effects of fire severity in a north Patagonian subalpine forest

Abstract. Question: What is the relative importance of fire‐induced canopy mortality, soil burning and post‐fire herbivory on tree seedling performance? Location: Subalpine Nothofagus pumilio forests at Challhuaco valley (41°13'S, 71°19'W), Nahuel Huapi National Park, Argentina. Methods: We fenced and transplanted soils of three burning severities along a fire severity gradient produced by a fire in 1996. Over two growing seasons we monitored soil water, direct incoming solar radiation, seedling survival, final seedling total biomass and root/shoot ratio. Additionally, we assessed severity‐related changes in soil properties. Results: Incoming radiation (an indicator of the amount of canopy cover left by the fire) was the primary factor influencing spring and summer top soil water availability, first and second‐year seedling survival and seedling growth. While seedling survival and soil water content were negatively affected by increased radiation, seedling final biomass was highest in very open microsites. Burned soils showed lower water holding capacity and soil carbon; however these changes did not affect topsoil water, and, contrary to expectation, there was a slight tendency toward higher seedling survival on more heavily burned soils. Herbivory significantly reduced seedling survival, but only under high‐radiation conditions. While the effect of radiation on final seedling biomass was not affected by herbivory, R/S ratios were significantly reduced by herbivory in high radiation micro sites. Conclusions: Despite inducing faster aerial growth, increased radiation and herbivory in severely burned sites may effectively prevent post‐fire regeneration in north Patagonian subalpine forest where seed sources are not limiting.     

Soil transfers from valley oak (Quercus lobata Nee) stands increase ectomycorrhizal diversity and alter root and shoot growth on valley oak seedlings

 Soils from valley oak (Quercus lobata Nee) riparian areas of the Cosumnes River Nature Conservancy Preserve near Sacramento, California were added to growth medium of valley oak seedlings grown in a greenhouse or in agricultural fields at Cosumnes which probably once supported valley oak trees and are now replanted with native riparian vegetation or allowed to revegetate naturally. Agricultural field soil from the Cosumnes River Preserve was presumed to be low or lacking in ectomycorrhizal inoculum. The study was designed to (1) determine whether valley oak stand soil transfer could cause mycorrhizal infection on valley oak seedlings in an agricultural field and in a greenhouse, (2) describe ectomycorrhizal morphological types formed on valley oak seedlings, and (3) determine whether seedling growth is enhanced more by transfer of natural valley oak stand soil than agricultural field soil. In the field study, transfer of forest soil increased average ectomycorrhizal diversity (2.4 types) more than transfer of agricultural field soil (1.2 types). Valley oak seedlings were responsive to ectomycorrhizal infection in the field study. With increase in mycorrhizal infection there was an increase in shoot growth at the expense of root growth. In the greenhouse study, both percent mycorrhizal infection and mycorrhizal diversity were increased more by transfer of oak forest and woodland soils than agricultural field soil. Eight morphotypes occurred on seedlings in forest and woodland soils but only three morphotypes in agricultural soil. This result strongly suggests that the agricultural field also harbors ectomycorrhizal propagules but forest and woodland soils support a more abundant and diverse ectomycorrhizal flora. Accepted: 17 August 1997     

Effects of topography,soil moisture,wind and grazing on Festuca seedlings in a Patagonian grassland

Abstract. The effects of topography, soil moisture, wind and grazing on the emergence and survival of seedlings of Festuca spp. were examined in the steppe zone of Patagonia, Argentina. Ungrazed and grazed field treatment plots were established on a plain and a north-facing slope at the Media Luna Ranch (43° 36′S, 71° 25′W). On the leeward and windward sides of each of 15 Festuca plants, 0.1 m × 0.4 m quadrats were censused bimonthly for seedling emergence and survival over three growing seasons. Three categories were distinguished: recently germinated and up to the first leaf, two to four leaves, and from five leaves up to one tiller. Soil moisture content, litter cover and frost heaving effects were also determined for each treatment at each sampling date. Festuca spp. showed two emergence peaks, one in late fall and the other in early to mid-spring. Seedling emergence was significantly correlated with soil moisture content in the 0–5 cm of the soil during the three growing seasons. Seedlings that emerged in the fall had higher survivorship than those that emerged in spring. Seedling emergence and survival was significantly (p < 0.01) lower on slopes, in the grazing treatment, and on windward sides of adult plants. In this grassland, an increase in the availability of safe sites for seedling emergence and survival might be achieved by protecting vegetation from grazing, particularly on north-facing slopes.     

Species richness and soil reaction in a Northeastern Oklahoma landscape

Although the relationship between soil reaction and species richness is of great theoretical interest, and has been relatively well studied in temperate latitudes, the southern Great Plains of North America have not received much attention. We analyzed the relationship between species richness and measures of soil reaction (pH, cation exchange capacity, Ca) in 418 10 m × 10 m samples (with nested smaller quadrats) in the Tallgrass Prairie Preserve, Osage County, Oklahoma. We found a negative correlation of richness with soil reaction that is consistent across scales and years in grasslands, with soil calcium exhibiting the strongest relationships. The relationship in woodlands is weakly positive. The proportion of (primarily Eurasian) exotic species is positively related to calcium concentrations in both woodlands and grasslands. Although our results are not inconsistent with species pool hypotheses, critical tests of such hypotheses are operationally difficult.     

干旱半干旱草地生态系统与土壤水分关系研究进展

研究干旱半干旱草地生态系统与土壤水分关系和相互作用机理对于揭示草地生态系统稳定性及其水土关键要素的变化过程具有重要意义。从不同界面、不同尺度综述了草地生态系统对土壤水分的影响及草地生态系统的响应与适应机制,总结了草地生态系统与土壤水分关系模型研究的相关进展,并分析了气候变化对草地生态系统和土壤水分关系的影响。草地生态系统通过影响水文过程和生态过程来影响土壤水分,土壤水分在植物生长发育、形态、生理生态过程、种间关系、群落组成和结构以及草地生态系统功能等方面对草地生态系统产生影响;充分揭示草地生态系统-土壤水分相互作用机理是模型研究的关键;气候变化对草地生态系统植物与土壤水分关系具有重要影响。今后应加强以下研究:1)开展草地不同优势种和植物功能型与土壤水分关系的研究,找出能反映植物对土壤水分响应的性状指标,阈值响应点及适应机制;2)注重对不同时间和空间尺度上的转换和比较;3)加强个体、群体和生态系统尺度草地植物生长模型的研究及其与土壤-植被-大气水分传输模型的耦合;4)加强草地生态系统与土壤水分关系对气候变化响应的研究。     

Effects of grassland intensification on Whinchats Saxicola rubetra and implications for conservation in upland habitats

Modern, intensive grassland management has led to strong declines in ground‐nesting grassland birds, and is now increasingly threatening the last remaining strongholds of the Whinchat Saxicola rubetra in the Central European uplands. In this study, we assess key threats to Whinchat populations in these uplands in order to suggest appropriate conservation measures. We compared the direct threat of early mowing as well as the indirect threat resulting from a deteriorating arthropod food source in an inner‐alpine valley. Five of our seven study sites were mown too early with respect to the chicks' fledging date. Such early mowing was particularly evident on the more intensively farmed, earlier mown valley bottoms than on the valley slopes. Arthropod abundance and biomass did not differ between valley bottoms and slopes. However, valley bottoms had a greater amount of unprofitable prey items such as flies. Breeding bird density was mainly determined by the degree of overlap between the mowing schedule and breeding phenology. These findings suggest that in upland grasslands at an early stage of intensification, early mowing is of greater importance for populations than possible negative effects of a reduced food source. We suggest that mowing is delayed until a sufficient proportion of nestlings are safely fledged.     

Seedling establishment of deciduous trees in various topographic positions

Abstract. We investigated the effect of topography‐related environmental factors (i.e., ground‐surface stability and soil moisture) on seedling establishment of 8 deciduous tree species. A field experiment was carried out using canopy species, which were classified into 3 groups based on the spatial distribution of adult trees (ridge, slope and valley). Demographic parameters were compared among species during the early stage of seedling establishment among 3 topographic positions in combination with gap and canopy conditions. The percentage of emerging seedlings tended to be lower on the ridge irrespective of the adult topographical distribution patterns. There was no clear trend in seedling emergence among the species groups classified by their spatial distribution. Seedling survival during 2 growing seasons was significantly different among species, topographic positions and light conditions. On the ridge, seedlings of the species dominating ridge tops had greater survival than those of other species, probably due to differences in demand for soil moisture. On the slope, frequent physical damage caused by surface material movement was observed and some species showed greater adaptability to the disturbed slope habitat. Survival of all seedlings was highest in the valley plots. Light conditions were the critical factor for seedling survival in some species. The results of this study suggest that topography creates diverse habitats for the establishment of tree seedlings. In addition to soil moisture, surface material movement may be a significant factor affecting seedling establishment.     

Hydration status influences seed fire tolerance in temperate European herbaceous species

Prescribed burning is an important management tool in many parts of the world. While natural fires generally occur during the driest and warmest period of the year, prescribed burning is often timed out‐of‐season, when there is higher soil moisture and lower biomass combustibility. However, fire season may influence seedling recruitment after fire, e.g. through the effect of seed hydration status on fire tolerance. In non‐fire‐prone temperate regions, anthropogenic fire may occur exclusively in periods outside the growing season with higher soil moisture, which may have negative consequences on seedling recruitment. Fire tolerance of moist and dry seeds of 16 temperate European herbaceous species belonging to four families was assessed using heat treatment of 100 °C for 5 min and subsequent germination trials. Moist seeds of Asteraceae, Poaceae and Brassicaceae had a predominantly negative reaction to the heat treatment, while those of Fabaceae tolerated it or germination was even enhanced. The reaction of dry seeds was completely different, with positive responses in three species of the Fabaceae and fire tolerance in species of other families. Our results point out that hydration status may significantly influence the post‐fire germination of seeds. Dry seeds were found to tolerate high heat, while moist seeds were harmed in more than half of the species. This implies that if prescribed burning is applied in temperate grasslands of Europe, it should be timed to dry periods of the dormant season in order to protect seeds from negative effects of fire.     

Scale‐dependence of vegetation‐environment relationships in semi‐natural grasslands

Questions: Which environmental and management factors determine plant species composition in semi‐natural grasslands within a local study area? Are vegetation and explanatory factors scale‐dependent? Location: Semi‐natural grasslands in Lærdal, Sognog Fjordane County, western Norway. Methods: We recorded plant species composition and explanatory variables in six grassland sites using a hierarchically nested sampling design with three levels: plots randomly placed within blocks selected within sites. We evaluated vegetation‐environment relationships at all three levels by means of DCA ordination and split‐plot GLM analyses. Results: The most important complex gradient determining variation in grassland species composition showed a broad‐scale relationship with management. Soil moisture conditions were related to vegetation variation on block scale, whereas element concentrations in the soil were significantly related to variation in species composition on all spatial scales. Our results show that vegetation‐environment relationships are dependent on the scale of observation. We suggest that scale‐related (and therefore methodological) issues may explain the wide range of vegetation‐environment relationships reported in the literature, for semi‐natural grassland in particular but also for other ecosystems. Conclusions: Interpretation of the variation in species composition of semi‐natural grasslands requires consideration of the spatial scales on which important environmental variables vary.     

Transitional slopes act as hotspots of both soil CO<Subscript>2</Subscript> emission and CH<Subscript>4</Subscript> uptake in a temperate forest landscape

Forest soils are an important component of CO₂ and CH₄ fluxes at the global scale, but the magnitude of these fluxes varies greatly in space and time within a landscape. Understanding the spatial and temporal distributions of these fluxes across complex landscapes remains a major challenge for researchers and land managers alike. We investigated the spatiotemporal variability of soil-atmosphere CO₂ and CH₄ fluxes and the relationships of these fluxes to chemical and physical soil properties distributed across a topographically-heterogeneous landscape. Soil CO₂ and CH₄ fluxes were measured along with soil temperature, moisture, bulk density, texture, carbon, sorption capacity, and dissolved organic matter quality over 2 years along hillslope transects spanning valley bottom, transition zone, and upland landscape positions in a temperate forest watershed. Transition zone soil CO₂ efflux was 54–160% higher than low-lying valley bottoms, and 15–54% higher than uplands. Net seasonal CH₄ uptake was 58–150% higher in transition zone soils than in uplands, while valley bottoms were occasionally large net sources (up to 19 nmol CH₄ m^?2 s^?1). Soil CO₂ efflux and net CH₄ uptake were both positively associated with seasonal temperature, and were highest in soils with relatively high carbon and clay content, and relatively low bulk density, moisture, and sorption capacity. We concluded that: (1) transition zone soils act as landscape hotspots for net CH₄ uptake in addition to CO₂ efflux, and (2) that this spatial distribution is more consistent across seasons for net CH₄ uptake than for CO₂ efflux.     

Invasion of woody species into temperate grasslands: Relationship with abiotic and biotic soil resource heterogeneity

Question: Invasion of woody species into grasslands is a global phenomenon. This is also topical in semi‐natural temperate grasslands that are no longer profitable for agricultural management. Trees and grasses interact through harsh root competition, but below‐ground processes have been neglected in the dynamics of semi‐natural grasslands. Trees are thought to have a competitive advantage in resource‐rich and heterogeneous soils. We tested whether soil resource quantity and heterogeneity differ between paired temperate semi‐natural grasslands and forests (former grasslands), and whether this was caused abiotically by varying soil depth or biotically by fine roots. Location: Thin‐soil calcareous alvar grasslands with overgrown parts (young Pinus sylvestris forests) in W. Estonia. Methods: The quantity and spatial heterogeneity of soil resources (moisture and nutrients), soil depth, and root parameters (mass, length and specific length) were measured in 1‐m transects of 11 samples in 26 paired grasslands and forests. The quantity and heterogeneity of soil resources were compared between vegetation types and related to soil depth and root parameters. Results: Soil resources were lower and more heterogeneous in forests than in grasslands. The invasion of woody species was enhanced abiotically by deeper soil. Root mass was larger in the forests, but root length was longer in the grasslands. Both root mass and specific root length were more heterogeneous in the forests. Forest root length was negatively correlated with transient soil moisture patches and positively correlated with more persistent nutrient‐rich patches. No such relationship was found in grasslands. Conclusions: Abiotic soil heterogeneity (local deep‐soil patches) supports woody species invasion, but the trees themselves also biotically make soils more heterogeneous, which further enhances woody species invasion. Large trees use soil resources patchily, making soils biotically poorer and more heterogeneous in resources. The dynamics of temperate semi‐natural grasslands are strongly linked to below‐ground ecological processes, and high soil heterogeneity can be both the cause and the outcome of woody species invasion.     

A new species of Atheroides Haliday (Hemiptera,Aphididae) native to North America

We report and describe the first species of Atheroides Haliday presumed to be native to North America, collected at the Valles Caldera National Preserve, New Mexico, USA. We hypothesize its placement among the Siphini based on morphological, phylogenetic analysis and extend the distribution of the genus to the Holoarctic. We expand the key of the known Atheroides to include the new species and discuss the current hypotheses of the geographic distribution of the type species, Atheroides serrulatus Haliday.     

Historical dendroarchaeology of two log structures in the Valles Caldera National Preserve,New Mexico,USA

We used dendroarchaeological techniques to determine the year of construction of two historic structures in the Valles Caldera National Preserve of New Mexico, USA. Historical documents date some structures in the headquarters area of the Preserve, but the Commissary Cabin and Salt Barn were lacking conclusive construction dates. Both structures were originally thought to have been built by the Otero family who bought the property in 1899. We found that the structures were built from two tree species, white fir (Abies concolor (Gordon) Lindl. ex Hildebr.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), surprising given that ponderosa pines are also found in great numbers in the adjacent forest. Tree rings from 20 logs were confidently crossdated both graphically and statistically and provided cutting dates of trees in both structures of 1940 and 1941 when compared against the Fenton Lake reference chronology (Commissary Cabin: r = 0.69, t = 15.54, p < 0.0001, n = 263 years; Salt Barn: r = 0.77, t = 11.7, p < 0.0001, n = 232 years). By combining the cutting date years and terminal ring attributes, we suggest that both structures were built in the spring or early summer of 1941 using freshly cut logs and logs that had been cut the previous spring (1940, before or during the growing season) and stockpiled. The cutting dates of 1940 and 1941 indicate that these buildings were constructed during the Franklin Bond (1939–1945) era and associated with the transition from sheep ranching to more modern cattle grazing. These new dates provide a more distinct understanding of the cultural resources at the Valles Caldera National Preserve and provide interpretative staff with more accurate information that can be given to the public.     

Prediction of herbage yield in grassland: How well do Ellenberg N‐values perform?

Question: How useful are Ellenberg N‐values for predicting the herbage yield of Central European grasslands in comparison to approaches based on ordination scores of plant species composition or on soil parameters? Location: Central Germany (11°00′‐11°37’E, 50°21‐50°34’N, 500–840 m a.s.l.). Methods: Based on data from a field survey in 2001, the following models were constructed for predicting herbage yield in montane Central European grasslands: (1) Linear regression of mean Ellenberg N‐, R‐ and F‐values; (2) Linear regression of ordination scores derived from Non‐metric Multidimensional Scaling (NMDS) of vegetation data; and (3) Multiple linear regression (MLR) of soil variables. Models were evaluated by cross‐validation and validation with additional data collected in 2002. Results: Best predictions were obtained with models based on species composition. Ellenberg N‐values and NMDS scores performed equally well and better than models based on Ellenberg R‐ or F‐values. Predictions based on soil variables were least accurate. When tested with data from 2002, models based on Ellenberg N‐values or on NMDS scores accurately predicted productivity rank order of sites, but not the actual herbage yield of particular sites. Conclusions: Mean Ellenberg N‐values, which are easy to calculate, are as accurate as ordination scores in predicting herbage yield from plant species composition. In contrast, models based on soil variables may be useful for generating hypotheses about the factors limiting herbage yield, but not for prediction. We support the view that Ellenberg N‐values should be called productivity values rather than nitrogen values.     

Resilience,persistence and relationship to standing vegetation in soil seed banks of semi‐arid Australian old fields

Questions: Do soil seed banks of semi‐arid grasslands reassemble after abandonment from cultivation? Do seeds of native and exotic species persist in the soil? Does time since abandonment affect compositional similarity between the vegetation and seed bank? Does the seed bank contribute to resilience in the vegetation? Location: Native grasslands in northern Victoria, Australia. Methods: Seed bank sampling was conducted in spring and autumn over 3 yrs, across a 100‐yr chronosequence. Species richness, composition and germinant density were determined using the seedling emergence method. Seed persistence was assessed by comparing seed densities in spring and autumn. Seed bank composition was compared with the vegetation. Results: The spring seed bank was dominated at all stages by sedges and rushes; hence, native species richness and seed density were largely unaffected by abandonment. In autumn, grassland species contributed more to the seed bank, but richness was reduced after abandonment and showed little recovery, although seed density partially recovered. Seed bank composition showed some recovery in both seasons. Most species had low persistence in the soil. Compositional similarity between the vegetation and seed bank was greater in old fields than uncultivated grasslands in spring, but not autumn. Conclusions: Resilience varied among seed bank parameters and seed banks had low functional importance. Patterns in the seed bank followed, rather than caused, those in the vegetation. Thus, vegetation recovery cannot rely on the seed bank and persistent seeds were not the key mechanism of resilience in the vegetation.     

Litter accumulation alters the abiotic environment and drives community successional changes in two fenced grasslands in Inner Mongolia

Fencing is an effective and practical method for restoring degraded grasslands in northern China. However, little is known about the role of excess litter accumulation due to long‐term fencing in regulating abiotic environment and driving changes in community structure and function. We conducted a three‐year field experiment in two fenced grasslands in Inner Mongolia, and monitored light quantity, soil temperature, and soil moisture continuously, and determined community height, community aboveground net primary productivity (ANPP), and the relative dominance of different plant functional groups. Litter accumulation reduced light quantity and soil temperature but increased soil moisture. The regulating effects of litter accumulation on soil temperature and soil moisture fluctuated temporally and gradually weakened over the growing season. Litter accumulation also altered community vertical structure and function by increasing community height and ANPP. The increase in soil moisture increased the relative dominance of rhizome grasses but suppressed bunch grasses, thereby shifting bunch grass grasslands to rhizome grass grasslands. Our findings provide a potential mechanism for community succession in the context of litter accumulation in fenced grasslands and indicate that the vegetation and ecosystem services of degraded grasslands are improved after appropriate fencing.     

Small vegetation gaps increase reseeded yellow-flowered alfalfa performance and production in native grasslands

Reseeding yellow-flowered alfalfa (YFA) in degraded grasslands may require a vegetation-free microsite for germination and subsequent establishment. This study aimed to examine the role of microclimates of different-sized vegetation gaps on seedling performance and adult plant production of YFA. Field microsites were established in the meadow steppe of Hulunber, Inner Mongolia, China. Seedling performance, plant production, the microclimate within vegetation gaps, and soil nutrients (plant-available N, P, and K, total N concentration) were assessed at the end of each growing season from 2013 to 2015. Our results indicate light availability, and topsoil temperature of each gap were significantly increased as gap size increased, while topsoil moisture and air relative moisture were decreased in larger gaps. Small gaps (diameter ≤10 cm) improved seedling emergence, survival, biomass, and root nodulation, as compared with seedling performance associated with the larger gaps, presumably in response to increased shade and moisture. Additionally, large gaps (>20 or >40 cm) were characterized by significantly lower plant-available P, total N concentrations, plant-available K, and soil pH. However, root exclusion treatments did not improve overall seedling performance, plant production, or soil properties, as compared to corresponding microsites with root presence, regardless of gap size. Our results suggest that reseeding YFA into grasslands where disturbance, such as light grazing, has resulted in small gaps will be more effective than in highly degraded grasslands.     

Forest encroachment into a Californian grassland: examining the simultaneous effects of facilitation and competition on tree seedling recruitment

Competition and facilitation are both considered major factors affecting the structure of plant assemblages, yet few studies have quantified positive, negative, and net effects simultaneously. In this study, we investigated the positive, negative, and net effects of tree saplings on the encroachment of two tree species, Douglas fir (Pseudotsuga menziesii) and tanoak (Lithocarpus densiflora), into a coastal California grassland. The study involved three components: sampling the spatial distributions of P. menziesii and L. densiflora in the grasslands, a field experiment examining seedling survival in different grassland environments, and a greenhouse experiment examining the effects of soil moisture on early seedling performance. The field experiment was conducted over a 2-year period, using Pseudotsuga in 2002 and both species in 2003. Seedlings were separated into four treatment groups: those planted in open grassland, in shaded grassland, under artificial (plastic) conifer saplings, and under natural Pseudotsuga saplings. Air temperature, relative humidity, soil moisture, incident radiation levels and fog water inputs were measured for each treatment group in 2003. In the greenhouse experiment, Pseudotsuga and Lithocarpus seedlings were grown for 13 weeks in watering treatments simulating the summer soil moisture conditions of the open grasslands and under Pseudotsuga saplings. Surveys of naturally established seedlings found that Lithocarpus occurred only under Pseudotsuga saplings, while most Pseudotsuga seedlings were located near but not directly under conspecific saplings. In the field experiment, positive effects of tree saplings were much larger than negative effects, resulting in strong net facilitation of seedling establishment. Survival for both species was always higher under the plastic and live trees than in the open or shade plots. The primary mechanism facilitating seedling survival appeared to be increased soil moisture caused by input of fog precipitation coupled with reduced microsite evaporation. The greenhouse experiment further showed that soil moisture strongly affected seedling performance, with both species having much higher photosynthetic rates in the higher moisture treatment. In the lower moisture treatment, Pseudotsuga seedlings had higher photosynthetic rates and stomatal conductance than Lithocarpus, suggesting they may be able to better tolerate the environmental conditions found in the open grasslands. Our combined results suggest that rate and patterning of woody plant encroachment can be strongly influenced by facilitation and that fog precipitation may play a key role in plant interactions.