Species richness and canopy productivity of Australian plant communities

The species richness (number of vascular plants per hectare) of Australian plant communities (containing a mosaic of gap, regeneration, maturation and senescent phases) is correlated with the annual biomass productivity of the overstorey canopy.The annual production of leaves and stem in the canopy of the plant community is shown to be limited by the requirements of photosynthesis (particularly light and the availability of water) and the length of the growing season.The species richness of Australian plant communities is the product of the blance between the dominance of the overstorey and the response of the understorey to the shading of the overstorey. For all climatic regions and zones the species richness of the overstorey of the plant community is shown to be exponentially related to the annual shoot growth of the overstorey canopy, until the latitudinal or altitudinal tree line is reached. With latitudinal increase outside the tropics, overstorey canopies of forest communities absorb increasingly more of the incident solar radiation. markedly reducing the species richness of the understorey strata. In contrast, in these latitudes the overstorey of plant communities with widely spaced trees or tall shrubs will absorb far less solar radiation, thus enabling the species richness of the understorey to be maintained.     

Overstorey Control of Understorey Species Composition in a Near-natural Temperate Broadleaved Forest in Denmark

Little is known about the importance of the forest overstorey relative to other factors in controlling the spatial variability in understorey species composition in near-natural temperate broadleaved forests. We addressed this question for the 19 ha ancient forest Suserup Skov (55°22′ N, 11°34′ E) in Denmark, one of the few old-growth temperate broadleaved forest remnants in north-western Europe, by inventorying understorey species composition and environmental conditions in 163 100 m² plots. We use unconstrained and constrained ordinations, variation partitioning, and Indicator Species Analysis to provide a quantitative assessment of the importance of the forest overstorey in controlling understorey species composition. Comparison of the gradients extracted by unconstrained and constrained ordinations showed that the main gradients in understorey species composition in our old-growth temperate broadleaved forest remnant are not caused by variability in the forest overstorey, but are related to topography and soil, edge effects, and unknown broad-scale factors. Nevertheless, overstorey-related variables uniquely accounted for 15% of the total explained variation in understorey species composition, with the pure overstorey-related (R_po), topography and soil (R_pt), edge and anthropogenic disturbance effects (R_pa), and spatial (R_ps) variation fractions being of equal magnitude. The forward variable selection showed that among the overstorey-related variables understorey light availability and to a lesser extent vertical forest structure were most important for understorey species composition. No unique influence of overstorey tree species identity could be documented. There were many indicator species for high understorey light levels and canopy gap centres, but none for medium or low light or closed canopy. Hence, no understorey species behaved as obligate shade plants. Our study shows that, the forest overstorey has a weak control of understorey species composition in near-natural broadleaved forest, in contrast to results from natural and managed forests comprising both conifer and broadleaved species. Nevertheless, >20% of the understorey species found were indicators of high light conditions or canopy openings. Hence, variability in canopy structure and understorey light availability is important for maintaining understorey species diversity.     

Within‐stand spatial structure and relation of boreal canopy and understorey vegetation

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed‐wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad‐leaved deciduous trees differ in their spatial relationships with understorey vegetation? Location: EMEND experimental site, Alberta, Canada. Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets. Results: Boreal mixed‐wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non‐vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales. Conclusions: The spatial structure and relation of boreal mixed‐wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed‐wood forests.     

Phenological mismatch with trees reduces wildflower carbon budgets

Interacting species can respond differently to climate change, causing unexpected consequences. Many understorey wildflowers in deciduous forests leaf out and flower in the spring when light availability is the highest before overstorey canopy closure. Therefore, different phenological responses by understorey and overstorey species to increased spring temperature could have significant ecological implications. Pairing contemporary data with historical observations initiated by Henry David Thoreau (1850s), we found that overstorey tree leaf out is more responsive to increased spring temperature than understorey wildflower phenology, resulting in shorter periods of high light in the understorey before wildflowers are shaded by tree canopies. Because of this overstorey–understorey mismatch, we estimate that wildflower spring carbon budgets in the northeastern United States were 12–26% larger during Thoreau's era and project a 10–48% reduction during this century. This underappreciated phenomenon may have already reduced wildflower fitness and could lead to future population declines in these ecologically important species.     

The influence of Prosopis canopies on understorey vegetation: Effects of landscape position

Abstract. The influence of canopy trees and shrubs on under‐storey plants is complex and context‐dependent. Canopy plants can exert positive, negative or neutral effects on production, composition and diversity of understorey plant communities, depending on local environmental conditions and position in the landscape. We studied the influence of Prosopis velutina (mesquite) on soil moisture and nitrogen availability, and understorey vegetation along a topographic gradient in the Sonoran Desert. We found significant increases in both soil moisture and N along the gradient from desert to riparian zone. In addition, P. velutina canopies had positive effects, relative to open areas, on soil moisture in the desert, and soil N in both desert and intermediate terrace. Biomass of understorey vegetation was highest and species richness was lowest in the riparian zone. Canopies had a positive effect on biomass in both desert and terrace, and a negative effect on species richness in the terrace. The effect of the canopy depended on landscape position, with desert canopies more strongly influencing soil moisture and biomass and terrace canopies more strongly influencing soil N and species richness. Individual species distributions suggested interspecific variation in response to water‐ vs. N‐availability; they strongly influence species composition at both patch and landscape position levels.     

Carbon dioxide concentrations within forest canopies—variation with time, stand structure, and vegetation type

Vertical CO₂ profiles (between 0.02 and 14.0 m) were studied in forest canopies of Pinus contorta, Populus tremuloides, and in a riparian forest with Acer negundo and Acer grandidentatum during two consecutive growing seasons. Profiles, measured continuously during 1- to 13-day periods in four to five stands differing in overstorey canopy area index (CAI < 4.5; including leaves, branches and stems), were well stratified, with highest [CO₂] just above the forest floor. Canopy [CO₂] profiles were influenced by stand structure (CAI, presence of understorey vegetation), and were highly dependent on vegetation type (deciduous and evergreen). A doubling of CAI in Acer spp. and P. tremuloides stands did not show an effect on upper canopy [CO₂], when turbulent mixing was high. However, increasing understorey biomass in Acer spp. stands had a profound effect on lower canopy [CO₂]. In open stands with a vigorous understorey layer, higher soil respiration rates were offset by increased understorey gas exchange, resulting in [CO₂] below those of the convective boundary layer (CBL). Midday depletions up to 20 ppmv below CBL values could be frequently observed in deciduous canopies. In evergreen canopies, [CO₂] stayed generally above the CBL background values, [CO₂] profiles were more uniform, and gradients were smaller than in deciduous stands with similar CAI. Seasonal changes of canopy [CO₂] reflected changes in soil respiration rates as well as plant phenology and gas exchange of both dominant tree and understorey vegetation. Seasonal patterns were less pronounced in evergreen than in deciduous forests.     

Spatial dynamics of regeneration in a conifer/broad‐leaved forest in northern Japan

Abstract. This study deals with stand dynamics over a 6‐yr period in a conifer/broad‐leaved mixed forest in Hokkaido, northern Japan. The annual rates of gap formation and recovery were 81.3 m²/ha and 66.7 m²/ha, respectively and turnover time of the canopy was 125 yr. The recruitment processes of the component species in this cool‐temperate forest were governed by different canopy types: gap, canopy edge and closed canopy. Magnolia obovata regenerated in canopy edges, and Acer mono and Prunus ssiori regenerated in canopy edges and gaps. The results suggested that the mosaic structure made up of closed canopy, canopy edge and gap was related to various regeneration niches. Abies sachalinensis had high mortality rates, initiating gap expansion. The transition probabilities from closed canopy or canopy edge to gap for deciduous broad‐leaved trees were lower than for A. sachalinensis, which implies that the difference in degeneration patterns of conifer and broad‐leaved canopies contributes to the heterogeneity of spatial structure in the mixed forests. Spatial dynamics were determined by a combination of gap expansion by A. sachalinensis (neighbour‐dependent disturbance) and gap formation by deciduous broad‐leaved trees (random disturbance).     

Competitive interactions between overstorey proteas and sprouting understorey species in South African mountain fynbos

Abstract. Previous studies in the mountain fynbos of South Africa have demonstrated that short fire cycles favour the establishment of dense covers of understorey sprouters while longer fire intervals enable the establishment from seed of overstorey proteas and the formation of a overstorey. One consequence of these differences between fire cycle lengths is the effect that understorey sprouters and an overstorey protea canopy have on species richness. In the case of short fire intervals, species richness is decreased while longer intervals between fires allow species richness to decrease or increase depending on the patchiness of the overstorey canopy. Such results are suggestive of competitive effects between understorey sprouters and overstorey canopy proteas. In this study, data were collected from several pyric successional stages in mountain fynbos to study the effect of overstorey proteas on the growth and flowering of understorey sprouters since the last fire. Data were also collected to determine the effect that understorey sprouters had on the establishment and fecundity of overstorey protea species. Competitive interactions between overstorey proteas and sprouting understorey species were evident at all the sites studied. The vegetative growth and seed production of understorey sprouters, which grew under a canopy of overstorey proteas during the current interfire period, were significantly lower than that for plants growing in the open. In addition, the postfire growth and seed production of understorey sprouters were significantly lower for individuals, which grew under an overstorey protea canopy during the previous fire cycle, than for those individuals which grew in the open. The fecundity of overstorey proteas, which grew near understorey sprouters, was lower than that of plants which grew in the open. This effect was evident for up to the first 15 years after a fire. However, not all understorey sprouters affected the overstorey proteas equally. Also, seedlings of overstorey proteas established significantly less successfully in close proximity to understorey sprouters after a fire than in the open or under proteas. Finally, the results demonstrate that complex species‐specific, understorey–overstorey interactions are important in mountain fynbos. For example, some overstorey species depend on trophically similar species to reduce potential competition from understorey sprouters for their successful establishment at a site.     

Pine savanna overstorey influences on ground‐cover biodiversity

Question: Does the overstorey of pine savannas influence plant species biodiversity in the ground cover? Location: Camp Whispering Pines (30°41’N; 90°29’W), eastern Louisiana (USA). Methods: We used ecologically sensitive restoration logging to remove patches of Pinus palustris (longleaf pine) in a second‐growth loess plain Pinus palustris savanna managed using frequent lightning season fires. Five years later, we measured numbers of vascular plant species and transmitted light in replicated 100‐m² plots. Treatments involved three different overstorey conditions: no overstorey for 5 years, no overstorey for several decades, and overstorey pines present for decades. Results: Both recent and long‐term openings contained, on average, about 100 vascular plant species per 100 m², 20% more than in similar‐sized areas beneath overstorey trees. Responses varied with life form; more herbaceous species occurred in recent and older overstorey openings than beneath overstorey trees. Total numbers of all species and of less abundant forb species were positively and linearly related to light transmitted to ground level. Those species responding to openings in the overstorey and positively associated with increased transmitted light levels were monocarpic and shortlived perennial forb and grass species with a seed bank in the soil. In addition, community structure, as reflected in species composition and abundances, appeared to vary with canopy condition. Conclusions: Restoration involving ecologically sensitive removal of patches of overstorey pines in frequently burned pine savannas should benefit the ground cover and increase plant species biodiversity as a result of increased abundance of seed bank species.     

Factors affecting species richness of tree regeneration in mixed‐wood stands of central Maine

Question: Two questions about within‐stand spatial variability are addressed in this paper. How does species richness of tree regeneration respond to small‐scale ecological gradients, and what effect does natural Abies balsamea abundance have on the species richness of other tree regeneration? Location: A long‐term, gap‐silviculture experiment, Acadian mixed‐wood forest, Maine, USA. Methods: Eight stands treated with and without gap harvesting were sampled to capture sub‐stand heterogeneity of understorey tree regeneration concurrently with patterning of local stand conditions. Spatial and non‐spatial models were developed to test the relationships between two response variables [species richness of small (height ≥0.1 m, but <0.75 m) and large (height ≥0.75 m, but <1.4 m) regeneration] and five explanatory variables (depth to water table, percentage canopy transmittance, A. balsamea regeneration density, and overstorey basal area and species richness). Results: Despite high unexplained variance for all models, consistent associations among variables were found. Negative associations were found between: (1) the species richness of small regeneration and A. balsamea regeneration density and (2) the species richness of large regeneration and overstorey basal area. Positive associations were found between: (1) the species richness of small regeneration and both overstorey basal area and species richness and (2) the species richness of small and large regeneration and canopy transmittance. Conclusions: Promoting tree species diversity in Acadian mixed‐wood stands may not be achievable through the use of gap‐harvesting alone if the density of understorey Abies balsamea is not reduced either naturally or through silvicultural intervention.     

Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands

Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands? Location: Minnesota, USA. Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long‐term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover. Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4‐grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3‐grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N‐fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover. Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire‐resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light‐demanding grasses and forbs.     

The balance between the foliage projective covers of overstorey and understorey strata in Australian vegetation

Foliage Projective Cover of the overstorey (canopy) of a‘climax’community appears to reach an equilibrium value determined largely by the prevailing climate. Overstorey FPC decreases in‘climax’communities in a graded series from humid to arid regions. Understorey cover (of all strata below the canopy) in‘climax’communities attains a balance with overstorey FPC. Disturbance (gaps, microhabitats, fire, overgrazing, invasion of woody weeds, etc.) may reduce the overstorey cover which will be compensated by an increase in understorey cover. Secondary succession back to the‘climax’structure will follow a path maintaining an inverse linear relationship between understorey cover and overstorey cover. At the same time, species diversity appears to decrease as overstorey cover increases.     

A dendro‐ecological study of forest overstorey productivity following the invasion of the non‐indigenous shrub Lonicera maackii

Question: Will a non‐indigenous, invasive, understorey shrub, such as Lonicera maackii (Amur honeysuckle) have an impact on the productivity of overstorey trees in hardwood forests? Location: Trees from 12 invaded and four non‐invaded sites were sampled in hardwood forests of southwestern Ohio, US. Methods: Changes in radial and basal area tree growth in the ten years prior to L. maackii invasion vs. ten years after invasion were examined using dendrochronological techniques. Intervention analysis was used to detect growth changes 25 years prior to and 25 years following invasion, and estimates of load impacts for L. maackii population and biomass were also calculated. Results: We found that the rate of radial and basal area growth of overstorey trees was reduced significantly in eleven out of twelve invaded sites. Non‐invaded sites did not exhibit this consistent pattern of reduced growth. For invaded vs. non‐invaded sites, the mean basal area growth was reduced by 15.8%, and the overall rate of basal area growth was reduced by 53.1%. Intervention analysis revealed that the first significant growth reductions were 6.25 ± 1.24 (mean ± SE) years after invasion with the greatest frequency of negative growth changes occurring 20 years after invasion. In invaded stands, 41% of trees experienced negative growth changes. In terms of invasive load estimates per 1000 L. maackii individuals, radial tree growth was reduced by 0.56 mm.a^?1, and basal area growth was reduced by 0.74 cm².a^?1, Given these findings, significant economic losses could occur in hardwood forests of Ohio. Conclusions: To our knowledge, this is the first study using dendrochronological techniques to investigate the impact of a non‐indigenous, understorey plant on overstorey tree growth. Active management will likely be needed to maintain forest productivity in L. maackii impacted landscapes.     

Drivers of native species regeneration in the process of restoring natural forests from mono‐specific,even‐aged tree plantations: a quantitative review

A substantial proportion of the existing tree plantations has been established following clearing of native forests. This form of conversion has become widely unaccepted, and there are increasing demands to reverse it through ecological restoration. Yet, there is a lack of integrated knowledge on how best to restore. Here, we reviewed 68 studies to identify the main factors determining establishment success of regeneration of native woody species when restoring natural forests from plantation forests using active and passive approaches, beneath existing canopies, and following their removal. According to the evidence collected, herbivory, within‐gap position, soil properties, and ground cover type and structure had limited influence on regeneration, showing significant effects in less than 26% of cases in which their influence was tested. In contrast, spatial landscape configuration, overstorey structure, ground vegetation structure, overstorey composition, and climate and geomorphology had significant effects in 67, 47, 47, 52, and 63% of cases, respectively. Regeneration diversity and abundance increased with proximity to natural vegetation remnants and seed sources. Lower canopy and understorey stocking levels positively influenced regeneration, as did interventions to reduce them. Canopy cover reduction proved especially effective in warmer regions, in stands of broadleaved species, younger ages (<30 years), higher densities (>1,000 trees/ha), and taller canopies (>20 m). Restoration of native forests can be optimized by adopting interventions that prove most effective, and prioritizing more responsive stand types. However, the specific stand attributes and environmental factors described should be further studied to understand the mechanisms underlying their influence on regeneration.     

Disappearance of overstorey and understorey litter in an open eucalypt forest

Litter accumulation dynamics and first year rates of disappearance were investigated for leaves of overstorey and understorey species in the mixed eucalypt forest in the Griffith University study area, Queensland, Australia. The average biomass of the litter layer, was 10.2 t/ha. The wood and overstorey leaf litter formed 62.7% of the accumulated litter biomass, and were spread continuously across the ground. The distribution of shrub litter was discontinuous, being concentrated in localized patches beneath individual plants. The litter bag and tethered leaf techniques were used to measure the rate of disappearance of overstorey and shrub leaves. The ‘pairedquadrat’ technique was used to measure the weight loss of the grasses. The small leaves of the dominant shrub, Pultenaea villosa Willd. disappeared most rapidly, followed by the overstorey leaves, grasses and Acacia leaves. Fragmentation by physical factors and litter fauna appeared to be the major factors responsible for the disappearance of the overstorey leaves during the first year of exposure. The data suggest that removal of leaf constituents by leaching and microbial decomposition were more important for the shrub litter than the overstorey leaves. Fractional disappearance rates (loss constants) obtained from the short term weight loss measurements, and calculated using the steady-state model of litter accumulation (k=L/X), overestimated the rate of litter disappearance and litter decomposition. For the overstorey leaves in particular, the loss constants also overestimated the rate of loss of material from the litter layer since the fragmented and consumed tissues accumulated in the fraction of comminuted fragments before moving into the humus/soil subsystem. A compartment model of the components of the litter layer in the mixed eucalypt forest is presented. It incorporates overstorey and understorey litter accession, accumulation and disappearance data. The adoption of a two dimensional decomposition/accumulation matrix is suggested as an appropriate framework within which to simulate the dynamics of the litter subsystem in mixed eucalypt forest ecosystems.     

Altered N,P and C dynamics with absence of fire in Eucalyptus forests affected by premature decline

Absence of fire is increasingly recognized as an important driver of soil nutrient budgets in Eucalyptus forest, especially in forests affected by premature Eucalyptus decline, due to the effects of soil nutrient accumulation on nutrient balances and forest community dynamics. In this study, we present a dataset of soil and foliar nutrient analyses, and vegetation measurements from a fire chronosequence survey in native E. delegatensis forest. Measured indices include total soil and extractable soil nitrogen (N), or phosphorus (P), soil organic carbon (C), soil acid‐phosphatase (PME) activity, foliar N and foliar P, and understorey and overstorey vegetation canopy height. We show that in some cases indices are strongly linked to time since fire (2–46 years). Time since fire correlated positively with foliar N, total and extractable soil N, soil organic C, and also soil PME activity; the latter an indicator of biotic P demand. Differences in the strength of these relationships were apparent between two geology types, with stronger relationships on the potentially less‐fertile geology. The strong positive correlation with time since fire and understorey canopy height reflected increasing shrub biomass and thickening of the shrub layer. The strong positive correlation for soil or foliar N, but not P, with time since fire, indicates that P does not increase relative to N over time. P may, therefore, become limiting to growth in this plant community. Similarly, the significantly higher concentrations of soil N but not P, also found in both older and long‐unburnt forest stands (>100 years since management), may exacerbate a situation of soil nutrient limitation over several decades. A characteristic feature of long unmanaged stands is a developing tea tree (Leptospermum sp.) understorey, which may benefit from elevated soil N availability and increasing organic C accumulation with prolonged fire absence. This increased shrub biomass would outcompete Eucalyptus for resources, including soil nutrients and water.     

Variations in resistance to canopy disturbances and their interactions with the spatial structure of major species in a cool‐temperate forest

Question: How does typhoon‐related disturbance (more specifically, disturbance in the understorey due to tree‐fall and branch‐fall) affect different species mortality rates in a vertically well‐structured forest community? Location: Cool‐temperate, old‐growth forest in the Daisen Forest Reserve, Japan. Methods: We investigated the canopy dynamics and mortality rate trends of trees ≥5 cm diameter at breast height in a 4‐ha study plot, and analysed the effects of tree diameter and spatial structure on the mortality risks for major tree species in the understorey. Results: Significant differences were found in the mortality rates and proportions of injured dead stems between census periods, which were more pronounced in the understorey than in the canopy. Acer micranthum, which showed increased mortality during typhoon disturbance periods, had a clumped distribution. In contrast, Acer japonicum and Viburnum furcatum, which showed similar mortality rates between census periods, had a loosely clumped spatial distribution and a negative association with canopy trees, respectively. In the understorey stems of Acanthopanax sciadophylloides and Fagus crenata, whose spatial distribution patterns depended on canopy gaps, significant increases in mortality rates were observed only during severe typhoon‐related disturbance periods. Conclusions: The sensitivity of trees to typhoon‐related canopy disturbance is more pronounced in the lower layers of vertically structured forest communities. Differences in mortality patterns generated through the combined effects of spatial variation in disturbance regime and species‐specific spatial distribution patterns (spatial aggregation, association with canopy trees, and canopy gap dependency) contribute to the co‐existence of understorey species in forest communities that are subject to typhoon‐related disturbance.     

The influence of savanna trees on nutrient,water and light availability and the understorey vegetation

In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.     

Long‐term dynamics in a planted conifer forest with spontaneous ingrowth of broad‐leaved trees

Question: What are the age structure and growth trends in a 160‐year old not‐managed Pinus sylvestris plantation with spontaneous development of Quercus robur and can recruitment of Q. robur be related to the radial growth pattern of the P. sylvestris overstorey? Location: Mattemburgh forest reserve, The Netherlands. Methods: Throughout the forest, we sampled 103 oaks and 102 pines with an increment corer. Tree ring widths were measured and cross‐dated to produce mean ring width series. With these data we determined tree ages, investigated growth trends and identified growth releases and suppressions. Results: Q. robur is uneven‐aged: some individuals recruited around 1925, but most reached coring height in the 1940s. The latter recruitment period related to a transition from stressed to released growth of the overstorey pines, growth releases of the oldest Q. robur and occurrence of P. sylvestris regeneration. No further recruitment has taken place since 1950. Conclusions: This study demonstrates that an old pine plantation can develop spontaneously into well‐structured pine forest with an understorey of oak and pine. However, understorey recruitment in these forest types is not a continuous process and in this case a single allogenic canopy disturbance triggered its establishment.     

Primary forest dynamics in lowland dipterocarp forest at Danum Valley, Sabah, Malaysia, and the role of the understorey

Changes in species composition in two 4-ha plots of lowland dipterocarp rainforest at Danum, Sabah, were measured over ten years (1986-1996) for trees > or = 10 cm girth at breast height (gbh). Each included a lower-slope to ridge gradient. The period lay between two drought events of moderate intensity but the forest showed no large lasting responses, suggesting that its species were well adapted to this regime. Mortality and recruitment rates were not unusual in global or regional comparisons. The forest continued to aggrade from its relatively (for Sabah) low basal area in 1986 and, together with the very open upper canopy structure and an abundance of lianas, this suggests a forest in a late stage of recovery from a major disturbance, yet one continually affected by smaller recent setbacks. Mortality and recruitment rates were not related to population size in 1986, but across subplots recruitment was positively correlated with the density and basal area of small trees (10-< 50cm gbh) forming the dense understorey. Neither rate was related to topography. While species with larger mean gbh had greater relative growth rates (rgr) than smaller ones, subplot mean recruitment rates were correlated with rgr among small trees. Separating understorey species (typically the Euphorbiaceae) from the overstorey (Dipterocarpaceae) showed marked differences in change in mortality with increasing gbh: in the former it increased, in the latter it decreased. Forest processes are centred on this understorey quasi-stratum. The two replicate plots showed a high correspondence in the mortality, recruitment, population changes and growth rates of small trees for the 49 most abundant species in common to both. Overstorey species had higher rgrs than understorey ones, but both showed considerable ranges in mortality and recruitment rates. The supposed trade-off in traits, viz slower rgr, shade tolerance and lower population turnover in the understorey group versus faster potential growth rate, high light responsiveness and high turnover in the overstorey group, was only partly met, as some understorey species were also very dynamic. The forest at Danum, under such a disturbance-recovery regime, can be viewed as having a dynamic equilibrium in functional and structural terms. A second trade-off in shade-tolerance versus drought-tolerance is suggested for among the understorey species. A two-storey (or vertical component) model is proposed where the understorcy-overstorey species' ratio of small stems (currently 2:1) is maintained by a major feedback process. The understorey appears to be an important part of this forest, giving resilience against drought and protecting the overstorey saplings in the long term. This view could be valuable for understanding forest responses to climate change where drought frequency in Borneo is predicted to intensify in the coming decades.