Tree Demography Research Articles

Changes in ash tree demography associated with emerald ash borer invasion, indicated by regional forest inventory data from the Great Lakes States

The emerald ash borer (EAB) ( Agrilus planipennis Fairmaire) is a nonnative phloem-feeding beetle that was accidentally introduced near Detroit, Michigan, two to three decades ago. North American ash ( Fraxinus spp.) exhibit little or no resistance, and as this insect species expands its range, extensive mortality results. Previous studies of the impacts of EAB, typical of most insect and disease impact studies, utilized data acquired from sites with known infestations and cannot be used to make regional estimates of change on forest land. By contrast, this study investigated the regional impacts of EAB on the affected resource using information from a large-scale forest inventory (Forest Inventory and Analysis program of the US Department of Agriculture, Forest Service) previously implemented to estimate regional forest resources. Results indicate that since 1980, ash has been increasing throughout many of the Great Lakes States but EAB is reversing this trend in recently invaded areas. Within 50 km of the epicenter of the EAB invasion, a major decline was observed after 2004. For growing stock (trees at least 12.7 cm diameter at breast height), average ash volume decreased from 12.7 to 3.2 m3·ha–1and mortality increased from 0.1 to 1.4 m3·ha–1·year–1on timberland between the 2004 and 2009 inventories.

Did historical tree removal promote woody plant encroachment in Australian woodlands?

AbstractQuestionWoody plants have increased in density in many ecosystems, but the factors promoting encroachment are often debated. Since European colonization, Callitris glaucophylla has recruited abundantly in many Eucalyptus–Callitris woodlands in eastern Australia following changes to disturbance regimes analogous to changes in many other ecosystems globally. We used a dynamic stand model to disentangle effects of disturbances on Callitris encroachment and asked, to what extent was Callitris encroachment enhanced by historical removal of pre‐settlement trees and subsequent thinning of regrowth?LocationWoodlands dominated by Eucalyptus sp. and C. glaucophylla, which originally occupied ca. 100 000 km2 in eastern Australia.MethodsWe used a process‐driven stand simulation model to simulate tree demography, with growth, survival and recruitment mediated by annual rainfall, competition and disturbance. Following parameter calibration and model testing, we orthogonally manipulated historical removal of pre‐settlement trees and thinning of regrowth to identify how both processes may have influenced structural changes over 120 yr of European settlement.ResultsRemoval of pre‐settlement trees had little effect on modelled encroachment, as trees increased to comparable densities whether pre‐settlement trees were retained or removed. In unthinned scenarios, Callitris regeneration formed ‘locked’ stands of high density but low basal area. Thinning promoted growth of retained Callitris, increased total stand basal area, and together with direct removal of large pre‐settlement Eucalyptus and thinning of Eucalyptus regrowth, transformed stands from Eucalyptus to Callitris dominance.ConclusionsRemoval of pre‐settlement trees does not appear to have been a necessary precursor to modelled encroachment in Eucalyptus–Callitris woodlands in eastern Australia, perhaps because initial tree cover was low and Callitris can regenerate beneath isolated trees. Manual thinning was required to effect major structural change because Callitris self‐thins extremely slowly; thinning will be less important in other ecosystems that self‐thin more rapidly. The impact of historical tree removal on encroachment is likely to vary according to initial tree cover, with greatest impacts in dense ecosystems with high tree cover. These results highlight the value of simulation models for disentangling the effects of multiple disturbances on tree encroachment and other ecosystem dynamics.

The more stems the merrier: advantages of multi‐stemmed architecture for the demography of understorey trees in a temperate broadleaf woodland

Summary1. Multi‐stemmed trees are an understudied but common component of temperate broadleaf forests that can provide insight into how plants persist and regenerate within communities. In particular, multi‐stemmed architecture may be an important trait for the growth and survival of trees in forest understoreys that have to cope with low light levels, but this idea has been rarely tested using long‐term individual‐level data.2. We use measurements of 8527 individual woody stems from 1985, 1996 and 2008 to model the growth, survival and recruitment of hazel (Corylus avellana) and hawthorn (Crataegus laevigata and C. monogyna) trees as a function of neighbourhood competition in the understorey of a minimum‐intervention, mixed‐ash woodland in England. We test the effects of browsing by deer (Muntiacus reevesi) on woodland dynamics by comparing demographic rates during a period of high deer densities (1985–1996) with rates recorded from 1996 to 2008, when sustained culling substantially reduced deer densities.3. Growth and survival of hazel and hawthorn trees increased with the number of stems they possessed, demonstrating clear benefits to multi‐stemmed architecture. Surviving trees continued to accumulate basal area and stems without any clear upper limit after 23 years. However, increasing numbers of stems generally reduced the growth and recruitment of stems within multi‐stemmed trees.4. Temporal differences in deer browsing appeared to influence the strength of intra‐specific (i.e. inter‐stem) competition and, consequently, the growth, survival and recruitment of multi‐stemmed trees. Most notably, stem survival declined with the number of stems in a tree only after deer culling, but not during a period of high deer densities, suggesting that intense deer browsing reduced resource competition among stems. Inter‐specific neighbourhood competition had no detectable effect on hazel or hawthorn demography.5. Synthesis. Multi‐stemmed architecture is an advantageous trait for understorey trees in temperate woodlands relative to the allocation of resources towards the growth of a single stem. We suggest that the low light levels of forest understoreys favour ‘persistence’, through multi‐stemmed growth, rather than ‘regeneration’ niches (i.e. periodic recruitment through seed), with the advantage of this life‐history strategy influenced by herbivory and intra‐specific competition.

Links between biomass and tree demography in a northern hardwood forest: a decade of stability and change in Hubbard Brook Valley, New Hampshire

We resurveyed a network of sampling plots (n = 371) 10 years after its establishment in Hubbard Brook Experimental Forest (New Hampshire, USA) to quantify recent trends in tree biomass and demography. We found no significant change in live-tree biomass during the decade. Total biomass was 246 Mg·ha–1 (95%CI = 235–258) in 1995–1996 and 245 Mg·ha–1 (95%CI = 234–256) in 2005–2006. Annual mortality during the period for trees ≥ 10 cm diameter at breast height (1.37 m) averaged 9.7 trees·ha–1·year–1 (95% CI of annual mortality rate = 1.36%–1.84%·year–1). Tree recruitment into the census pool was 8.4 trees⋅ha–1·year–1 (95% CI = 5.8–10.6). Although overall forest biomass remained constant, there were marked shifts in the relative dominance of the canopy species. For example, the live biomass of Betula alleghaniensis Britton declined by 7%, whereas the live biomass of Picea rubens Sarg. increased by 6% and that of Acer saccharum Marshall increased by 4%. There was no instance of recruitment significantly exceeding mortality for the major species. Relative growth rates ranged from 1.03%·year–1 for Betula papyrifera Marshall to 1.99%·year–1 for Abies balsamea (L.) Mill. Our results confirmed earlier reports that the forest at Hubbard Brook is no longer aggrading. Current live-tree biomass is lower than expected. Although effects of novel disturbances documented on a regional level have not led to directional changes in tree demography at Hubbard Brook, we suggest that these novel stressors are depressing the biomass potential of the forest.

Tree demography in an undisturbed Dipterocarp permanent sample plot at Uppangala, Western Ghats of India

We provide a data set on demography of trees monitored over 20 years in Uppangala permanent sample plot (UPSP) in undisturbed, old‐growth wet evergreen Dipterocarp forest located within the Pushpagiri Wildlife Sanctuary in India's Western Ghats biodiversity hotspot. During 1989–1990, all trees ≥30 cm girth at breast height (gbh) were sampled in five north–south transects 20 m wide and 180 to 370 m long covering a total area of 3.12 ha. In 1992–1993, additional rectangular plots were established, bringing the total area sampled to 5.07 ha. In all, 3870 trees were identified, tagged, mapped, and provided with permanent dendrometer bands. Since then, the sampled area has been regularly censused at 3–5 year intervals, recording tree recruitment, mortality, and growth. We present data from censuses conducted in 1990–1993, 1994, 1997–1998, 2001–2002, 2007, and 2010. These data have been used to study the natural forest dynamics and to calibrate spatially explicit simulation models.

Longleaf pine ( Pinus palustris) and hardwood dynamics in a fire-maintained ecosystem: A simulation approach

Longleaf pine ( Pinus palustris) savannas of the southeastern U.S. represent an archetype of a fire dependent ecosystem. They are known to have very short fire return intervals (∼1–3 years) that perpetuate understory plant diversity (up to 50 species m −2), support pine recruitment, and suppress fire sensitive hardwoods. Understanding the relationships that regulate longleaf and southern hardwoods is especially critical. With decreased fire frequency, insufficient intensity, or lack of underground competition, a woody mid-story rapidly develops, dominated by fire sensitive trees and shrubs that in-turn suppress more fire dependent species (including pine seedlings). This may occur in forest gaps, where pine-needle abundance is diminished, reducing fire spread potential. The interactions between longleaf pine, hardwoods, forest fuels, and fire frequency are complex and difficult to understand spatially. The objective of this study was to develop a spatially explicit longleaf pine–hardwood stochastic simulation model (LLM), incorporating tree demography, plant competition, and fuel and fire characteristics. Data from two longleaf pine study sites were used to develop and evaluate the model with the goal to incorporate simple site-specific calibration parameters for model versatility. Specific model components included pine seed masting, hardwood clonal sprouting, response to fire (re-sprouting, mortality), and tree density driven competition effects. LLM spatial outputs were consistent with observed forest gap dynamics associated with pine seedling establishment and hardwood encroachment. Changes in fire frequency (i.e., fire probability = 0.35–0.05) illustrated a shift in community structure from longleaf pine dominated to a hardwood dominated community. This approach to assessing model response may be useful in characterizing longleaf ecosystem resilience, especially at intermediate fire frequencies (e.g., 0.15) where the community may be sensitive to small changes in the fire regime. Height distributions and population densities were similar to in situ findings (field and LIDAR data) for both study sites. Height distributions output by the LLM illustrated fluctuations in population structure. The LLM was especially useful in determining knowledge gaps associated with fuel and fire heterogeneity, plant–plant interactions, population structure and its temporal fluctuations, and hardwood demography. This is the first known modeling work to simulate interactions between longleaf pine and hardwoods and provides a foundation for further studies on fire and forest management, especially in relation to ecological forestry practices, restoration, and site-specific applications.

History matters: tree establishment variability and species turnover in an African savanna

Demographic variability in the tree component of savannas, arising from both spatial and temporal variability in fire, herbivory, and climate, is thought to be a key driver maintaining tree-grass co-existence in savannas. Modeling work has shown that this variability can be vital to the persistence of savanna as a biome and that variability in climate, herbivory and fire affect tree establishment. However, empirical evaluations of the idea that savanna tree establishment is variable have so far been limited to tests of the effects of major events (such as the East African rinderpest epidemic) on tree establishment. The degree to which tree establishment varies in savannas under less extreme variability in climate, herbivory and fire remains unknown. We tested whether establishment of adult trees was constant or episodic using tree ring counting of three Acacia species in Hluhluwe iMfolozi Park in South Africa. Patterns of establishment of adult trees were analyzed at three spatial scales for temporal and spatial patchiness. Establishment of adult trees was not constant for any of the three species of Acacia, and all three species showed significant spatial patchiness and larger-scale temporal variability in establishment patterns. Spatial patchiness highlights the importance of landscape processes like fire and herbivory, while larger-scale temporal variability indicates that climatic variability also structures tree demography. These results suggest that current tree cover in savannas is a product of ecological history and is not necessarily determined by current ecological processes. An historical perspective is essential for interpreting patterns of tree cover in savannas, in models of the effects of climate, fire and herbivory (especially elephants), in experimental work, and in analyses of continental patterns of tree cover alike.

Population dynamics and management of Amazon tidal floodplain forests: Links to the past, present and future

Although tidal floodplain forests represent the oldest commercial logging frontier in the Amazon, tree demography analyses are lacking. Consequently, the accurate evaluation of impacts of past use and the development of ecologically sound forest management has lagged. To address that gap, we combine matrix model methods with data from interviews with tidal floodplain smallholders regarding land use and ecological knowledge. Here we examine the population dynamics of 8 common tidal floodplain species and explore the link between species population ecology and past, present, and future timber use.Despite the generally high timber stocks due to the dominance of Mora paraensis, these tidal floodplain forests offer challenges to management similar to those seen in tropical forests elsewhere, including low recruitment and species-specific tree growth rates, population densities and size distributions. Furthermore, species population ecology and survey results indicate that the long legacy of forest use in the tidal floodplain forests will influence the future of forest use and management, negatively impacting some species (e.g., Virola surinamensis) while likely benefiting others (e.g., M. paraensis). The observed diameter distributions of species least harvested in the past (e.g., M. paraensis, Licania heteromorpha) follow those derived from stable stage distributions. For other historically harvested species (e.g., Carapa guianensis, V. surinamensis), a larger number of harvest-sized trees for most species would be expected under unlogged conditions, a result corroborated by survey results indicating that maximum log size of long-used timber species has decreased over the past decades.While timber extraction in the Amazon Estuary has endured for centuries, our results suggest long-term timber use does not necessarily entail sustainability; a long history of timber harvesting can also indicate a gradual process of resource depletion as preferred species are sequentially exhausted. If current practices are left unchanged, the prospects for long-term management are likely to decrease further as the densities of preferred high-value species (C. guianensis, V. surinamensis, P. filipes) fall to levels that make management economically unattractive.

Differential effects of climate variability on forest dynamics along a precipitation gradient in northern Patagonia

Summary 1. The understanding of the impacts of climate anomalies on demographic processes involved in forest dynamics is becoming a major global change research objective, as climate‐driven demographic changes have the potential to alter ecological processes and change the composition of existing communities. 2. Here, we test whether the effects of climatic variability on Nothofagus dombeyi forest dynamics differ along the W–E precipitation gradient of northern Patagonia, Argentina, due to different controlling factors along the gradient. 3. Using a dendroecological approach, we sampled six N. dombeyi stands along the gradient. Tree death, growth release and sapling birth dates were obtained and used to reconstruct stand mortality, canopy disturbance and establishment patterns in each stand. Discrete extreme climatic events were obtained by applying thresholds to meteorological time series. Bivariate event analysis was performed to examine the temporal relationships between climate events and forest demographic and canopy disturbance patterns. 4. In the xeric region, forest dynamics is driven by drought/heat and humid events. Following drought/heat events we observed standing‐dead tree mortality, releases in growth, and tree establishment episodes. Regional synchrony of these events was coincident with droughts registered for northern Patagonia. However, successful establishment was related to wet periods that occurred after drought events, showing the dependence on favourable periods for growth. For wet regions, demographic patterns showed a temporally uniform pattern but with synchronies at the regional and local scales. Canopy openings produced by fallen trees, and consequent release in growth and establishment, were related to both drought and snowy/windy years. The effect of drought in wet region forests was related mainly to the extreme 1998–99 drought in northern Patagonia. 5. Synthesis. Climate in northern Patagonia influences N. dombeyi forest dynamics differentially along a precipitation gradient. In xeric forests, strong but relatively short climatic fluctuations impact forest structure through direct effects on tree demography. In wet forests, climatic‐induced mechanical disturbance prevails, driving mortality and subsequent growth and establishment. Considering that recent extreme droughts have the strongest negative impact on N. dombeyi populations at the eastern distributional limits, a change in distribution of this species as a consequence of projected climate change is expected.

Effects of abiotic and biotic factors and stochasticity on tree regeneration in a temperate forest community

How deterministic or stochastic is tree regeneration in a temperate forest? To answer this question, we analyzed the relative effects of abiotic and biotic factors on seed and seedling demography of tree species in an old-growth temperate forest, Ogawa Forest Reserve, Japan. The working hypothesis was that seed and seedling demography of trees would be controlled more deterministically in comparison with tropical forests and that the magnitude of deterministic effects of the abiotic and biotic factors would depend on regeneration traits relating to seed dispersal ability. We examined 5 abiotic factors (litter and humus cover, vegetation cover, light level, soil moisture, and relative elevation) and 2 species-specific biotic factors (distance from the nearest conspecific adult and density of conspecific seeds or seedlings) to quantify their contribution to the spatial variation in seedling emergence and survival for 18 principal tree species in the community. The results showed that conspecific seed density had a negative effect on seedling emergence for almost all species, as suggested by the Janzen- Connell model. On the other hand, various factors were detected for seedling survival of each species. We also found that regeneration traits relating to seed dispersal ability were significantly correlated with some of the abiotic factors. However, the 7 factors tested were less important than expected; seed and seedling demography appeared to be determined rather stochastically in this stable forest community. Even in a temperate forest, seed and seedling demography appeared to be rather stochastic, and niche partitioning among species during these stages was of limited importance.

Integral Projection Models for trees: a new parameterization method and a validation of model output

Summary 1. Matrix models are popular tools for plant demographic studies, but their application to long‐lived, slow‐growing species is hampered by the fact that (i) model output is highly sensitive to category width and (ii) growth variation between individuals can only be partially accounted for. Integral Projection Models (IPMs) – an extension of matrix models – offer a solution to these problems. 2. Here, we introduce a new method to parameterize IPMs for trees – the ‘integration method’– which allows constructing IPMs for long‐lived, slow‐growing species. This approach is more suitable than the ‘midpoint rule’, which is customarily used. 3. We built IPMs for six tree species from Vietnamese (sub)tropical forests. For four of these species, population growth rate (λ) was highly sensitive to the number of categories in the transition matrix. Population growth stabilized for IPMs with 100–1000 categories, corresponding to categories of 0.1–1 cm in trunk diameter. This preferred width is much narrower than the 10‐cm‐wide categories customarily used in tree models. 4. The distribution of elasticity values over transition types (stasis, progression to next and further categories) is also highly sensitive to matrix dimension in IPMs. In addition, elasticity distribution is influenced by including or excluding growth variation. 5. Age estimates obtained from IPMs were also highly sensitive to matrix dimension: an IPM with 1000 size categories yielded 2–4 times higher age estimates for large trees than one with 10 size categories. Observed ages obtained from tree ring analyses for four of the study species allowed validating these estimates. IPMs with 10 categories strongly underestimated age, while those with 1000 categories yielded slight age overestimates. Underestimating age in small matrices is caused by the occurrence of unrealistically fast pathways through the life cycle and is probably widespread among tree models with broad categories. Overestimating ages in IPMs with narrow categories may be due to temporally autocorrelated growth or errors in fitting growth curves. 6. Synthesis. IPMs are highly suitable tools to analyse tree demography. We recommend that tree IPMs (and classical matrix models) apply narrow diameter categories (0.1–1 cm width) to obtain reliable model output.

Altered resource availability and the population dynamics of tree species in Amazonian secondary forests

Despite research demonstrating that water and nutrient availability exert strong effects on multiple ecosystem processes in tropical forests, little is known about the effect of these factors on the demography and population dynamics of tropical trees. Over the course of 5 years, we monitored two common Amazonian secondary forest species-Lacistema pubescens and Myrcia sylvatica-in dry-season irrigation, litter-removal and control plots. We then evaluated the effects of altered water and nutrient availability on population demography and dynamics using matrix models and life table response experiments. Our results show that despite prolonged experimental manipulation of water and nutrient availability, there were nearly no consistent and unidirectional treatment effects on the demography of either species. The patterns and significance of observed treatment effects were largely dependent on cross-year variability not related to rainfall patterns, and disappeared once we pooled data across years. Furthermore, most of these transient treatment effects had little effect on population growth rates. Our results suggest that despite major experimental manipulations of water and nutrient availability-factors considered critical to the ecology of tropical pioneer tree species-autogenic light limitation appears to be the primary regulator of tree demography at early/mid successional stages. Indeed, the effects of light availability may completely override those of other factors thought to influence the successional development of Amazonian secondary forests.

Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration

AbstractTropical savannas are typically highly productive yet fire‐prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long‐term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr−1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 t C ha−1 yr−1, in the case of mild fires, and 0.51 t C ha−1 yr−1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1–2 t C ha−1 yr−1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long‐term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought.

Comment on “Has fire suppression increased the amount of carbon stored in western U.S. forests?” by A. W. Fellows and M. L. Goulden

Comment on “Has fire suppression increased the amount of carbon stored in western U.S. forests?” by A. W. Fellows and M. L. Goulden

Changing stock of biomass carbon in a boreal forest over 93 years

The growing stock more than doubled from 1.6 to 3.4 million m 3 between 1912 and 2005 in forests on an area of 387 km 2 in southern Finland. The stock expansion continued for 93 years noting interim results, which were available for 1959, 1982, 1994 and 1999. Forested area in the region hardly changed. Carbon sequestration was mainly a result of a long-term recovery from forest degradation, a legacy of land use in the 18th and 19th centuries. Tree demography responded to management change especially of mature stands: Average tree size and stocking density of stands increased. On average the expanding biomass stock sequestered 18 tons C annually per km 2 (18 g C per m 2). In comparison, the emissions of fossil carbon in the region were estimated at 12 tons C per km 2 (12 g C per m 2) on average. However, fossil CO 2 emissions exceeded biomass sequestration in recent decades. The powerful and persistent expansion of the carbon stock was an unintended co-benefit of forestry, which was motivated by the intention to improve timber yield. On the more negative side the change in management introduced clear-cuts, and a loss of diverse elements of the pre-industrial biota.

Fire history of a central Nevada pinyon–juniper woodland

Our study reconstructed fire history (1445–2006) from tree rings for a Great Basin single-needle pinyon pine ( Pinus monophylla Torr & Frém.) – Utah juniper ( Juniperus osteosperma (Torr.) Little) woodland. Information from multiple lines of evidence, including dateable fire scars (n = 83), tree demography, and charred coarse woody debris, was used to quantify fire frequency, severity, and extent. Fire cycle models were developed using survivorship analysis of time-since-fire estimates. We investigated the spatial and temporal variation in historical fire regime, addressing the plausibility of postsettlement fire exclusion as an explanation for increased woodland area and density since the late 1800s. Historical fire regime was characterized by infrequent, small, high-severity fires. Estimated fire cycle (1570–1880) was 427 years, with no evidence of postsettlement stand-replacing fires. Topographic analyses indicated that in this drought-prone landscape, more mesic conditions favor continuous fuels that lead to more frequent or extensive fire. Superposed epoch analysis showed increased fire occurrence during drought years but with no influence of antecedent climatic conditions. More frequent grassland and shrubland fires were recorded by fire scars near valley floors. Thus, anthropogenic fire exclusion in adjacent, shrub-dominated communities presents a plausible mechanism for woodland expansion in the study area. However, there is little ecological justification for reintroducing fire to areas of historic woodland, where effects of fire exclusion have been minimal.

Effects of life history strategies and tree competition on species coexistence in a sub-boreal coniferous forest of Japan

We examined the effects of different life history strategies and tree competition on species coexistence in a northern coniferous forest. We investigated the growth and demography of trees with stems ≥1 cm dbh in a 2-ha study plot in the Taisetsu Mountains of northern Japan. Three species, Abies sachalinensis, Piceajezoensis, and Piceaglehnii, were found to be dominant in the forest. A. sachalinensis was the most dominant species in the understory, while the two Picea spp. were more abundant in the larger dbh size classes. The turnover rate of A. sachalinensis was about twice that of the Picea spp. The relative growth rate of understory trees in each species did not differ between different canopy conditions (closed canopy or canopy gap). The competitive advantage between A. sachalinensis and P. glehnii switched as they grew from understory (A. sachalinensis superior competitor) to canopy trees (P. glehnii superior competitor). Meanwhile, A. sachalinensis and P. jezoensis exhibited different environmental preferences. We propose that reversal in competitive superiority between different growth stages and trade-off between longevity and turnover are more important factors to promote their coexistence than regeneration niche differentiation related to canopy gaps in this sub-boreal coniferous forest.

Spatially and Temporally Variable Fire Regime on Rincon Peak, Arizona, USA

Spatial and temporal patterns of fire history are affected by factors such as topography, vegetation, and climate. It is unclear, however, how these factors influenced fire history patterns in small isolated forests, such as that found on Rincon Peak, a “sky island” mountain range in southern Arizona, USA. We reconstructed the fire history of Rincon Peak to evaluate the influences of broad-scale (i.e., climate) versus local-scale (i.e., topographic) factors on fire occurrence and extent. We evaluated both fire scars and tree demography (natality and mortality) to investigate surface fire and crown fire events. The fire history of a 310 ha study area surrounding the top of Rincon Peak was reconstructed by tree-ring sampling in 21 plots. Between 1648 and 1763, spreading fires on Rincon Peak were controlled primarily by regional climate. Widespread surface fires occurred during drought years, and were generally synchronized with regional fire events known from an extensive network of other fire history studies. After 1763, fire extent was apparently limited by local factors (i.e., fuels) as frequent fires continued to burn, but were limited to the southern part of the study area until a widespread fire occurred in 1819. Landscape fires (i.e., fires that scarred ≥2 plots) were absent from the entire study area between 1819 and 1867 despite continued burning in adjacent mountain ranges. Multiple lines of evidence indicate that the 1867 fire was both a surface and a stand-replacing event that killed most trees within a 60 ha patch. Our findings suggest that past climatic variations had important effects on fire regimes and age structures of small, fragmented ponderosa pine (Pinus ponderosa) landscapes like Rincon Peak. Given anticipated climate changes, the rich biodiversity harbored in these steep, isolated landscapes will be critical habitat in the migration of species and should therefore be considered high conservation priority.

Grass competition suppresses savanna tree growth across multiple demographic stages

Savanna ecosystems, defined by the codominance of trees and grasses, cover one-fifth of the world's land surface and are of great socioeconomic and biological importance. Yet, the fundamental question of how trees and grasses coexist to maintain the savanna state remains poorly understood. Many models have been put forward to explain tree-grass coexistence, but nearly all have assumed that grasses do not limit tree growth and demography beyond the sapling stage. This assumption, however, has rarely been tested. Here I show that grass can strongly suppress the growth of trees. I removed grass around trees of three size classes in an Acacia drepanolobium savanna in Laikipia, Kenya. For even the largest trees, grass removal led to a doubling in growth and a doubling in the probability of transitioning to the next size class over two years. These results suggest that grass competition in productive (nutrient-rich) savannas may limit tree growth as much as herbivory and fire (the main factors thought to determine tree demography within a rainfall region) and should be incorporated into savanna models if tree-grass coexistence and savanna dynamics are to be understood.

Tree growth inference and prediction when the point of measurement changes: modelling around buttresses in tropical forests

Abstract:Estimation of tree growth is generally based on repeated diameter measurements. A buttress at the height of measurement will lead to overestimates of tree diameter. Because buttresses grow up the trunk through time, it has become common practice to increase the height of measurement, to ensure that measurements remain above the buttress. However, tapering of the trunk means that increasing measurement height will bias estimates of diameter downward by up to 10% per m of height. This bias could affect inference concerning species differences and climate effects on tree demography and on biomass accumulation. Here we introduce a hierarchical state space method that allows formal integration of data on diameter taken at different heights and can include individual variation, temporal effects or other covariates. We illustrate our approach using species from Barro Colorado Island, Panama, and La Selva, Costa Rica. Results include trends that are consistent with some of those previously reported for climate responses and changes over time, but differ in relative magnitude. By including the full data-set and accounting for bias and variation among individuals and over time, our approach allows for quantification of climate responses and the uncertainty associated with measurements and the underlying growth process.