Based on the framework of attribute diversity (a generalization of Hill numbers of order q), we develop a class of functional diversity measures sensitive not only to species abundances but also to ...trait-based species-pairwise functional distances. The new method refines and improves on the conventional species-equivalent approach in three areas: (1) the conventional method often gives similar values (close to unity) to assemblages with contrasting levels of functional diversity; (2) when a distance metric is unbounded, the conventional functional diversity depends on the presence/absence of other assemblages in the study; (3) in partitioning functional gamma diversity into alpha and beta components, the conventional gamma is sometimes less than alpha. To resolve these issues, we add to the attribute-diversity framework a novel concept: τ, the threshold of functional distinctiveness between any two species; here, τ can be chosen to be any positive value. Any two species with functional distance ≥ τ are treated as functionally equally distinct. Our functional diversity quantifies the effective number of functionally equally distinct species (or "virtual functional groups") with all pairwise distances at least s for different species pairs. We advocate the use of two complementary diversity profiles (τ profile and q profile), which depict functional diversity with varying levels of τ and q, respectively. Both the conventional species-equivalent method (i.e., τ is the maximum of species-pairwise distances) and classic taxonomic diversity (i.e., τ is the minimum of non-zero species-pairwise distances) are incorporated into our proposed τ profile for an assemblage. For any type of species-pairwise distance matrices, our attribute-diversity approach allows proper diversity partitioning, with the desired property gamma ≥ alpha and thus avoids all the restrictions that apply to the conventional diversity decomposition. Our functional alpha and gamma are interpreted as the effective numbers of functionally equally distinct species, respectively, in an assemblage and in the pooled assemblage, while beta is the effective number of equally large assemblages with no shared species and all species in the assemblages being equally distinct. The resulting beta diversity can be transformed to obtain abundance-sensitive Sørensen-and Jaccard-type functional (dis)similarity profiles. Hypothetical and real examples are used to illustrate the framework. Online software and R codes are available to facilitate computations.
The classical environmental control model assumes that species distribution is determined by the spatial variation of underlying habitat conditions. This niche-based model has recently been ...challenged by the neutral theory of biodiversity which assumes that ecological drift is a key process regulating species coexistence. Understanding the mechanisms that maintain biodiversity in communities critically depends on our ability to decompose the variation of diversity into the contributions of different processes affecting it. Here we investigated the effects of pure habitat, pure spatial, and spatially structured habitat processes on the distributions of species richness and species composition in a recently established 24-ha stem-mapping plot in the subtropical evergreen broad-leaved forest of Gutianshan National Nature Reserve in East China. We used the new spatial analysis method of principal coordinates of neighbor matrices (PCNM) to disentangle the contributions of these processes. The results showed that (1) habitat and space jointly explained ~53% of the variation in richness and ~65% of the variation in species composition, depending on the scale (sampling unit size); (2) tree diversity (richness and composition) in the Gutianshan forest was dominantly controlled by spatially structured habitat (24%) and habitat-independent spatial component (29%); the spatially independent habitat contributed a negligible effect (6%); (3) distributions of richness and species composition were strongly affected by altitude and terrain convexity, while the effects of slope and aspect were weak; (4) the spatial distribution of diversity in the forest was dominated by broad-scaled spatial variation; (5) environmental control on the one hand and unexplained spatial variation on the other (unmeasured environmental variables and neutral processes) corresponded to spatial structures with different scales in the Gutianshan forest plot; and (6) five habitat types were recognized; a few species were statistically significant indicators of three of these habitats, whereas two habitats had no significant indicator species. The results suggest that the diversity of the forest is equally governed by environmental control (30%) and neutral processes (29%). In the fine-scale analysis (10 × 10 m cells), neutral processes dominated (43%) over environmental control (20%).
Native species that forage in farmland may increase their local abundances thereby affecting adjacent ecosystems within their landscape. We used two decades of ecological data from a protected ...primary rainforest in Malaysia to illutrate how subsidies from neighboring oil palm plantations triggered powerful secondary 'cascading' effects on natural habitats located >1.3 km away. We found (i) oil palm fruit drove 100-fold increases in crop-raiding native wild boar (Sus scrofa), (ii) wild boar used thousands of understory plants to construct birthing nests in the pristine forest interior, and (iii) nest building caused a 62% decline in forest tree sapling density over the 24-year study period. The long-term, landscape-scale indirect effects from agriculture suggest its full ecological footprint may be larger in extent than is currently recognized. Cross-boundary subsidy cascades may be widespread in both terrestrial and marine ecosystems and present significant conservation challenges.
Experimental and observational studies on seedling dynamics posit mechanisms that can influence forest diversity, structure and function. However, high mortality and slow growth of seedlings make it ...difficult to evaluate the importance of this life‐history filter to total tree life history. Quantifying the duration and transition of the seedling phase would help us understand this ‘black box’ in tree population biology.
We used a 16‐year dataset of comprehensive seedling‐to‐sapling demography from a subtropical rainforest to construct population models that capture temporal demographic fluctuations for eight major tree species. We used data‐driven demographic models and simulations to estimate the transition ratios from newly recruited seedlings to saplings of 2‐m height and the time taken to attain 2‐m height for a newly recruited seedling conditional on its survival.
Projections among species estimated that as few as 57 to more than 40,000 seedlings (with a median of 2,087) were required to make a single 2‐m high sapling. Furthermore, it would take 22–200 years (with a median of 47) for a newly recruited seedling to become a 2‐m high sapling. We found that temporal variation in demographic rates could greatly reduce the number of seedlings per established sapling, but not passage times (PTs). We also identified the importance of consistently fast growth rates for seedlings to escape the high mortality of early stages.
Synthesis. Our findings demonstrate that high mortality in the very early seedling stage severely limits the probability that a newly recruited seedling will transition to the sapling stage. Although the PTs vary, we found this to be true across species with a range of life‐history strategies. Only seedlings with consistently fast growth rates are expected to pass through this life‐history filter. Findings from seedling studies should consider how short‐term studies of seedling demography might capture the rare exceptional individuals and exceptional conditions that might define the dynamics of this seedling bottleneck.
摘要
許多小苗的動態監測與實驗研究均假定此階段的動態變化會影響森林的多樣性、結構與功能。然而，受限於小苗的高死亡率與低生長率，我們很難評估小苗階段的篩選對於樹木整體生活史動態的重要性。具體量化樹木個體在小苗階段的停留時間與轉換比例，可以幫助我們了解這個樹木族群動態中的「黑盒子」。
我們使用台灣福山亞熱帶雨林長達16年的小苗動態監測資料，來建構該森林中8種主要樹種的族群模式，此一模式可以忠實呈現樹木小苗生長率、死亡率隨時間變化之特性。我們透過此一族群模式進行數值模擬，估算樹木從剛萌發的新增苗成長到2 m高小樹之轉換比例，以及若小苗可以順利存活，其成長到2 m高所需的時間。
依據模式所推估的結果，有的樹種平均只需要57株新增苗就能有一株能長成2 m高的小樹，但也有樹種需要超過40,000株新增苗才能有一株小樹 (中位數為2,087株小苗)。而樹木個體在這個階段的停留時間，則介於22到200年 (中位數為47年)。當模式中納入小苗存活率與生長率之年間變化時，可以大幅地提高新增苗成長為小樹的成功率，但並不會影響在此階段的停留時間。我們也發現，樹木小苗持續性的快速生長是其能否突破小苗建立初期高死亡率的關鍵。
我們的研究顯示樹木小苗建立初期的高死亡率是新增苗能否成長為小樹的主要限制。儘管具有不同生活史策略的樹種從小苗成長為小樹時間有所差異，但其小苗同樣都受到這個限制的影響，只有能持續快速生長的小苗可以突破這層篩選。未來我們在詮釋小苗研究成果時，必須考慮小苗調查中能否涵蓋這些稀有但是能快速生長的個體，以及能讓小苗快速生長的條件，這兩者是決定樹木是否能突破「小苗瓶頸」的關鍵因子。
We used a demographic process model to quantify the seedling‐to‐sapling transition in a subtropical rainforest. Our results show that consistently fast growth is critical for tree seedlings to escape the high mortality of early stages, suggesting that the rare exceptional individuals (and exceptional conditions) might define the dynamics of this life‐history bottleneck.
Seedling dynamics are critical for determining community composition and future forest structure. Two mechanisms are crucial drivers of seedling vital rates: biotic interactions and environmental ...filtering. Biotic interactions at the neighbourhood scale, such as the Janzen–Connell effect, are considered among the main mechanisms maintaining species coexistence. These effects can show considerable temporal variation because of changes in neighbourhood density (i.e. seedling recruitment following masting years) and weather conditions. However, there is limited evidence of whether interannual variation in these conditions modulates biotic interactions and its potential cascading effects on seedling vital rates.
We used seedling data collected over 17 years in a subtropical rainforest in northern Taiwan. We modelled interannual variation in first‐year survival and growth of 12,830 seedlings of 36 species and 47 cohorts using (generalized) linear mixed‐effects models with several crowding indices and abiotic conditions (edaphic and topographic factors) as predictors.
The first‐year seedling survival and growth depended on both weather conditions and neighbourhood density. Seedling survival was greater when recruitment was more abundant and in years with greater rainfall, and this was mostly mediated by the interaction with conspecific and heterospecific seedlings. Seedling growth decreased with the increasing abundance of new recruits and increased with annual rainfall. Growth was negatively affected by the basal area of conspecific and heterospecific trees and by the density of conspecific seedlings when the number of new recruits was greater. The abiotic factors had a limited effect on seedling survival and growth compared to biotic factors.
Synthesis. Our results show that variation in recruitment abundance due to masting and annual rainfall significantly affects seedling vital rates by changing biotic interactions. These findings are in accordance with the economy of scale hypothesis, which predicts higher per‐capita reproductive success in years of higher seed production. Furthermore, the results support the hypothesis that fluctuations in rainfall can alter biotic interactions in forests. Efforts to predict forest community responses to climate changes should consider the temporal variations in biotic interactions.
Using data from 12,830 seedlings collected over 17 years in a subtropical rainforest, our study supports predictions of the Economy of Scale hypothesis, providing support for the expectations that higher seed production is reflected in higher recruitment success over the first year. We also found significant influence of rainfall fluctuations on biotic interactions. Climate changes might significantly affect future forest dynamics and composition through altering biotic interactions.
Summary
The relative importance of tree mortality risk factors remains unknown, especially in diverse tropical forests where species may vary widely in their responses to particular conditions.
We ...present a new framework for quantifying the importance of mortality risk factors and apply it to compare 19 risks on 31 203 trees (1977 species) in 14 one‐year periods in six tropical forests. We defined a condition as a risk factor for a species if it was associated with at least a doubling of mortality rate in univariate analyses. For each risk, we estimated prevalence (frequency), lethality (difference in mortality between trees with and without the risk) and impact (‘excess mortality’ associated with the risk, relative to stand‐level mortality).
The most impactful risk factors were light limitation and crown/trunk loss; the most prevalent were light limitation and small size; the most lethal were leaf damage and wounds. Modes of death (standing, broken and uprooted) had limited links with previous conditions and mortality risk factors.
We provide the first ranking of importance of tree‐level mortality risk factors in tropical forests. Future research should focus on the links between these risks, their climatic drivers and the physiological processes to enable mechanistic predictions of future tree mortality.
1. In a unique phenomenon restricted to the ever wet forests of Southeast Asia, hundreds of species from dozens of plant families reproduce synchronously at irregular, multi-year intervals. The ...proximate environmental cues that synchronize these general flowering events have not been evaluated systematically because there have been no long-term, high temporal resolution, species-level records from the region. 2. We present 13 years of weekly flowering records for five Shorea species as well as daily temperature and rainfall records from the Pasoh Forest Reserve, Peninsular Malaysia. We constructed models to evaluate hypothesized relationships between flowering and cool temperature, drought, and additive and synergistic effects of cool temperature and drought for each species. Model parameters include periods of time for floral cue accumulation and flower development and temperature and/or rainfall thresholds required for floral initiation. Parameters estimated using flowering observations from 2001 to 2011 were used to forecast flowering for 2011-2014. 3. We show that drought and cool temperatures acting synergistically best explain the timing of flowering events for all Shorea species in the section Mutica and forecast the largest general flowering event accurately. Periods estimated for signal accumulation ranged from 54 to 90 days among species. Periods estimated for flowers to develop ranged between 43 and 96 days and closely followed the interspecific sequence of flowering in the Shorea species. Drought and temperature thresholds also varied among species, with Shorea maxwelliana requiring the most severe drought and Shorea leprosula the lowest temperatures. 4. Synthesis. Our results indicate that cool temperatures and low rainfall occurring on seasonal time-scales of about 2-3 months rather than brief cold snaps or brief droughts best explain general flowering in Shorea species at the Pasoh Forest Reserve. Low rainfall is equally likely in winter (December-February) and summer (July–August) and cool temperatures are most likely in winter at Pasoh, which explains why general flowering events are restricted to spring and fall, with more frequent and stronger flowering in spring. In addition, species-specific sensitivity to environmental cues suggests that future climate change will have differential impacts on the frequency of reproduction, with potential consequences for regeneration of these dominant species of lowland tropical forests.
Accurate estimates of forest biomass stocks and fluxes are needed to quantify global carbon budgets and assess the response of forests to climate change. However, most forest inventories consider ...tree mortality as the only aboveground biomass (AGB) loss without accounting for losses via damage to living trees: branchfall, trunk breakage, and wood decay. Here, we use ~151,000 annual records of tree survival and structural completeness to compare AGB loss via damage to living trees to total AGB loss (mortality + damage) in seven tropical forests widely distributed across environmental conditions. We find that 42% (3.62 Mg ha−1 year−1; 95% confidence interval CI 2.36–5.25) of total AGB loss (8.72 Mg ha−1 year−1; CI 5.57–12.86) is due to damage to living trees. Total AGB loss was highly variable among forests, but these differences were mainly caused by site variability in damage‐related AGB losses rather than by mortality‐related AGB losses. We show that conventional forest inventories overestimate stand‐level AGB stocks by 4% (1%–17% range across forests) because assume structurally complete trees, underestimate total AGB loss by 29% (6%–57% range across forests) due to overlooked damage‐related AGB losses, and overestimate AGB loss via mortality by 22% (7%–80% range across forests) because of the assumption that trees are undamaged before dying. Our results indicate that forest carbon fluxes are higher than previously thought. Damage on living trees is an underappreciated component of the forest carbon cycle that is likely to become even more important as the frequency and severity of forest disturbances increase.
Tree mortality is typically considered the only source of biomass loss in forest systems. A pervasive but commonly neglected biomass loss is the damage to living trees (i.e., branchfall, trunk breakage, wood decay). We show that 42% of total aboveground biomass loss is due to damage to living trees across seven tropical forests. Our results contrast with the typically low forest biomass losses estimated only from tree mortality and suggest that forest carbon turnover may be higher than previously thought. Since forest disturbance rates are expected to increase under climate change, biomass loss to damage is likely to become more important
An important goal in plant community ecology is to understand how species traits determine demographic performance. Several functional traits have been shown to correlate with growth and mortality ...rates in trees, but less is known about how the relationships between functional traits and demographic rates change with tree size. We examined the associations of functional traits with growth and mortality across 43 tree species in the Fushan 25‐ha subtropical rain forest plot in northern Taiwan. We estimated the 95th percentile maximum stem diameter, wood density and six leaf functional traits (leaf area, specific leaf area, thickness, succulence, and mass‐based nitrogen and phosphorus contents) obtained from leaves on juvenile and adult individuals of each species. To quantify size‐dependent changes in growth and mortality, relative growth rate (RGR) and mortality were estimated as a function of stem diameter using hierarchical Bayesian models. These rate estimates were then correlated with functional traits at a range of stem diameter classes. Relationships between functional traits and demographic rates varied with tree size. Maximum size was positively correlated with RGR across a wide range of tree sizes. Wood density was negatively correlated with RGR and mortality for small‐sized trees. Leaf traits such as leaf area and specific leaf area at juvenile and adult stages were associated more strongly with demographic rates for corresponding sizes than from other sizes. Synthesis. The observed size‐dependent changes in the trait–demography relationships are possibly due to the effects of developmental and environmental changes with increasing tree size. The underlying effects of functional traits on demographic performance vary with tree size, and this should influence dynamics in a tree community.
•Herbivory did not differ between plantation and natural forests.•Herbivory decreased with increasing species richness and increased with basal area of conspecifics and heterospecifics.•Damage by ...chewers was the most common, followed by miners.•Leaf thickness mediated the contribution of chewers and miners to total damage.•Allowing plantations to grow a diverse understory can favor ecosystem processes.
Conversion from natural forest to timber plantation is an important cause of pristine forests loss. These changes alter community structure and cause a decrease in the diversity of plant communities. Leaf functional traits, which are an important defensive strategy, might also be influenced by such alterations. Altogether, shifts from natural forest to timber plantation may lead to profound changes in plant-plant and plant-animal interactions, including herbivory and the relative abundance of different feeding guilds, an important aspect of forest ecosystem function and community dynamics. In this study, we assessed insect herbivore damage in saplings of four native tree species in the understory of natural forests and two types of ca. 80-year-old timber plantations. We measured leaf damage by the most common feeding guilds in over 9000 leaves from 200 saplings. We first compared herbivory between natural forest and plantations, and we then analyzed the drivers of herbivory and of the two most common guilds – chewers and miners – using several indices of forest structure and leaf traits as predictors. Total herbivory did not change among forest types, but it showed some variation in two of the four species. Herbivory decreased with increasing species richness and increased with increasing basal area of conspecific and heterospecific trees. Leaf damage caused by chewers increased with leaf thickness, while damage caused by miners showed the opposite relationship. On an ecological level, our findings support expectations from the resource concentration hypothesis and on the role of leaf traits in mediating the impact of insect feeding guilds on leaf herbivory. On an applied level, these results suggest that timber plantations can support similar levels of ecosystem processes than natural forests decades after their establishment. Allowing the regeneration of a diverse understory in plantation forests should be implemented to favor ecosystem dynamics close to those of the original forests.
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