Fire Intensity Affects the Relationship between Species Diversity and the N Utilization Stability of Dominant Species

Quantitative evaluation of the drivers of species richness in a Mediterranean ecosystem (Cape, South Africa)

BACKGROUND AND AIMS

Mediterranean ecosystems have a high vascular plant species richness (SR) relative to their surface area. This SR, representing the balance between speciation and extinction, has been attributed to multiple mechanisms that result in both high rates of speciation and/or low rates of extinction. An abiding question is, however, what is special about Mediterranean ecosystems that enables this high SR? Apart from the long-term climatic stability of the region, SR has also been related to resource availability, the many individuals hypothesis, resource spatial heterogeneity, temporal heterogeneity and biotic feedbacks.

METHODS

Spatial patterns of species richness were related to climatic, edaphic and biotic variables and to spatial variability within the Greater Cape Floristic Region (GCFR) of South Africa. Boosted regression tree models were used to explore the strength of relationships between SR and environmental predictors related to each hypothesized mechanism.

KEY RESULTS

Water availability (i.e. precipitation) was a stronger predictor of SR than potential evapotranspiration or temperature. Scarcity of nutrients was also related to SR. There was no indication that SR was related to the density of individuals and only temporal heterogeneity induced by fire was related to SR. Spatial heterogeneities of climatic, edaphic and biotic variables were strongly associated with SR. Biotic interactions remain difficult to assess, although we have some evidence for a putative role in regulating SR.

CONCLUSIONS

While the lack of ecosystem-resetting disturbances (e.g. glaciation) is undoubtedly a key requirement for high species accumulation, predictably, no one explanation holds the key to understanding SR. In the GCFR high SR is the product of a combination of adequate water, nutrient scarcity, spatial and temporal heterogeneity, and possibly biotic feedbacks.

Stable isotopes in tree rings record physiological trends in Larix gmelinii after fires

Fire is an important regulator of ecosystem dynamics in boreal forests, and especially has a complicated association with growth and physiological processes of fire-tolerant tree species. Stable isotope ratios in tree rings are used extensively in eco-physiological studies for evaluating the impact of past environmental (e.g., drought, air pollution) factors on tree growth and physiological processes. Yet, such studies based on carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings are rarely conducted on fire effect, especially not well explored for fire-tolerant trees. In this study, we investigated variations in basal area increment and isotopes of Larix gmelinii (Rupr.) Rupr. before and after three moderate fires (different fire years) at three sites across the Great Xing'an Mountains, Northeastern China. We found that the radial growth of L. gmelinii trees has significantly declined after the fires across study sites. Following the fires, a simultaneous increase in δ13C and δ18O has strengthened the link between the two isotopes. Further, fires have significantly enhanced the 13C-derived intrinsic water-use efficiency (iWUE) and largely altered the relationships between δ13C, δ18O, iWUE and climate (temperature and precipitation). A dual-isotope conceptual model revealed that an initial co-increase in δ13C and δ18O in the fire year can be mainly attributed to a reduction in stomatal conductance with a constant photosynthetic rate. However, this physiological response would shift to different patterns over post-fire time between sites, which might be partly related to spring temperature. This study is beneficial to better understand, in a physiological perspective, how fire-tolerant tree species adapt to a fire-prone environment. We also remind that the limitation of model assumptions and constraints may challenge model applicability and further inferred physiological response.

Allocation Strategies of Carbon, Nitrogen, and Phosphorus at Species and Community Levels With Recovery After Wildfire

Plant stoichiometry and nutrient allocation can reflect a plant's adaptation to environmental nutrient changes. However, the allocation strategies of carbon (C), nitrogen (N), and phosphorus (P) between leaf and fine root in response to wildfire have been poorly studied. Our primary objective was to elucidate the trade-off of elemental allocation between above- and belowground parts in response to the soil nutrient changes after a wildfire. We explored the allocation sloping exponents of C, N, and P between leaf and fine root at the species and community levels at four recovery periods (year 2, 10, 20, and 30) after moderately severe wildfire and one unburned treatment in boreal forests in Great Xing'an Mountains, northeast China. Compared with the unburned treatment, leaf C concentration decreased and fine root C increased at year 2 after recovery. The leaf N concentration at year 10 after recovery was higher than that of unburned treatment. Plant growth tended to be limited by P concentration at year 10 after recovery. Nutrient allocation between leaf and fine root differed between species and community levels, especially in the early recovery periods (i.e., 2 and 10 years). At the community level, the nutrient concentrations of the leaf changed more as compared to that of the fine root at year 2 after recovery when the fine root nutrients changed more than those of the leaf. The different C, N, and P allocation strategies advanced the understanding of plant adaptation to soil nutrient changes during the postfire ecosystem restoration.

Relationship between Tree Richness and Temporary Stability of Plant Communities: A Case Study of a Forest in Northeast China

The relationship between diversity and stability is a classic issue in ecology, but no general consensus has been achieved. To address this relationship, a field survey of a forest in Northeast China was conducted. The temporary stability was defined from the perspective of community characteristics. The results showed that communities with the highest temporary stability value were characterized by a single dominant species. A significant linear relationship with a low R2 value was observed between temporary stability and tree richness. When dominant and non-dominant tree species were studied, no significant linear relationship was obtained between temporary stability and non-dominant tree richness. However, the relationship between temporary stability and dominant tree richness was significant with a high R2 value, and the temporary stability decreased with increasing dominant tree richness. This study demonstrates that dominant tree richness is closely related to temporary stability, and temporary stability can serve as a stability indicator. The results provide a new perspective for understanding stability and additional information for revealing the relationship between diversity and stability in forest ecosystems.

Changes in plant nutrient utilization during ecosystem recovery after wildfire

Wildfire is the primary natural disturbance in boreal forest ecosystems. It substantially changes soil nutrient conditions and plant nutrient dynamics. After a wildfire, various plant strategies of nutrient utilization are fundamental to ecosystem recovery processes. Stability of plant nutrients reflects the ability of plants possessing relatively constant elemental concentrations in the face of nutrient changes, which can be calculated by the value of "nutrient homeostasis". However, the mechanism of how nutrient homeostasis mediates plant community recovery in post-fire ecosystems remains unknown. The dominant tree species that survived after fire and the new emergence of regenerated tree species are the important components of a plant community during the recovery process. Our primary objective was to elucidate the nutrient homeostasis trade-off between dominant and regenerated species over years after recovery. Five treatments, namely, 2 year, 10 year, 20 year, 30 years after moderate burning severity, and unburned forests, were designed in the boreal forests of Great Xing'an Mountains, Northeast China. Compared with unburned forests, wildfire lowered the average value of homeostasis of plant nutrients (N and P). Moreover, the mean homeostasis value of the dominant species (i.e., Larix gmelinii) was higher than that of the regenerated species (i.e., Betula platyphylla). The slope of relationship between nutrient homeostasis and recovery years of the regenerated species was higher than that of the dominant species, suggesting that the nutrient homeostasis in the regenerated species recovered more quickly than dominant species after recovery. Compared with the dominant species, changes in the regenerated species' homeostasis can explained more to the changes of species diversity during the years after recovery. This study revealed plant nutrient adaptation in different species and different plant organs with years after wildfire and highlighted the importance of nutrient homeostasis in plant adaptation strategies and the recovery of plant community.

Optimal Community Assembly Related to Leaf Economic- Hydraulic-Anatomical Traits

Multi-dimensional trait mechanisms underlying community assembly at regional scales are largely unclear. In this study, we measured leaf economic, hydraulic and anatomical traits of 394 tree species from tropical to cold temperate forests, from which we calculated the leaf trait moments (mean, variance, skewness, and kurtosis) using community-weighted methods. Economic and hydraulic traits were decoupled at the species level, but coupled at the community level, and relationships between leaf traits in observed communities were stronger than that in null communities, suggesting that the adaptive mechanisms of plant species may be different. Furthermore, leaf economic traits were distributed more evenly across species occupying communities with lower temperature and precipitation, whereas hydraulic traits were distributed more evenly under lower water availability. This suggests that limiting similarity of specific leaf traits within communities would be enhanced when related-resources are limited, and highlights the independent assembly of leaf economics and hydraulic traits in terms of functional evenness. Importantly, the moments of leaf economic and hydraulic traits of observed communities explained more variation in ecosystem productivity than that of null communities, indicating ecosystem productivity depended on trait-based community assembly. Our results highlight the principles of community assembly regarding multi-dimensionsional traits in natural forests at a regional scale.

N-P utilization of Acer mono leaves at different life history stages across altitudinal gradients

The relationship between plants and the environment is a core area of research in ecology. Owing to differences in plant sensitivity to the environment at different life history stages, the adaptive strategies of plants are a cumulative result of both their life history and environment. Previous research on plant adaptation strategies has focused on adult plants, neglecting saplings or seedlings, which are more sensitive to the environment and largely affect the growth strategy of subsequent life stages. We compared leaf N and P stoichiometric traits of the seedlings, saplings, and adult trees of Acer mono Maxim and different altitudes and found significant linear trends for both life history stages and altitude. Leaf N and P content by unit mass were greatly affected by environmental change, and the leaf N and P content by unit area varied greatly by life history stage. Acer mono leaf N-P utilization showed a significant allometric growth trend in all life history stages and at low altitudes. The adult stage had higher N-use efficiency than the seedling stage and exhibited an isometric growth trend at high altitudes. The N-P utilization strategies of A. mono leaves are affected by changing environmental conditions, but their response is further dependent upon the life history stage of the plant. Thus, this study provides novel insights into the nutrient use strategies of A. mono and how they respond to the environmental temperature, soil moisture content along altitude and how these changes differ among different life history stages, which further provide the scientific basis for the study of plant nutrient utilization strategy on regional scale.