Distribution of twelve moist forest canopy tree species in Liberia and Côte d'Ivoire: response curves to a climatic gradient

Abstract. The occurrence and abundance of 12 canopy tree species from the moist tropical forests of West Africa have been studied in relation to a climatic gradient. We focused on environmental factors related to water availability: annual amount of rainfall, the length of the dry season, and cumulative water deficit. Species occurrence and abundance data are used for 39 forest sites in Liberia and southwest Côte d'Ivoire. Species responses are modelled using a set of five increasingly complex models, ranging from a no-trend model to a skewed bell-shaped response curve. The study species show different distribution patterns. Most of them suggest a close relationship to climatic conditions. Fitting of species occurrence data to each of the three climatic factors results in most cases in simple models. In only one out of 36 cases a bell-shaped response curve is needed to describe the data. Four of the 12 species show no response to the climatic factors when only occurrence is evaluated. When abundance data are used, in 33 of the 36 cases significant response models are found. In general these are much more complex than in the cases of species occurrence data: in 10 of the 36 cases a bell-shaped response model is found to describe the data best. This is in contrast with the widespread belief that species response curves are bell-shaped: within the forest zone in the area studied this is not generally the case. The importance of the three climatic factors for the distribution of the species is evaluated: for four species mean annual rainfall is the most important variable, for four species the length of the dry period, and for one species cumulative water deficit. Consequently, the assumption that mean annual rainfall is the most important factor determining tree species distribution in West African forests is not correct. Species response models to climatic factors show where species have their geographical optima. Implications for forest management are briefly discussed.     

Anthropogenic and climatic impacts on surface pollen assemblages along a precipitation gradient in north‐eastern China

Aim To understand the scenarios of ‘anthropogenic biomes’ that integrate human and ecological systems, we need to explore the impacts of climate and human disturbance on vegetation in the past and present. Interactions among surface pollen, modern vegetation and human activities along climate and land‐use gradients are tested to evaluate the natural and anthropogenic forces shaping the modern vegetation, and hence to aid the reconstruction of vegetation and climate in the past. This in turn will help with future predictions. Location The North‐east China Transect (NECT) in north‐eastern China. Methods We analysed 33 surface pollen samples and 213 quadrats across four vegetation zones along the moisture/land‐use gradients of the NECT. Detrended correspondence analysis (DCA) and redundancy analysis (RDA) of 52 pollen taxa and three environmental variables were used to distinguish anthropogenic and climatic factors that affect surface pollen assemblages along the NECT. Results The 33 surface samples are divided into four pollen zones (forest, meadow steppe, typical steppe and desert steppe) corresponding to major vegetation types in the NECT. Variations in pollen ratios of fern/herb (F/H), Artemisia/Chenopodiaceae (A/C) and arboreal pollen/non‐arboreal pollen (AP/NAP) represent the vegetation and precipitation gradient along the NECT. DCA and RDA analyses suggest that surface pollen assemblages are significantly influenced by the precipitation gradient. Changes in the abundance of Chenopodiaceae pollen are related to both human activities and precipitation. Main conclusions Surface pollen assemblages, fossil pollen records, archaeological evidence and historical documents in northern China show that a large increase of Chenopodiaceae pollen indicates human‐caused vegetation degradation in sandy habitats. The A/C ratio is a good indicator of climatic aridity, but should be used in conjunction with multiple proxies of human activities and climate change in the pollen‐based reconstruction of anthropogenic biomes.     

Coexistence and geographical distribution of Leguminosae in an area of Atlantic forest in the semi‐arid region of Brazil

Abstract Understanding the factors that affect plant species distribution and coexistence in areas with high plant species diversity is a challenge for ecologists. According to some authors, species occupy specific niches, but for others, species coexistence and geographical distribution patterns are random. Floristic composition of the family Leguminosae was studied on moist and dry slopes of the Baturité mountains in semi‐arid northeastern Brazil and was compared with findings for other plant formations elsewhere in Brazil. Substantial floristic differences were found between the moist windward and dry leeward slopes of the Baturité mountains despite their close geographical proximity. The leeward slope was slightly more diverse than the windward slope. Similarity analyses showed that the windward face is floristically allied to the Amazon forest, whereas the leeward slope is similar to other dry‐area formations of northeastern Brazil, such as thorny woodland (caatinga) and seasonal forests. The strong floristic differences that were observed between the windward and leeward slopes corroborate the theory of ecological niche conservatism, which holds that species occurrence is closely linked to environmental factors, such as temperature and precipitation.     

Soil carbon and nitrogen stocks in forests along an altitudinal gradient in the eastern Himalayas and a meta‐analysis of global data

High‐altitude soils potentially store a large pool of carbon (C) and nitrogen (N). The assessment of total C and N stocks in soils is vital to understanding the C and N dynamics in terrestrial ecosystems. In this study, we examined effects of altitude and forest composition on soil C and N along a transect from 317 to 3300 m a.s.l. in the eastern Himalayas. We used meta‐analysis to establish the context for our results on the effects of altitude on soil C, including variation with depth. Total C and N contents of soils significantly increased with altitude, but decreased with soil depth. Carbon and N were similarly correlated with altitude and temperature, and temperature was seemingly the main driver of soil C along the altitudinal gradient. Altitude accounted for 73% of the variation in C and 47% of the variation in N stocks. Soil pH and cation exchange capacity were correlated with both soil C and N stocks. Increases in soil C and N stocks were related to forest composition, forest basal area as well as quantity of leaf litter that were in turn influenced by altitude and temperature. Concentrations of C in foliage increased by 2.1% for every 1000 m rise in altitude, while that in leaf litter increased by 2.3%.     

Using a climatic niche model to predict the direct and indirect impacts of climate change on the distribution of Douglas‐fir in New Zealand

Climate change is likely to have major impacts on the distribution of planted and natural forests. Herein, we demonstrate how a process‐based niche model (CLIMEX) can be extended to globally project the potential habitat suitable for Douglas‐fir. Within this distribution, we use CLIMEX to predict abundance of the pathogen P haeocryptopus gaeumannii and severity of its associated foliage disease, Swiss needle cast. The distribution and severity of the disease, which can strongly reduce growth rate of Douglas‐fir, is closely correlated with seasonal temperatures and precipitation. This model is used to project how climate change during the 2080s may alter the area suitable for Douglas‐fir plantations within New Zealand. The climate change scenarios used indicate that the land area suitable for Douglas‐fir production in the North Island will be reduced markedly from near 100% under current climate to 36–64% of the total land area by 2080s. Within areas shown to be suitable for the host in the North Island, four of the six climate change scenarios predict substantial increases in disease severity that will make these regions at best marginal for Douglas‐fir by the 2080s. In contrast, most regions in the South Island are projected to sustain relatively low levels of disease, and remain suitable for Douglas‐fir under climate change over the course of this century.