Abstract
Long-term studies have revealed that forest species composition was shifting under environment change and disturbance induced by loss of large trees. Yet, few studies explicitly analyzed their impacts on composition concurrently. To learn more about impacts of environment change and disturbance on driving forest community, we investigated shifts in functional composition over past 24 years in an old-growth subtropical forest in southern China. We analyzed nine traits that are mainly related to leaf nutrients, photosynthetic capacity, hydraulic conductivity, and drought tolerance of plants and examined hypotheses: (1) The functional composition change over time was directional instead of random fluctuation, (2) drought-tolerant species increased their abundance under soil dryness, (3) both environmental change and disturbance related to changes of functional composition significantly, and (4) initial trait values of quadrats strongly influenced their subsequent change rates in quadrat level (10 × 10 m). We found that species composition had shifted to favor species with high leaf nutrient content, high photosynthesis rate, high hydraulic conductivity, low water-use efficiency, and high drought tolerance traits, which was due to soil dryness and disturbance. These two factors explained 47–58% of quadrats’ trait value changes together. Considering rapidly increasing stem density, this pattern may indicate ecological processes of which disturbance provided numerous recruits of resource-acquisition strategy species and soil dryness conducted a selecting effect on shaping composition in the forest. Additionally, quadrats with initial trait values at the far end of change direction shifted faster in three traits, which also indicated that functional composition changes in quadrats were directional and homogenized. Our results implied that environment change and accompanied disturbance events possibly drove species composition change along a different trajectory in the subtropical forest that experienced high climatic variability.
Highlights
Climate changes have altered the physical environments of plant communities and shifted community species composition due to the fact that different functional groups, plant genera, and species respond divergently to climate change [1,2,3,4,5]
We aim to test the following four hypotheses: (1) The species composition change over time was directional instead of random fluctuation, (2) drought-tolerant species increased their abundance under soil dryness, (3) both environmental change and disturbance related to changes of functional composition significantly, and (4) initial trait values of quadrats strongly influenced their subsequent change rates in this old-growth subtropical forest
The DHS plot experienced stem density (SD) increasing and loss of basal area (BA) over the past 24 years and it could be divided into two distinct periods: 1992–2004 and 2004–2015
Summary
Climate changes have altered the physical environments (e.g., soil moisture, temperature, and water vapor pressure deficit) of plant communities and shifted community species composition due to the fact that different functional groups, plant genera, and species respond divergently to climate change [1,2,3,4,5]. Based on long-term monitoring forest plots, several recent studies reported that climate change, especially drought, induced community composition to be shifting towards more dry-tolerant individuals and genera in Central American forests [1,6], West African forests [7], Amazon forests [2,8], and some temperate forests [9]. Despite these well-documented changes, disturbance induced by tree mortality almost follows climate change and is possibly one of the most important factors of restructuring forests, but few studies have explicitly analyzed impacts of disturbance and environmental change on functional composition concurrently. This pattern was observed in more than 100 forest plots in South America [2]
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