Abstract
Understanding climatic influences on the proportion of evergreen versus deciduous broad‐leaved tree species in forests is of crucial importance when predicting the impact of climate change on broad‐leaved forests. Here, we quantified the geographical distribution of evergreen versus deciduous broad‐leaved tree species in subtropical China. The Relative Importance Value index (RIV) was used to examine regional patterns in tree species dominance and was related to three key climatic variables: mean annual temperature (MAT), minimum temperature of the coldest month (MinT), and mean annual precipitation (MAP). We found the RIV of evergreen species to decrease with latitude at a lapse rate of 10% per degree between 23.5 and 25°N, 1% per degree at 25–29.1°N, and 15% per degree at 29.1–34°N. The RIV of evergreen species increased with: MinT at a lapse rate of 10% per °C between −4.5 and 2.5°C and 2% per °C at 2.5–10.5°C; MAP at a lapse rate of 10% per 100 mm between 900 and 1,600 mm and 4% per 100 mm between 1,600 and 2,250 mm. All selected climatic variables cumulatively explained 71% of the geographical variation in dominance of evergreen and deciduous broad‐leaved tree species and the climatic variables, ranked in order of decreasing effects were as follows: MinT > MAP > MAT. We further proposed that the latitudinal limit of evergreen and deciduous broad‐leaved mixed forests was 29.1–32°N, corresponding with MAT of 11–18.1°C, MinT of −2.5 to 2.51°C, and MAP of 1,000–1,630 mm. This study is the first quantitative assessment of climatic correlates with the evergreenness and deciduousness of broad‐leaved forests in subtropical China and underscores that extreme cold temperature is the most important climatic determinant of evergreen and deciduous broad‐leaved tree species’ distributions, a finding that confirms earlier qualitative studies. Our findings also offer new insight into the definition and distribution of the mixed forest and an accurate assessment of vulnerability of mixed forests to future climate change.
Highlights
Evergreen and deciduous broad-leaved tree species can coexist across a variety of landscapes around the globe and play important roles in forest structure and functions (Kikuzawa, Onoda, Wright, & Reich, 2013; Ouédraogo et al, 2016; Wang, Kent, & Fang, 2007)
We found that the estimated latitudinal range of the mixed forests in our study site was ca. 29.1–32°N and the optimum distribution for this forest type occurred at ca. 31.39°N, where evergreen species were equal in dominance to deciduous species (Figure 1)
Our analyses have assessed the geographical distribution of evergreen and deciduous broad-leaved tree species in relation to key climatic factors
Summary
Evergreen and deciduous broad-leaved tree species can coexist across a variety of landscapes around the globe and play important roles in forest structure and functions (Kikuzawa, Onoda, Wright, & Reich, 2013; Ouédraogo et al, 2016; Wang, Kent, & Fang, 2007). Long-lived leaves of evergreen species potentially allow for a longer photosynthetic season than the leaves of deciduous species allow, while deciduous species reduce transpiration and respiration during drought or low temperature and usually possess higher photosynthetic rates per unit leaf area (Aerts, 1995; Givnish, 2002; González-Zurdo, Escudero, Babiano, García-Ciudad, & Mediavilla, 2016; Kikuzawa et al, 2013) These pronounced advantages and disadvantages of the respective leaf types are closely coupled with the distribution of evergreen and deciduous tree species in divergent climatic conditions (Condit et al, 2000; Weng et al, 2016; Woodward, Lomas, & Kelly, 2004). Other studies have shown a tight relationship between forest deciduousness and/or evergreenness and temperature-related variables such as mean annual temperature (Kikuzawa & Lechowicz, 2011; Zhang, Luo, Zhu, Daly, & Deng, 2010)
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