RESULTS AND DISCUSSION

To illustrate the assembly process of East Asian EBLF, we selected 21 clades with 2028 species, belonging to 14 orders and 24 families across the angiosperm tree of life (Fig. S1 and Table S1). A total of 680 evergreen species (406 endemic) and 381 deciduous species (206 endemic) growing in subtropical East Asia are included. This sampling strategy maximized taxon representation in East Asian EBLF so far, especially covering the members of the first five dominant families in this biome, i.e. Fagaceae, Lauraceae, Magnoliaceae, Theaceae and Hamamelidaceae [5]. By integrating results of molecular dating, biogeographic and habit analyses for each of the 21 clades (Fig. S2), we compiled age credibility intervals of the transition from deciduous to evergreen habits in subtropical East Asia, and calculated the maximal number of observed transition events (MTE) per million year (Myr) (Fig. S3). Considering that the species diversity in a biome arises from both in situ diversification and immigration, we further compiled credibility intervals of age estimates for in situ diversification and dispersal events of evergreen lineages and calculated their maximal number of observed events (MDivE & MDisE) per Myr, respectively. In addition, we performed generalized least squares multiple regression analyses to test the possible correlation between evolutionary dynamics of evergreen lineages and environmental changes, i.e. East Asian annual precipitation and global temperature, over time and employed breakpoint regression analyses to test whether Asian monsoon climate change impacted the MTE, MDivE and MDisE.

Our biogeographic and habit analyses identified 48 transition events from deciduous to evergreen habits in subtropical East Asia. A total of 788 biogeographic events related to subtropical East Asian evergreen species examined here were found, including 773 in situ diversification events and 15 dispersal events. In situ diversification events overwhelmingly predominated over dispersal events by ∼52 times (773/15), implying that indigenous elements contributed far more than immigrants to the current biodiversity of East Asian EBLF. Considering the relatively low sampling for some clades, this hypothesis needs to be tested in the future by sampling more taxa. The transition from deciduous to evergreen habits and in situ diversification initially occurred in the Lower Cretaceous (Fig. S3). Transition events accelerated at ∼56, 35 and 27 Ma, and peaked around 25–23 Ma and 8–7 Ma (Fig. 2a). In situ diversification events accelerated at ∼48, 23 and 16 Ma, and peaked around 8–6 Ma (Fig. 2b). Dispersal events into subtropical East Asia started at ∼36 Ma and peaked around 26–23 Ma and 6–5 Ma, which were from two source regions: temperate Eurasia and tropical Asia (Fig. 2c). Our multiple regression analyses support the positive relationship between evergreen lineage occurrence, in situ diversification, dispersal and East Asian annual precipitation, respectively (Fig. 3 and Table S2), highlighting the important roles of the precipitation regime in promoting the diversification of evergreen lineages.

Figure 2.: Evolutionary dynamics of evergreen lineages in subtropical East Asia and potential driving factors. (a) Transition rates from deciduous to evergreen habits based on the MTE per Myr. (b) In situ diversification rates of evergreen lineages based on the MDivE per Myr. Details for transition and in situ diversification dynamics in a broader timeframe can be found in Fig. S3. Arrowheads indicate estimated inflection points. (c) Dispersal rates of evergreen lineages from other regions to subtropical East Asia based on the MDisE per Myr. (d) Elevation changes of different parts of the QTP [35] and the Qilian Shan [56], global mean temperature difference to today [39], Asian climatic records [38] and suitable area (grid cell) dynamics of East Asian EBLF. EECO, Early Eocene Climatic Optimum; Pli, Pliocene; Q, Quaternary.

Figure 3.: Partial regression plots showing correlations between the three events related to East Asian subtropical evergreen lineages and annual precipitation in East Asia. The solid line and gray shading indicate the regression line and 95% credible interval separately, with circles showing the raw data.

In situ diversification began at ∼125 Ma, by ancestral lineage of the Altingiaceae–Hamamelidaceae clade (Figs S2 and S3). Transition started at ∼93 Ma, mainly by ancestral lineage of Magnolia (Figs S2 and S3 and Table S3). These suggest that some ancient elements of East Asian EBLF might have occurred in the Lower to Upper Cretaceous, implying that the EBLF functions as a museum for plant species diversity in East Asia. During this period, global average temperature was considerably high without large and permanent icecaps [46], and angiosperm-dominated forests started to arise [47,48] along with the Cretaceous Terrestrial Revolution [49].

The MTE curve initially accelerated at ∼56 Ma (Fig. 2a), which temporally coincides with the transition from the ‘Warmhouse’ to ‘Hothouse’ climate state (Fig. 2d) [39]. The MDivE curve first accelerated at ∼48 Ma (Fig. 2b), in line with the Early Eocene Climatic Optimum (Fig. 2d). Geological data indicate the uplift of southern (∼55 Ma) and central (∼45 Ma) parts of the QTP caused by the India–Eurasia collision [35]. The orogenic events in the QTP, as well as the migration of Pacific warm pool back to Asia, resulted in the dominance of ITCZ-type monsoonal climate in East Asia and thereby brought more precipitation [37]. We also identified an increased point of the MTE curve in the late Eocene (∼35 Ma; Fig. 2a). During this period, subtropical monsoon climate gradually prevailed in southern China and expanded northward (Fig. 2d), along with significant increase in precipitation [38,45,50,51]. Based on our ancestral range and habit reconstructions (Fig. S2), most evergreen lineages that originated in the Eocene are currently distributed in southern China to Southeast Asia, such as some species of Lithocarpus (Fagaceae), Actinodaphne henryi and Sinosassafras flavinervium (Lauraceae) and the Viburnum punctatum–V. lepidotulum clade (Viburnaceae). Thus, EBLF might have only appeared in low latitudes of subtropical East Asia during the Eocene. Paleobotanical data also indicate that dominant genera in southern Chinese EBLF started to occur in the Paleocene–early Eocene, and peaked in the middle Eocene [16].

Transition further accelerated in the late Oligocene (∼27 Ma) and later reached the first peak around 25–23 Ma (Fig. 2a). The MDivE curve sharply increased at ∼23 Ma (Fig. 2b), and the MDisE curve peaked around 26–23 Ma (Fig. 2c). Moreover, MDivE and MDisE curves that were calculated based on the evergreen lineages only from the first five dominant families in East Asian EBLF significantly increased around the OMB (Fig. S4). In the late Oligocene–early Miocene, the dramatic uplift of central and southern QTP occurred (Fig. 2d), which largely reorganized Asian climate and shaped the modern-like Asian monsoon system (Fig. 2d) [38,41]. During this period, East Asian summer monsoon started to establish [52], and South Asian summer monsoon intensified [53], contributing to more summer precipitation [54] and the emergence of spring persistent rainfall [40] in East Asia. Meanwhile, the weakening of Asian winter monsoon increased winter precipitation in East Asia [19]. The breakpoint regression analyses supported that the formation of modern-like Asian monsoon system influenced all of the MTE, MDivE and MDisE (Table 1). Our modelling analysis indicates significant increase of suitable area of East Asian EBLF at ∼25 Ma (Fig. 2d), in agreement with the result of Guo et al. [55] that a wide humid belt similar to the modern one occurred in subtropical East Asia near the OMB. Importantly, considering the credibility intervals of estimated times, 85%, 88% and 93% of transition, in situ diversification and dispersal events took place after the OMB (∼23 Ma). We also found that ∼95% of the evergreen species in East Asian EBLF and ∼96% of the evergreen species endemic to this biome occurred after ∼23 Ma (Fig. 4), suggesting that East Asia EBLF acted as an evolutionary cradle for East Asian biodiversity. Thus, our data show that the modified monsoon system resulted in the humidification of East Asia and thereby promoted the modernization of the EBLF in this region around the OMB, as supported by the results of modelling and fossil data [19].

Figure 4.: Distribution of estimated origination ages of evergreen species in East Asian EBLF.

 | ELPD difference ± standard error
Model | MTE | MDivE | MDisE
Null | −36.8 ± 6.6 | −41.4 ± 5.8 | −13.2 ± 4.6
E | −7.0 ± 8.8 | −27.1 ± 5.4 | N/A
M | −4.6 ± 3.2 | 0.0 ± 0.0 | −0.2 ± 1.7
P | −37.8 ± 6.6 | −40.9 ± 5.8 | −14.0 ± 4.5
EM | −16.9 ± 7.8 | −26.7 ± 5.4 | N/A
EP | −5.8 ± 9.1 | −27.8 ± 5.8 | N/A
MP | 0.0 ± 0.0 | −0.2 ± 1.3 | 0.0 ± 0.0
EMP | −5.8 ± 9.0 | −27.8 ± 5.8 | N/A

The MDivE curve shows a virtually linear increase between 23 and 8 Ma (Fig. 2b) and peaked in the late Miocene (∼8–5 Ma), during which MTE and MDisE curves also reached their high peaks (Fig. 2a–c). This temporally coincides with the uplift of Qilian Shan (Fig. 2d) [56]. From ∼12 to 4 Ma, an intensified Asian monsoon dominated East Asia, which might have contributed to the more persistent wet season through the year [37,51], and the East Asian spring persistent rainfall reached its modern-day geographic distribution [40]. The EBLF thereby flourished continuously in East Asia throughout the Miocene.

From ∼4 Ma onwards, the MTE, MDivE and MDisE curves all decreased (Fig. 2a–c), which temporally coincides with a marked decrease of precipitation in East Asia [37,40] and the transition from the ‘Coolhouse’ to ‘Coldhouse’ climate state (Fig. 2d). The freezing temperature is a key factor limiting the distribution of evergreen broadleaved species [5,28]. The breakpoint regression analyses also found that this decrease in precipitation had an impact on the MTE and MDisE curves (Table 1). In addition, our modelling results suggest the significant decrease of suitable areas of East Asian EBLF occurred at this time (Fig. 2d). Thus, temperature and precipitation drop could have hampered the diversification of evergreen lineages and have progressively deteriorated East Asian EBLF since the Pliocene.

Among the referred 15 dispersal events of evergreen lineages into subtropical East Asia (Table S4), most of them were from tropical Asia (∼73%), and the remaining were from temperate Eurasia (∼27%), suggesting the great contribution of tropical Asia to species diversity of East Asian EBLF. In addition, we also identified 225 dispersal events out of East Asian EBLF spanning from 90.42 to 0.29 Ma, most of which (∼81.3%) colonized tropical Asia, followed by temperate Eurasia (∼14.7%), North America (∼2.2%), South America (∼1.3%) and Africa (∼0.4%) (Fig. S5). Altogether, these results suggest that East Asian EBLF serves as not only a sink, but also an important source region for species diversity of other regions of the world, and thus promotes biotic exchanges between temperate and tropical regions, highlighting the importance of its conservation.

In summary, our multi-taxon study provides new insights into the historical assembly of East Asian EBLF over a broad period of ∼125 Myr. All of the MTE, MDivE and MDisE curves experienced a dynamic process. The occurrence of in situ diversification is much earlier than that of dispersal, and the former overwhelmingly predominated over the latter. Our results support that the modernization of East Asian EBLF did not take place until the OMB, which was driven by the formation of modern-like Asian monsoon climate system. East Asian EBLF has functioned as both evolutionary museums and cradles for evergreen species diversity of East Asia, and served as a transfer station for biotic exchanges between temperate and tropical regions, emphasizing the conservation priority for this world’s largest EBLF. Many herbaceous and deciduous species also inhabit East Asian EBLF. Whether they have similar trends through time to evergreen species needs to be investigated in the future.