Lianas As Structural Parasites: a Re-Evaluation
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Review Ecology February 2012 Vol.57 No.4: 307312 doi: 10.1007/s11434-011-4690-x SPECIAL TOPICS: Lianas as structural parasites: A re-evaluation TANG Yong1,2*, Roger L. KITCHING2 & CAO Min1 1 Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla 666303, China; 2 Griffith School of the Environment, Griffith University, Nathan, Queensland 4111, Australia Received April 28, 2011; accepted July 7, 2011; published online December 2, 2011 Lianas are a principal physiognomic component of tropical and subtropical forests and are typically considered to be parasites of trees. In contrast, the substantial contribution of lianas to rainforest leaf litter production (up to 40%) suggests that they play im- portant roles in nutrient cycles and may benefit their host trees. Lianas contribute disproportionately to total forest litter produc- tion at least partially because lianas invest relatively little in support structures and proportionately much more to leaf production when compared with trees. In contrast to tree leaves, liana leaves are higher in nutrient concentrations, relatively short-lived, and decompose more rapidly. In addition, the special life form of lianas allows them to grow vertically and horizontally in the forest and relocate nutrients, mainly towards their host trees, through the production of leaf litter. Consequently, lianas may contribute substantially to the high rainforest productivity, and the roles they play in liana/tree associations and rainforest dynamics needs to be re-evaluated. Liana, compensatory effect, nutrient dynamics, leaf litter, rainforest Citation: Tang Y, Kitching R L, Cao M. Lianas as structural parasites: A re-evaluation. Chin Sci Bull, 2012, 57: 307312, doi: 10.1007/s11434-011-4690-x Lianas are woody vines that climb other plants to ascend Equator, such as the Amazonian lowland rainforest, co- forest canopies [1]. The non-self-supporting life form of occurring with the highest tree species diversity [1,6]. Addi- lianas has been considered as an evolutionary adaptation tional factors, including soil type, seasonality of rainfall and driven by competition for light and allows lianas to invest disturbance, have been proposed as being important in de- more in transportation and photosynthetic organs, princi- termining local liana distribution [6–8]. The high liana pally leaves [2]. Lianas occur in almost all forest types but abundance in seasonal rainforests reflects their ability to are more abundant in tropical forests [1]. The number of maintain growth during dry seasons by accessing ground species and the abundance of lianas decrease with an in- water via deep roots and efficient vascular conducting sys- crease in latitude and show a sharp drop near the Tropic of tems [6]. This ability to obtain water during dry periods Cancer where the climate changes northwards from tropical allows lianas to take advantage of the high solar radiation to temperate. The low occurrence of lianas in temperate environment associated with dry seasons. Thus, lianas grow regions has been ascribed to frequent low winter tempera- much faster than trees during the dry season in seasonal tures that may cause freezing-induced embolisms in lianas’ rainforests [6]. long vessel systems and lead to unrecoverable damage [3]. Lianas play important roles in forest dynamics. They add As a principal physiognomic component of tropical and substantially to baseline levels of plant diversity in forests subtropical rainforests, lianas comprise around 25% of the and maintain tree diversity through their role in gap dynam- diversity of woody plants in these ecosystems [1,4,5]. Their ics [9]. A high liana load in tree canopies may cause high abundance and diversity increase along a low to high rain- tree-fall rates, thus maintaining rainforests in a perpetual fall gradient, and peak in tropical rainforest sites near the disclimax and thereby potentially maintaining diversity by reducing dominance levels among tree species [10]. Many *Corresponding author (email: [email protected]) studies have found that lianas increase in both abundance © The Author(s) 2011. This article is published with open access at Springerlink.com csb.scichina.com www.springer.com/scp 308 Tang Y, et al. Chin Sci Bull February (2012) Vol.57 No.4 and diversity following natural and anthropogenic disturb- impacts on the litter decomposition process in rainforests. ance [8,11,12]. Consequently, they play an important role in Liana leaves have significantly lower mass per unit leaf area regulating forest regeneration processes [12]. The increase (LMA) than tree leaves, because of the faster growth and in the abundance of lianas reflects their ability to regenerate different resource allocation strategies of lianas [24–26]. vegetatively and grow both vertically and horizontally in Furthermore, liana leaves have a much higher nitrogen response to changes in the light environment. Thus, as rain- concentration than those of their host trees (Table 1). Alt- forest fragmentation increases across the tropics, an increase hough most studies compared only green leaves, we expect in the dominance of lianas in rainforests is expected [8,11]. that leaf litter follows the same pattern because the nutrient Indeed, this effect may be enhanced through a positive content of green and senesced leaves are highly correlated feedback loop driven by increases in atmospheric carbon [27]. Low LMA and high leaf nitrogen concentration have dioxide [13,14]. Finally, lianas, perhaps through their lower been suggested to be economical trade-offs [28] and may levels of chemical foliar defenses, are the preferred hosts of generate faster rates of litter decomposition than leaves with many herbivores, particularly insects, and liana occurrence high LMAs and low nitrogen concentrations. Moreover, in forests may substantially increase animal diversity leaves with low LMA and high nitrogen contents, such as through food-web effects in canopies and leaf litter [15,16]. those of lianas, tend to have a shorter life span than those of Lianas have long been considered as parasites of trees many trees [29–31], thereby maintaining a rapid nutrient [11,17]. This hypothesis is based on the proposition that cycling system. they have detrimental effects on the growth and reproduc- Liana leaves also have a high concentration of phospho- tion of their host trees by causing physical damage and by rus [31–33], a major plant growth element generally limited competing for limited resources such as water, nutrients and in many rainforests [34]. A recent study has shown that lia- light [6,18,19]. Lianas may also significantly delay rainfor- nas have a lower phosphorus resorption rate from senescing est regeneration during the gap phase by competing with leaves than do trees [32], which may be caused by the high trees, especially those of shade-tolerant species [20]. The nitrogen content [30] and the short lifespan of liana leaves. majority of studies on lianas, especially those conducted in Thus, we expect that the leaf litter of lianas is higher in a silvicultural context, have only focused on negative ef- phosphorus concentration than that of trees. Given the per- fects on their host trees [21]. However, many liana-bearing vasive phosphorus deficiency in many rainforests, lianas trees are actually large canopy trees that live for many years potentially play an important role in phosphorus recycling and readily grow and produce seeds [22]. Presently, there in rainforests by producing leaf litter with a high concentra- are still no explanations for how liana-bearing trees are able tion of this element [25,32,35]. Moreover, phosphorus to maintain their growth in highly competitive rainforest availability also has been considered to be an important while incurring negative impacts from lianas. Although the factor regulating microbial decomposition processes in importance of lianas in forests has been widely accepted rainforests [36–38]. Leaf litter with a high phosphorus con- [23], the possibility that trees might benefit from the pres- centration may potentially promote microbial processes and ence of lianas has been overlooked. By reviewing published thus make more of the element available to plants through research, we demonstrate that lianas may play important microbial phosphorus immobilization [36]. That is, phos- roles in nutrient dynamics by producing a large amount of phorus is taken up by microbes and then slowly released nutritious and easily decomposed leaf litter and can benefit back into the system as they die, thereby preventing rapid their host trees via nutrient transportation. loss of phosphorus. The microenvironment created by liana leaf litter could promote the decomposition of associated tree leaf litter, thus 1 Characteristics of liana leaves providing an efficient nutrient recycling system. Plants and soils nutrients interact in a manner such that correlations Lianas produce a larger quantity of leaves than trees, and between them may be caused by nutrient limitations to plant their leaves are structurally and nutritionally different from growth or by plant effects on soils [39,40]. The unresolved those of trees (Table 1). These differences have profound association of lianas with richer soils [1,4], especially those Table 1 Comparison of leaf nutrient contents of lianas and trees Liana Tree Forest type Reference N (mg g–1) P (mg g–1) LMA (g m–2) N (mg g–1) P (mg g–1) LMA (g m–2) Mountain rainforest 15.3 0.7 94.9 11.7 0.4 170.5 [25] Lowland rainforest 24.1 – 68.2 18.4 – 97.9 [24] Tropical mountain rainforest 24.6 1.24 101 22.8 1.21 115 [32] Tropical seasonal rainforest 28.79 1.58 49.1 22.22 1.12 63.2 [31] Tang Y, et al. Chin Sci Bull February (2012) Vol.57 No.4 309 with high phosphorus concentrations [8,41], may be be- rainforest in Barro Colorado Island, Republic of Panama. cause of the high nutrient content of liana leaf litter and its This dramatic increase has been proposed to be because high rate of decomposition, and not the inherent nutrient- of increasing atmospheric levels of carbon dioxide [14] rich soil per se.