Does soil moisture availability explain liana seedling distribution across a tropical rainfall gradient?

Abstract

Liana density tends to increase with decreasing rainfall and increasing seasonality. However, the pattern of liana distribution may be due to differences in soil water retention capacity, not rainfall and seasonality per se. We tested the effect of rainfall and soil substrate with respect to the distribution of liana seedlings in six sites across a rainfall gradient from the wet Atlantic to the dry Pacific in central Panama. Soils were either limestone, with low water‐holding capacity, or laterite, with higher water‐holding capacity. We sampled liana seedlings at each site using three 1 × 100 m transect. We found that relative liana seedling density was higher on limestone soils compared to laterite soils regardless of the amount of rainfall. Furthermore, liana community composition on limestone soils was more similar to dry forest sites than to adjacent wet and moist forest sites. Liana seedling species diversity relative to trees was significantly higher in a low‐fertility dry forest site compared to a high‐fertility forest, but did not differ from the other sites. Thus, liana seedling density and community structure may be driven more by soil type and thus by soil moisture availability than strictly by mean annual rainfall and the seasonality of rainfall.

Lianas have a seasonal growth advantage over co‐occurring trees

“…, , Manzané‐Pinzón et al. ). By contrast, trees should be favored over lianas in forests that lack a dry season, since trees grow well during rainy periods.…”

“…Further, many liana species grow like tree saplings in the understory (Manzané‐Pinzón et al. ), and thus sapling physiology and growth (e.g., van der Sande et al.) may not accurately represent the liana‐tree comparison for canopy individuals.…”

“…By contrast, the abundance of lianas, a common tropical plant group that is both taxonomically and functionally diverse (Schnitzer and Bongers 2002, Wyka et al 2013, Gianoli 2015, deviates from this common trend. Specifically, the density of lianas in tropical forests, both in absolute terms and relative to trees, increases with the strength of seasonal drought and decreases with increasing mean annual precipitation and the availability of soil moisture (e.g., Schnitzer 2005, DeWalt et al 2010, Manzan e-Pinz on et al 2018.…”

“…The seasonal growth advantage hypothesis has been tested primarily in studies that measured the physiological responses of lianas and trees during wet and dry seasons. These studies reported that lianas generally have better access to water, experience less water stress, have higher levels of photosynthesis, and have greater water-use efficiency and osmotic adjustment than do co-occurring trees during the dry season relative to the wet season (Cai et al 2009, Chen et al 2015, Mar echaux et al 2017. For example, in a common garden study with six replicated tree species and six replicated liana species in central Panama, C. M. Smith-Martin, C. L. Bastos, O. R. Lopez, J. S. Powers, and S. A. Schnitzer, unpublished data found that, compared to trees, lianas had 44% higher predawn leaf water potential, 61% higher intrinsic water-use efficiency, and 28% higher photosynthesis in the dry season compared to the wet season.…”

Effects of dry‐season irrigation on leaf physiology and biomass allocation in tropical lianas and trees

“…The absolute rooting depth of plants may not be the most important predictor of the ability of plants to take up water, and rooting depth likely depends on where the extractable water is located at a given site, which can change considerably with soil type and the geology of the site (Manzané‐Pinzón et al. ).…”