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

Abstract

Lianas are more abundant in seasonal forests than in wetter forests and are thought to perform better than trees when light is abundant and water is limited. We tested the hypothesis that lianas perform better than trees during seasonal drought using a common garden experiment with 12 taxonomically diverse species (six liana and six tree species) in 12 replicated plots. We irrigated six of the plots during the dry season for four years, while the remaining six control plots received only ambient rainfall. In year 5, we measured stem diameters for all individuals and harvested above‐ and belowground biomass for a subset of individuals to quantify absolute growth and biomass allocation to roots, stems, and leaves, as well as total root length and maximum rooting depth. We also measured rate of photosynthesis, intrinsic water use efficiency (iWUE), pre‐dawn and midday water potential, and a set of functional and hydraulic traits. During the peak of the dry season, lianas in control plots had 54% higher predawn leaf water potentials (ΨPD), and 45% higher photosynthetic rates than trees in control plots. By contrast, during the peak of the wet season, these physiological differences between lianas and trees become less pronounced and, in some cases, even disappeared. Trees had higher specific leaf area (SLA) than lianas; however, no other functional trait differed between growth forms. Trees responded to the irrigation treatment with 15% larger diameters and 119% greater biomass than trees in control plots. Liana growth, however, did not respond to irrigation; liana diameter and biomass were similar in control and irrigation plots, suggesting that lianas were far less limited by soil moisture than were trees. Contrary to previous hypotheses, lianas did not have deeper roots than trees; however, lianas had longer roots per stem diameter than did trees. Our results support the hypothesis that lianas perform better and experience less physiological stress than trees during seasonal drought, suggesting clear differences between growth forms in response to altered rainfall regimes. Ultimately, better dry‐season performance may explain why liana abundance peaks in seasonal forests compared to trees, which peak in abundance in less seasonal, wetter forests.

Allometric scaling laws linking biomass and rooting depth vary across ontogeny and functional groups in tropical dry forest lianas and trees

“…Smith‐Martin et al . () found that lianas appeared to explore more soil volume per stem diameter than did co‐occurring trees, yet the present study did not find that lianas had greater root extensions per stem diameter, shoot biomass, or total biomass. Moreover, the measurements herein did not quantify fine root biomass (typically defined as roots < 2 mm diameter), which may play a role in differentiating trees and lianas, if lianas have more fine root mass.…”

“…Whether these patterns hold in other types of tropical forests is not known. The present findings that lianas do not have deeper roots than co‐occurring trees are consistent with harvests of 4‐yr‐old lianas and trees in a common garden in Panama (Smith‐Martin et al ., ). Furthermore, a recent study using indirect methods to assess this question for wet forest in French Guiana showed that lianas do not use deeper sources of water than trees, even during the dry season (De Deurwaerder et al ., ).…”

“…The limitations of the present study are evident: it is difficult to harvest entire rooting systems of mature plants, the study of coarse roots does not allow us to distinguish roots designed for anchoring and support vs those designed for water uptake, and there was a limited sample size from only one location where lianas were deciduous and thus there may not have been the need for exceptionally deep roots. Nevertheless, the data herein do suggest no evidence that lianas have deeper rooting depths than co‐occurring trees in this dry forest, consistent with findings from another common garden study (Smith‐Martin et al ., ), and thus the assumption of deeper liana rooting depths is not supported.…”

“…This increase in lianas had been attributed to their better performance than trees during seasonal drought; part of the explanation for this pattern is the assumption that lianas have deeper roots than co‐occurring tree, and thus can access deeper sources of water. However, based on the present study in Costa Rican dry forest and a previous study in moist forest in Panama (Smith‐Martin et al ., ), direct evidence from harvests is accumulating and is overturning the common assumption that lianas are deeper‐rooted than trees. How lianas maintain similar or better water status during seasonal drought (Smith‐Martin et al ., ) and are able to grow more during the dry season whereas co‐occurring trees grow more during the wet season in semi‐moist forest still remains unknown (Schnitzer & van der Heijden, ).…”

“…In support of the hypothesis that lianas are relatively deeprooted, one study showed that 15-yr-old liana saplings of one species had roots that reached depths of ≤ 10 m; however, that study focused only on one species, and co-occurring tree rooting depths were not examined (Restom & Nepstad, 2004). By contrast, in a common garden experiment with 4yr-old lianas and trees, rooting depths of lianas and trees were similar (Smith-Martin et al, 2019). Additionally, De Deurwaerder et al (2018) found that in a rain forest in French Guiana, lianas used more superficial water than trees during the dry season based on stable isotopes of hydrogen and oxygen.…”