Limits...
Do differences in understory light contribute to species distributions along a tropical rainfall gradient?

Brenes-Arguedas T, Roddy AB, Coley PD, Kursar TA - Oecologia (2010)

Bottom Line: Within sites, the effect of understory light availability on species performance depended strongly on water availability.Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment.The slower growth rates of the dry-distribution species, possibly due to trade-offs associated with greater drought tolerance, may exclude these species from wetter forests.

View Article: PubMed Central - PubMed

Affiliation: Smithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama.

ABSTRACT
In tropical forests, regional differences in annual rainfall correlate with differences in plant species composition. Although water availability is clearly one factor determining species distribution, other environmental variables that covary with rainfall may contribute to distributions. One such variable is light availability in the understory, which decreases towards wetter forests due to differences in canopy density and phenology. We established common garden experiments in three sites along a rainfall gradient across the Isthmus of Panama in order to measure the differences in understory light availability, and to evaluate their influence on the performance of 24 shade-tolerant species with contrasting distributions. Within sites, the effect of understory light availability on species performance depended strongly on water availability. When water was not limiting, either naturally in the wetter site or through water supplementation in drier sites, seedling performance improved at higher light. In contrast, when water was limiting at the drier sites, seedling performance was reduced at higher light, presumably due to an increase in water stress that affected mostly wet-distribution species. Although wetter forest understories were on average darker, wet-distribution species were not more shade-tolerant than dry-distribution species. Instead, wet-distribution species had higher absolute growth rates and, when water was not limiting, were better able to take advantage of small increases in light than dry-distribution species. Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment. The slower growth rates of the dry-distribution species, possibly due to trade-offs associated with greater drought tolerance, may exclude these species from wetter forests.

Show MeSH

Related in: MedlinePlus

The effect of water addition on the light response of seedlings planted in the dry site. Lines represent the mean response for all species combined: a new leaf production (NewLfPr), b net leaf growth (NetLfGr), c stem height growth (StmHtGr) and d mortality. The mean response was determined using mixed-effects models (linear for growth and logistic for mortality) for uncaged seedlings only. The length of the line represents the range of light levels for each set of plots. In a and b, the line at zero is for reference. The P values in the panels represent the significance of the main effect of watering (W) on seedling performance and the water × light interaction (W × L). See Table 2 for relevant statistics
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC3094538&req=5

Fig3: The effect of water addition on the light response of seedlings planted in the dry site. Lines represent the mean response for all species combined: a new leaf production (NewLfPr), b net leaf growth (NetLfGr), c stem height growth (StmHtGr) and d mortality. The mean response was determined using mixed-effects models (linear for growth and logistic for mortality) for uncaged seedlings only. The length of the line represents the range of light levels for each set of plots. In a and b, the line at zero is for reference. The P values in the panels represent the significance of the main effect of watering (W) on seedling performance and the water × light interaction (W × L). See Table 2 for relevant statistics

Mentions: When water was a limiting resource, such as in the control plots at the dry site, higher light had a negative effect on seedling growth and mortality (Table 2d; Fig. 3). This was most striking with respect to mortality, because in the absence of water supplementation both dry- and wet-distribution species had higher mortality at higher light, although this was significant only for the latter (Table 2d). A 1 mol m−2 day−1 increase in PPFU increased seedling mortality by 40% for dry-distribution species, nearly fivefold for wet-distribution species, and nearly three-fold for all species combined (Table 2d). For all species combined there was a significant light × water treatment interaction with respect to mortality (Fig. 3). With respect to the leaf growth variables, performance also decreased with increasing light, but these responses were significantly negative only when wet-distribution species were analyzed separately (Table 2d). While, on average, the dry-distribution species did not have a negative response to increasing light, their light responses were less steep when water was limiting (Table 2c, d), and the light × water treatment interactions of all species combined were significant for all growth variables (Fig. 3).Fig. 3


Do differences in understory light contribute to species distributions along a tropical rainfall gradient?

Brenes-Arguedas T, Roddy AB, Coley PD, Kursar TA - Oecologia (2010)

The effect of water addition on the light response of seedlings planted in the dry site. Lines represent the mean response for all species combined: a new leaf production (NewLfPr), b net leaf growth (NetLfGr), c stem height growth (StmHtGr) and d mortality. The mean response was determined using mixed-effects models (linear for growth and logistic for mortality) for uncaged seedlings only. The length of the line represents the range of light levels for each set of plots. In a and b, the line at zero is for reference. The P values in the panels represent the significance of the main effect of watering (W) on seedling performance and the water × light interaction (W × L). See Table 2 for relevant statistics
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3094538&req=5

Fig3: The effect of water addition on the light response of seedlings planted in the dry site. Lines represent the mean response for all species combined: a new leaf production (NewLfPr), b net leaf growth (NetLfGr), c stem height growth (StmHtGr) and d mortality. The mean response was determined using mixed-effects models (linear for growth and logistic for mortality) for uncaged seedlings only. The length of the line represents the range of light levels for each set of plots. In a and b, the line at zero is for reference. The P values in the panels represent the significance of the main effect of watering (W) on seedling performance and the water × light interaction (W × L). See Table 2 for relevant statistics
Mentions: When water was a limiting resource, such as in the control plots at the dry site, higher light had a negative effect on seedling growth and mortality (Table 2d; Fig. 3). This was most striking with respect to mortality, because in the absence of water supplementation both dry- and wet-distribution species had higher mortality at higher light, although this was significant only for the latter (Table 2d). A 1 mol m−2 day−1 increase in PPFU increased seedling mortality by 40% for dry-distribution species, nearly fivefold for wet-distribution species, and nearly three-fold for all species combined (Table 2d). For all species combined there was a significant light × water treatment interaction with respect to mortality (Fig. 3). With respect to the leaf growth variables, performance also decreased with increasing light, but these responses were significantly negative only when wet-distribution species were analyzed separately (Table 2d). While, on average, the dry-distribution species did not have a negative response to increasing light, their light responses were less steep when water was limiting (Table 2c, d), and the light × water treatment interactions of all species combined were significant for all growth variables (Fig. 3).Fig. 3

Bottom Line: Within sites, the effect of understory light availability on species performance depended strongly on water availability.Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment.The slower growth rates of the dry-distribution species, possibly due to trade-offs associated with greater drought tolerance, may exclude these species from wetter forests.

View Article: PubMed Central - PubMed

Affiliation: Smithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama.

ABSTRACT
In tropical forests, regional differences in annual rainfall correlate with differences in plant species composition. Although water availability is clearly one factor determining species distribution, other environmental variables that covary with rainfall may contribute to distributions. One such variable is light availability in the understory, which decreases towards wetter forests due to differences in canopy density and phenology. We established common garden experiments in three sites along a rainfall gradient across the Isthmus of Panama in order to measure the differences in understory light availability, and to evaluate their influence on the performance of 24 shade-tolerant species with contrasting distributions. Within sites, the effect of understory light availability on species performance depended strongly on water availability. When water was not limiting, either naturally in the wetter site or through water supplementation in drier sites, seedling performance improved at higher light. In contrast, when water was limiting at the drier sites, seedling performance was reduced at higher light, presumably due to an increase in water stress that affected mostly wet-distribution species. Although wetter forest understories were on average darker, wet-distribution species were not more shade-tolerant than dry-distribution species. Instead, wet-distribution species had higher absolute growth rates and, when water was not limiting, were better able to take advantage of small increases in light than dry-distribution species. Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment. The slower growth rates of the dry-distribution species, possibly due to trade-offs associated with greater drought tolerance, may exclude these species from wetter forests.

Show MeSH
Related in: MedlinePlus