The Influence of Variable Rainfall Frequency on Germination and Early Growth of Shade-Tolerant Dipterocarp Seedlings in Borneo

<div><p>Climate change induced alterations to rainfall patterns have the potential to affect the regeneration dynamics of plant species, especially in historically everwet tropical rainforest. Differential species response to infrequent rainfall may influence seed germination and seedling establishment in turn affecting species distributions. We tested the role of watering frequency intervals (from daily to six-day watering) on the germination and the early growth of Dipterocarpaceae seedlings in Borneo. We used seeds that ranged in size from 500 to 20,000 mg in order to test the role of seed mass in mediating the effects of infrequent watering. With frequent rainfall, germination and seedling development traits bore no relationship to seed mass, but all metrics of seedling growth increased with increasing seed mass. Cumulative germination declined by 39.4% on average for all species when plants were watered at six-day intervals, and days to germination increased by 76.5% on average for all species from daily to six-day intervals. Final height and biomass declined on average in the six-day interval by 16% and 30%, respectively, but the percentage decrease in final size was greater for large-seeded species. Rooting depth per leaf area also significantly declined with seed mass indicating large-seeded species allocate relatively more biomass for leaf production. This difference in allocation provided an establishment advantage to large-seeded species when water was non-limiting but inhibited their growth under infrequent rainfall. The observed reduction in the growth of large-seeded species under infrequent rainfall would likely restrict their establishment in drier microsites associated with coarse sandy soils and ridge tops. In total, these species differences in germination and initial seedling growth indicates a possible niche axis that may help explain both current species distributions and future responses to climate change.</p></div

Relative response to infrequent water as a function of seed mass.

<p>Response to infrequent watering (i.e. the relative difference between daily and six-day watering) as a function of seed mass for cumulative germination, days to germination and seedling height and biomass (mean ±95% CI). No relationship existed between cumulative germination and seed mass. Germination of large seeds was more resistant to infrequent watering, but after germination large-seeded species had significantly greater declines in growth due to infrequent watering. Points represent mean values for each species. (SM: open circle, PT: upward triangle, DL: closed square, PM: downward triangle, HN: open diamond, SB: closed triangle, SP: closed diamond, SA: closed circle).</p

Relationship between growth and seed mass in daily watering.

<p>The relationship of baseline biomass and leaf area as a function of seed mass in the daily watering treatment (mean ±95% CI). Unequal variance was accounted for using a weighted variance for each species. Leaf area was based on the last harvest after 60 days. Points represent individual seedlings. (SM: open circle, PT: upward triangle, DL: closed square, PM: downward triangle, HN: open diamond, SB: closed triangle, SP: closed diamond, SA: closed circle).</p

Soil C depth profile.

<p>Measured soil organic C (SOC) and bulk density with depth for the Sabah Biodiversity Experiment (Saner 2009). Overlayed points are soil organic C typical of the Kretam soil association (SFD 2008), which is found at Malua Forest Reserve. The line is a smoothed loess curve of mean SOC (±95% CI).</p

Comparison of basal area, volume and selected stocks and fluxes related to C turnover processes.

<p>Means ± SEM from selectively logged forest (Sabah Biodiversity Experiment) and unlogged forest sites (Danum Valley, except for soil respiration reported from Lambir Hills, Sarawak, North Borneo). Difference (± SED) is reported with the corresponding t-value and level of significance. SDW: Standing deadwood, TAGC: Total aboveground carbon.</p>a<p>Gale 2000.</p>b<p>Green <i>et al.</i> 2005.</p>c<p>Burghouts <i>et al.</i> 1992.</p>d<p>Katayama <i>et al.</i> 2009.</p

Total aboveground C stocks (TAGC) and basal area in selectively logged an unlogged forest.

<p>All species (square), dipterocarps (circle) and pioneer species only (triangle) (mean ± SEM; n = 4). Decreases in dipterocarp basal area but not in carbon stocks were compensated by the pioneers.</p

Pre-enrichment planting C balance for the Sabah Biodiversity Experiment.

<p>The area is located in selectively logged forest of the Malua Forest Reserve. All components are given as mean ± SEM Mg C ha⁻¹, except for C fluxes (*), which are reported as Mg C ha⁻¹ yr⁻¹. Values in parenthesis indicate the percentage of single C stocks to total organic C (167.9 Mg C ha⁻¹).</p

Summary table of seed traits of dipterocarp species ordered from largest to smallest seed mass.

a<p>Descriptions and germination range taken from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070287#pone.0070287-Ng1" target="_blank">[30]</a> and recalcitrance taken from KEW database.</p

Map of Sabah and forest quality map of Malua Forest Reserve.

<p>(A) Allocated production forest is shown across Sabah (light green, also including Malua Forest Reserve). The study sites (marked in red) are located in the Malua Forest Reserve and the Danum Valley Conservation Area (dark green). (B) Forest quality map of the Malua Forest Reserve (visual interpretation of aerial photographs, 1∶17,500). The boundary of the Sabah Biodiversity Experiment (500 ha) is outlined (red). Forest classification is based on number of trees ≥60 cm DBH derived from crown size. Cloud forest: >16 trees ha⁻¹, Moderate: 9–16 trees ha⁻¹, Poor: 5–8 trees ha⁻¹, Very Poor: 1–4 trees ha⁻¹, Shrubs/Bare Land/Grassland: none, Plantation: Oil palm monoculture.</p

Local rainfall at Malua forest during the experiment.

<p>Daily rainfall over the period of the experiment. Total rainfall was approximately 724 mm over the course of 90 days with 44 near rainless days (<1 mm day⁻¹).</p