Biotic and Abiotic Controls on Tree Colonization in Three Early Successional Communities of Chiloe´Island, Chile
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Journal of Ecology 2011, 99, 288–299 doi: 10.1111/j.1365-2745.2010.01737.x Biotic and abiotic controls on tree colonization in three early successional communities of Chiloe´Island, Chile Marcela A. Bustamante-Sa´nchez1,2*, Juan J. Armesto1,2 and Charles B. Halpern3 1Departamento de Ecologı´a, Center for Advanced Studies in Ecology and Biodiversity (CASEB), Pontificia Universidad Cato´lica de Chile, Casilla 114-D, Santiago, Chile; 2Institute of Ecology and Biodiversity (IEB), Casilla 653, Santiago, Chile; and 3School of Forest Resources, University of Washington, Seattle, WA 98195-2100, USA Summary 1. Most studies of tree regeneration are limited to particular environments and may not capture variation in the biotic or abiotic factors that regulate recruitment at larger spatial scales. Critical processes such as competition and facilitation can vary spatially, along gradients in resource avail- ability and environmental stress, and temporally, with plant development. 2. We examined patterns of natural tree recruitment and experimentally followed germination and seedling survival of five tree species (pioneer to late seral) in three early successional communities of contrasting bio-physical environments in a rural landscape on Chiloe´Island, Chile. 3. We quantified natural recruitment of juveniles and saplings and assessed relationships between tree density and local environment. We used a removal experiment to test the influence of early suc- cessional vegetation on seed germination and early survival of tree species. In each community, seeds and seedlings were placed in paired experimental plots from which vegetation was removed or left intact (control). To identify potential correlates of germination and seedling survival, we mea- sured light transmittance and soil properties in each plot. 4. In all communities, established vegetation had either a positive or neutral effect on germination and ⁄or survival although responses varied among life stages and species. Germination and survival were correlated with the lower levels of light in controls, consistent with negative correlations between natural tree densities and light. Vegetation cover was not dense enough to facilitate survival of late successional species, but not too dense to inhibit survival of shade-intolerant or mid-tolerant species. Among communities, natural densities of juveniles were greatest under conditions where experimental germination rates were highest. Seedling height growth was lowest in the community characterized by waterlogged soils, consistent with the naturally low transition rate from juveniles to saplings and a negative correlation between density of shade-intolerant trees and soil moisture. 5. Synthesis. Our experiments indicate strong, mostly positive controls (facilitation) on tree recruit- ment in early seral shrublands with differing bio-physical environments. Benefits of shading are manifested at different stages in the life history. However, community context is critical: variation in seasonal patterns of soil moisture may explain spatial variation in the density and size structure of natural tree recruitment. Key-words: community assembly, ecological filters, facilitation, plant–plant interactions southern temperate rain forest, species’ interactions, tree regeneration, vegetation heterogeneity the regional species pool (Weiher & Keddy 1995; Dı´az, Cabido Introduction & Casanoves 1998). These filters can vary in strength in time Plant community assembly following disturbance is regulated and space, selecting individuals from among the species and by abiotic and biotic filters that select potential colonists from life stages upon which they operate (Armesto & Pickett 1986; Walker & Chapin 1987; De Steven 1991a; b; Gill & Marks *Correspondence author. E-mail: [email protected] 1991). Abiotic filters that affect seedling establishment include Ó 2010 The Authors. Journal of Ecology Ó 2010 British Ecological Society Controls on tree colonization in shrublands 289 well-known resource and non-resource constraints such as winteri J.R. et G. Foster, Winteraceae) on elevated surfaces, light, soil moisture, temperature and substrate availability. such as woody detritus (Aravena et al. 2002). Our objectives Biotic filters include competition and facilitation, the pro- are threefold: (i) to quantify patterns of natural tree recruit- cesses by which plants interact for resources or modify their ment and environmental variation among three floristically environments in ways that reduce or enhance germination, and structurally distinct early successional communities; (ii) to establishment and growth (Connell & Slatyer 1977; Callaway determine whether early seral vegetation inhibits or promotes & Walker 1997). The relative importance of these processes seed germination and early survival of trees, and whether these can vary spatially with resource availability and environmental effects vary among communities and ⁄ or tree species with dif- stress (Pugnaire & Luque 2001; Tewksbury & Lloyd 2001; fering life histories and (iii) to isolate the abiotic factors (e.g. Callaway et al. 2002; Kuijper, Nijhoff & Bakker 2004) or light, soil moisture and nutrient availability) that could explain temporally with succession or plant ontogeny (developmental variation in patterns of germination and early survival. For stage; Holmgren, Scheffer & Huston 1997; Miriti 2006; shade-intolerant (pioneering) tree species, we hypothesized a Schiffers & Tielborger 2006). How these processes contribute shift from neutral effects in open shrublands (Berberis commu- to natural reassembly of plant communities, or to restoration nity), to inhibitory effects in denser shrublands with lower light of damaged or degraded systems, requires an understanding of availability (Baccharis community). In contrast, for shade-tol- the physical and biotic context in which species interact, and erant (late-seral) species, we hypothesized facilitative effects of the nature, strength and timing of these interactions (Clark increasing strength from more open to more closed communi- et al. 1999; Garcı´a, Obeso & Martı´nez 2005; Go´mez-Aparicio, ties (Berberis to Baccharis), reflecting the beneficial effects of Go´mez & Zamora 2005; Brooker et al. 2008). Despite the shading (reduced light and temperature stress). Finally, we importance of context dependency for many ecological pro- hypothesized that for all species, rates of germination and cesses (Jones & Callaway 2007), relatively few studies, mainly seedling survival would be lowest where soils are anoxic and in the northern hemisphere and tropics, have considered the the potential for fungal infectionishighduetoseasonalwater- role of community context in regulating the establishment of logging of soils (Baccharis community; Piper et al. 2008). woody plants during early succession (Berkowitz, Canham & Kelly 1995; Burton & Bazzaz 1995; Go´mez-Aparicio et al. Materials and methods 2004; Benitez-Malvido 2006; Acacio et al. 2007). Community context may be critical to this process because variation in STUDY AREA environment and disturbance, combined with the stochastic nature of dispersal, can lead to significant heterogeneity in The study was conducted within the rural landscape surrounding the Senda Darwin Biological Station (SDBS), which lies 15 km northeast community structure (Halpern 1988; Traveset et al. 2003; of Ancud, northern Chiloe´Island (42° S; Fig. 1). The vegetation in Zavaleta, Hulvey & Fulfrost 2007). this region is a mosaic of remnant fragments of Valdivian and North- The rural landscape over much of southern Chile, and in Patagonian evergreen forest (Veblen et al. 1996), grazed pastures and other regions of South America, is a mosaic of vegetation small agricultural fields (Willson & Armesto 1996). The climate is patches of varying size, human influence and successional stage wet-temperate with a strong oceanic influence (Di Castri & Hajek (Echeverrı´a et al. 2007). In the Lake District, and on Chiloe´ 1976). Mean annual precipitation averages 2000–2500 mm; 20% falls Island (39–42° S), widespread logging and farming during the between December and March, during the growing season (austral 20th century (Lara, Donoso & Aravena 1996), resulted in con- version of native evergreen forests to pastures, croplands and seral shrublands. Recolonization of these shrublands by trees Puerto has been slow and highly variable, spurring interest in the fac- Montt tors that limit establishment and growth (Aravena et al. 2002; Dı´az & Armesto 2007; Dı´az, Bigelow & Armesto 2007). Here, we experimentally test how early seral communities of differing 42°S Ancud Senda Darwin composition, structure and physical environment influence the Biological Station germination and ⁄ or survival of native tree species with Argentina differing life histories and successional roles (shade tolerance). This is the first study in a temperate ecosystem of the southern Castro hemisphere to compare controls on tree establishment among Pacific Ocean seral scrubland communities with differing bio-physical envi- ronments. These include: (i) open shrubland (<25% cover) 43°S dominated by Berberis buxifolia Lam. (Berberidaceae); (ii) denser shrubland (c. 50% cover) with seasonally waterlogged soils, dominated by Baccharis patagonica (H. et A.) Chile (Asteraceae), (Dı´az & Armesto 2007; Dı´az, Bigelow & 0 25 50 km Armesto 2007) and (iii) moderately dense shrubland (c.30% 74°W 73°W 72°W cover) also dominated by B. patagonica, with significant estab- lishment of small trees (mainly 20- to 30-year-old