Density-dependent mortality versus spatial segregation in early life stages of Abies densa and Rhododendron hodgsonii in Central Bhutan

Abstract

In order to improve the understanding on the coexistence of rhododendron species and canopy trees in old growth forests with less anthropogenic influence, we studied the role of spatial segregation versus density-dependent mortality of regeneration of Abies densa and Rhododendron hodgsonii in Central Bhutan. The spatial patterns and the microsite preferences of A. densa and R. hodgsonii were studied on 12 permanent sample plots in 1996 and 2001 in Central Bhutan. We characterized survival of the two species on different microsites and tested for deviations of random mortality. Intraspecific spatial correlations were highly positive for individuals of both species in adjacent size classes. Higher positive intraspecific as compared to interspecific correlations suggested spatial segregation of patches of different species and of plants in different size classes. For A. densa seedlings, we detected density-independent mortality as a response to environmental heterogeneity. Spatial distribution of A. densa trees was random or overdispersed showing that in later life stages competition gets stronger. Seedlings of R. hodgsonii showed higher densities than seedlings of A. densa. Density-dependent mortality started at earlier life stages, when density in clumps was high. We found seedlings of both species on all microsites; temporal changes of percentages of seedlings on different microsites showed a similar trend with increases of seedlings on bryophyte mats and decreases on all other microsites. Decreases on litter and nurse logs, however, were much stronger for A. densa than for R. hodgsonii. R. hodgsonii was more frequent on nurse logs and showed better survival on this microsite. Survival of A. densa on nurse logs was low; nurse logs do not play an important role as safe site for A. densa in stands with an understory of R. hodgsonii. Percentages of R. hodgsonii established on litter showed only small decreases and were still higher than its spatial coverage. A. densa had seedlings in a seedling and sapling bank. Survival in a storage stage allows A. densa to outweigh the limitations of narrower regeneration niches and may, in combination with spatial segregation in early life stages, contribute to the coexistence of the two species.

Introduction

Density-dependent processes are important in structuring plant communities. Plants respond to their local neighborhood and effect resources in their close vicinity. Neighborhood processes can influence growth and survival of species positively by facilitation (Callaway et al., 1996, Berkowitz et al., 1995, Kelly et al., 1998) and negatively through competition for resources (Tilman, 1997, Begon et al., 1996), allelopathy (Mallik, 1995), and—indirectly—by specialist herbivores and pathogens (Janzen, 1970, Connell, 1971, Augspurger, 1983, Howe, 1990). Negative neighborhood processes (sensu Stoll and Weiner, 2000) in general and competition in particular lead to more regular spatial patterns over time (Pielou, 1960, Kenkel, 1988, Kenkel, 1988, Duncan, 1991, Moeur, 1997, He and Duncan, 2000).

The presence and intensity of competition depends on the degree of environmental heterogeneity and the response of the plants to this heterogeneity (Lehman and Tilman, 1997, Pacala et al., 1996). Spatial segregation, caused by differential response of plant species to their local environment reduces interspecific competition (Nakashizuka, 2001, Pacala, 1997, Rees et al., 1996). In the early life stages, differential response of plants to different microsites leads to different probabilities of seedling survival, thus qualifying certain sites as regeneration niches (Grubb, 1977) or safe sites (Harper, 1977). Competition and segregation are not mutually exclusive in most ecosystems but interact to varying extents (Brokaw and Busing, 2000). Particularly in plants with large ontogenetic variation in size, e.g. trees, spatial segregation and intra- and interspecific competition may play different roles in different life stages. This is the case with spatial segregation through regeneration niches in early life stages of trees before they increase in size, extend their rooting systems and crowns beyond the spatial extent of the niches and start competing with neighboring trees. Knowing the relative importance of segregation and competition in different life stages helps to understand species coexistence and vegetation dynamics.

Niche partitioning as a mechanism for explaining coexistence was mainly studied in canopy tree species (Szewczyk and Szwagrzyk, 1996, Gray and Spies, 1997, Narukawa and Yamamoto, 2003). Even though the importance of understory species in influencing establishment and early growth and survival of tree species is widely recognized (Coates et al., 1991, Mallik, 1995, Clinton and Vose, 1996, Taylor et al., 1995, Kneeshaw and Bergeron, 1996, Gratzer et al., 1999), few studies characterize mechanisms allowing for coexistence of canopy tree species with long-lived understory species (Manabe and Yamamoto, 1997, Svenning, 2000).

Among these understory species, ericaceous understory plants in general and rhododendron species in particular were shown to compete with, and, in some cases, totally inhibit canopy tree regeneration in temperate forest ecosystems throughout the northern hemisphere (Stainton, 1972, Messier and Kimmins, 1991, Mallik, 1995, Clinton and Vose, 1996, Baker and Van Lear, 1998, Nilsen et al., 1999, Nilsen et al., 2001, Peterken, 2001, Lei et al., 2002). However, most of the evidence stems from forests where anthropogenic influence influenced density and growth of the rhododendron species (in North America: Phillips and Murdy, 1985, Monk et al., 1985, Clinton and Vose, 1996; in the Himalayas: Yoda, 1968, Stainton, 1972, Schmidt-Vogt, 1990, Holzner and Kriechbaum, 1998). Knowledge on the coexistence of rhododendron species and canopy trees in old growth forests with less anthropogenic influence is poor.

R. hodgsonii is an important understory tree species of A. densa forests in the Bhutan Himalayas (Grierson and Long, 1991). Anthropogenic impact on Abies densa forests in the Bhutan Himalayas is low, since commercial use of these forests was limited so far due to restricted access. However, in some areas where logging operations were carried out, severe problems of natural regeneration were reported and attributed to competition with R. hodgsonii (Godi, 1995, Burgi et al., 1992). A better understanding of the mechanisms of coexistence of the two species is a prerequisite of sustainable forest management.

We present the results of a 5-year study on the role of spatial segregation versus density-dependent mortality of regeneration of A. densa and Rhododendron hodgsonii. Our specific objectives were (i) to assess the spatial structure and spatial association of seedlings, saplings and trees, (ii) to test whether mortality in seedlings and saplings were density-dependent or -independent, and (iii) to identify microsite preferences of the two species.