Spatial patterns of Avicennia germinans during natural recolonization: inferences on the importance of intraspecific facilitation

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1 TOPIC Spatial patterns of Avicennia germinans during natural recolonization: inferences on the importance of intraspecific facilitation Aor Pranchai 1,2*, Juliane Vogt 1, Yue Lin 3 and Uta Berger 1 1 Institute of Forest Growth and Computer Science, Dresden University of Technology, Tharandt, Germany; 2 Department of Silviculture, Faculty of Forestry, Kasetsart Univeristy, Bangkok, Thailand; 3 Department of Ecological Modelling, UFZ Helmholtz Centre for Environmental Research, Leipzig, Germany

2 INTRODUCTION Interspecific plant facilitation Many studies of plant communities in deserts and shrublands provides evidence that plant facilitate each other under harsh conditions It has been suggested that plant facilitation can be used for rehabilitation of degraded sites Maybe facilitation can even be used for the rehabilitation of degraded mangroves However, most facilitation studies are looking at interspecific interaction among different plant species

3 KNOWLEDGE GAP Intraspecific plant interaction In mangroves the number of different tree species is very low Especially, under harsh conditions, usually only one species is able to grow and survive That means that we cannot use restoration techniques based on current facilitation research directly So, to understand how we can use facilitation in mangrove restoration, We have to find out if mangrove trees of the same species facilitate each other.

4 OBJECTIVES To detect plant facilitation in recolonizing mangrove stands 1. We analyzed under if small trees of a same size reduce the environmental stress on their neighbors 2. To find out if larger mangrove trees have a nurse plant affect on small surrounding trees

5 STUDY SITE Bragança peninsula In the past, the study site was covered by mangroves until the construction of highway in 1974.

6 STUDY SITE Recolonized by A. germinans The road disturbed the water regime by interrupting the tidal flow As a result, large mangrove areas died Nowadays, this degraded area is recolonized by A. germinans The environmental condition are very harsh The mangrove trees are affected by high salinity and seasonal droughts, which cause crown diebacks Because of these conditions, A. germinans trees grow more like a shrub than a tree

7 FIELD MEASUREMENTS Early recolonization 2 stands in two different stages of successional development In each stand 3 plots with a size of 20 m x 20 m were set up Late recolonization x-y position of all trees and seedlings was mapped during the rainy season, 2011 diameter at ground level height crown diameter

8 g(r) ANALYSIS Spatial statistic Pair correlation function R-package Spatstat (Baddeley & Turner 2005) K'( r) g( r) Univariate analysis 2 r (allowed us to find out at what distance small trees cluster) Distance (cm)

9 g 12 (r) ANALYSIS Spatial statistic Bivariate analysis g 12 K12'( r) ( r) 2 r (we could find out if small trees aggregate around larger ones) Monte Carlo Simulations (to assess the departure from the null hypothesis complete randomness) Distance (cm) Index of association (IA) (the distance from the envelope to assess the strength of association, this allowed us to compare multiple graphs) (Fajardo, Goodburn & Graham 2006) IA( r) g( r) 1 CE 1

10 RESULTS Plant density Seedling Adult plants The early recolonization stand had harsher environmental conditions than the late recolonization stand We did not measure the environmental conditions directly but concluded by looking at the different forest structure In early recolonization stand, there were less seedlings than adult plants This changed with increasing density and crown area That means that with higher tree density and crown cover more seedlings could establish

11 K/d RESULTS Crown dieback (Crown diameter/collar diameter) (Hemery, Savill, and Pryor 2005) Stressed Early Late recolonization stage Vigorous K/d as an indicator of crown dieback (The lower the ratio is, the more crown dieback happened) Trees in the early recolonization stand suffered more crown dieback than in the later stage To study the effect of neighboring plants on each others vitality, we separated stressed and vigorous trees (We classified in each plot 50 percent of the trees with a lower K/d ratio as stressed, and the remaining with a higher K/d ratio as vigorous) We also separated 20% of the smallest trees from the other 80% which are larger trees

12 IA(r) IA(r) RESULTS Both small stressed nor vigorous tress are randomly distributed Only the seedlings are clustered Univariate vigorous trees are more densely clustered than stressed plants Early recolonization Late recolonization The neighboring small trees facilitate each other by improving the harsh conditions more than they compete for resources Distance (cm) Distance (cm) If smaller trees do not have a close neighbor they are fully exposed to the harsh conditions and experienced more crown dieback.

13 Crown radius (cm) RESULTS IA(r) Bivariate Seedlings Early recolonization stage Stressed trees Vigorous trees Distance (cm) Our question was: Are vigorous trees more aggregated around large trees than stressed trees?, which would indicate a nurse plant effect by the larger trees Under harsh conditions there is no nurse plant effect

14 RESULTS Crown radius (cm) IA(r) Bivariate Vigorous trees Stressed trees Late recolonization stage Seedlings When the conditions improve vigorous trees are much more aggregated around larger trees than stressed trees Distance (cm) Clearly larger trees act as nurse plants under more benign condition Amelioration Vigorous trees were aggregated under the crown edge of larger trees, not very close to their stem The aggregation of the stressed trees is aligned with the seedling distribution (which implies that the aggregation of stressed trees in a result of seed dispersal and not facilitation) We found more vigorous than stressed trees under the crown of a larger tree (this means that larger tree ameliorate the harsh environmental conditions and improve the fitness of smaller tree)

15 CONCLUSIONS Intraspecific facilitation in recolonizing mangrove forest Mutualistic interactions in both stand, because small vigorous trees were clustered Nurse plant effect of larger trees in late recolonization Plants grow better under the tree crown edges where the trade-off between environmental amelioration and light radiation is optimal

16 RECOMMENDATIONS Facilitation should be considered to improve mangrove rehabilitation Restoration on degraded area are difficult (Samson & Rollon 2008) High seedling mortality (Matsui et al. 2010) Extreme environmental condition (salinity, temperature, radiation, low water availability) (Milbrandt & Tinsley 2006) This new approach in mangroves might increase the survival rate of planted seedlings

17 RECOMMENDATIONS For the application of nurse plants in the restoration of degraded mangrove forests more information about the size and shape of nurse plants is necessary Experimental studies are necessary to investigate how nurse plants affect beneficiaries in mangrove communities

18 ACKNOWLEDGEMENTS CREC project (EU IRSES # ) Mr. Peter Frohberg, Dr. Moirah Menezes and Dr. Ulf Mehlig Department of Silviculture, Faculty of Forestry, Kasetsart University, Thailand Center for Advanced Studies in Tropical Natural Resources, Faculty of Forestry, Kasetsart University Faculty of Foresty

19 THANK YOU FOR YOUR ATTENTION